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Water extraction by grain sorghum in a sub-humid environment. I. Analysis of the water extraction pattern
Field Crops Research, 33 ( 1993 ) 81-97
81
Elsevier Science Publishers B.V., Amsterdam
Water extraction by grain sorghum in a subhumid environment. I. Analysis of the water extraction pattern M.J. Robertson a, S. Fukai a, M.M. L u d l o w b a n d G.L. H a m m e r c ~Department of Agriculture, University of Queensland, St. Lucia, Qld., Australia bCSIRO Division of Tropical Crops and Pastures, St. Lucia, Qld., Australia CQueensland Department of Primary Industries, Toowoomba, Qld., Australia (Accepted 1 July 1992)
ABSTRACT Robertson, M.J., Fukai, S., Ludlow, M.M. and Hammer, G.L., 1993. Water extraction by grain sorghum in a sub-humid environment. I. Analysis of the water extraction pattern. Field Crops Res., 33:8197. Under water-limiting conditions, water extraction by a dryland crop is limited by the depth of the root system, and by the rate and degree of water extraction. The water extraction pattern of 6 crops of grain sorghum under continuous soil drying in a sub-humid sub-tropical environment was analysed in terms of two components: the rate of descent of the extraction front down the soil profile (extraction front velocity), and the time required to extract 90% of the extractable water from each depth after the extraction front arrived ( l/klgo). Extractable water content (0a), at each depth, was defined as the difference between the stable water content (0) at the start of extraction and the lower asymptote of the exponential decay curve of 0 versus time (lower limit ). The crops varied in genotype, level of evaporative demand, degree of tillering, and plant population density, and were grown on the same soil type over two seasons. The aim of the study was to test the stability of the extraction front velocity, 0, and of l/klgo under different agronomic and environmental conditions, to assess their usefulness for modelling water extraction of sorghum. Extraction front velocity varied little among the 6 crops with an overall average of 3.43 cm day- I. The extraction front of crops that were grown under long, terminal drying cycles continued to descend until early grain-filling, reaching a maximum depth of 190 cm. The value of 1/klgo in the upper 100 cm of the profile varied considerably across crops. It is shown that this variation can be explained by variation in root length density and level of evaporative demand. For crops exposed to a long, terminal drying cycle, the actual water extracted below 150 cm depth was less than 0a in the soil layers above 150 cm. This was due to both a lower 0a below 150 cm, associated with low root length density, and also insufficient time between the arrival of the extraction front and maturity, for the crop to extract all the water above the lower limit.
Correspondence to: M.J. Robertson, C S I R O Division o f Tropical Crops and Pastures, Private Bag P.O., Aitkenvale, Qld. 4814, Australia.
82
M.J. ROBERTSONET AL.
INTRODUCTION
Water extraction is a key component of crop growth simulation models, which are a powerful tool for assessing crop performance particularly in environments where water supply is variable and relatively unpredictable. The aim of this series of two papers is to identify conservative quantities that characterise root growth and the water extraction process, and which can be used in crop growth simulation models. The degree to which water availability limits crop production depends upon the balance between the supply of water from the root system and demand from the atmosphere. When water is in ample supply from the soil, extraction is largely controlled by demand. As soil water is depleted by a crop, extraction becomes limited by water supply from the root system. Extraction of water from the soil is determined by the depth of the root system, and by the rate and degree to which it can extract water. Despite being severely stressed, many droughted crops leave a substantial amount of apparently available water in the subsoil at maturity (Hamblin, 1985). What prevents crops from fully utilising this water? Are the dimensions of the root system inadequate to fully utilise the stored water quickly and thoroughly enough (Taylor, 1984)? Or is it that while crop root systems are extensive enough to be able theoretically to extract all the available water, poor extraction is due to less effective root function at depth in the profile (Passioura, 1983)? The factors that limit water extraction by annual crops in water-limiting environments, need clearer identification. A framework, which can be used to analyse these questions, has been proposed by Monteith (1986). It is basically a combination of a mathematical function describing the descent of the extraction front with time and an exponential equation that describes the decline in water content with time after the arrival of the extraction front at each depth. This framework is being used as a basis for simulating water extraction in crop growth models (Monteith et al., 1989; Chapman et al., 1990). The basic aspects of the framework have been validated for sorghum and millet on two soil types in India during the dry season (Monteith, 1986), and for sunflower on a wide range of soil types in semi-arid Australia (Meinke et al., 1993). Both of these environments had high evaporative demand. The first aim of this paper is to further test the Monteith framework and to establish the robustness of the principal parameters in environments of milder evaporative demand, such as the sub-humid subtropics. Under an environment of milder demand, the extraction front may descend at a slower rate and the exponential decline in water content may be slower. Also, little is known of how the values of the parameters of the framework vary due to agronomic factors such as plant population density, genotype and degree of tiUering. Accordingly, the second objective of this paper is to examine the response of the principal parameters of the framework
WATER EXTRACTION BY GRAIN SORGHUM. I.
