Radiation Use Efficiency

RADMTIONUSEEFFICIENCY Thomas R. Sinclair' and Russell C. Muchow2 'USDA-ARS Agronomy Physiology & Genetics Laboratory University of Florida Gainesville, Florida 3261 1-0965 *CSIRO, Tropical Agriculture Cunningham Laboratory Brisbane, Qld. 4067, Australia

I. Introduction A. Time-Based Growth Analysis B. Light-Based Growth Analysis C. Terminology 11. Theoretical Analyses of RUE A. Initial Analysis of Crop Productivity B. Leaf Photosynthetic Rates and RUE C. Radiation Environment and RUE D. Conclusions from Theoretical Analyses 111. Experimental Determination of RUE A. Determination of Crop Mass B. Determination of Solar Energy C. Calculation of RUE IV Experimental Measures of RUE A. C+Species B. C , Species V. Sources of Variability in RUE A. Species B. CO, Assimilation Rate C. Seasonal Variation D. Radiation Environment VI. Conclusions References

I. INTRODUCTION Light levels have a profound influence on plant growth, as readily observed in plants in the shady areas of a garden compared with plants in areas of full sunlight. Somewhat surprisingly then, only in recent times has considerable attention been 215 Adwmmr in Armnomy, Vuhme 65

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THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

given to investigating the quantitative relationship between crop growth and light levels. The influence of light levels on crop photosynthesis and mass accumulation was not considered explicitly until the late 1950s (DeWit, 1959). Prior to this time, the focus on classical crop growth analysis seems to have inhibited a more mechanistic appraisal of crop mass accumulation.

A. TIME-BASEDGROWTH ANALYSIS Early investigators sought to analyze crop growth as a function of time (Blackman, 1919), although it was clear that growth is not dependent on time at all. Consequently, the accepted and popular approach of classical crop growth analysis was to describe growth by the change in crop mass (m) as a function of time (f). Crop growth rate (dm/df) could be readily estimated from successive harvests through the growing season. Crop growth rates were calculated and compared for various crops and locations in spite of the fact that these results were confounded by varying amounts of light intercepted by the growing crop as a result of differences in incident light or changing crop leaf area. The misleading approach of expressing crop growth as a function of time was muddled further in crop growth analysis by calculating the derived variables of relative growth rate and net assimilation rate. Relative growth rate (RGR), as defined by Watson (1952) in the following equation, was a more generalized expression of the efficiency index proposed by Blackman (1919): RGR = l/m dm/dt. The RGR equation required the additional assumption that crop mass increased as an inverse function of current crop mass. This type of expression is used to describe, for example, the growth of microbes in an environment of unlimited resources. Such an assumption in the analysis of crop growth requires, of course, that there is a constant relationship between plant mass and leaf area. Additionally, it is implicit that there is no self-shading among leaves. Clearly, the RGR assumption can only be approximated during the early stages of plant development. Blackman (1919) recognized for individual plants that RGR declines as plant mass increases. According to Watson (1952), in 1917 Gregory, recognizing light interception as critical in plant growth, developed net assimilation rate (NAR) as an approach to incorporate leaf area (L) explicitly in crop growth analysis: NAR

=

l/L dm/dt.

In addition to the problem of assuming that growth is a function of time, this equation failed to consider the uneven pattern of light interception by leaves in a crop canopy. The NAR calculation assumed a uniform light distribution over leaves.

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This assumption grows worse as shading in the leaf canopy increases. In spite of the potential for instability resulting from the assumption of uniform light distribution, fairly stable values of NAR were obtained in much of the early research using this approach (Watson, 1952). The basis for this stability is likely traced to the fact that much of the data was from situations where leaf area index was low for much of the season and the amount of shading was not great. In modern crops with high plant densities and rapid development of leaf area, there is a high degree of leaf shading. Calculations of NAR do not offer much mechanistic insight about the development of crop mass and yield.

B. LIGHT-BASEDGROWTH ANALYSIS Some of the first, cogent analyses of increases in crop mass in response to the amount of light available to a crop were done by DeWit (1959) and by Loomis and Williams (1963). DeWit (1959) assumed a constant efficiency of light use at the leaf level up to a saturating light value and examined the consequences of light distribution in a leaf canopy. Canopy photosynthetic capability was calculated to be proportional to the amount of solar radiation incident to the canopy. Loomis and Williams (1963) advanced this mechanistic perspective by considering the quantum nature of light and by attempting to express the efficiency in the use of light in terms of accumulated plant mass. Assuming a maximum value for leaf quantum efficiency of 10 quanta per CO, fixed, they calculated that the limit for crop efficiency was 3.34 g CH,O MJ-' solar radiation. Early experimental data also confirmed a close linkage between the amount of light intercepted by a crop and its growth. Shibles and Weber (1965) found a linear relationship between dry matter increase and the fraction of radiation intercepted through the entire growing season for soybean. Williams et al. (1965) found for maize that 1.71 g of plant mass was produced for each MJ of intercepted solar radiation. During the 1970s, however, several studies analyzed efficiency on the basis of incident radiation rather than intercepted radiation. These estimates were used by ecologists and agronomists to compare plant productivity under different systems of land use and management and in different climates. For example, Cooper (1970) estimated an efficiency in the use of incident radiation by assuming that 45% of the total incoming radiation is in the visible spectrum and available for photosynthesis, and that the production of 1 g of dry matter corresponds to the fixation of 4250 calories of chemical energy. He calculated that the average efficiency in the use of incident radiation for pasture production of up to 3% was possible over the whole year with up to 5% during the most productive part of the growing season. Cooper also estimated annual dry matter production potential in contrasting climatic regions based on differences in annual energy input. Analysis based on in-

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cident radiation was also done by Austin et al. (1978), who found that sugarbeet was less efficient than sngarcane. Jong et al. (1982) showed that grain yield of maize in Hawaii was linearly related to incident radiation and this accounted for 78.5% of the yield variation. The limitation in relating productivity to incoming solar radiation is that only a proportion of the incident radiation is intercepted by plants during their life cycle and available for photosynthesis (Squire, 1990). Differences in leaf canopy development among crops and climates can confound comparison of efficiency based only on incident radiation. In fact, Monteith (1972) commented that classifying ecosystems on the basis of efficiency in relation to incident radiation is becoming a popular form of taxonomy but it contributes little to our understanding of how plants respond to their environment. His analysis went a stage further, relating efficiencies of dry matter production to the physical and biological factors that determine growth rates such as the fraction of radiation intercepted by a leaf canopy, the irradiance of individual leaves, the diffusion resistance of stomata, and the behavior of the photochemical system. Monteith (1972) concluded that the interception efficiency, defined as the ratio of the actual rate of gross photosynthesis to the maximum rate estimated for a stand of identical plants with enough leaves to intercept all the incident light, emerges as a major discriminate of dry matter production. This efficiency on the one hand accounts for differences in productivity under different conditions of climate and management and, on the other, for differences between the mean and maximum rates of production within a particular stand. In 1977 John Monteith published a paper that fully established both experimental and theoretical grounds for the relationship between accumulated crop dry matter and intercepted solar radiation. He concluded from a composite of experimental results obtained under good growth conditions that for most crops approximately 1.4 g of crop mass was accumulated per MJ of intercepted solar radiation. Monteith also presented a theoretical curve for the response of radiation use efficiency (RUE) to changes in maximum rate of leaf photosynthesis. While RUE was highly sensitive to maximum leaf photosynthesis rates at low rates, the sensitivity of RUE to photosynthetic rates was much less in the usual range of leaf photosynthetic rates for nonstressed crops. Monteith’s 1977 paper was particularly important in pointing to RUE as a robust and theoretically appropriate approach for describing crop growth. Subsequent to Monteith’s paper, a number of studies incorporated estimates of radiation interception so that RUE could be calculated. Several papers summarized RUE estimates obtained under a range of conditions (Gallagher and Biscoe, 1978; Gosse et al., 1986; Kiniry et al., 1989). Gallagher and Biscoe (1978) compared the growth of cereals under a range of conditions at two locations in the United Kingdom. Unfortunately, the amount of intercepted radiation was not measured but estimated based on crop leaf area. Nevertheless, RUE was essentially stable and

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equivalent for wheat and barley when grown under good conditions. They reported that the RUE value was about 3 g MJ-’ of photosynthetically active radiation (PAR; or equivalently, 1.35 g MJ- intercepted solar radiation as discussed in Section 111). Unfertilized and drought-stressed conditions resulted in a decrease in RUE. In a comparison among various crop species, Gosse et al. (1986) concluded that, in fact, important differences in RUE existed among species. They found that C, species had the highest RUE, followed by nonleguminous species, and leguminous species had the lowest. Variation among species was also reported by Kiniry et al. (1989) in their comparison of five crop species. In their study also, RUE was generally calculated from estimates of light interception based on leaf area. Maize had the highest RUE at 3.5 g MJ-’ of intercepted PAR (or 1.75 g MJ-’ intercepted solar radiation), and sorghum had only 2.8 g MJ- (or 1.4 g MJ- solar radiation) intercepted PAR. The estimates of RUE for sunflower, rice, and wheat were 2.2, 2.2, and 2.8 g MJ- intercepted PAR (or 1.1, 1.1, and 1.4 g MJ- solar radiation), respectively.

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C. TERMINOLOGY “Conversion efficiency” has sometimes been used as the preferred terminology instead of radiation use efficiency (Horie et al., 1997). The concept of solar energy conversion arose from thermodynamic considerations where ecosystems are likened to machines supplied with energy from an external source, that is, solar energy. The available energy input in any environment is determined by the seasonal distribution of solar radiation, and provided that water and nutrients are not limiting, this sets the ultimate limit to productivity (Cooper, 1970; Loomis et al., 1971; Monteith, 1972).Dividing the useful energy of a thermodynamic process by the total energy involved gives a figure for the efficiency of the process and this procedure has been used to analyze the flow of energy in ecosystems, particularly for pastures that are grown primarily as a source of digestible energy and other nutrients for the ruminant. This approach was used with grain crops in evaluating performance in terms of intercepted radiation (Allen and Scott, 1980; Muchow and Coates, 1986; Marshall and Willey, 1983). The terminology of “radiation conversion efficiency,” however, is inappropriate when measuring plant mass accumulated per unit of radiation because it implies that there is a process of direct conversion of radiant energy to mass. This is, of course, an erroneous view because photosynthesis and plant mass accumulation involve no direct conversion of energy to plant mass. At the energy levels of solar radiation, it is not even possible to directly condense radiant energy to atoms such as carbon. The actual photosyntheticprocess involves the absorption of radiant energy by pigments resulting in the establishment of new energy levels in these mol-

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ecules. The higher energy states of these pigments are then used to assimilate carbon dioxide and synthesize plant constituents. Hence, crop mass accumulation relative to light levels is appropriately referred to as radiation use efficiency.

II. THEORETICAL ANWYSES OF RUE Before examining in detail the various experimental measures of RUE, those studies that examined theoretically the nature of RUE will be reviewed. These theoretical studies help to give a background and framework for evaluating potential sources of variation in experimental measures of RUE. Specifically, theoretical studies help to identify variations in the environment and crops that might influence RUE.

A. INITIALANALYSISOF CROPPRODUCTMTY As discussed previously, one of the first attempts to estimate theoretically crop productivity based on solar radiation incident to a crop was published by Loomis and Williams (1963). The basis of their calculation was an assumed quantum efficiency for individual leaves of 10 mol per mole of assimilated CO,. Loomis and Williams calculated for the crop that 14 pg CH,O could be assimilated per calorie of incident solar radiation. Assuming a 0.7 conversion factor from CH,O to plant mass, then the equivalent RUE on a total solar radiation basis was 2.34 g MJ-’. Because of the essentially optimum value assumed for the quantum efficiency, this theoretical estimate offered a maximum limit to RUE. DeWit (1965) expanded substantially on the calculation of Loomis and Williams by considering the geometry of light interception by a leaf canopy. The model of DeWit accounted for variations in solar elevation, leaf angles, and fraction of diffuse radiation. Using a Michaelis-Menten equation to express the response of individual leaves to light level, an approximately linear increase in canopy assimilation was calculated in response to absorbed light by the leaf canopy. In addition, DeWit’s analysis indicated that an increasing diffuse component in the incident light resulted in increased canopy productivity. Calculations of RUE from the results presented by DeWit (DeWit’s Tables 6 and 7) for daily canopy CO, assimilation gives RUE values that are essentially stable through the growing season (i.e., time of year) and over latitude. Increasing maximum leaf photosynthetic rate resulted in increasing values in calculated RUE. Goudriaan (1982) subsequently expanded DeWit’s analysis on the latitude response and showed little variation in RUE over a wide range of latitudes. Duncan et al. (1967) also undertook an analysis of canopy photosynthesis based

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on the geometry of light interception. Unfortunately no data were presented that allow direct calculations of RUE. They did, however, analyze the importance of leaf angle on daily net photosynthesis. They compared canopies with extreme, uniform leaf angles of 0 and 80" as compared to the more usual angle of 40". Lower leaf angles were advantageous below a canopy LAI of 3 to 4, and more erect leaf angles were advantageous above this LAI. Overall, there was little difference between canopies of differing leaf angles when LA1 was less than 6. Duncan (197 1) extended this analysis by considering canopies with leaf layers that were either completely horizontal or vertical. An advantage in daily photosynthesis resulting from a hypothetical mixture of leaves with vertical and horizontal leaves occurred only when LAI was greater than 4. For most crop species, these analyses indicated that RUE would not be sensitive to leaf angle even with extreme leaf angles. Overall, these early analyses gave important suggestions about the relative importance of individual variables on RUE. A number of variables were indicated to have only a small influence on RUE, including solar altitude, latitude, time of year, and leaf angle. Quantum efficiency and maximum leaf photosynthetic rate appeared to have the potential to alter RUE values substantially.

