Effect of timing and severity of water deficit on four diverse rice cultivars II. Physiological responses to soil water deficit

ELSEVIER

Field Crops Research 37 (1994) 215-223

Field Crops Research

Effect of timing and severity of water deficit on four diverse rice cultivars II. Physiological responses to soil water deficit J.M. Lilley*, S. Fukai Department of Agriculture, The Universityof Queensland, Qld. 4072, Australia Received 29 December 1993; accepted 21 June 1994

Abstract Rice shows significant genotypic variation in physiological response to water deficit. This work examines whether this variation is due to genotypic differences in water extraction capability. Leaf elongation rate, leaf rolling, leaf death, and predawn leaf water potential were studied for four rice (Oryza sativa L.) cultivars during water deficit imposed either before panicle initiation (vegetative-stress), or after panicle initiation (reproductive-stress). The four rice cultivars chosen, CPIC8, Lemont, Rikuto-Norin 12 (RN), and Todoroki-Wase (TW), were known to have differing responses to water deficit when grown under upland conditions. Paper I of this series showed differences in rooting pattern and soil water extraction. The cultivar differences in water extraction resulted in differing rates of development of stress. Cultivar RN had poor water extraction and was most sensitive to water deficit, with most rapid decline in leaf elongation, most rapid leaf rolling, and greatest leaf death. TW also had poor water extraction but plants were small and this cultivar escaped severe stress, particularly in the vegetative phase. CPIC8 and Lemont extracted more soil water and were less sensitive to water deficit. After accounting for differences in water extraction ability, cultivar differences in sensitivity of physiological processes to water deficit were small. Leaf elongation rate was more sensitive to water deficit than leaf rolling while pre-dawn leaf water potential, measured after overnight recovery of water status, declined only after those processes almost ceased. Duration of physiological activity was determined to a large extent by the ability of the cultivars to extract water from deep soil layers. Greater extraction prolonged physiological activity and therefore crop growth. This characteristic could be particularly important in an environment where several short periods of water deficit occur.

Keywords: Drought tolerance; Leaf elongation rate; Leaf rolling; Rice; Water stress

1. Introduction It is currently thought that physiological processes are poorly linked to leaf water potential (Sinclair and Ludlow, 1985), and leaf expansion has been shown to * Corresponding author. Present address: Division of Tropical Crops and Pastures, CSIRO, CunninghamLaboratory, St. Lucia, QId. 4067, Australia. 0378-4290/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0378-4290(94)OOO41-A

respond to changes in soil water status before changes appear in bulk leaf water potential (Passioura, 1988). In rice, studies o f genotypic variation in physiological response during water deficit support this general view. For example, Cutler et al. (1980) found that cultivars were inconsistent in their relationship between leaf elongation rate and leaf water potential. Similarly, Turner et al. (1986) found that leaf rolling began at higher midday leaf water potential and turgor pressure

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J.M. Lilley, S. Fukai / FieM Crops Research 37 (1994) 215-223

in upland- than in lowland-adapted cultivars. The greater sensitivity of leaf rolling to water deficit in the upland cultivars delayed development of severe water deficit by 1-3 days. O'Toole and Cruz (1980) also found genotypic variation in sensitivity of leaf rolling to water potential. However, genotypic variation has not been shown in the response of physiological characters such as leaf water potential, leaf rolling and leaf elongation rate to the level of plant-extractable water. In the preceding paper (Lilley and Fukai, 1994), large differences were shown in root development among four cultivars and corresponding differences existed in water extraction capability. In this paper, physiological response to soil water deficit is examined. The aim of this work was to determine if differences among cultivars in their physiological response to water deficit were a function of ability to extract water from the soil. That is, if genotypic variation exists in the response of four physiological traits, leaf elongation rate, leaf rolling, leaf death and leaf water potential, to change in the amount of plantextractable water. Water deficit of varying severity was induced during either vegetative or reproductive stages of growth. Differences among cultivars are discussed in terms of duration of water deficit and soil water availability.