83
to variation in agronomic conditions. This paper reports the analysis of extraction patterns by crops of grain sorghum grown in two seasons at a subhumid location, under a variety of agronomic conditions. A related issue, of the relation between water extraction and root growth, is addressed in a companion paper (Robertson et al., 1993 ). MATERIALS AND METHODS
Framework for analysing water extraction The analytical framework, proposed by Monteith (1986), is basically a combination of a function describing the downward movement of an extraction front and a function that describes the extraction behaviour of a static root system. The root system of an annual crop growing in a drying soil effectively descends as a two-dimensional front. As roots grow downward, water is extracted progressively from deeper layers. When roots enter a new layer and begin to extract water, the water content (0) in that layer starts to decline. An abrupt change in 0 identifies the time of arrival of the extraction front. The downward velocity of the extraction front can be determined by plotting the depth of each layer against the time after sowing when extraction starts. Extrapolation of this relationship to the time axis gives the intercept, which represents the lag phase for the descent of the extraction front. One assumption in the framework is that when a crop is growing solely on stored water, the extraction front corresponds closely to the root front with a few leading roots advancing ahead of the extraction front. This assumes that extraction begins in a layer of soil as soon as the root front reaches it. Once extraction begins in a layer, 0 usually declines exponentially with time, so long as demand exceeds supply. The exponential decay follows the relation described by Passioura ( 1983 ): 0= 01+ 0aexp ( - k l ( t - t c ) )
(l)
where 0 is water content (cm 3 cm -3 ), 0~is the lower limit of extractable water content (cm 3 cm-3), t - tc is duration (days) of the exponential decay, which starts in a layer at time tc, I is root length density (cm of root per c m 3 of soil) and k (cm 2 day - ~) is a constant relating to the diffusivity of water flow to and through the roots. In reality, I changes with time, so the value of k represents an "average" diffusivity for the duration of extraction in the layer. The product kl can be regarded as an extraction decay constant, i.e. the fraction of remaining extractable water that is extracted each day. 0a is the maximum amount of water that roots are capable of extracting from surrounding soil, and is usually taken as the volumetric water content above the lower limit when the exponential decay begins ( t = to). 01 is defined as the lower asymptote of the exponential curve of 0 with time. It is not always automatically
84
M.J. ROBERTSON ET AL.
equal to the lowest value of 0 achieved by a crop under a drying cycle. Whether or not 0 approaches the extrapolated 0t is dependent on the length of the drying cycle and other constraints such as root length density, or the time available for extraction before maturity. The derivative of eqn. ( 1 ) with respect to time gives an expression for the extraction rate (cm 3 c m - 3 d a y - l ) ,
-dO/dt=klOaexp(-kl( t-tc) ) =kl(O-Ol)
(2) (3)
indicating that the extraction rate decreases linearly with 0 above the lower limit. Dividing the extraction rate by the root length density gives the uptake rate per unit root length, q (cm 3 cm - ~ day - ~) as a function of 0
q=k( O-O~)
(4)
This analytical framework assumes that once the root system arrives at a given depth there is relatively little change in root length and therefore kl can be taken as close to constant for a soil layer. The parameter k will be conservative within a soil type across sites and seasons, for a crop growing on stored water in an environment where evaporative d e m a n d nearly always exceeds the supply from the roots. This is because the roots are extracting water from the soil as fast as physically possible and k assumes its m a x i m u m value. The series of experiments reported here were undertaken to derive the values of the parameters to, kl, Oaand k as a function of soil depth, for grain sorghum (Sorghum bicolor (L.) Moench) growing on stored soil water. Factors such as genotype, tillering pattern, plant population, and level of d e m a n d were varied among the experiments to test the stability of these parameters.