B. LEAFPHOTOSYNTHETIC RATESAND RUE As mentioned in the Introduction, Monteith (1 977) presented the first theoretical analysis leading explicitly to predictions of RUE. Although the details of his model were not presented, the input conditions included a leaf quantum efficiency of 10 mol PAR per mole CO, fixed, a photorespiration rate equivalent to 0.3 of photosynthesis rate, and a dark respiration rate equal to 0.4 of photosynthesis. Essentially a linear relationship was calculated between dry matter accumulation and intercepted solar radiation (Monteith's Fig. 3) for any given maximum leaf photosynthesis rate. A curvilinear response of RUE to maximum leaf photosynthesis rate, as anticipated in the analysis of DeWit (1965), was predicted (Monteith's Fig. 4). RUE increased from 0 at 0 g (CH,O) m-* h- leaf photosynthetic rate to about 1.6 g MJ-' at 5 (CH,O) mp2 h p ' maximum leaf photosynthetic rate. At a leaf photosynthetic rate of 3 g (CH,O) mP2h- I , RUE was calculated to be 1.4 g MJ-', which was the value Monteith (1977) concluded was generally representative of C, crops. Murata (1981) calculated RUE using a single equation for calculating daily gross canopy CO, assimilation. RUE was calculated based on differing leaf photosynthetic rates of various crop species and varying levels of solar radiation. In a comparison of species, higher rates of leaf photosynthesis for C, species resulted in higher calculated values of RUE (average of 2.48 g mass MJ- solar radiation) than for C, species (average 1.82 g MJ-I). Murata's estimates of RUE, however, seem somewhat inflated because he assumed a very high conversion coefficient

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from CO, to plant dry matter (0.61). A more realistic conversion decreases the RUE values to about three-fourths of his original estimates. Considering the conversion of CO, to CH,O is 0.68 (30/44) and that the conversion of CH,O to plant material high in carbohydrate is roughly 0.7 (Sinclair and DeWit, 1975), a more realistic coefficient is approximately 0.48 (= 0.68 X 0.7). Consequently, more realistic estimates of RUE using Murata's results are seemingly 1.95 g MJ- for C, species and 1.43 g MJ-' for C, species. Sinclair and Horie (1989) used a model of canopy photosynthesis that calculated separately the CO, assimilation of leaves exposed to direct beam radiation and those in the shade in the canopy. Leaf photosynthetic rates were calculated using an asymptotic exponential equation that was defined by a quantum efficiency of 5 g CO, MJ- solar radiation, and by the light-saturated CO, assimilation rate. The main intent of this model was to examine the influence of light-saturated photosynthetic rates and leaf nitrogen contents on RUE. Results of calculations from this model were similar to previous analyses in that RUE was closely linked to leaf carbon exchange rate (Fig. 1). In addition to variations among species as a result of differences in leaf photosynthetic rates, variations in the energy content of the plant products also were calculated to alter RUE. For example, soybean with plant products high in proteins and lipids was calculated to have a lower RUE at equivalent leaf photosynthetic rates than cereal crops (Fig. 1). The saturating nature of the RUE response to leaf photosynthetic rate at high leaf photosynthetic rates was concluded to be important in explaining potential sta-

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CER (mg C02 m-2d) Figure 1 Calculated RUE as a function of light-saturated leaf photosynthetic rate (Sinclair and Hone, 1989). The cereals, maize and rice, were plotted separately from soybean to account for differences in the energy content of the plant mass.

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bility in RUE. Large variations in leaf photosynthetic rate at the high values were required to result in substantial variations in RUE (Fig. 1). Any factor, however, that decreased leaf photosynthesis to the range of low photosynthetic rates had a direct consequence in lowering RUE. Therefore, low photosynthetic rates resulting from various stresses were predicted to result in decreased RUE. For example, Sands (1996) calculated that temperature might influence RUE depending on leaf photosynthetic response to temperature. The sensitivity of leaf photosynthetic rates to changes in leaf nitrogen was explored by Sinclair and Hone (1989) as a potentially important source of variation in RUE. In their model, maximum leaf CO, assimilation rate was argued to be a direct function of leaf nitrogen content per unit leaf area. They assumed that the leaf nitrogen content was uniform throughout the leaf canopy. From this model, Sinclair and Horie (1989) calculated substantial differences among crop species in the relationship between RUE and leaf nitrogen content (Fig. 2). At high leaf nitrogen contents, the calculated RUE response curves were saturated and there were only small changes in RUE with changes in leaf nitrogen. The maximum values of RUE from this analysis for each species were about 1.8 g MJ-' for maize, 1.5 MJ-' for rice, and 1.3 g MJ-' for soybean. At more realistic levels of leaf nitrogen, the RUE estimates were slightly less than these maximal values. For situations where leaf nitrogen content decreased to lower levels, the estimates of RUE became very sensitive to these changes in nitrogen content (Fig. 2). Certainly, under conditions where leaf nitrogen content is fairly low, changes in

"

00.2

0.6

1.0

1.4 1.8 2.2

2.6

3.0

Leaf N (g m-') Figure 2 Calculated RUE as a function of the mean leaf N per unit area for a crop canopy (Sinclair and Horie, 1989).The differences in crop species result from differences in the energy content of the plant mass and the relationshipbetween leaf photosynthetic rate and leaf N content.

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these nitrogen contents were predicted to have the possibility of resulting in large changes in RUE. Both Hammer and Wright (1 994) and Sands (1996) calculated a similar sensitivity of RUE to leaf nitrogen content. Sinclair and Shiraiwa (1993) expanded the investigation of the sensitivity of RUE to leaf nitrogen content by investigating the sensitivity of RUE to a nonuniform distribution of nitrogen in the leaf canopy. Commonly, the leaves at the top of the canopy, which are exposed to higher light levels, tend to have the highest leaf nitrogen contents. The calculations of Sinclair and Shiraiwa (1993) showed important increases in RUE as a result of the nonuniform distribution of nitrogen, particularly at low mean leaf nitrogen levels. The analysis of Hammer and Wright (1994) showed a similar improvement in RUE from a nonuniform leaf nitrogen content at low average leaf nitrogen levels. In summary, theoretical analyses have consistently indicated a dependence of RUE on leaf photosynthetic activity. The example presented in Fig. 1 is illustrative of most of the calculated relationships between RUE and maximum leaf photosynthesis rates. Increasing leaf photosynthesis at high rates results in only small increases in RUE. For C, species, maximum RUE was generally calculated to be in the range 1.4 to 1.5 g MJ- intercepted solar radiation. Depressed leaf photosynthesis rates, however, were calculated to result in large decreases in RUE.

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C. RADIATIONENVIRONMENT AND RUE The original calculations of DeWit (1965) indicated that RUE might be sensitive to the fraction of the diffuse component in the incident radiation but stable under differing latitudes. Murata (1981) included as a variable in his calculations of RUE the levels of total daily radiation. Decreasing solar radiation resulted in increased RUE, particularly at daily radiation levels that are less than half of that on a bright day. Hammer and Wright (1994) provided a detailed analysis of the importance of the radiation environment on RUE. In their model they simulated changes in radiation level by altering the atmospheric transmission ratio. They concluded that the atmospheric transmission ratio had the greatest effect on RUE of any abiotic variable they examined. RUE increased by about 0.4 g MJ-' when going from a clear day to a very cloudy day. In a less sophisticated analysis, Sinclair et al. (1992) had previously calculated about the same increase in RUE when the amount of diffuse light was held constant and the total amount of incident light was increased by increasing only the direct component. In segregating the influence of the change in proportion of diffuse radiation, Hammer and Wright (1994) concluded that the increasing fraction of diffuse component on the cloudy day accounted for a RUE increase of 0.15 g MJ-I.

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Norman and Arkebauer (199 1) calculated the response of RUE on an hourly basis in response to the changing radiation conditions. When they plotted the calculated RUE values against the diffuse:direct radiation ratio of the incident radiation, they obtained nearly a linear increase in RUE as the diffuse:direct ratio increased. These results were confounded, however, by decreases in total solar radiation associated with an increasing diffuse component. Analyses of the influence of solar elevation, and therefore latitude, on RUE have indicated no substantial influence, consistent with the original analysis of DeWit (1965). The lack of sensitivity of RUE to latitude was noted in the RUE calculations of Sinclair and Horie (1989) and Hammer and Wright (1994). Finally, the sensitivity of RUE to varying LA1 has been examined theoretically. Horie and Sakuratani (1985) showed little response in calculated RUE for rice as a result of varying LAI. Similarly, Sinclair and Horie (1989) found that RUE was stable over a range of LAI, except at LA1 less than 1.O. At the low LAI, RUE estimates were calculated to decrease. Sands (1996) also reported a lack of sensitivity in RUE to varying LAI. Overall, the radiation environment has been calculated to be important in determining RUE but the effects are fairly small. Higher RUE values were estimated for low-radiation, high-diffuse component conditions than for high-radiation, low-diffuse component conditions. These differences in RUE are evident during the diurnal cycle and might be of some significance when comparing RUE across environments.

D. CONCLUSIONS FROM THEORETICAL ANALYSES Overall, the various studies of RUE have been consistent in the analysis of RUE sensitivity to various variables. Clearly, maximum leaf photosynthetic capability is the main theoretical variable that influences RUE. Important differences among crop species and locations may well result from differences in leaf photosynthetic activity. Theoretical analyses of RUE also indicated that stress resulting in decreased leaf photosynthetic rate would result in major decreases in RUE. Consequently, in reviewing and comparing experimental results, it is likely that the more meaningful comparisons among experiments will be those where the crops were not subjected to stresses. Among the remaining factors that potentially influence RUE, the level of radiation was calculated to have an important, albeit a fairly modest influence on RUE. Potential sources of variation in RUE among locations indicated by the theoretical analyses are the level of solar radiation and the fraction of diffuse radiation. Other factors such as latitude and LA1 appear to have only a very minor role in explaining variation in RUE.

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III. EXPERIMENTAL DETERMINATION OF RUE RUE is a derived variable based on measurements of the ratio of accumulated crop mass and intercepted radiation. Whereas this definition of RUE seems rather straightforward,there has been a wide range of experimental approaches used to estimate RUE. Similarly, a wide range of units have been used to express RUE. For example, RUE estimates differ because i. some are expressed on a PAR basis while others are expressed on a shortwave radiation basis and the transmission of PAR through canopies differs from that of short-wave radiation (Marshall and Willey, 1983); ii. some are expressed on an intercepted basis while others are on an absorbed basis; iii. some are based on net aboveground dry matter production with variable leaf losses while others are expressed on a total dry matter production basis including roots; iv. some are based on differences between two discrete samplings at different stages of crop growth and are subject to large sampling errors in contrast to those based on the fitted slope from many crop growth samplings. To further complicate measurements of RUE, the various experimental approaches used in measuring accumulated crop mass and radiation are subject to a number of errors and bias. Therefore, in comparing RUE among experiments, the reliability in the RUE estimates can vary to a large extent. In the following sections some of the experimental issues associated with measuring the components of RUE are discussed.

A. DETERMINATION OF CROP MASS The crop mass used in the calculation of RUE is usually based on net aboveground biomass production. Inclusion of roots will result in higher RUE but it is difficult to estimate root biomass. In cereals, root mass at anthesis is commonly 10 to 20% of the total crop mass (Gregory, 1994). A recent study on sugarcane in Hawaii showed that belowground mass decreased from 17% of total crop mass at 6 months to 11% from 12 to 24 months (Evensen et al., 1997). Variable losses of mass due to leaf senescence and even shoot death under some stress situations can lead to an underestimate of RUE. In sugarcane, for example loss of trash material can lead to underestimates of up to 15% in RUE (Muchow et al., 1997), and stalk death associated with lodging further reduces RUE (Muchow et al., 1994). Of more importance in the precision of biomass estimates, however, is field and sampling variability. Given that RUE estimates are based on consecutive field

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samplings, RUE estimates can be highly variable in nonuniform stands especially when based on the difference between two discrete samplings. For well-managed crop stands, crop mass can usually be estimated to an accuracy of 5 to 15% (Gallagher and Biscoe, 1978). However, use of small plots and small sampling areas can lead to edge effects and biased estimates. For example, Monteith (1978) found that several reports of high maximum growth rates were associated with significant amounts of lateral radiation interception in small plots. Laboratory standards such as calibration of balances and care in partitioning biomass samples contribute to the rigor in estimates of crop biomass and hence RUE. When comparing different crop species, it is also necessary to consider the energy content of the plant mass. For example, Muchow et al. (1993) adjusted soybean biomass upward to take account of the higher energy content of the grain with the energy content of the grain taken as 1.3 times that of the vegetative material (Sinclair and DeWit, 1975; Penning de Vries et al., 1983). In the Muchow et al. (1993) study, energy contents of the vegetative material and of the grain in mung bean and cowpea were assumed the same. Other studies have corrected for the presence of energy-rich substances that accumulate during reproductive growth in several species including peanut (Bell et al., 1992; Wright et al., 1993) and sunflower (Hall et al., 1995; FlCnet and Kiniry, 1995). The variation in nutrients in plant tissue contributes little to estimates of RUE (Squire, 1990). In summary, the use of crop biomass as the basis of RUE calculation may need special attention in dealing with a productive oil crop.

B. DETERMINATION OF SOLARENERGY The amount of radiation intercepted as used in the calculation of RUE has two components (i) the input of solar radiation and (ii) the fraction that is intercepted by the leaf canopy. Incoming radiation can be expressed as either total solar (0.4 to 3 pm) or that in the wavebands of PAR (0.4 to 0.7 pm). Radiation absorbed by green foliage is sometimes substitutedfor interceptedradiation. An important consideration is how incident and intercepted radiation are measured.