2. Material and methods

2.1. Experimental An experiment was conducted in the 1987/88 summer at The University of Queensland Research Farm, Redland Bay, in south-east Queensland, Australia. A general description of the experimental site and climatic conditions, water stress treatments, and cultivars studied is given in Part I of this series (Lilley and Fukai, 1994). To briefly recap, an experiment was conducted under upland conditions (freely drained soil and no standing water) and contained five water treatments which each contained four cultivars. The four cultivars, CPIC8, Lemont, Rikuto-Norin 12 (RN) and TodorokiWase (TW), differed in root development and consequently in their ability to extract water. The cultivars had different phenological development and sowing was staggered to achieve a similar anthesis date. Cultivars Lemont, RN and TW were sown 7, 18 and 21

days, respectively, after CPIC8. The five treatments were: VS (vegetative-stress), a treatment in which rainfall was excluded with a rain shelter for 24 days during the vegetative stage (54-78 days after sowing); VI (vegetative-irrigated), a corresponding fully-irrigated treatment; RSS (reproductive-severe-stress), rainfall was excluded for 30 days during the reproductive stage (65-95 days after sowing); RMS (reproductive-mild-stress), rainfall was excluded during the same period as RSS but supplemented with four small (12.5 mm) irrigations after 6, 12, 17 and 20 days of water exclusion to create an intermittent stress; RI (reproductive-irrigated), a corresponding fully-irrigated treatment. Vegetative treatments were sown 3 weeks before reproductive treatments to stagger the periods of use of the rain shelter. Due to different ages of the crops when the water deficit was imposed, RSS and RMS reached full canopy cover at the beginning of the stress, while in VS radiation interception was 60-76% for CPIC8, Lemont and RN and 10-13% for TW (Lilley and Fukai, 1994).

2.2. Measurements Evapotranspiration rate was calculated from soil water extraction (Lilley and Fukai, 1994). Mean pan evaporation during each interval of soil water measurement was similar, ranging from 6.1 to 8.2 mm dayin VS and 7.3 to 9.7 mm day- ~ in RSS and RMS, and this small variation was found to have a minimal effect on evapotranspiration rate. Canopy transpiration rate was calculated by excluding water losses below wilting point in the surface soil, and all water loss below a depth of 1.0 m (both were assumed not to involve plant uptake). In the surface layer, wilting point was reached during the first measurement period in all stress treatments; however, there may have been some subsequent upward water movement from deeper layers due to evaporative losses. Canopy transpiration was not calculated for RMS where surface water content fluctuated due to the four small ( 12.5 mm) irrigations. Leaf rolling was assessed visually in each plot, in all treatments. Several plants were assessed and the plot was given a mean leaf-rolling score, scale 1 to 5, 1 being fiat and 5 a tightly rolled leaf (O'Toole and Moya, 1978). Ratings were made near midday about twice per week during the period of water deficit of all treatments.

ii.M. Lilley, S. Fukai/ Field Crops Research37 (1994) 215-223

217

Leaf elongation rate was measured on three tillers in each plot in all treatments. Tillers were selected and tagged several days before water was withheld. Leaf length was measured from the tip of the youngest leaf to the ligule of the leaf below. Measurements were made every 2-4 days from tagging until the flag leaf emerged or for 12 days after rewatering. Daily elongation rate of the youngest leaf was calculated. The mean of the 10 largest leaf elongation rates recorded under irrigated conditions was used to calculate the maximum leaf elongation rate for each cultivar (37, 41, 57 and 36 mm d a y - ~ for CPIC8, Lemont, RN and TW, respectively). Leaf death was measured at the end of the water deficit period in all treatments. Five tillers were selected randomly from a sample of harvested plant material. The length of the dead leaf portion and of the whole leaf blade of the three youngest leaves of each tiller was recorded. Mean percentage of dead leaf was calculated for each tiller. Plant water status was monitored several times during the water deficit period in all treatments. Leaf water potential was measured before dawn with a pressure chamber on three occasions in VI and VS at 9 and 22 days of water exclusion and 6 days after rewatering. In RI, RMS and RSS, pre-dawn leaf water potential was measured at 10, 17 and 24 days of water exclusion. All four replicates were sampled and two samples were taken per plot. The fraction of total extractable water remaining in the soil (FTEW) was determined to compare cultivars on the basis of relative water availability. For each cultivar, extractable water was determined in each layer (the difference between the upper and lower limits of water extraction) and total extractable water was obtained by accumulating extractable water over the profile (Lilley and Fukai, 1994). Fraction of total extractable water remaining in the soil (FTEW) was calculated for each soil water measurement date and interpolated values were used for physiological measurement dates.