Experimental Location. All experiments reported in this series of papers were conducted at the University of Queensland Research Farm at Redland Bay (latitude 27 ° 37'S, longitude 153 ° 19'E, 5 m a.s.l. ), south-east Queensland, Australia. The soil was a deep friable red soil (oxisol) with a clay content 60-80% throughout the profile and a pH range of 5.5-6.2. The dry bulk density ranged from 1.03 g c m - 3 at l 0 cm to 1.4 g c m - 3 below 130 cm. No obvious chemical or physical constraints to root growth were present in the profile to the maxi m u m rooting depth of sorghum. The climate at this location during the s u m m e r growing season is characterised by a mean temperature of 23-25 oC, incident solar radiation of 20-25 MJ m -2 d a y - 1, a daytime saturation deficit of 1.0 kPa and wind speed of 14 m s-1. The level of evaporative d e m a n d is moderate for the subtropics. Solar radiation, wind run, m i n i m u m and m a x i m u m air temperature, rainfall,
WATEREXTRACTIONBY GRAIN SORGHUM. I.
85
wet and dry bulb temperature were recorded daily at 0900 h local time, at a location 200 m from the experimental site. The mean daytime saturation deficit was calculated from wet bulb temperature depression at 0900 h, and maximum temperature. Potential evaporation was estimated by the Penman (1948) equation using the algorithms of Meyer et al. (1987), and a wind function for grain sorghum from Rosenthal et al. (1989).
Treatments and design. In four experiments an automatic, mobile rain shelter, of the the type described by Foale et al. (1979), was used to exclude rainfall from the plots in two adjoining areas at different times during each experiment. Experiment l: The aim was to establish the validity of the analytical framework for describing the extraction patterns. The plots were subjected to a preanthesis drying cycle. Experiment 2: Genotypic variation in the extraction parameters was investigated by comparing the extraction of two cultivars, cv. E57 (regarded as highly drought-resistant) and cv. Pride (regarded as moderately drought-resistant), during a pre-anthesis drying cycle. Experiment 3: The effect of tiller removal on water extraction was investigated during a long drying cycle extending from 22 days after sowing (das) TABLE 1
Sowing date, length of the drying cycle, crop growth characteristics, and mean weather conditions during the drying cycle for each experiment Experiment
1
Sowingdate
17 Nov. 1987 20Oct. 1988
2
Treatment
Duration of drying cycle (das)
3 16 Jan. 1989
16 Jan. 1989
E57
Pride Uniculm +Tillers
28-66
21-61
2161
22-102
22-102
22-102
56 90 4.22 1199 532
65 55 59 102 95 102 3.63 2.85 1.52 1247 969 683 663 515 367
61 104 1.55 698 386
60 103
Crop growth Days from sowing to: Anthesis Maturity LAIat anthesis Maturity biomass ( g m -2) Grain yield (g m -z)
1.96 758 373
Weather conditions Solar radiation (MJ m -2 day- ~) Min. temperature (°C) Max. temperature (°C) Saturation deficit (kPa) Penman evaporation ( m m d a y -~)
24 21 28 1.1 4.18
23 19 28 1.2 3.91
23 17 19 19 28 27 1.2 0.8 3.91 2.81
17 19 27 0.8 2.81
17 19 27 0.8 2.81
86
M.J. ROBERTSON ET AL.
to maturity. The two tillering treatments were: ( 1 ) tillers were prevented from developing, by removing them whenever they appeared ("uniculm"), and (2) a freely-tillering treatment where tillers were left intact (" + tillers" ). The crop was grown at a low plant population density ( 10 plants m -2) to stimulate tillering. Experiment 4: This experiment was sown at the same time as Experiment 3 at a higher density (20 plants m - 2 ) to see if there was any impact of plant population density on the extraction parameters. Experiments 1 and 2 were sown early and Experiments 3 and 4 late in the summer growing season resulting a higher level of evaporative demand in Experiments 1 and 2 than Experiments 3 and 4. The sowing date, length of the drying cycles, crop growth characteristics, and environmental conditions in each experiment are presented in Table 1. In all experiments, except where otherwise stated, cultivar Pride was sown. Treatments (e.g. genotype or tiller manipulation) were randomised. There were 4 replicates, except in Experiment 1 where there were 6. Plot size was 8 × 4 m in Experiment 1, and 6 × 3 m in all other experiments.