1. Incident Radiation Outside the earth’s atmosphere, a surface kept at right angle to the sun’s rays receives energy at a mean rate of 1.36 kJ m-* s-’, a figure known as the solar constant (Monteith, 1972).The proportion of this radiant energy received at the earth’s surface is determined by the geometry of the earth’s surface with respect to the sun and depends on latitude and season. The annual average value of this geometrical factor decreases from about 0.3 in the tropics to 0.2 in temperate latitudes (Mon-

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teith, 1972). The greatest annual input to solar radiation occurs in subtropical regions in latitudes 20 to 30°,in climates with little cloud cover and correspondingly low rainfall (Cooper, 1970). Humid tropical regions show somewhat lower values while temperate Oceanic regions, which currently have the most intensive grassland production, are distinguished by remarkably low radiation input. Solar radiation penetrating the earth's atmosphere is absorbed and scattered by gases, by clouds, and by aerosols in the form of soil and salt particles, smoke, insects, and spores. Accordingly, the incident radiation available for crop growth varies from day to day and data are usually obtained from direct measurements using solarimeters or from indirect measurements based on sunshine hours. For a regularly calibrated and well-maintained weather station, the uncertainty in incident radiation will be about 22%. At sites where it is necessary to estimate incident radiation from records of cloudiness or sunshine hours, the uncertainty in monthly averages of incident radiation will be on the order of ? 10% (Monteith, 1972). However, given the short crop cycles of many annual crops and the fact that as leaf area develops the proportion of incident radiation that is intercepted increases nonlinearly, aberrations in RUE estimates can occur unless reliable daily estimates of incident radiation are obtained from correctly sited and calibrated sensors. Attention to incident radiation data is frequently overlooked in many experimental measures of RUE. The ratio of PAR to total radiation in the direct solar beam is between 0.44 to 0.45 when the sun is more than 30" above the horizon (Moon, 1940) and a figure of 0.45 has often been used by biologists to calculate the receipt of PAR from the flux of total radiation recorded with a solarimeter (Monteith, 1965; Meek et al., 1984).This estimate ignores a contribution of diffuse radiation, which is scattered by gas molecules in the atmosphere and which contains a much higher proportion of PAR than the direct beam. When the solar elevation exceeds 40°, the estimated ratio of PAR to total radiation and the diffuse component is about 0.60. Combining the direct and diffuse components in appropriate proportions, the ratio of PAR to total solar radiation is close to 0.5. Monteith (1972) suggests an average of 0.5 is probably appropriate in the tropics as well in the temperate latitudes. Chlorophyll absorbs very strongly in the blue and red regions of the spectrum so that the light reflected and transmitted by leaves is predominantly green. Integrating over the whole spectrum from 0.4 to 0.7 nm, the fraction of PAR absorbed by leaves is usually between 0.80 to 0.90, the precise figure depending on factors such as the amount of chlorophyll per unit area of lamina. Gallagher and Biscoe (1978) used a figure of 0.90. Using an average figure of 0.85, the fraction of total solar radiation absorbed by green leaves is therefore about 0.5 X 0.85 = 0.425. Accordingly, to convert RUE values to a total solar radiation basis, it is appropriate to multiply those estimates based on intercepted PAR by 0.5, and those based on absorbed PAR by 0.425.

229

RADIATION USE EFFICIENCY

2. Intercepted Radiation The amount of radiation intercepted by the leaf canopy can be determined from the radiation that it receives and transmits. This can be measured using tube solarimeters beneath the crop canopy compared to a measurement of the incident radiation above the canopy (Szeicz et al., 1964; Monteith et al., 1981). The tube solarimeter is a device for measuring irradiance received by a thermopile that provides an electrical output proportional to the difference between the temperature of black and white segments of its surface. This electrical output can be logged to provide continuous diurnal measurement of the fraction of incoming radiation that is intercepted (4) (Muchow and Davis, 1988). Care must be taken in solarimeter placement to ensure that the portions of the canopy “seen” by a set of replicated instruments are not anomalous. When calculating RUE, it is important that the interception is that from the green leaf canopy and not overestimated due to interception by dead leaves. A good practice is to remove dead leaves at weekly intervals from areas where tube solarimeters are placed (Muchow and Davis, 1988; Muchow et al., 1993, 1994). Uniform stands are essential to get reliable cost-effective measures of radiation interception using tube solarimeters, but even in a well-managed nonuniform crop such as sugarcane, reliable estimates of intercepted radiation required four solarimeters per plot (Muchow et al., 1994). The need for replication, the cost and fragility of tube solarimeters, and the need for continuous data-logging have resulted in many surrogate measurements for the fraction of incident radiation intercepted by the crop canopy Commonly, spot measurements are taken below the canopy using a line sensor around solar noon and compared to incident radiation above the canopy (Gallo and Daughtry, 1986). Spot measurements are usually confined to sunny days to avoid measurement difficulties associated with transient clouds. Unfortunately, restricting measurements to sunny days with high fractions of direct radiation results in biased4 estimates that are low. Interpolation between spot measurements also contributes to error inversely proportional to the frequency of measurement. Monteith (1994) also highlights the temporal error associated with spot measurements when done at one time of the day, usually midday, because4 is a function of time of day. Charles-Edwards and Lawn (1 984) have reported that& is underestimated by up to 10%when measurements are taken in the middle of the day instead of being integrated during the day, and FlCnet et al. (1996) found that east-west row orientation minimized time-of-day effects. Muchow (1985) measured diurnal variation inf; in a range of grain legumes and showed that the lower the value off, at solar noon, the greater its diurnal variation. The underestimate in& based on noon measurement decreased from 40 to 3% when& at solar noon increased from 0.2 to 0.9. RUE estimates based on spot measurements must be examined cautiously.

q).

230

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

A second method used for estimating& is based on measuring the reflected radiation from the leaf canopy in two spectral bands. These techniques are based on a comparison of the reflected radiation in a red wavelength band (630 to 690 nm) and a near infrared wavelength band (760 to 900 nm). Generally, a normalized difference vegetation index (NDVI) is calculated as the difference in reflectance between these two wavelength bands (near infrared minus red) divided by the sum of the reflectance in these two wavelength bands. Empirical relations between & and NDVI have been obtained for wheat (Garcia et al., 1988) and for maize and soybean (Daughtry et al., 1992). Major et a/. (1991) calculated& for maize from the reflectance data in these two wavelength bands by using a model that considered canopy radiative properties in detail. Unfortunately, Major er al. (1991) offered no direct comparison betweenfi calculated using the reflectance data in the model and& obtained directly from radiation interception results. All experiments using spectral reflectance data to estimate fi have focused on essentially cloud-free days near solar noon. Therefore, the problem of one-time, spot measurements also arises with attempts to estimate interception from the spectral composition of reflected radiation (Garcia et al., 1988; Rudorff et al., 1996; Major et al., 1992). Extensions of this technique to estimate& with NDVI estimated from satellites are similarly difficult because they are spot measures that are taken at various angles relative to the crop surface (Hanan et al., 1995). A third method of estimatingfi is to estimate the foliage cover by photographing the canopy either from above or looking upward from the soil surface. This is done by fitting a standard camera with a fisheye lens (Anderson, 1971). In addition to being less expensive than tube solarimeters, the technique has the advantage that photographs sample a relatively large area, may be taken rapidly in the field, are easily reproducible, and provide a permanent record of the state of the crop. Gregory and Marshall (1980), as cited by Monteith et al. (1981), compared estimates of& using a fisheye lens to the value calculated from tube solarimeters placed above and below canopies of millet and peanut. The hemispherical photographs were taken from the soil surface and the estimates agreed well both in the early and late stages of development. However, there was a discrepancy of about 20%during the period when the canopies were developing rapidly, probably as a consequence of the unavoidable positioning of the camera between the rows. Steven et al. (1986) compared foliage cover estimates of& with& estimates from solarimeters in sugarbeet, field beans, and barley with variable results, which were particularly poor for barley. Haverkort et al. (1991) compared several indirect methods for determining&, and observed that ground cover was associated with fewer errors than methods based on leaf area index and extinction coefficient, and on infrared reflectance. These surrogate measures, while useful in some circumstances, must be treated with caution in calculating RUE given the spatial and temporal errors discussed above.

RADIATION USE EFFICIENCY

23 1

C. CALCULATION OF RUE There are many methods used for estimating the components of RUE with many associated errors. Regularly calibrated and well-sited sensors are required to measure daily incident radiation. Fractional interception is best measured continuously using tube solarimeters in uniform crop stands to avoid temporal errors. Dead leaves should be removed periodically so that interception by green leaves is measured. Similarly biomass needs to be well defined (i.e., shoot plus or minus roots with or without senesced material) and taken from representative areas without edge effects. There are also important issues in how the component data are used to calculate RUE. RUE can be calculated from the difference in biomass between two consecutive harvests divided by the corresponding amount of radiation intercepted. This method suffers from large errors associated with calculated differences. A more appropriate measure is to fit a linear relationship between cumulative biomass accumulation and cumulative radiation interception, with RUE calculated as the slope of the linear relationship. This method has been widely used. This cumulative approach has been criticized by Demetriades-Shah et d. (1992) but the criticisms were appropriately refuted by Arkebauer et al. (1994), Kiniry (1994a), and Monteith (1994). Care needs to be taken in obtaining RUE estimates by regression, as in some species RUE declines during reproductive growth, even under high input conditions (Muchow and Davis, 1988), associated with N mobilization to the grain and subsequent reduction in leaf photosynthetic capacity (Muchow and Sinclair, 1994). Perhaps most important is the need for the establishment of good crop stands and good husbandry with well-recorded inputs, as a prerequisite for obtaining reliable estimates of RUE. Given the wide variation in experimental methods used to measure RUE, we now present a case study on calculating RUE over a range of environmental conditions. Muchow (1994) reports a study in which maize was grown in both a tropical and a subtropical environment at different sowing dates under different N fertilizer rates. The same hybrid (Dekalb XL82) was sown with the same row spacing, and identical management inputs with the same sprinkler irrigation regime in all crops. Crops were thinned to the same density and biomass accumulation was determined by sampling 2 m2 every 7 to 10 days from thinning to maturity. Radiation interception was measured by placing a tube solarimeter diagonally across the two inner rows of each replicate plot at ground level. These solarimeters were used to record at 2-min intervals the radiation (0.35 to 2.4 km) transmitted through the crop canopy, using the data collection system described by Muchow and Davis (1988). Dead leaves (>50% blade area senesced) were removed at about weekly intervals from plants around the tube solarimeters, so that radiation transmission through green leaf area only was recorded. Another tube solarimeter was placed above the crop and the incident radiation was recorded. Dai-

232

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

ly totals and individual tube calibration factors were used to calculate the fraction of incident radiation intercepted (&) in each plot. Since the readings from individual solarimeters were found to vary by up to 20%from the nominal calibration, the absolute incident radiation (S) was recorded with a regularly calibrated pyranometer. The amount of radiation intercepted (Si) was calculated as the cumulative product of the daily& and S. Radiation use efficiency was calculated both as the average value from sowing to maturity and as the maximum value before photosynthetic capacity declined during grain-filling. The average RUE was calculated as the ratio of net aboveground biomass at maturity to cumulative Si from sowing to maturity. The maximum RUE was derived as the fitted slope of the linear relationship between net aboveground biomass and Si (Fig. 3) using a stepwise regression procedure (Muchow and Sinclair, 1994). Starting at crop maturity, data points were progressively removed from the fit until no further improvement was gained in the proportion of variance accounted for by the regression. Under high N supply, maximum RUE was relatively stable across sowings and environments, indicating no response to temperature, absolute incident radiation level, or water vapor saturation deficit for the range of conditions experienced (Fig. 3). The difference in biomass production could be largely explained by differences in Si. Where N supply limited yield, the decrease in biomass production was associated with a much larger decrease in RUE than in Si. Under both low and high N sup-

“0

A

0

600 1200 1800 Intercepted radiation (MJ m-’)

600 1200 1800 Intercepted radiation (MJ m-’)

B

Figure 3 Relationship between net aboveground biomass accumulation and radiation interception for maize where 0 and 24 g N m-2 were applied for (A) 29 January 1986 sowing at Katherine and (B) 28 August 1990 sowing at Lawes (Reprinted from Field Crop Res., Vol. 38, R. C. Muchow, “Effect of nitrogen on yield determination in irrigated maize in tropical and subtropical environments,” pp. 1-13, with permission from Elsevier Science).The slope of the fitted linear relationship is the maximum radiation use efficiency and values are (A) 0.87 g MJ-’ for 0 g N m-2 and 1.65 g MJ- I for 24 g N m-2 and (B) 0.52 g MJ-l for 0 g N m-* and 1.64 g MJ-’ for 24 g N m-*.

RADIATION USE EFFICIENCY

233

ply, the grain demand for N could not be met solely by soil N uptake during grain filling, and there was significant mobilization of vegetative N to grain N. Consequently leaf N and RUE declined during grain filling in all situations, highlighting the difference between maximum and seasonal average RUE.

W. EXPERIMENTAL MEASURES OF RUE The objective of this section is to compare RUE reported for various species and experiments. The focus is particularly on data collected under optimum, usually control conditions, in order to compare observations on potential RUE from each study. The collection of a baseline of potential RUE data could serve as a useful reference in developing a realistic perspective on the upper limits of RUE that might be reasonably expected for individual crop species. Fortunately, in the past 10 years there has been a large expansion in the number of studies that included estimations of RUE, so there is a large data resource on which to make these comparisons. This large data resource also allows some selectivity in the data to be included in the comparison among species. As indicated in the previous section, there are a number of possible sources of error or bias as a result of various methodologies used in estimating RUE. We have established two criteria in selecting data sources to be included in this comparison. Our narrowing of the data sources to be included in this comparison does not mean, however, that the excluded data are necessarily in error or biased. We have introduced this selectivity among the published data sources in order to focus on those reports that used more exhaustive methodology in developing estimates of RUE. The two criteria used to select data for this comparison of RUE are based on experimental issues discussed previously. One criterion was that we included only estimates of RUE based on direct measures of canopy radiation interception during the growing season. Several studies calculated radiation interception based on measures of leaf area index and an assumed radiation extinction coefficient. Errors in measuring leaf area index and uncertainty of transferring an extinction coefficient to a new situation can introduce substantial uncertainty in RUE estimates. The second criterion was that RUE must have been obtained from several periodic observations through the growing season. RUE values that were calculated based only on the difference between two observations were not included because of the greater level of uncertainty in the RUE estimates when only two measures are made during the growing season. After selecting those data sources that met the two above criteria, there was also a problem in making comparisons among various experiments because of differences in how RUE was expressed. To facilitate comparison of RUE among varous sources, an attempt was made to convert estimates of RUE to a common unit.