changes with time in parameter values. Student' s t-tests were conducted to compare cultivars across treatments. Canopy transpiration was linearly regressed against FLEW, and a sigmoid regression was made of leaf rolling and relative leaf elongation rate against FTEW. Coefficients of regressions were tested to determine if there were significant differences among cultivars.

2.3. Statistical analysis

Leaves remained unrolled (leaf-rolling score = 1) at all measurement occasions in RI, while minor water stress caused slight leaf rolling on two occasions for cultivar RN in VI. In all stress treatments, leaf-rolling measured at midday occurred first in RN, which had the largest leaf-rolling score at subsequent measure-

All treatments were analysed separately. At each measurement date an analysis of variance was conducted for each physiological parameter. Analysis of variance was also conducted to allow comparisons of

3. Results 3.1. Evapotranspiration

Initially, evapotranspiration was larger in the reproductive-stress treatments where plants were larger than in vegetative-stress treatments. In both VS (vegetativestress) and RSS (reproductive-severe-stress), evapotranspiration declined in the first few days and then increased around 10-14 days before slowly declining again (Fig. 1). The increase in evapotranspiration was mostly due to an increase in rate of water loss from the surface soil layer and coincided with leaf rolling (Fig. 2) and therefore increased soil exposure and increased evaporative water loss. In RMS, evapotranspiration rate of all cultivars increased between 10 and 20 days of water deficit due to the irrigations and decreased in the last 10 days of the water deficit period. Cultivar differences in evapotranspiration rate were small in VS except during the first measurement period when evapotranspiration rate of TW was less than the other three cultivars due to its small leaf area. In RMS, evapotranspiration rates of CPIC8 and Lemont tended to be largest, although there was no consistent ranking of the cultivars. In RSS, CPIC8 had a larger evapotranspiration rate than all other cultivars for the first 10 days of water exclusion, after which both CPIC8 and Lemont had larger evapotranspiration rates than the Japanese cultivars, RN and TW. The latter difference was significant when averaged across all measurement periods. 3.2. Leaf rolling

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Fig. 1. Evapotranspirationrate of four cultivars (CPIC8 D, Lemont O, RN A, TW V) in (a) VS, (b) RMS and (c) RSS treatments during a period of water deficit. Arrows show timing of 12.5 mm irrigations in the RMS treatment. Bar is LSD (P=0.05) for comparing cultivars within time and changes within cultivars over time.

Fig. 2. Middayleaf-rollingscoreof fourcultivars (CPIC8 I-q,Lemont O, RN A, TW ~7) in (a) VS, (b) RMS and (c) RSS treatments during a period of water deficit. Arrows show timing of 12.5 mm irrigations in the RMS treatment. Bar is LSD (P=0.05) for comparing cultivars within time and changes within cultivars over time.