Crop management. All crops were grown at 20 plants m -2 with 0.5 m row spacing, except in Experiment 3 where the plant population was l0 plants m -2. Before sowing, all crops received a broadcast application of nitrogen, phosphorus and potassium in a commercial fertiliser formulation, at a rate of 8 g (each element) m -2. The crops were thinned to the required plant populations two weeks after sowing, and irrigated frequently to aid establishment. A side dressing of 8 g m - 2 of nitrogen was broadcast as urea at about 20 days after sowing, and the plots were then irrigated to aid incorporation of the fertiliser. Helicoverpa spp., aphids and sorghum midge (Contarinia sorghicola) were controlled by applications, respectively, of 400 g ha -1 dimethoate (75% BASF Perfektion E240), 225 g h a - 1 methomyl ( 1% Du Pont Lannate L) and 1 g ha -1 diazinon 80. Leaf rust (Puccinia purpurea) was controlled by an application of 80% mancozeb (Crop King Dithane M-45 ) at the rate of 0.2 g m -2. Developing panicles were protected from bird damage by enclosing sample areas in 16-mm nylon mesh netting intercepting 10% of solar radiation at anthesis. Weeds were controlled by hand-hoeing. Irrigation was applied from 1-m-high sprinkler lines at a rate of about 15 m m h - t, and applied every week to plots not currently in a drying cycle, at a rate sufficient to replace water lost by evapotranspiration. Measurements. 0 was measured by neutron moderation (CPN, Model 503 DR). The access tubes were sited at the mid-interrow position. Readings were made using a 16-s count at 10- or 20-cm depth intervals, depending on the experiment, every 3-4 days during the drying cycle. A calibration equation to determine 0 from neutron moisture meter counts was determined from field
WATER EXTRACTION BY GRAIN SORGHUM. I.
87
measurements made at the same site during crop growth at various stages of soil water depletion. 0 in the 0-20 cm layer was determined gravimetrically by taking a 36-mm core within 1 m radius of the access tube. Dates of anthesis (exsertion of anthers on more than 50% of the panicle) and maturity (presence of black layer on more than 75% of the grains of the panicle) was determined by regular inspection and scoring 8 adjacent plants per plot. Above-ground biomass at anthesis and maturity was sampled by cutting plants in a 1.0-m 2 quadrat at ground level from the inner rows of each plot. A distance of at least 1.0 m was left between successive quadrats. The number o f # a n t s in each sample was recorded. A subsample of 5 plants was taken and the fresh weight of the subsample and of the remainder of the sample was determined. The subsample was partitioned into green lamina, stem plus sheath plus dead lamina, and panicle and then dried at 80°C. At anthesis the green lamina area was measured with a planimeter and leaf area index was calculated from the weight of the green lamina sub-sample as a proportion of above-ground biomass, and specific leaf area. At maturity, five panicles per plot were threshed and grain yield determined. All above-ground biomass is expressed on an oven-dry basis. Roots were sampled from each layer of 10 or 20 cm thickness to 200 cm depth, with a 36- or 42-ram i.d. coring tube. The cores were obtained at 66, 67, 102 and 102 days after sowing in Experiments l, 2, 3 and 4, respectively. Two cores per plot were taken, one at the mid-interrow position and the other in the row. 'Background' roots were sampled from about four cores taken in an adjacent fallow area. The soil cores were stored at 4°C until they were washed. The washed roots were stored in water at 4 °C until they were read with a digital scanner that had been calibrated with known lengths of cotton thread. The scanner detected only live, white roots with a diameter exceeding 0. l ram. Repeated measurements on the same sample showed that the estimated length had a coefficient of variation of about 5%. The treatment mean of the background roots was subtracted from the measured mean to obtain the value of root length density.
Data analysis.
In each experiment, for each access tube and at each depth of measurement, equations were fitted using an iterative fitting procedure to the time course of 0 during the drying cycle:
O=Ou
t<~tc
(6)
0=0~+((0u-0~) X e x p ( - k l x ( t - t c ) ) )
t>tc
(7)
where 0 is volumetric water content (cm 3 cm-3 ), 0u is the initial soil water content, 0~ is the lower limit of extractable soil water and therefore 0u-01 is
88
M.J. ROBERTSON ET AL.