234

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

The common unit selected for comparing RUE was mass (usually shoot only) per unit of intercepted solar radiation. Indeed, most experiments were directly dependent on measurements obtained in these units. Deviations in the expression of RUE in many cases resulted from decisions by investigators to convert the measurement units into alternate units. Fortunately, many papers reported the methods used to make these conversions so that it was straightforward to recalculate the units as shoot mass per unit of intercepted solar radiation. In those cases where the conversion technique is not explicitly available in a paper, there were two assumptions that may have been invoked. The first assumption was used to convert those reports of RUE based on intercepted PAR into units of total solar radiation. As discussed in the previous section, it was assumed that the level of PAR was 0.5 of total solar radiation. The second assumption was used to convert from absorbed PAR radiation to intercepted PAR radiation. When a conversion was unavailable in the original data source, it was assumed that 0.85 of intercepted PAR radiation was absorbed, as discussed previously. The estimates of RUE were collated by crop species (Table I) because of the important differences that have previously been suggested among species (Gosse et al., 1986; Kiniry et al., 1989).An attempt was made to include some of the key information about each experiment in Table I, and this table is the basis for making comparisons of potential RUE among species.

1. Maize

There have been extensive reports on RUE in maize. An important consideration arising from these data is that maximum RUE occurs during vegetative growth and there is a tendency for RUE to decrease during grain filling associated with mobilization of leaf nitrogen to the grain and consequent reduction in RUE (Muchow and Davis, 1988). The maximum value reported was 1.86 g MJ-' by Otegui et al. (1995). However, radiation interception was measured by spot readings around solar noon at 14-day intervals and since the fraction of radiation intercepted varied from 0.59 to 0.79 over the 2 years of the study, this may result in an underestimate of radiation interception and an overestimate of RUE. A number of studies have shown maximum RUE in the range 1.6 to 1.7 g MJ-' (Table I). In fact, there is a great deal of consistency around these values for a large number of studies. Interestingly, the study of Andrade et al. (1992, 1993) showed lower maximum RUE associated with the lower temperature environment. Also, Tollenaar and Aguilera (1992) observed higher RUE in new compared with old hybrids. The main outlier in Table I for maize is Bolanos and Edmeades (1993) with a maximum RUE of 1 g MJ-'. The authors noted that while the low RUE values were

Table I Summary of Maximum RUE Reported for Various Crop Species, Including an Estimate of These RUE in the Common Units of Plant Mass per Unit of Intercepted Solar Radiation Mas,

Source

Lacation

Expenmental variables

Samplearea

Stage

hdiauon Shoot\

Spot

Intercepted

or

01

01

total

continuous

RUE

absorbed

PAR or solar

Maximum reported value

Intercepted

Solar

1.6Og MJ-' I..U)gMJ-'

Estimated ddjusment (gMJ;;)

Comments

Maim Katherine, Australia

N. species

2 m-?

Muchow and Davis (1988) Tollenaar and Bruulsema (1988)

Elora, Canada

Hybrids. density

2.0.2.3, and and 2.1 mz

Muchow (1989a)

Katherine, Ausualia

Sowing date, species

Andrade er al. (1992)

Balcarce, Argentina

Year,CUltiVar

Daughvy er 01 ( 1992) Tollenaar and Aguilera (1992) Andrade er 01. (1993)

West Lafayette, IN,USA Elora. Canada

Balcarce, Argentina

Bolanos and Edmeades (1993)

Tlaltiapan. Mexico

Shoots Shoots

Continuous

Absorbed

PAR

3.46gMJ-'

1.40 1.47

2 m2

Vegetative. season

Shoots

Continuous

Intercepted

Solar

1.59 g MJ-' 1.27gMJ-'

1.59 1.27

10 plants (6.1

Vegetative. season

Shoots

Spot (?)

Intercepted

PAR

3.03 g MI2.96gMJ-'

0.54111~

Season

Shoots

Absorbed

PAR

4.26gM.-'

1.81

Hybrids. density

3.4 to 3.6 m2

4-6 weeks postsilking

Shwts

Spot (reflected) Continuous

Absorbed

PAR

3.78 g MI-'

1.61

Year.cultivar.

10 plants

Vegetative

Shoots

Spot (-15-day intervals)

Intercepted

PAR

3.17 g MJ-'

1.52

2.25 m2

Anthesis, maturity

Shoots

Spot(10- to 12day intervals)

Intercepted

PAR

1.99gMJ-' 1.48gMJ-'

1.00 0.74

to 9.1 plants

Continuous

1.60

Vegetative. season Vegetative

'

1.45 1.42

m-Z)

sowing date

Selection cycles, water regime

Reduction in RUE under N stress. Growth rate 30.5 g m-2 day-'. PAR absorbed 4.07 mol photon W Z day-', RUE = 3.46 PAR absorbcd 0.425 = 1.47. RUE during vegetative growth fitted by regression. Decrease In RUE after silkiing;RUE maize higher than for sorghum. Low-temperature study with mean 15 to 18°C during vegetative gmwth.

Silking to 6 weeks postsilking; higher DM in new cf old hybrid atmbuted to higher RUE. Max RUE ranged from 2.27 to 3.17gM.-'PARover5years dependent on temperature; RUE varied with sowing date (temperature). Noted low RUE under well-watered conditionchigh VPD. Water applied only every 10 days.

continues

Table I-confinued Mass

Source Kiniry (19946)

Muchow (1994) Otegui el a/.

Lacation Temple, TX, USA

w N

m

Coates (1986) Muchow and Davis (1988) Hammer and Vanderlip (1989) Muchow (1989a)

Year, species, competition

Sample area -0.3 mz

Katherine & Lawes. Australia R o p . Argentina

Location. year. N

2 m-2

Hybrids, sowing date

1 m2

ICRISAT. India

Year, soil type. hybrids Sowing date. cultivars. row spacing, density N. species

Periodic. 3 mL Penodic,

(1995) Sorghum Sivakumar and Huda (1985) Muchow and

Experimental variables

Kununurra, Ausvalia Katherine. Australia Manhattan, KS. USA

Katherine, Austrdlia

Greenhouse. genotype X temperature Sowing date, species Location. year. N

Stage Vegetative

Radiation Shoots

Spot

01

Or

total

continuous

Shoots

Spot ( I - to 23day intervals)

Periodic. 2

m2

PAR or solar

Maximum reported value

Estimated adjustment

Intercepted

PAR

3.42 g MJ-'

1.54

(g

Comments

(1991)

3.75gM1-' (1992) 1.68 g MI-' 1.67 g MI-' 4.14 g MJ-' 3.39 g h%-'

1.69

Shoots

Continuous

Intercepted

Solar

Shoots

Spot (14-day intervals)

Intercepted

PAR

Season

Shoots

Intercepted

PAR

2.74 g MI-'

1.37

Season

Shoots

Spot (7- to 10day intervals) Spot (7-day intervals)

Intercepted

PAR

2.40 g MJ-'

1.20

Vegetative. season Vegetative

Shoots

Continuous

Intercepted

Solar

Shoots

Continuous

Intercepted

PAR

I .25 1.12 2.16

Shoots

Continuous

Intercepted

Solar

Vegetative

Shoots

Continuous

Intercepted

Solar

1.25 g MI-' I.I2gMJ-' 4.31 g MI-' at 25°C 2.99 g MI-' at 17°C 1.29 g id-' I.IOgW-' 1.26 g MJ-'

Vegetative, season

1.68 1.67 1.86 1.53

1.29 1.10 1.26

Katherine & Lawes. Australia

Westgate PI a/. (1997)

Moms, MN, USA

Hybrids, row spacing

Periodic, 1 m2

Vegetative

Shoots

Spot (7- to 10 day intervals)

Intercepted

PAR

3.02 g MJ-'

1.51

Ayr. Australia

Growth analysis, plant crop

Periodic.

Season

Shoots

Continuous

Intercepted

Solar

1.75 g MJ-'

1.75

2 m2

15 mz

Reduction in RUE under N stress. No IoCation effect on max RUE. Fraction PAR intercepted varied from 0.59 to 0.79 over 2 years, 4 sowing dates. and 4 hybrids.

Decrease in RUE after anthesis. Cultivar difference.

IS O

Muchow and Sinclair (1994)

Sugarcane Muchow er al. ( 1994)

Periodic.

Intercepted or absorbed

Vegetative, season Vegetative. season

I m' Periodic, 2 m' 15 potc

RUE

Decrease in RUE after anthesis. RUE independent of temperature, solar radiation. and water vapor saturation deficit.

If assume 15%underestimate of biomass due to nonrecovery of all trash (Evensen er a/., 1997) then RUE = 2.0 g W - ' .

Robenson cr a/.

Munchow el al. (1997)

Potato Allen and Scott (1980) Burstall and Hams ( 1986) Jeffenes and Mackerron ( 1989) Kenaf Muchow (1992)

1.72 g MI-' plant 1.59gM.-' ratoon 1.87 g MI-' Hawaii 1.96 g MI Australia

I .72 1.59

RUE lower during late growth due to winter temperature and biomass loss due to stalk death. a h .

1.87

Includes estimates of 15% wash loss in biomass estimates for Ausualia: Hawaii measured biomass estimates for 1st-year growth.

Ingham. Australia

Variety, plant, ratoon crop

Periodic. I5 m2

Season

Shoots

Continuous

Intercepted

Solar

Kunia. Hawaii, USA; Ingham and Ayr. Australia

Variety

Periodic. 18.9 m2 and 15 m2

Season

Shoots

Contiuous (and Intercepted calculated using k = 0.4)

Solar

Sutton Bonnington. UK Sonning-on-Thames, England Scotland

NA

Periodic, NA Penodic. 1.4rn' Periodic, 1.08 m2

Season

Total

Continuous

Intercepted

Solar

I .6 g MI-'

1.6

Season

Total

Continuous

Intercepted

Solar

1.76 g M.-'

1.76

Varietal differences in RUE.

Total

Continuous

Intercepted

Solar

1.75 g MI-'

I75

Includes tubers

Katherine. Australia

Water. N

2 m2

Season

Shoots

Continuous

Intercepted

Solar

1.20 g h W '

1.20

RUE decreased more than RI under water and N stress.

Lincoln. New Zealand

Season, cultivar

Season

Shoots

'?

Intercepted

PAR

2.38 g MI-'

1.19

All results gave common RUE.

Sutton Bonnington.

N. year

-0.2

Vegetative

Total

Continuous

Absorbed

PAR

1.28

N. stage location

12 plants

Vegetative and reproductive

Shoots

Spot (reflected)

Absorbed

PAR

4.07 mmol hex. mo1-I 3.82 g MI-'

1.62

RUE decreaed with decreased N application. No location difference. Decrease = 1 low N. Decreaye in reproductive stage.

0.525 m2

Vegetative

Shoots

Intercepted

PAR

1.46g MJ-'

0.73

Decrease during reproductive p w h .

N

0.3 m2

Vegetative

Shoots

Spot(6 occasions) Spot (9- to 20day intervals)

Absorbed

PAR

3.36gM.-'

1.51

Penh. Ausrralia

Cultivar

0.32 m2

Vegetative

Shoots

Continuous

Intercepted

PAR

2.93 g MI-'

I .46

No difference in N treatment identified. RUE increased up to 10 days after anthesis. Cultivar and season difference.

East Beverley. Australia

Season. sowing dare

1.068 m2

Season

Shoots

Contmuour

Intercepted

PAR

I .68 g MI-'

0.84

(19%)

Variety Water supply

~

'

I .96

w N

Wheat Wilson and lamteson (1985) Green ( 1987) Garcia el a/. (1988)

Gregoory er a/

(1992) Fischer (1993)

Yunusa er a / (1993) Gregoory and Eastham (1996)

UK Mandan. ND. USA; Manhattan, KS, USA; Lubbock, TX.USA East Beverley. Austmlia Griffith. Australia

m2

per ueatmenl

~~

~

continues

Table I-continued Mass

Source

Calderini era/. (1997) Barley Gregory el al. (1992) Goyne n al (1993) Jamieson el a/. (1995)

w N Q)

Rice Hone and Sakuratani (1985) Inhapan and Fukai (1988) Sunflower Trapani cr o/. (1992)

Experimental variables

Lncation

Buenos Aires. Argentina

Cultivar

East Beverley. Australia Warwick, Australia

CUltiVZS

Lincoln, New Zealand

Cultivars, irrigation Irrigation

Tsukub4 Japan

Sowing date, shading cultivar

Redland Bay, Australia

CUltiVar.

Buenos Aires, Argentina

Cultivar

Samplearea

0.075 m2

0.525 mz

Radiation Shoots or total

Vegetative and reproductive

Shoots

Spot (3- 104-

Vegetative

Shoots

Stage

RUE

Spot

Intercepted

01

Or

continuous

absorbed

PAR or solar

Intercepted

Solar

day intervals)

Spot (6 wca-

Maximum reported value

1.25 g MI-' I .02 g MI-

'

Estimated adjustment (g MJLd,)

I .02

Cultivar variation. RUE decreased post-anthesis.

1.25

Intercepted

PAR

1.79 g MJ-'

0.90

Decrease during reproductivegrowth.

Absorbed

PAR

2.9OgMJ-'

1.30

Some cultivar differences

Intercepted

PAR

2.33 g MJ-'

1.16

Some irrigation differences

0.36 m2

Season

Shoots

0.1 m2

Season

Shoots

sions) Spot(-7-day intervals) Spot (once)

Season

Shoots

Continuous

Absorbed

PAR

3.28gMJ-'

1.39

Shading increased RUE

0.15 m2

Entire season

Shoots

Spot (-74.9 intervals)

Intercepted

Solar

0.93 g MI-'

0.93

No cultivar difference. RUE decreased for dry.

2-3 plants

Vegetative

Shoots and total

Spot(4- to7day intervals)

Intercepted

PAR

3.13g,,, MI-'

1.56 I .63

Decreased at emergence and photo synthesis. No cultivar difference.

1.14

Decreased during early stages. Derreased with low N. No effect of density. Decreased with less N. Decreased late in season. No N difference.

irrigation

Gimenez el 01. (1994)

Cordoba, Spain

N. density

4 4 plants

Vegetative

Shoots

Spot (3 occasions)

Intercepted

PAR

3.26 g_, MI-' 2.29 g MI-'

Halletal. (1995)

Buenos Aires. Argentinia Temple, TX.USA

N. density

3-4 plants

Season

Total

Intercepted

Solar

1.24gMJ-'

1.24

N

1.4 m2

Season

Shoots and total

Spot(l0a'casions) Spot (9 a'casions)

Intercepted

PAR

1.77 g5,, MI-'

0.88 0.97

1.33

Henet and Kiniry (1995)

Bange et al. (1997a) Bange et al.