ments indicating this cultivar was the most sensitive to water deficit (Fig. 2). Leaf rolling occurred later in CPIC8 than other cultivars but it quickly reached maximum score, while leaves of Lemont tended to roll more gradually. In VS, in which T W intercepted much less radiation than other cultivars (Lilley and Fukai, 1994), leaves of T W rolled significantly less than the other cultivars, reaching a maximum score of 4. TW had greater leaf area in the reproductive-stress treatments (RMS and RSS) than in the vegetative-stress treatment, consequently the greater radiation load caused more rapid soil water depletion and leaves rolled more quickly. In RMS, there was almost no leaf rolling during the first 15 days of water exclusion, except for RN, the most sensitive cultivar, which showed some leaf rolling between irrigations. After 15 days, leaves of all

cultivars were partially rolled at midday, with RN rolling most and T W and Lemont least. In all treatments, leaves of all cultivars unrolled (score of 1 ) within two days of relief from stress. At 14 days after water exclusion in VS and 17 days for RSS and RMS, diurnal measurements indicated leaf rolling increased during the day (Fig. 3 ). In RSS, mean leaf-rolling score was already near maximum at 5.30 am for RN and TW but leaves were almost fiat for CPIC8 and Lemont. Leaf rolling increased only slightly after 10 am in all treatments. 3.3. L e a f elongation rate

To compare cultivars, relative leaf elongation rate (RLER) was calculated by dividing each measured

J.M. Lilley, S. Fukai / Field Crops Research 37 (1994) 215-223

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value by the maximum rate observed for each cultivar. In VS, RLER decreased rapidly for CPIC8, Lemont and RN, but more slowly for TW (Fig. 4a). In RMS, RLER of all cuitivars declined as water deficit developed, fluctuating slightly as the small irrigations were added. These fluctuations allowed differences among cultivars to be expressed more in RMS than VS and RSS (Fig. 4). In general, TW had the largest RLER throughout the water deficit period. In RSS, decline in RLER was very rapid and there was less variation among cultivars (Fig. 4c). Leaf elongation ceased after 9-11 days of water exclusion in RSS and 15 days in VS (except for TW). Leaf elongation recommenced immediately after rewatering for all cultivars in all treatments.

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3.4. Leaf death

There was no death of young leaves in the irrigated treatments (VI and RI) during the treatment period. In VS, about 50% of the length of young leaves of RN were dead but only 8-23% for the other three cultivars (Table 1 ). Leaf death was greater in RSS than VS for all cultivars; RN again had the most dead leaf (87%), followed by TW (59%) with CPIC8 (48%) and Lemont (49%) having the least. In RMS, dead leaf was significantly less than in RSS. Again, dead leaf was greatest for RN (35%), compared to 7-13% for the other three cultivars.

J.M. Lilley, S. Fukai / Field Crops Research 37 (1994) 215-223

220

Table 1 Dead leaf ( % ) (percent d e a d l e a f length o f the three y o u n g e s t leaves) for four cultivars, at the e n d o f the w a t e r deficit period in vegetative-stress ( V S ) , reproductive-mild-stress ( R M S ) and reproductive-severe-stress ( R S S ) treatments Cultivar

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3.5. Leaf water potential Pre-dawn leaf water potential in the irrigated treatments did not differ significantly among cultivars at any measurement occasion, and data are not presented. In each stress treatment, leaf water potential of all cultivars generally declined as water deficit developed (Table 2). Decline in leaf water potential was more rapid in RSS than VS and was slowest in RMS. Pre-