the plant-extractable water content, t is time after sowing (days), tc is time after sowing at which the exponential decline begins, k l i s the extraction decay constant ( d a y - l ). The equation was not fitted to the data from the 0-20 cm layer because of soil evaporation and the associated diffusion of water towards the surface as it dries. The function was also not fitted to some of the deep layers because of lack of points in the exponential phase. For ease of comprehension, values of k l were converted to a time constant for extraction of 90% of the extractable water - hereafter called 1/kl9o - with units of days. A linear regression was fitted to tc and depth of each layer from the soil surface to determine the downward progress of the extraction front. In the experiments with long drying cycles extending until maturity, step-wise linear regression was used to determine the final depth of the extraction front. Treatment means were compared in a randomised design using least significant differences (LSD). Where error indices occur in the text and on figures, they refer to the standard error of the mean (sem). RESULTS
The crops in this study all produced biomass at maturity in excess of 650 g m - 2 and grain yield higher than 350 g m -2 (Table 1 ). LAI at anthesis was greater than 1.5. The crops in Experiments 1 and 2, which were droughted only for the pre-anthesis period, produced the highest biomass and grain yield at maturity. A typical example of the water extraction pattern is presented for selected layers from one access tube in the uniculm treatment of Experiment 3 (Fig. 1 ). The pattern followed the general behaviour predicted by the Monteith (1986) framework. At the start of the drying cycle (22 days after sowing), extraction started simultaneously in all layers down to 50 cm (data not shown), presumably because roots were already present to this depth. In deeper layers, 0 remained constant at 0u, until the extraction front arrived. Once extraction started at each depth 0 declined exponentially with time. Extraction was observed to 190 cm, although in this experiment, the exponential curves could only be fitted with confidence to layers at and above 170 cm. The descent of the extraction front was similar across experiments for depths down to 150 cm. The combined regression for all experiments and treatments gave an extraction front velocity of 3.43 c m d a y - I starting 14.8 days after sowing (Fig. 2 ). Most of the variation in the extraction front velocity among experiments was due to variation in tc for layers below 150 cm and values of the extraction front velocity ranged across the experiments and treatments from 3.27 to 4.92 cm day -1 (Table 2). The variation was probably due the errors in estimating tc in deep layers where the decline in 0 was slow. The length of the lag phase varied little, being between 14.8 and 19.3 days (Table 2 ). In Experiment 3, the regression gave a final depth of extraction of 190 cm,
89
WATER EXTRACTION BY GRAIN SORGHUM. I. 0.38
i
Depth (cm) :'o i
lgO 170 150
0.36
B 0.34 © "*~
0,32
O L,
0.30
-~
0 28
"~ 0
0.26
~,
vv~7
110
024 0
90
V V v
50
D D [Z
...
"E].D
I
I
I
I
I
20
40
60
80
1 O0
20
Time after sowing (days) Fig. 1. The water extraction pattern and fitted curves for selected layers measured from one access tube o f the +tillers treatment o f Experiment 3. A curve was not fitted to the data at 190 cm.
reached at 73 das, 14 days after anthesis. In Experiment 4, the maximum depth of extraction was also 190 cm, reached at 64 days after sowing. There was no significant difference in extraction front velocity between E57 and Pride in Experiment 2, and no effect of tiller removal in Experiment 3. There was little effect on the extraction front velocity of the lower plant population density in Experiment 3, compared to the higher density crop in Experiment 4, which was sown at the same time. Values of the extractable water content (0a), are presented in Table 3. In the upper 100 cm of the profile 0a varied little across experiments, being between 0.042 and 0.072 cm a cm -3 (Table 3 ). There was no significant difference in 0, at any depth, between E57 and Pride in Experiment 2 and between the uniculm and + tillers treatments in Experiment 3. In the experiments with long drying cycles (3 and 4), 0a below about 150 cm declined with depth to be negligible at 170 cm. Total extractable water in the 200-cm profile was 81 _+5, 85 _+5 and 90+ 5 mm for the uniculm and +tillers crops in Experiments 3 and 4, respectively. In all three crops, the proportion of 0a that was actually extracted at maturity declined from about 90% at 130 cm to less than 40% at 170 cm. The decline in 0a with depth below 150 cm was associated with sparse root length density at depth. Figure 3 shows the relation between root length density measured during early grain-filling, and 0~ measured in the treatments subject to terminal drying. 0, was 0.04-0.06 cm 3 cm-3 for root length densi-
90
M.J. ROBERTSONET AL.
Time after sowing 20 0
40 p
(days)
60 i
80 i
1 O0 i
Experiment 20
I
O
2 3 unieulm
v D
+Tillers
•
40
60 v
0
J:=
80
oJ 100 0
120 0 0
140
X r-..4
160
A \Dill ~O\D• z~ D \ •
180
A
200 I
I
I
I
I
Fig. 2. Depth o f the extraction front with time using mean values at each depth from each experiment. The fitted line is Y= 3.43 ( + 0.20) X X - 50.7 ( + 10.1 ) (R 2 = 94%, n = 81, P < 0.001 ).