Ganon College, Australia Ganon College.

Comments

N

7 plants

Season

Shoots

Continuous

Intercepted

Solar

I .94 &md MJ-' 1.47 g MJ-'

Shading

7 plants

Season

Shoots

Continuous

Intercepted

Solar

1.33 g MJ-'

I .47

Decreased with decreased N treatmentto 1.25gMJ-'. Higher RUE with shading.

soybean Nakaseko and Gwth (1983) Leadley ef ol. (1990) Daughuy ef a/.

Sapporo. Japan Raleigh. NC. USA Beltaville. MD

(1992)

Muchow et al. (1993) Sinclair and Shiraiwa (1993) Rochette pf a/. (1995)

Peanut Bell erol. (1987)

Katherine, Australia Gainesvtlle, FL, USA, Shiga, Japan Ottawa, ON, Canada

Kununurra, Australia

Species

Season

Solar

I I .8 mg kcal-' 0.86 g MJ-I

0.86

Absorbed

PAR

2.34gMJ-'

0.99

Intercepted

Solar

0.86 g MJ-'

0.86

spot (7-daY intervals)

Intercepted

Solar

0.66 g M.-' 1.15gMI-l

0.66

Shoots and total

Continuous

Intercepted

2.04 g
1.02

2.09 g,, MJ-'

I .I4

Total

Spot (7day intervals)

1.37

0.98

Increased greatly from shading.

Shoots

Spot (3 occa-

Intercepted

PAR

sions) Continuous

Intercepted

spot (reflected) Continuous

0,. inside open-

4 plants

Season

Shoots

top chambem Row spacing and direction Season. location

0.45 or 1.9 mL

Season

Shoots

1.70r2rn2

Seaon

Shoots

CUlt!"ar. location

0.56 or I rnz

Comparison of biomass harvest to co2flux density

0.48 m2

Density

6 plants

Shoots

Season

PAR

Stirling er ol. (1992) Bell erol. (1992)

Hyderabad, India

Kingaroy and Bundaberg, Australia Gainesville. FL. Bennett er n/ (1993) USA Kingaroy and Wright er ol. Bundaberg, (1993) Australia Bell el 01. (1994~) Delhi, Canada Faba bean Fasheun and Dennett (1982) Slim and Saxena (1992) Madeira er ul. (1994)

Decreases with increasing 0,.

Differences among cultivm.

1.15

Only small vanation in RUE when w > 2

Shading

10 plants

Season

Total

Spot (10 occa. sions)

Intercepted

Solar

3.04 g M1- I (veg.1 2.27 g M.-' (sea.) 0.98 g M.-'

Location, cultivar

1.2or1.8rn'

SCaSO"

Total

spot ( 7 h Y

Intercepted

Solar

1.12 g MJ-'

1.12

No cultivar difference. Location difference.

Intercepted

Solar

1.01 g MJ-'

1.01

No cultivar difference.

Intercepted

Solar

1.07 g MI-'

I .07

RUE sensitive to leaf N content.

Vegetative and season

Intercepted

PAR

w N

a

1.26

No density difference.

1.02

intervals) Cultivar

1.22 m2

Season

Total

N

1.2 or I .8 rn2

Season

Total

Cultivar, season

0.6 m2

Season

Total

spot (Fday intervals)

Intercepted

Solar

2.24 g MJ-'

1.12

Small cultivar differences.

Reading, UK

Faba bean

3 plants

Season

Shoots

Continuous

Absorbed

PAR

4.8gMJ-1

2.04

Difference with sowing date.

ICARDA. Syria

Faba bean. cultivar, season, density Faba bean disease

0.225 m2

Season

Shoots

Spot (7-day

Intercepted

PAR

2.06 g MJ-'

1.03

Shoots

intervals) Continuous

Intercepted

Solar

1.45 g M1-'

I .45

Cultivar and sowing density variation. Disease decreased RUE.

Sutton, Bonnington, UK

10 plants

Season

spot (7-day intervals) Spot (7-day intervals)

continues

Table I-continued Mass

Source Other legumes Hughes er nl. (1981) Muchow and CharlesEdwards (1982) Nakaseko and Gotoh (1983) Heath and Hebblethwate (1985) Leach and Beech (1988) Singh and Sri Rama (1989) McKenzie and Hill (1991)

Location

Experimental variables

Trinidad

Pigeon pea

ffimbedey, Australia

Mung bean

Sapporo, Japan

Phaseolus. Azuki

Samplearea

Stage

Radiation Shoots or total

Spot

RUE Maximum repofled value

Estimated adjusment (gMJ;I)

Intercepted or absorbed

PAR or solar

Intercepted

Solar

1.91%

1.09

Variation due to watering.

Absorbed

PAR

2.17 g MJ-'

0.92

RUE less during @-filling.

Spot (3 occasions) Spot (2- to 3day intervals)

Intercepted

PAR

0.98

Species differences.

Intercepted

PAR

9.1 mg kcd1.46 g MJ-'

1.46

RUE decreased by water shortage at one location.

Spot (14- to 28day intervals) Spot (3- to 4day intervals) Spot(14day intervals)

Intercepted

PAR

1.53 g h W '

0.76

Intercepted

Solar

0.67 g MJ-'

0.67

Cultivar variation lower for wide row spacing. Water deficit decreased RUE.

Absorbed

PAR

2.14gMJ-'

0.96

Sowing date difference

Some differences between species.

Or

continuous

Comments

Season

Shoots

I m'

Vegetative

Shoots

?

Season

Shoots

P a

0.25 m2

Season

Shoots

Chickpea cultivar. spacing Chickpea

7

Season

Shoots

0.3 m2

Season

Shoots

Lentil. sowing date. cultivar. irrigation Cowpea. mung bean Lupin, season

0.1 m2

Season

Shoots

1.7 or 2 m2

Season

Total

Continuous

Intercepted

Solar

1.09 g MJ-'

1.09

1.068 m2

Season

Shoots

Continuous

Intercepted

PAR

1.16 g MJ-'

0.58

Madrid, Spain

Pea

0.3 mz

Vegetative

Shoots

Intercepted

PAR

1.43 g MJ-'

0.72

No difference between two cultivars.

Merredii, Ausualia

Various swcies

0.5 m2

Vegetative

Shoots

Spot (7day intervals) Spot(lCday intervals)

Intercepted

PAR

1.09 g MJ-'

0.54

Some species differences.

bean Suuon Bonnmgton,

UK Dolby, Australia Hyderabad. India Cunterbury. New Zealand

Muchow et a/.

Katherine, Ausualia

(1993) Gregoty and Eastham

East Beverley,

Spot(peri0dically) Spol(7-day intervals)

'

Austda

(1996)

Manin er nl. (1%)

Thomson and Siddique (1997)

Nore. The only experimental reports included were those that (1) measured intercepted radiation and (2) measured plant mass accumulation at several times during the growing season.

RADIATION USE EFFICIENCY

241

obtained under apparently well-watered conditions, a high saturation deficit may have contributed to water stress where water was applied only every 10 days. There have been a number of published reports with maximum RUE in maize much higher than those shown in Table 1. Jones (198 1) cited by Kiniry et al. (1989) reported 4.5 g MJ-' PAR (or 2.25 g MJ- solar) but radiation interception was estimated from an extinction coefficient and not measured directly. Also, this value was obtained at a density of 6 plants m-*, whereas at 4 plants m--2 RUE was 3.6 g MJ-' PAR (or 1.8 g MJ-' solar), which is close to the maximum values reported in Table I. Indeed, it is difficult to explain such a marked response with a relatively small change in density, especially since the greater effect is more likely on the amount of intercepted radiation. Similarly, Cabelguenne (1987), as cited by Kiniry et al. (1989), and Kiniry (1987), as cited by Kiniry et al. (1989), obtained high RUE values of 2.05 and 1.95 g MJ- I , respectively. Here, either the extinction coefficient was estimated or there was high variability in RUE across plant densities where radiation interception was measured with a line sensor only in the middle of the day. Sivakumar and Virmani (1984) measured maximum RUE of 0.82 g per mole and Kiniry et al. (1989) converted this to 3.8 g MJ-' PAR. Here PAR interception was measured only at midday on noncloudy days and the conversion from PAR to solar was not given. If a conversion of 0.5 is used, this translates into a RUE of 1.9 g MJ-I solar. If a conversion of 0.45 is used, then RUE is 1.7 g MJ-', which is in line with the maximum reported values in Table I. In summary, the maximum RUE for maize is consistent in the range 1.6 to 1.7 g MJ-I during vegetative growth and the RUE during reproductive growth decreases such that the seasonal RUE ranges from 1.3 to 1.7 g MJ-' (Table I).

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2. Sorghum Fewer studies were available where maximum RUE was measured in grain sorghum. Values seem to be consistently in the range 1.2 to 1.4 g MJ-' during vegetative growth with seasonal values being slightly lower (Table I). Sivakumar and Huda (1985) reported a maximum value for RUE of 1.37 g MJ-', which may be slightly overestimated because PAR interception was only measured at midday on noncloudy days. An important outlier for sorghum is the study by Hammer and Vanderlip (1989) where a maximum of 2.16 g MJ-' was recorded. However, this study was conducted in a glasshouse and radiation interception was measured using a line sensor at solar noon. Additional reflected radiation that was not measured and an increased proportion of diffuse radiation may have contributed to the high RUE in this study. Several higher maximum RUE values have been reported for sorghum by Kiniry et al. (1989). Again, they cite Sivakumar and Virmani (1984) at 2.9 g MJ-I PAR. At a conversion of PAR to solar of 0.5, this gives a RUE of 1.45 g MJ-'. Other

2 42

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

higher values cited by Kiniry et al. (1989) are from studies where radiation interception was estimated based on an extinction coefficient. The exception is Steiner (1986) where Kiniry et al. (1989) quoted a value of 3.0 g MJ-I but no values are given in the actual paper by Steiner (1986). Rosenthal et al. (1993) also recorded a maximum RUE of 3.46 g MJ- PAR absorbed, or as converted 1.47 g MJ-'. However, here absorbed PAR was estimated from leaf area and there was a highly scattered relationship between biomass and absorbed PAR (? = 0.68). In summary, it appears that the maximum RUE for sorghum is in the range 1.2 to 1.4 g MJ-', with RUE from sowing to maturity being less. This range of potential RUE for sorghum is less than that of maize. In direct comparisons of RUE in sorghum and maize, sorghum had the lower RUE (Muchow and Davis, 1988; Muchow, 1989a; Muchow and Sinclair, 1994). Since sorghum and maize both have the C , photosynthetic pathway and similar potential leaf photosynthetic rates (Muchow and Sinclair, 1994), it is surprising that sorghum has not been commonly observed to have RUE values equivalent to maize.

'

3. Sugarcane Very few studies have measured RUE in sugarcane. Recent work by Muchow et al. (1994) and Robertson et al. (1996) have shown maximum values in the range 1.7 g MJ- based on net aboveground biomass. Sugarcane has a very long growth period (12 to 36 months) compared to most crops, and loss of mass in the senesced leaves (trash) is a major concern in the determination of RUE. Where all trash has been recovered, the value is closer to 2 g MJ-' (Muchow et al., 1997). Ratoon crops have a lower maximum RUE compared to plant crops (Robertson et al., 1996). These studies for sugarcane are particularly interesting given the long growth duration and large standing mass of sugarcane. In the study of Muchow et al. (1994), a maximum RUE of 1.75 g MJ-' was obtained where biomass was linearly related to intercepted radiation up to a biomass of 72 tons h-l. Not all the trash was recovered in this study and if it is assumed that the trash accounts for 15% of the biomass (Evensen et al., 1997), then the RUE is close to 2.0 g MJ-I as estimated for Hawaiian crops by Muchow et al. (1997). The reason that sugarcane has a higher RUE value than does maize has not been fully explored. One possibility, however, is that the ultimate product in sugarcane, sucrose, has a lower energy content than the seeds of maize. Maize seeds contain protein and lipids such that 0.7 1 g seed is produced per gram of photosynthate (Sinclair and DeWit, 1975). In sugarcane, the storage of sucrose is likely to result in even greater than 0.83 g carbohydrate produced per gram of photosynthate (estimated by Penning De Vries, et al., 1974). Therefore, much of the advantage in RUE of sugarcane over maize may simply be a result of the difference in energy content of the plant product of these two species. In all three of the sugarcane studies in Table I, RUE declined during late growth

RADIATION USE EFFICIENCY

243

due both to lower temperature during winter and to biomass loss associated with stalk death and lodging. Also, for Hawaiian crops, the apparent RUE during the second 24 months of growth was lower and this is likely to be associated with lodging and stalk death. However, it is extremely difficult to accurately measure radiation interception in lodged crops, and hence estimates of RUE in large lodged crops must be viewed with caution. In summary, the maximum RUE for sugarcane appears higher than those for maize or grain sorghum with values approaching 2 g MJ-' when the majority of dead leaf is recovered. Consequently, the RUE results from the three C, species indicate a large range among these species in the expression of maximum RUE.