dawn potential of RN and CPIC8 declined more rapidly than in other cultivars and for RN was significantly lower in RSS than in RI after only 10 days of water exclusion. In VS, Lemont and TW had consistently higher leaf water potentials than CPIC8 and RN, while in RSS Lemont had a higher potential than the other three cultivars. 3.6. Plant water stress and available soil water To determine if genotypic variation in physiological response was due to differences in the response to duration of water exclusion rather than severity of soil water deficit, physiological responses were compared on the basis of relative water availability for a cultivar. The fraction of total extractable water remaining in the soil (FTEW) was calculated for each cultivar at each soil water measurement (Lilley and Fukai, 1994) and physiological responses were plotted against interpolated values of FTEW. In VS and RSS, canopy transpiration rate (Lilley and Fukai, 1994) was linearly related to FTEW (r 2 = 0.92 and 0.79 for VS and RSS, respectively; Fig. 5). In RMS, canopy transpiration was not calculated due to difficulty in estimating canopy transpiration accurately with increased soil evaporation after the small irrigations. Leaf rolling did not occur when FTEW was greater than 0.6 in VS and 0.7 in RSS and RMS (Fig. 5). Maximum leaf rolling occurred at higher FTEW in RSS than in RMS and VS. In general, TW was less sensitive to soil water deficit than the other cultivars. At FTEW above 0.5, RN was more sensitive than other cultivars, with one point below the common curve in all three treatments. In RMS, Lemont had a similar response to that of TW, while in RSS, the response functions were similar for all cultivars. As the soil dried, relative leaf elongation rate declined most rapidly in RSS and ceased at higher FTEW (around 0.4) than in VS and RMS (0.2-0.3) (Fig. 5). In all stress treatments, CPIC8, Lemont and RN showed a similar response of leaf elongation rate to FTEW, while TW was less sensitive to drying soil. In VS, leaf elongation of TW continued at FTEW below 0.1, while it ceased around 0.3 in RSS and 0.1 in RMS. At FTEW greater than 0.3, pre-dawn leaf water potential was generally higher than - 1.0 MPa in all

221

J.M. LiUey, S. Fukai / Field Crops Research 37 (1994) 215-223

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treatments. Below an F T E W of 0.3, leaf water potentials as low as - 3.4 M P a were recorded.

4. Discussion This study shows genotypic differences in physiological response to water deficit. Firstly, it examines the rate at which water stress symptoms develop, an important issue in relation to the duration of water deficit. Secondly, it shows that most of the differences in rate o f stress development can be explained by differences in the capability of the cultivars to extract water from the soil.

4.1. Cultivar differences in physiological response to duration of water exclusion In VS, T W escaped severe water stress. Shoots of T W were small, with erect leaves, small radiation inter-

ception at the onset of stress (Lilley and Fukai, 1994) and hence smaller transpiration demand than other cultivars. T W had a higher leaf water potential and less leaf death, later leaf rolling and a faster relative leaf elongation rate than other cultivars throughout the water deficit period in VS. In the reproductive-stress treatments, where leaf area o f T W was greater, response to water exclusion was more rapid. Turner et al. (1986) found that leaf rolling increased with declining midday leaf water potential and that cultivars differed in their sensitivity to changes in leaf water potential and turgor pressure. Although RN intercepted the same amount of radiation as CPIC8 and Lemont (Lilley and Fukai, 1994), it was more sensitive to water deficit. Leaf elongation rate declined most rapidly, leaf death was greatest and leaves of RN were always more tightly rolled than those of the other three cultivars. It appears that RN, which extracted less water, was less able to prevent water loss

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J.M. Lilley, S. Fukai / Field Crops Research 37 (1994) 215-223

and therefore experienced a longer duration of dehydration. Lemont and CPIC8 responded more slowly to water exclusion than RN and TW (except TW, in VS). Less leaf death occurred, leaf elongation rate declined more slowly, and leaf rolling occurred later in the day and later in the water deficit period. This continued physiological activity is consistent with the larger transpiration rates observed for CPIC8 and Lemont. Of these cultivars Lemont tended to be less sensitive than CPIC8.

4.2. Cultivar differences in physiological responses to degree of soil water deficit In general, declining FTEW during the vegetative stage and reproductive stage resulted in similar responses among the four cultivars for canopy transpiration rate, leaf rolling, relative leaf elongation rate and leaf water potential (Fig. 5). However, leaf rolling began in wetter soil in RSS (FTEW = 0.6) than VS (FTEW=0.4), possibly due to larger plant size (older) in RSS which resulted in larger evaporative demand and more rapid development of stress symptoms. There was more scatter in the relationships between physiological responses and FTEW in RMS, indicating that plant response to available water is not consistent in intermittent stress conditions. The presence of a root signal (Passioura, 1988) may explain this inconsistent response to soil water content. CPIC8 and Lemont extracted a similar amount of water (Lilley and Fukai, 1994) and had a similar rate of decline in FI'EW, hence physiological responses to decreasing FTEW were similar for these cultivars. RN extracted less water than CPIC8 and Lemont (Lilley and Fukai, 1994), but at a similar transpiration rate, and therefore F l E W of RN declined more rapidly than for other cultivars in both RSS and VS. The relationships between leaf rolling, relative leaf elongation rate and leaf water potential, and FTEW for RN, CPIC8 and Lemont, were similar, and therefore it appears that the early response of RN to water deficit was due mostly to rapid use of available water. Leaf rolling and leaf elongation of TW were less sensitive to declining F l E W than for other cultivars (Fig. 5), although small leaf area in this cultivar in VS had two confounding effects. Firstly, smaller leaf area would have resulted in less transpiration demand for