TABLE2 Regression coefficients ( + sem) for the relationship between extraction front depth and time after sowing. The lag period for the descent of the extraction front, is the intercept of the regression with the time axis and is calculated from -c/m, where c and m are the intercept and slope of the regression, respectively; n = number of observations in the regression Experiment
1
Treatment Extraction front velocity (cm day -1 ) Lag period (days) R 2 (%) n
3.88 (0.15) 16.3 (3.05) 94 81
2
3
4
E57
Pride
Unicuim
+Tillers
3.97 (0.41) 17.8 (4.01) 74 34
4.92 (0.25) 14.8 (2.00) 87 45
3.62 (0.19) 19.3 (2.60) 88 50
3.27 (0.17) 15.9 (2.51) 88 51
4.39
(0.18) 18.0
(1.75) 91 72
91
WATER E X T R A C T I O N BY G R A I N S O R G H U M . I.
TABLE 3 Values of the extractable water content ( + sem) (cm 3 cm- 3), in selected layers for each experiment Experiment
1
2
Treatment Depth (cm) 50
0.058
(0.002) 70
0.053 (0.001) 0.052 (0.002) 0.046 (0.002)
90 110
3
4
E57
Pride
Uniculm
+Tillers
0.050 (0.003) 0.063 (0.007) 0.049 (0.010) 0.047 (0.082)
0.062 (0.011) 0.072 (0.006) 0.061 (0.009) 0.051 (0.006)
0.050 (0.005) 0.050 (0.004) 0.050 (0.001) 0.049 (0.004) 0.048
0.043 (0.001) 0.045 (0.005) 0.049 (0.005) 0.049 (0.004) 0.056 (0.002) 0.046 (0.025) 0.010 (0.004)
130
(0.011) 150
0.065 (-)' 0.049 (-)
170
0.042 (0.001) 0.053 (0.004) 0.055 (0.005) 0.051 (0.004) 0.048 (0.005) 0.037 (0.008) 0.037 (O.006)
qndicates only one observation per mean.
co I
E © co
C' 0.06
© 4.J 0.04 O O
[
•
.
0 02
T • ±
Expt 3 +tillers Expt 3 umculm Expt 4
.+..)
N f~
0.00 0.0
i
t
0.1
0.2
l
03
• Q •
[] 0
i
0.4
i
05
0.6
Root length density (era e m -3) Fig. 3. Relation between root length density during early grain-filling and the extractable water content. Data are from Experiments 3 and 4, which were subjected to a terminal drying cycle. Points from depths above 150 cm have open symbols, and those from below 150 cm have closed symbols. Standard errors are shown except where they are smaller than the symbol.
92
M.J. ROBERTSON ET AL.
ties above about 0.2 cm cm-3, but below this threshold, extractable water was less. While 0a was fairly stable across experiments, the extraction decay constant, kl, varied markedly, particularly in the upper profile (Table 4). The largest values in the top 120 cm of the profile were obtained in Experiment 1 (0.12-0.19 day - 1 ), followed by Experiment 2 (0.06-0.12 day - ~), and the smallest values were recorded in Experiments 3 and 4 (0.03-0.08 day-~ ). These values are equivalent to values of 1/k19o of 12-19 days for Experiment 1, 20-38 days for Experiment 2, and 28-62 days for Experiments 3 and 4. In the experiments with terminal drying, k/decreased with depth below 110 cm (i.e. the rate of decline in water content was slower) from 0.04-0.08 dayat 110 cm (1/kl9o=30-60 days) to about 0.01-0.02 day -~ at 170 cm (1/ k19o= 115-230 days). There were no significant differences in kl between E57 and Pride in Experiment 2, and neither between the tillering treatments in Experiment 3. The extraction front passed 120 cm about 50 days after sowing (see Fig. 2). As 1/klgo was greater than 50 days for the layers below the 120-cm depth, there was insufficient time to extract 90% of the total extractable water before physiological maturity at about 90-100 days after sowing. For instance, in TABLE 4 Values of the extraction decay constant, k l ( + sem) ( d a y - ' ), in selected layers for each experiment. Also shown are the mean values for layers between 50 and 110 cm Experiment
1
2
3
4
E57
Pride
Uniculm
+Tillers
0.12 (0.013) 0.19 (0.016) 0.14 (0.010) 0.14 (0.008)
0.08 (0.017) 0.07 (0.019) 0.14 (0.044) 0.07 (0.013)
0.09 (0.021) 0.07 (0.023) 0.12 (0.054) 0.06 (0.027)
0.06 (0.009) 0.05 (0.011 ) 0.06 (0.027) 0.04 (0.016) 0.03 (0.004) 0.02 (-)' 0.02 (-)
0.08 (0.008) 0.06 (0.006) 0.06 (0.020) 0.04 (0.008) 0.02 (0.004) 0.02 (0.009) 0.02 (0.001)
0.08 (0.014) 0.06 (0.009) 0.07 (0.021) 0.07 (0.029) 0.04 (0.012) 0.04 (0.008) 0.02 (0.003)
0.14
0.09
0.08
0.05
0.05
0.07
Treatment Depth (cm)
50 70 90 110 130 150 170
Mean
(50-110 cm)
qndicates only one observation per mean.