1. Potato

The several studies that examined RUE in potato have consistently obtained maximum values in the range 1.6 to 1.75 g MJ-' (Table I). These values seem particularly high for a C, species. Similar to comparison between sugarcane and maize, the reason for the high RUE values in potato relative to other C, species may be attributed partly to the biochemical composition of the plant product in potato. The tuber is up to 80% of the total plant weight (Burstall and Harris, 1986; Jefferies and Mackerron, 1989) and the tuber is extremely high in starch content. The conversion of photosynthate to carbohydrate in potato may be higher than 0.83 g carbohydrate per gram photosynthate because starch is the main carbohydrate product rather than cellulose (Penning De Vries, 1975). In contrast, seeds of wheat contain approximately 14% protein and have a conversion from photosynthate to seed mass of 0.7 1 (Sinclair and DeWit, 1975). This difference in the photosynthate costs for production of plant mass (17% advantage to potato) is consistent with potato having a high RUE. In contrast to variations in RUE through the season observed in other species, the three reported studies with potato all indicate an essentially constant RUE through the growing season. This may result from the fact that the growth of the tubers begins at an early stage of crop development and continues at essentially a constant fraction of total growth (Allen and Scott, 1980; Burstall and Harris, 1986; Jefferies and Mackerron, 1989). Consequently, there is no strong differentiation between vegetative and reproductive development stages in potato that might impose an important influence on the use of assimilate and nutrients within the plant. One study on potato in Hawaii (Manrique et al., 1991) obtained RUE values of only 1.17 g MJ-'. Manrique et al. (1991) suggested that the low RUE was associated with high vapor pressure deficit and high absolute solar radiation in the Hawaiian environment. However, radiation interception was estimated based on a

244

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

fixed extinction coefficient of 0.57 and hence it is difficult to be certain of the RUE values and the associated reasons for the apparent lower RUE. Overall, observed RUE values in potato are very high, with a likely explanation based on the high fraction of starch in the plant mass. As a basis for comparing to grain species, potential RUE for potato appears to be in the range of about 1.45 to 1.7 g MJ-'.

2. W h e a t There are more experimental reports for RUE of wheat than for any other species. Therefore, the range of locations and experimental conditions probably offers the greatest variation in growth conditions of any species. Not surprisingly, there is a wide range in the estimates of RUE (Table I). Two studies in Australia (Gregory et al., 1992; Gregory and Eastham, 1996) reported RUE values that were substantially lower than others, and may not reflect the potential RUE for wheat. Among the six other studies, the mean RUE was 1.38 g MJ-'. The two highest values of RUE were 1.51 and 1.62 g MJ- but these were based on spot measures of radiation and are likely to be overestimates. The values of 1.46 g MJ-I (Yunusa et al., 1993) based on continuous measures of radiation may more accurately reflect potential RUE for wheat. A number of factors were found to result in variation in potential RUE. One of the more important variations was the stage of crop development.As discussed later, during early vegetative growth and during reproductive growth, RUE was lower than during the middle and late stages of vegetative growth (Green, 1987; Fischer, 1993; Calderini et al., 1997). Cultivar differences in RUE have also been reported. Yunusa et al. (1993) compared three cultivars with a wide range of year of release and found that the most recently released cultivar had the greatest RUE. On the other hand, Calderini et al. (1997) compared seven cultivars released over the period from 1920 to 1990 and found no trend in a change in RUE with year of release.

',

3. Barley The three RUE values for barley (Table I) tend to be somewhat lower than those reported for wheat. However, the two reports with the lowest values of RUE for barley (Gregory et al., 1992; Jamieson et al., 1995) both included a comparison with wheat, and the RUE of barley was equivalent to or greater than that of wheat. Therefore, it seems unlikely that the RUE of barley is inherently inferior to that of wheat. The study of Goyne et al. (1993) reported the highest value of RUE for one cultivar of barley of 1.30 g MJ-', which is comparable to several studies with wheat. RUE of barley appeared to be reasonably stable throughout the growing season. No variation in RUE through the growing season was indicated in the study of

RADIATION USE EFFICIENCY

245

Goyne et af. (1993). On the other hand, Gregory et al. (1992) showed a decrease in RUE after flag leaf emergence as compared to before flag leaf emergence. Little variation in RUE among barley cultivars has been identified. RUE was equivalent among cultivars in the study of Gregory et al. (1992) and in the study of Goyne et af. (1993), except for the superiority of the cultivar Gilbert.

4. Rice Only two references could be identified for rice that included both measurements of radiation interception and biomass accumulation (Table I). A partial explanation for such a limited number of investigations may be the difficulty of measuring intercepted radiation in a paddy field. That is, measurement of radiation transmittance under a crop canopy at the water surface in a paddy field is more difficult than positioning an instrument on a soil surface. In any event, the maximum RUE value of 1.39 g MJ- reported by Horie and Sakuratani (1985) compares favorably with the RUE values obtained for other cereal crops. There was substantial consistency in RUE among cropping seasons in the study of Horie and Sakuratani (1985), although there was an indication of differences in RUE between genotypes. Kiniry et al. (1989) estimated RUE values for rice from four studies. Excluding the largest value, which was roughly twice the other three, the mean value was 1.1 g MJ-I.This estimate is consistent with the two direct measures of RUE. Nevertheless, there remains a need to document RUE fully in rice. Despite the difficulties of measuring intercepted radiation under paddy conditions, measures of RUE under a range of conditions need to be obtained as have been done in other species.

'

5. Sunflower

The mean value of RUE from the six experimental reports for sunflower (Table I) was 1.27 g MJ-I,the highest value being 1.56 g MJ-'measured in Argentina (Trapani et al., 1992) during the period of rapid growth before anthesis. No difference was found among the tested cultivars. Also, Bange et al. (1997a) observed RUE values in sunflower of 1.6 g MJ- or greater in individual replicates. There is no obvious explanation for these high values of RUE obtained during the period of rapid vegetative growth. Considerable variation in RUE for sunflower during the growing season was found in several studies. Trapani et al. ( I 992) found-thatRUE from emergence to a leaf area index of 1.7 was 45% of that from leaf area index of 1.7 to anthesis. Also, they reported that RUE during the postanthesis phase was 43% of the preanthesis stage. Similarly, Gimenez et af. (1994) and Bange et af. (1997a) obtained low RUE during the early periods of sunflower development. The most detailed description of the changes in RUE through the growing season was presented by

246

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

Hall et al. (1995). Their data clearly indicated a decreased RUE during crop establishment and during the later stages of seed fill. The high seasonal variation in RUE in sunflower might, consequently, explain some of the variation in RUE reported among the various investigations. Nitrogen fertilization of sunflower has also been considered for its influence on RUE. The sensitivity to the application of nitrogen has been variable and may reflect the original nitrogen status of the soil. Hall et al. (1995) and FlCnet and Kiniry (1995) found little influence of nitrogen application on RUE. On the other hand, Gimenez et al. (1994) and Bange et al. (1997a) found substantially decreased RUE in treatments of no nitrogen fertilization. Nitrogen availability also appears to be an important source of RUE variation among experiments.

6. Soybean RUE for soybean (Table I) tends to be lower than that reported for species that have already been reviewed. The mean maximum RUE value in the six studies is 1.02 g MJ-'. Curiously, the two highest RUE values (1.26 and 1.15 g MJ- I ) were both reported from Japan. The basis for the overall lower RUE values reported for soybean as compared to other C, species seems to result from two factors. First, the energy content of the constituents of the soybean plant, particularly in the seed, necessarily results in a decreased RUE (Fig. 1). The photosynthate costs for nitrogen accumulation by either symbiotic nitrogen fixation or nitrate reduction are high. Second, the maintenance of a high leaf photosynthetic rate in soybean to sustain a high RUE may be difficult because of the high nitrogen requirements in the leaves (Sinclair and Horie, 1989). Within individual studies a fairly high level of stability in RUE throughout the growing season has, however, been found. Plotting cumulative plant mass and radiation interception through the season has not indicated large changes in RUE. Only the detailed study of Rochette et al. (1995) indicated that RUE was low early in the season (LA1 < 2), and then remained fairly stable after that. Interestingly, including roots in the estimate of RUE, as in the study by Rochette et al. (1993, increased the seasonal estimate by only 8%.

7. Peanut Agreement in seasonal RUE among the six experimental reports for peanut is quite high (Table I). The range of RUE from these experiments is only from 0.98 to 1.12 g MJ-' with a mean value of 1.05 g MJ-'. These RUE values are more similar to those reported for soybean than for the other C, species. An especially important factor in evaluating the RUE of peanut is correction of the mass during seed fill for the energy content of the seed. Bell et al. (1987) and

RADIATION USE EFFICIENCY

247

Bennett et al. (1993) showed the sharp decrease in RUE during seed fill if the plant mass during this period is not corrected for seed energy content. When a correction is done for energy content, then RUE remains constant through much of the season. Energy content of the plant is also likely to be important in explaining the relatively low RUE of peanut. The vegetative component of the peanut plant is high in protein content, which results in a high requirement for assimilate and nitrogen input. The importance of nitrogen accumulation in determining peanut RUE was shown directly in the results of Wright et al. (1993) where RUE in a warm climate was closely linked to the leaf nitrogen content. A great deal of stability in RUE has also been found among peanut cultivars. No differences among cultivars were detected in comparisons of two cultivars (Bell et al., 1992), of three cultivars (Wright et al., 1993), and of four cultivars (Bennett et al., 1993). Bell et al. (1994c), however, did find that the cultivar Chico had a significantly lower RUE than the five other tested cultivars. Plant density also did not result in variation in RUE (Bell et al., 1987).

8. FabaBean The three reports of RUE in faba bean give highly divergent results (Table I). The values of 1.03 g MJ-' (Silim and Saxena, 1992) and of 1.45 g MJ- (Madeira et al., 1994) are in the range of RUE values reported for other C, species. In contrast, the RUE estimate presented by Fasheun and Dennett (1982) converts to 2.04 g MJ-'. This extremely high value for RUE was based on plant samples of only three plants, which may have resulted in an overestimation of plant mass per unit area. The calculated crop growth rate of 30 g m-* day-' obtained in this study also seems to be a very high estimate. Clearly, additional data are needed to resolve the potential RUE in faba bean.

9. Other Grain Legumes Overall, the values of RUE for a number of grain legumes (Table I) are consistent with the range of values obtained in soybean and peanut. The maximum RUE obtained in these additional studies was 1.46 g MJ- reported for pea (Heath and Hebblethwaite, 1985), followed by 1.09 g MJ-' reported for pigeon pea (Hughes et al., 1981) and for cowpea (Muchow et al., 1993). In a number of cases, RUE values were reported to be less than 0.8 g MJ-I. Such low estimates of RUE in many of these grain legumes seems inconsistent with the low energy content (i.e., high carbohydrate levels) of their grain (Sinclair and DeWit, 1975). Based on the energy content, it seems many grain legumes should have RUE values that are substantially greater than those of soybean and peanut. The experimental results to date indicate a need for intense studies of RUE in a

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248

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

number of these grain legumes. Important questions exist concerning whether RUE is inherently low within some of these grain legume species, or whether the cultivars that have been tested or the environment of the experiments limited the expression of a higher RUE. Compared to soybean and peanut, it appears that there is an opportunity to increase the RUE of some of these other grain legume species.

V. SOURCES OF VARIABILITY IN RUE Both the theoretical analyses and the experimental results clearly demonstrate that there is not a constant universal RUE value. The theoretical analyses indicate several factors that could be important in causing variation in the RUE that is achieved. In contrast to the conclusion of Demetriades-Shah et al. (1992), the variation in RUE has been found to be quantitatively linked to plant traits and environmental conditions. The intent of this section is to examine the important factors associated with variability in RUE.

A. SPECIES Table I and the discussion in the previous section indicate important variations among crop species. While variations in measured RUE within a species are noted, to a great extent there is reasonable consistency in the estimates of RUE within a species. C, species tend to have the higher RUE values, with sugarcane being the highest of all species with RUE values approaching 2.0 g MJ-' (Table I). Sugarcane is a good candidate for maximum RUE values because of its use of the C, photosynthetic pathway and its production of sucrose as the final plant product. Maize has RUE values somewhat lower than those of sugarcane, in the range 1.5to 1.7 g MJ- (Table I). Surprisingly, sorghum has fairly modest RUE values. The reason for the lower values of sorghum within the C, species is unknown, but the theoretical studies indicate the importance of the expression of leaf photosynthetic rate on RUE. Muchow and Sinclair (1994) found that the lower value of RUE in sorghum compared to maize was associated with a lower leaf nitrogen content. An important challenge is to understand the low values of RUE obtained for sorghum relative to other C, species. Important differences in experimental measures of RUE also exist among the C, species. Clearly, potato has RUE values substantially greater than any other C, species, in the range 1.6 to 1.75 MJ-' (Table I). As discussed previously, the fact that much of the plant product in potato is starch allows a higher accumulation of plant mass per unit of photosynthate than in most other species. The cereals (wheat, barley, and rice) seem to have similar RUE values in the

2 49

RADIATION USE EFFICIENCY

range of about 1.3 to 1.5 g MJ-I, as originally proposed by Monteith (1977). One important outcome of this review, however, is the fact that very few experiments with measures of intercepted radiation have reported RUE for rice. Considering its importance, it seems that there is a need for more extensive measurements of RUE in rice. Among those species that produce energy-rich seed components, sunflower has been found to have high RUE values, although the highest reported values for RUE in sunflower are restricted to the vegetative stage of plant development. Trapani et al. (1992) found potential RUE during vegetative growth to be 1.56 g MJ- which is equal to or greater than those of the C , cereal crops. Also, Bange et al. (1997a) presented data from individual replicates in which RUE was 1.6 g MJ-' or greater. These high RUE values during vegetative growth for a C , species are particularly intriguing. There appears to be no ready explanation for these high RUE values, which indicates a need for more intensive investigation. Over the whole season, however, sunflower is reported to have RUE in the range 1.25 to 1.35 g MJ(Table I). Soybean and peanut have RUE values lower than that of sunflower, approximately 1.05 g MJ-' 10% (Table I). It is likely that the lower RUE value for soybean and peanut is due to the high energy content of the constituents of both vegetative tissues and seeds. In particular, both of these species are composed of tissues with high protein contents. Grain legumes other than soybean and peanut could be expected to have higher RUE values than these two species because other grain legumes tend to produce plant material of lower energy content. Surprisingly, this is not the case, with most measures of RUE in these other grain legumes being less than 1.O g MJ- * (Table I). Only the experiment of Heath and Hebblethwaite (1985) with pea resulted in a large estimate of RUE (1.46 g MJ-') that is comparable to the C , cereals. Overall, these results indicate a potential for increasing RUE in many of the grain legumes. At the least, additional research would be helpful to document and to develop a better understanding of their apparently low RUE values. Only a few experiments have directly compared differences in RUE between C, and C , species. Of course, an inherent difficulty in such species comparisons is the possibility that all species in the comparison may not be fully adapted to the environmental conditions of the test. As a consequence, a particular species may not be able to fully express its potential RUE. Nevertheless, such comparisons generally have produced results that are consistent with comparisons across experiments. Inthapan and Fukai (1988) compared the RUE of rice against that of maize and sorghum. RUE values of maize and sorghum were equivalent, but the RUE value for these two C, species was 42% greater than that of rice. In a comparison of maize and soybean, Daughtry et al. (1992) found that RUE was 1.81 and 0.99 g MJ-I, respectively. While this RUE for maize is a bit higher than that i.n many

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250

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW

studies, this difference between species seems to be fully consistent with other data. Rudorff et al. (1996) compared wheat and maize, but unfortunately the study was done in open-top chambers. Particularly for wheat, the potential for altered radiation environment because of the plastic enclosure could have resulted in the measurement of an abnormally high RUE of 1.71 g MJ-' for wheat. Consequently, Rudorff et al. (1996) detected no difference in the RUE between wheat and maize. Only two direct comparisons have been made between species that have plant tissue differing in biochemical compositions.Muchow et al. (1993) compared the RUE of soybean, mung bean, and cowpea and demonstrated the importance of the difference in the energy content of soybean relative to the other two species when comparing the measured RUE values. A comparison of wheat and lupin was done by Gregory and Eastham (1996). While the RUE of both species tended to be low in this particular study, wheat had a maximum RUE that was 45% greater than that of lupin. This difference in RUE between species, however, is larger than expected based on biochemical composition alone. In summary, important differences in RUE have been confirmed among crop species. There is a tendency for potential RUE in C, species to be greater than those in C , species as indicated in the theoretical analyses. There are, however, overlaps between C, and C, species so that the clear distinction as identified by Gosse et al. (1986) has important exceptions.An important contributor to the variation in RUE species is differences in the energy content of the biochemical constituents of the plant products. For example, sugarcane and potato have low energy products (sucrose and starch, respectively) that appear to result in exceptionally high RUE compared to other species with the same photosynthetic capabilities. Therefore, it is necessary to define separately a potential RUE for each crop species.