this cultivar, therefore conserving water and enabling TW to avoid severe stress. Secondly, greater soil exposure, for the small TW, increased soil evaporation relative to other cultivars, so that transpiration and hence total plant-extractable water would be overestimated. Therefore, FTEW of TW may have been greater than estimated, thus exaggerating the differences between the cultivars. However, this is unlikely in the reproductive-stress treatments where canopy cover of TW was complete and total extractable water, evapotranspiration rate, and hence rate of decline in F I E W of TW were similar to RN (Lilley and Fukai, 1994). In RMS and RSS, TW was able to tolerate drought better than the other cultivars, having a slightly faster relative leaf elongation rate and less leaf rolling at similar b l E W . Small differences in susceptibility to water deficit and duration of physiological activity would be important if several cycles of water deficit and profile replenishment occurred, as this would magnify the differences in crop growth associated with greater physiological activity.

4. 3. Relative sensitivity of different physiological processes to water deficit Canopy transpiration rate and relative leaf elongation rate began declining, and leaves began rolling at FTEW below 1.0, 0.8 and 0.6, respectively (Fig. 5). It is concluded that leaf expansion was more sensitive to water deficit than leaf rolling. Leaf water potential, measured at dawn after the plants had opportunity to restore plant water status was the least sensitive indicator of soil water deficit, declining only after the other physiological processes had almost ceased. This is in agreement with Tanguilig et al. (1987) who found that leaf elongation rate declined sooner than pre-dawn leaf water potential. O'Toole and Cruz (1980) also found that leaf rolling began before pre-dawn leaf water potential declined, and that stomatal resistance began increasing around the same time as leaf rolling began.

5. Conclusion

Despite large differences between both severity and rate of stress development in VS, RMS and RSS, differences between cultivars in physiological responses to stress were consistent across the treatments. Geno-

J.M. Lilley, S. Fukai / FieM Crops Research 37 (1994) 215-223

typic variation in water extraction capability, which was partly due to differences in rooting pattern, resulted in different rates of stress development, and therefore differences between cultivars in the level of stress experienced. By comparing physiological responses to water stress at a given level of water availability ( F T E W ) , rather than at a given duration of water exclusion, it was possible to establish differences between cultivars in susceptibility to water deficit. T W was more tolerant o f water exclusion during reproductive stress, but avoided severe water deficit during vegetative stress due to small leaf area. RN was more susceptible to declining F T E W than other cultivars. F T E W declined more rapidly for some cultivars (particularly R N ) , and this resulted in more rapid stress development and a longer duration of severe stress. W e conclude that increased water extraction capability extends physiological activity and could therefore increase biomass production and yield. Physiological processes differed in sensitivity to soil water deficit, with leaf elongation and canopy transpiration being more sensitive than leaf rolling, while pre-dawn leaf water potential declined only after these physiological processes had almost ceased.

Acknowledgments W e thank Tim Hughes and staff at Redland Bay Research Farm for agronomic management of the experiment. Technical assistance of T. Eadie and J.

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Jackson was greatly appreciated. Valuable comments on the manuscript by K. Weaich and J.R. Wilson are gratefully acknowledged. This work was completed as part of a PhD thesis while JML was supported by a Commonwealth Postgraduate Research Award at The University of Queensland.

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