WATEREXTRACTION BY GRAIN SORGHUM. I.
93
the uniculm treatment in Experiment 3, the residual 0a, increased from 0.00 at 120 cm to 0.03 cm 3 cm -3 at 170 cm. Across the three crops subjected to a terminal drying cycle, the residual water was about 10 m m or 12% of the extractable water in the 200 cm profile. Differences among the experiments in the values of the extraction decay constant (kl) could be related in part to differences in root length density (l). The parameter k, when multiplied by ( 0 - 01) ( see equation 4 ), represents the uptake rate per unit root length, and may be similar among crops if variation in kl is due to variation in/. Taking the mean value for the layers between 50 and 110 cm, kl varied nearly 3-fold across experiments. Variation in I accounts for some of the variation in kl, because k varied only 1.5-fold across experiments (Table 5 ). In Experiments 1 and 2 values of k in the upper profile were similar, indicating that differences in root length density explain the differences in rate of extraction between these experiments. However, root length density can only partially explain the differences when Experiments 1 and 2 are compared to 3 and 4, i.e. the uptake rate per unit root length was higher for Experiment 1 and 2. The difference in k between these two groups of experiments is probably related to the level of evaporative demand. The mean rate of potential evaporation during the drying cycle, as estimated by the Penman (1948) equation was 3.9-4.2 m m day -~ for Experiments 1 and 2, and 2.8-3.0 m m d a y - l for Experiments 3 and 4. In Experiment 3, the value of k declined with depth below 110 cm, whereas in Experiment 4 it was relatively constant with depth. A decline in k with TABLE5 Values of k (cm 2 day -~ ) in selected layers for each experiment. The values of k were derived by dividing kl by the/, the root length density using treatment means. Also shown are the mean values for layers between 50 and 110 cm Experiment
1
Treatment Depth (cm) 50 70 90 110 130 150 170 Mean (50-110cm)
2
Uniculm
+ Tillers
4
E57
Pride
0.21 0.37 0.27 0.28
0.20 0.16 0.30 0.40
0.22 0.14 0.36 0.46
0.20 0.14 0.23 0.18 0.15 0.06 0.05
0.27 0.19 0.31 0.15 0.09 0.05 0.03
0.35 0.12 0.20 0.17 0.28 0.24 0.19
0.28
0.27
0.30
0.19
0.23
0.21
94
M.J. ROBERTSON ETAL.
depth suggests that the extraction rate per unit root length at depth was lower than at the surface. DISCUSSION
Extraction front The results of Experiments 3 and 4 show that when a sorghum crop is grown only on stored water, the extraction front descends at a constant rate until about 10 days after anthesis to a maximum depth of about 190 cm. Other studies have shown that if soil conditions do not seriously limit root growth, sorghum roots reach a maximum depth of 160-200 cm (Miller, 1916; Mayaki et al., 1976; Kaigama et al., 1977), although none of these studies showed at what stage of crop growth roots reach the maximum depth of penetration. The extraction front descended at rates that varied between 3.3 and 4.9 cm day-~, and was remarkably stable across experiments despite variation in genotype, tillering pattern, plant population density, and level of evaporative demand. Nakayama and van Bavel (1963) recorded a range in root front penetration rates in the field of 1.9-4.9 cm day-~ using a radiotracer technique. The values found in this study also agree with those measured for tropical cereals in India on vertisol and alfisol soil types (Monteith, 1986 ). This suggests that the extraction front velocity of sorghum is relatively insensitive to soil properties, as also found by Meinke et al. (1993) for sunflower. The extraction front velocity was also stable across experiments despite differences in evaporative demand in agreement with other studies (Squire et al., 1987). There was little effect of the lower plant population density on the extraction front velocity in Experiment 3. Extraction decay constant The variation in extraction rate (k/) among experiments shows that it is not a conservative parameter for describing water extraction for a particular combination of species and soil type. The value of kl varied between 0.04 and 0.19 d a y - i in the top 100 cm of the profile, became smaller with increasing depth below 100 cm, and varied among experiments due to differences in root length density and evaporative demand. The difference between Experiments 1 and 2 on one hand, and 3 and 4 on the other, is probably due to later sowing of the latter, when radiation, temperature and saturation deficit are lower (Table 1 ). This had the combined effect of reducing the mean rate of potential evaporation during the drying cycle from 3.9 (Experiment 1 ) and 4.2 m m day -~ (Experiment 2) to 2.8 (Experiment 3) and 3.0 m m day -~ (Experiment 4). This lower demand was probably associated with a lower extraction rate over a prolonged period of time. This suggests that the value of the extraction decay constant is determined by conditions over the length of the
WATER EXTRACTION BY GRAIN SORGHUM. I.