B. CO, A~SIMILATION RATE Theoretical analyses have all shown a clear dependence of RUE on leaf CO, assimilation rate (e.g., Fig. 1). Increasing leaf photosynthesis rate is directly linked to increasing RUE, although the response is curvilinear, approaching a maximum RUE at high photosynthesis rates. Due to the difficulty in experimentally relating CO, assimilation to RUE, only a few studies have generated data to examine this relationship. Bennett et al. (1993) measured the leaf CO, exchange rate through the growing season for peanut in conjunction with RUE measures. Both CO, exchange rate and RUE were found to be essentially constant through the bulk of the growing season. Rochette et al. (1995) did a careful examinationof the link between canopy CO, assimilation rate and RUE for a soybean crop. Canopy CO, flux density was mea-

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sured hourly by the eddy correlation technique over 60 days during the growing season. Daily values of net CO, exchange were linear for daily intercepted PAR radiation between 4 and 11 MJ mP2 day-'. Use of the linear relationship over the season introduced only a 4% error compared to use of a nonlinear relationship. Hence, the basic premise of RUE that a linear relationship exists between CO, assimilation and light interception was confirmed. Further, Rochette et al. (1995) calculated RUE for intervals through the growing season by summing the daily measures of CO, assimilation and light interception, and converting to an estimate of dry weight accumulation. They found that the data obtained by direct plant harvest agreed to within about 10% of the CO, assimilation estimate of RUE over the periods in the season when the LA1 was greater than 2.

1. Nitrogen Response Interestingly, the more exhaustive examination of the influence of CO, assimilation influence on RUE has resulted from the linkage between leaf CO, assimilation rate and leaf nitrogen content. In their review, Sinclair and Horie (1989) concluded that variation in leaf CO, was commonly associated directly with changes in leaf nitrogen content per unit leaf area. The theoretical study of Hammer and Wright (1994) provided a detailed analysis of the linkage between RUE and leaf nitrogen content. A number of experimental studies have examined the response of RUE to leaf nitrogen content. In soybean, Sinclair and Shiraiwa (1993) found a positive, curvilinear relationship between RUE and specific leaf nitrogen within each of their two experimental locations. The mean canopy leaf nitrogen content in each case was fairly low (less than 1.6 g mP2). In contrast, in peanut Wright et al. (1993) found little variation in RUE with specific leaf nitrogen except for a nonnodulating line grown in a warm environment. The specific leaf nitrogen content in their study was, however, generally high (greater than about 1.5 g mP2) in those situations where RUE was essentially constant. Therefore, these two contrasting studies are consistent with the theoretical conclusion that there is a saturating response in RUE to increasing leaf nitrogen content (Fig. 2). In sunflower, Gimenez et al. (1994) found that increased soil nitrogen fertility resulted in both an increased specific leaf nitrogen content and increased RUE. In a more detailed study with sunflower, Hall et al. (1995) measured average canopy leaf nitrogen content and RUE for seven periods during the growing season for two levels of soil nitrogen fertility. A curvilinear relationship between RUE and leaf nitrogen content was obtained when using all the data collected through the season, except for the first harvest of the season when RUE values were low. For much of the season RUE was essentially constant, but the last two harvests during seed-fill had both decreased RUE and decreased leaf nitrogen content. Bange et

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al. (1997a) undertook a similar study in sunflower with essentially the same results. In the period from bud visible to anthesis, leaf nitrogen content was high (greater than 1.5 g mP2) and RUE was fairly stable. Following anthesis when leaf nitrogen content decreased, RUE was also observed to decrease. The combined data of Bange et al. (1997a) revealed a curvilinear increase in RUE with leaf nitrogen content. In contrast to the above studies, in sunflower FlCnet and Kiniry (1995) found no changes in RUE among three levels of soil nitrogen applications during most of the growing season. Only during seed growth, when there was no further accumulation of plant mass in the 0 N application treatment, was there a difference in RUE among the treatments. Muchow and Davis (1988) subjected maize and sorghum to a range of nitrogen fertility treatments. During the later stages of vegetative growth, they found a close correlation between RUE and average canopy leaf nitrogen content per unit leaf area. A common linear relationship fitted both species over the range of leaf nitrogen values observed. Muchow and Sinclair (1994) examined the relationship between RUE and leaf nitrogen content in maize and sorghum over a wider range of leaf nitrogen content per unit leaf area. Their results showed a nonlinear, saturating response of RUE to increasing leaf nitrogen. Consistent with these results, Fischer (1993) was able to develop relationships for wheat between RUE and the percent N concentration in the tops of the plants for individual growth periods. Nearly all results, therefore, indicated that RUE achieves a saturated value at high leaf nitrogen contents and decreases curvilinearly with decreasing leaf nitrogen content below the saturating leaf nitrogen content. It is not surprising then that investigations of RUE under treatments of differing soil nitrogen fertility have shown important variations in RUE (Fig. 3). These observations have been done in a number of species, including wheat (Green, 1987; Garcia et al., 1988; Fischer, 1993), sunflower (Gimenez et al., 1994; Hall et al., 1995; Bange et al., 1997a), and maize and sorghum (Muchow and Davis, 1988; Muchow, 1994).

2. Drought Response Soil water deficits can have a major influence on leaf photosynthesis, and, consequently, it is hypothesized that RUE is also decreased under drought conditions. It seems likely that some of the studies that have reported decreased RUE may have been a result of periods of soil water deficit. A few studies that made direct comparisons of RUE under well-watered and drought-stressed conditions have been reported. Muchow (1985) compared the influence of soil water deficits on RUE measured in several grain legumes. Drought stress in all cases resulted in a decreased RUE relative to the well-watered treatments. The effect of the drought stress was particularly marked in a treatment when the crops were established with irrigation for the first 6 weeks and then received no further irrigation. In all comparison the

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drought-stressed treatments had RUE values that were only one-third or less of the controls. Inthapan and Fukai (1988) measured the RUE of rice, maize, and sorghum subjected to irrigated and water-deficit treatments. All three crops had a sharp decrease in RUE in the water-deficit treatment, with rice having the greatest reduction. Muchow (1989b) compared the RUE of maize, sorghum, and pearl millet in a drought study involving three experiments. Only maize consistently had RUE decreased in the water-deficit treatment of each experiment. In two of the experiments, RUE of sorghum and pearl millet was found not to be statistically decreased by the water-deficit treatments. These contrasting responses in RUE to drought stress were interpreted by Muchow (1989b) to indicate differences in the rate of water use from the soil and the severity of drought that was ultimately imposed on each species. The fact that RUE did not decrease in an experiment even though irrigation was withheld was concluded to indicate that a significant soil water deficit had actually not developed. Jamieson et al. (1995) also found that the timing and duration of a drought treatment were important in interpreting RUE. They compared the RUE of barley subjected to 12 irrigation treatments. RUE was decreased in those treatments with early drought such that there was a linear decline in RUE with a water deficit calculated from potential evapotranspiration. Those treatments designed to impose drought stress in middle or late periods during the growing season did not exhibit a decrease in RUE. Unfortunately, soil water content was not measured directly in this experiment so variation in the severity of the soil water deficit could not be compared among the treatments. Singh and Sri Rama (1989), however, did an especially useful RUE experiment in which they measured soil water content of irrigated and nonirrigated plots of chickpea. They generated a relationship between RUE and fraction of extractable soil water whereby RUE was independent of fraction of extractable soil water when extractable soil water was greater than about 30%. At fraction of extractable soil water of less than 30%, however, there was a marked decline in RUE that was represented by an exponential equation. These results, therefore, demonstrated that the response of RUE to water deficits was quantitatively dependent on the severity of the soil water deficit.

3. Other Environmental Factors RUE seems to be fairly stable across environments under optimal growing conditions. For example, Muchow et al. (1993) concluded that the RUE values measured for three grain legumes in two locations were similar in spite of the fact that the environments varied in terms of temperature, solar irradiance, and vapor pressure. Nevertheless, there are circumstances where direct influences of the environment on leaf CO, assimilation rate have been linked to changes in RUE.

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The effect of temperature on the photosynthetic activity of leaves has the potential to result in altered RUE in some species. Andrade et al. (1992) concluded that low RUE values for maize grown at Balcarce, Argentina, were a result of low temperature. In additional experiments with maize using varying sowing dates, Andrade et al. (1993) found a linear decrease in RUE associated with a decrease in mean temperature from 21 to 16°C. They found variation in RUE during vegetative growth of 1.05 to 1.52 g MJ-' over five years that was associated with yearly differences in temperature. The influence of temperature on RUE is especially well documented for peanut. The leaf photosynthetic rate of peanut decreases in an approximately linear relationship with night temperature lower than 16°C (Sinclair et al., 1994; Bell et al., 1994a,b). This response is consistent with the decrease in RUE associated with minimum daily temperature measured at a number of locations, in that there was a linear relationship between the decrease in RUE and night temperature of less than about 20°C (Bell ef al., 1992).In addition, differences in peanut RUE between two locations in Australia (Wright et al., 1993) and between two years in Canada (Bell et al., 1994c) have been attributed to differences in the night temperature environment. Vapor pressure deficit has been indicated as having the potential for influencing RUE (Stockle and Kiniry, 1990). To the extent that a large vapor pressure deficit may result in decreased photosynthetic rates, then a decrease in RUE would be expected. The influence of vapor pressure deficit on leaf photosynthetic rate tends, however, to develop at fairly high vapor pressure deficits (greater than 2 Wa), but even then the decreases in photosynthetic rate are fairly modest (Fig. l b of Stockle and Kiniry, 1990). Nevertheless, Stockle and Kiniry (1990) presented regressions for sorghum and maize using a number of data sources for RUE showing a decrease in RUE with increasing vapor deficit. Their analysis indicated a large effect that was greater than predicted by leaf behavior. Since decreasing vapor pressure deficit is likely to be associated with a number of other environmental variables (e.g., level of solar radiation and fraction of diffuse radiation), a simple regression of RUE against vapor pressure deficit is confounded. In comparing RUE of barley between two seasons, Goyne et al. (1993) reached a conclusion concerning the influence of vapor pressure deficit on RUE similar to Stockle and Kiniry (1990). In one season of 1.06 kPa vapor pressure deficit a RUE of 0.67 g MJ-' was observed, while in another season of 0.68 kPa the RUE was 1.30 g MJ-I. Again, the large difference in RUE between seasons was much greater than anticipated response in leaf photosynthesis at these low values of vapor pressure deficit. Other factors seem likely to have contributed to the large differences in RUE observed between seasons. In contrast to the above, Muchow and Sinclair (1994) found no variation in RUE with vapor pressure deficit for maize and sorghum across six environments of differing vapor pressure deficit. Consideringthe reasonable stability observed in most

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crops across environments, it seems likely that vapor pressure deficit has only a small influence on RUE in most cropping situations. Other factors that inflict damage on leaves have also been shown to decrease RUE. Infection of faba bean plants with Ascochyta fabae resulted in decreased RUE (Madeira et al., 1994). Exposure of leaves to ozone has been found to be associated with decreases in RUE of soybean (Leadley et al., 1990) and of wheat and maize (Rudorff et al., 1996).

C. SEASONAL VARIATION Considering the importance of leaf photosynthetic rates on RUE, variation in photosynthetic capacity through the growing season would be anticipated to have an important influence on variation in RUE. In a number of cases, maximum RUE values are reported for the vegetative period when it is anticipated that the production of new leaves results in much of the radiation intercepted by young leaves with relatively high photosyntheticcapacity. There have been, in fact, several studies that have examined specifically the question of variation in RUE through the growing season. Two periods that potentially may have relatively lower RUE because of diminished leaf photosynthetic capacity are during crop establishment and during seed growth.

1. Crop Establishment Early in crop establishment, when the first leaves may have decreased photosynthetic capacity relative to later leaves, there exists the possibility that RUE may be decreased. This hypothesis is, however, difficult to study experimentally. Substantial errors are possible in measuring radiation interception in crop canopies composed of small plants. The plants can be widely dispersed, and it is difficult to position sensors under the leaf canopy when the leaves are developing near the soil surface. Nevertheless, several studies offer observations on RUE in the early stages of crop development. In a study of wheat RUE, Garcia et al. (1988) found that RUE was lower for the period from double ridge to terminal spikelet than for periods later in the growing season. Similarly, data presented by Fischer (1993) for the wheat cultivars with the highest nitrogen content revealed that growth intervals early in crop development had RUE lower than that achieved later in the season. In sunflower, Trapani et al. (1992) found a lower RUE during the emergence phase than during the period following this period. The point where there was an increase in RUE occurred when LA1 reached about 1.7. They attributed the difference in RUE between the two development periods to differences in leaf photosynthesis capability.Gimenez et al. (1994) also found a lower RUE in sunflower

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during the period from 9 to 42 days after sowing compared to that during the periods including 43 to 7 l days after sowing. These results were further confirmed by Hall et al. (1995), who found that the first harvest period for sunflower yielded a RUE lower than that measured in subsequent harvest periods prior to seed growth. Similar to other species, Rochette et al. (1995) found for soybean that during the early stages of crop development when LA1 was less than 2, RUE was lower than that achieved later in growth. They attributed this lowered RUE to a lowered leaf photosynthetic capacity.

2. SeedGrowth The dynamic interaction between nitrogen storage in leaves and the development of seeds commonly leads to a decrease in leaf nitrogen through the seed growth period. The close linkage between leaf nitrogen and photosynthetic capacity, consequently, raises the important possibility that RUE will decrease during the seed growth period. Fortunately, there have been a number of studies that have segregated RUE data for the vegetative and reproductive growth periods so that the effect of this hypothesis can be examined. Muchow (1985) found that RUE from emergence to 41 days after sowing in six grain legumes was greater than that from 42 days after sowing to maturity. The decrease in RUE was attributed to a loss in leaf photosynthetic activity during grain fill for each of these crops. On the other hand, the results of Leadley et al. (1990) revealed that RUE of soybean grown in open-top chambers was greater during reproductive development than during vegetative development. Lower RUE during the postanthesis period relative to the rapid growth phase prior to anthesis was observed in sunflower by Trapani et al. (1992). Subsequently, Hall et al. (1 995) also reported for sunflower a dramatically decreased RUE at the end of the seed growth period. Bange et al. (1997a) confirmed these previous observations and related the decrease in RUE during seed growth to a decrease in leaf nitrogen content and decreased leaf photosynthetic activity. In maize and sorghum, there are several studies that clearly demonstrated a decline in RUE following anthesis (Muchow and Davis, 1988; Muchow 1989a, 1994). This is illustrated in Table I where much higher RUE values are reported for vegetative growth than for seasonal growth. Calderini et al. (1997) found that RUE for the postanthesis period in wheat was substantially lower than RUE measured for the preanthesis period.

D. RADIATIONENVIRONMENT Theoretical analyses indicate that differences in RUE may result from differences in the radiation environment. Hammer and Wright (1994) concluded from

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such a theoretical analysis that the radiation level on a cloudy as compared to a clear day could result in a 0.4 g MJ-' increase in RUE. An increasing fraction of diffuse radiation on a cloudy day contributed a 0.15 g MJ- I increase in RUE. There are several experimental studies that offer direct evidence concerning the influence of the radiation environment on RUE. Horie and Sakuratani (1985) imposed shading treatments on rice that resulted in shading of 46 and 72%. The resulting RUE values were 1.26 g MJ-' for the unshaded treatment, 1.66 g MJ-' for the 46% shade, and 1.98 g MJ-' for the 72% shade. In an experiment with peanut, Stirling et al. (1990) positioned bamboo weave over plots to give about a 75% shade throughout reproductive development. As a result of this severe shading, the RUE increased from 0.98 g MJ-' in the unshaded treatment to 2.36 g MJ-' for the shaded treatment. Consequently, in experiments with both rice and peanut, severe shading resulted in dramatic increases in RUE. Bange et al. (1997b) placed plastic films over sunflower giving differences in the radiation environment that more realistically matched natural variations in the radiation environment. One of the plastic films resulted in a radiation level that was 86% of incident radiation and an increase in the proportion of diffuse radiation relative to unshaded conditions of 14% on clear days. The second plastic resulted in a radiation level of 80% of incident radiation and a 13% increase in diffuse radiation. In these treatments of fairly modest adjustments in the radiation environment, RUE was increased only by 0.15 and 0.19 g MJ- (1 1 and 14%) in the two treatments, respectively. Plants grown in controlled environments have also resulted in unusually high estimates of RUE. Hammer and Vanderlip (1989) measured RUE of sorghum grown in a greenhouse at 2.16 g MJ-I, which substantially exceeded all other sorghum measures (Table I). Similarly, Rudorff et al. (1996) reported a RUE of 1.71 g MJ- for wheat grown in open-top chambers, which is much greater than other measures of RUE for wheat (Table I). The decrease in total radiation and increase in the proportion of diffuse radiation resulting from the surrounding greenhouse or chamber structure are likely to have been important factors in contributing to elevated RUE in these studies. Overall, the theoretical and experimental results indicate that variations in the natural radiation environment can have an important influence on RUE. In particular, substantial decreases in the total radiation level can result in increased RUE values. Since decreased radiation is almost always associated with an increased proportion of diffuse radiation, the increase in RUE is further enhanced. In the natural environment, however, daily variations in irradiance are averaged by the plant over long time periods so that the small daily variations induced in RUE are not apparent. Only when comparing large, sustained differences in irradiance level are there likely to be differences in RUE. This possibility was examined by Sinclair and Shiraiwa (1993) in a comparison of RUE for soybean measured in Japan and in Florida. The lower radiation environment in Japan, and apparently greater dif-

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fuse component,compared to Florida was offered as the explanationfor the greater RUE measured for soybean in Japan.

VI. CONCLUSIONS In little more than 20 years, radiation use efficiency has become an important, if not essential, approach for understanding crop growth and yield. Because crop yield is linked directly to the capacity of the plants to use the intercepted solar energy in the accumulation of crop mass, RUE provides the measure that directly reflects the efficiency in the use of radiant energy. Considerable confusion has resulted about the RUE concept simply because a number of experimental approaches and units of expression have been presented. As with all experimental results, considerable care is needed to ensure the collection of data with a minimum of error and bias. Among some of the important considerationsin various experimental approachesare direct measures of radiation interception and adequate sampling. considerable caution is needed in considering RUE estimates that are not obtained from primary data. Periodic sampling through the growing season is important so that data are available to do regression analyses and that changes in RUE through the growing season can be detected. The results of both theoretical analyses and experiments are consistent about the major factors that influence RUE. Certainly, RUE varies among species because of differences in the biochemical components of the plant products and in photosynthetic capacity. Among species with similar photosynthetic rates, those plants that produce energy-rich plant products have lower RUE because of the limitation on the production of plant mass. Those species with high leaf photosynthetic activity, especially species with the C, photosynthetic pathway, tend to have higher RUE. There are, however, important exceptions to a clear distinction in RUE between C, and C, species. Sorghum is commonly found to have RUE that is substantially lower than other C, species, with RUE values in the same range as those for C , cereals. In addition, potato and sunflower during vegetative development have been found to have high RUE that are equivalent to maize. These exceptions to a clear distinction in RUE between C, and C, species may be especially important in gaining additional insights about factors influencing the expression of RUE. An important use of the RUE concept is to understand those circumstances in which RUE may not match the expected potential RUE value. Certainly, theoretical analyses have highlighted the importance of decreased photosynthetic activity resulting in decreased RUE. A number of factors, therefore, result in decreased RUE, including water deficits, lowered leaf nitrogen content, and temperature. Changes in leaf photosynthetic capacity through the life cycle of the crop can have a direct influence on changes in RUE. In addition, the environment can have im-

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portant consequences on RUE other than the direct influence on photosynthesis. The radiation environment in particular has been shown to influence RUE. Although not usually a major factor, some of the variation in RUE among various locations may be a consequence of differences in the radiation level or the amount of diffuse radiation. In general, RUE is an independent measure that can be used to benchmark crop performance and highlight yield limitations. RUE is a valuable approach for interpreting large variations in crop yield from season to season and across locations resulting from climatic variations, particularly cumulative solar radiation incident to a crop. Estimates of RUE can also identify crop management and husbandry limitations and can be used to assess the scope for yield improvement in different cropping systems. An important context in which to assess environmental factors on crop growth and RUE is crop models that are based on RUE. The use of RUE in models offers a relatively simple, mechanistically based description of the key factors influencing the accumulation of crop mass. For example, Sinclair (1986) presented one of the first crop models that was fully based on RUE. The model was designed to simulate soybean development and growth by assuming a constant potential RUE of 1.2 g MJ-'. Importantly, the actual RUE in the model was computed daily from potential RUE multiplied by functions that decreased RUE based on simulated leaf N per unit leaf area and soil water content. Hence, the influences of inadequate N or water-deficit stress on RUE were simulated in the model and offered a basis for assessing experimental results (Muchow and Sinclair, 1986). We anticipate that RUE will continue to have increasing importance in the assessment of crop performance and will lead to enhanced efficiency in the conduct of field experimentation. Certainly, measurements of RUE can help to resolve whether intercepted radiation is being used by the crop at its potential efficiency, or whether other factors are limiting crop growth. Careful data collection for evaluations of RUE offers a powerful tool in further understanding of crop growth and yield.

ACKNOWLEDGMENTS The assistance of Heidi Vogelsang (CSIRO, Brisbane,Australia) and Annette Prasse (ARS-USDA, Gainesville, FL, USA) in searching the literature and in organizing and preparing this paper are gratefully acknowledged.

REFERENCES Allen, E. J., and Scott, R. K. (1980).An analysis of growth of the potato crop. J. Agric. Sci. Cambridge 94.583-606.

THOMAS R. SINCLAIR AND RUSSELL C. MUCHOW Anderson, M. C. (1971). Indirect estimates of radiation and stand structure. In “Plant Photosynthetic Production, Manual of Methods’’ (Z. Sestak, J. Catsky, and P. G. Jarvis, eds.), pp. 447-45 1. Junk, The Hague. Andrade, F. H., Uhart, S. A,, Arguissain, G. G., and Ruiz, R. A. (1992). Radiation use efficiency of maize grown in a cool area. Field Crops Res. 28,345-354. Andrade, F. H., Uhart, S. A., and Cirilo, A. (1993). Temperature affects radiation use efficiency in maize. Field Crops Res. 32, 17-25. Arkebauer, T. J., Weiss, A,, Sinclair, T. R., and Blum, A. (1994). Discussion: In defense of radiation use efficiency: A response to Demetriades-Shah et al. (1992). Agric. Foresr Mereorol. 68,221-227. Austin, R. B., Kingston, G., Longden, P. C., and Donovan, P. A. (1978). Gross energy yields and the support energy requirements for the production of sugar from beet and cane: A study of four production areas. J. Agric. Sci. 91,667-675. Bange, M. P., Hammer, G. L., and Rickert, K. G. (1997a). Effect of specific leaf nitrogen on radiation use efficiency and growth of sunflower. Crop Sci. 37, I20 1- 1207. Bange, M. P., Hammer, G. L., and Rickert, K. G. (1997b). Effect of radiation environment on radiation use efficiency and growth of sunflower. Crop Sci. 37, 1208-1214. Bell, M. J., Muchow, R. C., and Wilson, G. L. (1987).The effect of plant population on peanuts (Arachis hypogaea) in a monosoonal tropical environment. Field Crops Res. 17,91- 107. Bell, M. J., Wright, G. C., and Hammer, G. L. (1992). Night temperature affects radiation-use efficiency in peanut. Crop Sci. 32, 1329-1335. Bell. M. J., Michaels, T. E., McCullough, D. E.,and Tollenaar, M. (1994a). Photosynthetic response to chilling in peanut. Crop Sci. 34, 1014-1023. Bell, M. J., Gillespie, T. J., Roy, R. C., Michaels, T. E., and Tollenaar, M. (1994b). Peanut leaf photosynthetic activity in cool field environments. Crop Sci. 34, 1023-1029. Bell, M. J., Roy, R. C., Tollenaar, M., and Michaels, T. E. (1994~).Importance of variation in chilling tolerance for peanut genotypic adaptation to cool, short-season environments. Crop Sci. 34, 1030-1039. Bennett, J. M., Sinclair, T. R., Ma, L., and Boote, K. J. (1993). Single leaf carbon exchange and canopy radiation use efficiency of four peanut cultivars. Peanut Sci. 20, 1-5. Blackman, V. H. (1919). The compound interest law and plant growth. Ann. Bot. 33,353-360. Bolanos, J., and Edmeades, G. 0. (1993). Eight cycles of selection for drought tolerance in lowland tropical maize. I. Responses in grain yield, biomass and radiation utilisation. Field Crops Res. 31, 233-252. Burstall, L., and Harris, P. M. (1986). The physiological basis for mixing varieties and seed ‘‘ages’’ in potato crops. J. Agric. Sci. Cambridge 106,411-418. Calderini, D. F., Dreccer, M. F., and Slafer, G. A. (1997). Consequences of breeding on biomass, radiation interception and radiation-use efficiency in wheat. Field Crops Res. 52,27 1-281. Charles-Edwards, D. A., and Lawn, R. J. (1 984). Light interception by grain legume crops. Plant Cell Environ. 7,247-25 1. Cooper, J. P. (1970). Potential production and energy conversion in temperate and tropical grasses. Herbage Abst,: 40, 1-13. Daughtry, C. S. T., Gallo, K. P., Goward, S. N., Prince, S. D., and Kustas, W. P. (1992). Spectral estimates of absorbed radiation and phytomass production in corn and soybean canopies. Remote Sens. Environ. 39, 14 1- 152. Demetriades-Shah, T. H., Fuchs, M., Kanemasu, E. T., and Flitcroft, I. (1992). A note of caution concerning the relationship between cumulated intercepted solar radiation and crop growth. Agric. Forest Mereorol. 58, 193-207. DeWit, C. T. (1959). Potential photosynthesis of crop surfaces. Nerherlands J. Agric. Sci. 7 , 141-149. DeWit, C. T. (1965). Photosynthesis of leaf canopies. Agricultural Research Rep. No. 663. Institute for Biological and Chemical Research on Field Crops and Herbs, Wageningen, The Netherlands.

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