95
drying cycle, which makes it difficult to determine values of kl for supplylimiting conditions. Across experiments the m a x i m u m value of k, the root-soil diffusivity, in the upper profile was about 0.4 c m 2 day - 1. From equation 4, it can be shown that if the m a x i m u m value of k is 0.4 cm 2 day - 1, and 0 above the lower limit is 0.05 c m 3 c m - 3 , then q has a value of 0.02 cm 3 c m - i d a y - ~ when 0 is at the upper limit. This value is close to the m a x i m u m potential uptake rate per unit root length of 0.03 cm 3 cm - ~ day - l quoted by Ritchie ( 1985 ) for wheat and maize. M a x i m u m instantaneous rates of 0.19 cm 3 c m - 1 d a y - t for sorghum growing in solution culture have been recorded (Meyer and Ritchie, 1980). However, as Ritchie ( 1985 ) points out, m a x i m u m uptake rates in the field will be lower, because flow resistance in unsaturated soil near the roots is always higher than in solution culture, and uptake rates in the field are an average for a day rather than instantaneous. Limitations to poor extraction at depth Extrapolation of the exponential decay in 0 to the lower limit allows one to distinguish between extractable water (0a) and water actually extracted by the end of the growth period: Under long drying cycles, most of the extractable water was extracted above the 150-cm depth in the profile. However, below 150 cm the residual water content increased with depth. This lack of extraction was due to two limitations. Firstly, below 150 cm, 0a declined with depth, which was correlated with a decline in root length density. This type of response is a similar to that observed by Barraclough and Weir ( 1988 ), who found for wheat on a clay loam that the a m o u n t of water extracted was limited by root uptake capacity below a threshold root length density of 1.0 cm c m - 3. Secondly, the actual water extracted below 150 cm was considerably less than the potentially extractable because of lack of time before maturity. For example, in those experiments with terminal drying cycles, 70% of the extractable water at 150 cm was actually extracted by maturity. Another 10 days past maturity was needed for the roots at this depth to take up 90% of the extractable water. Other studies have also reported incomplete extraction at depth despite crops being severely stressed (e.g. Jordan and Miller, 1980; Barraclough and Weir, 1988 ), but did not attempt to define quantitatively the limitations to poor extraction. Application of the Monteith framework allows analysis of the reasons for poor extraction at depth in terms of a poorly developed root system or insufficient time to extract all the water before maturity. At depths below about 110 cm in Experiment 3, k decreased with depth, implying that the potential uptake rate per unit root length (q) declined with depth, presumably because some component of the root-soil hydraulic resistance increased with depth. However, in Experiment 4, k and hence q was
96
M.J. ROBERTSON ET AL.
relatively stable with depth. The variation o f k with depth shows that q cannot be assumed to be always constant for all parts o f the root system.
Usefulness of the Monteith (1986)framework The results o f this study has shown that the framework o f Monteith (1986) can be used to analyse the extraction pattern o f sorghum growing on stored soil water in a sub-humid environment, and add to the evidence that the extraction front velocity is a conservative quantity for a crop species across a range o f soil types, environments, genotypes and agronomic conditions. However, variation in root length density and evaporative d e m a n d among crops means that the extraction decay constant determined under continuous drying, is not necessarily a stable quantity that describes the potential rate o f uptake. This is especially the case where d e m a n d is can be low, such as in the subh u m i d tropics. A strength o f the Monteith ( 1986 ) approach to analysing water extraction is that the possible reasons for limited extraction at depth can be more easily identified as being due either to inadequate time for extraction or lack o f roots. In this study, poor extraction below 150 cm was due both to lack o f effective roots and insufficient time before maturity. Despite the growing use o f the Monteith ( 1 9 8 6 ) framework to analyse the extraction pattern (Meinke et al., 1993), there is little understanding of the relationship between the growth o f the root system and the framework parameters. This topic is examined in a c o m p a n i o n paper ( R o b e r t s o n et al., 1993). ACKNOWLEDGMENT M.A. Foale, o f C S I R O Division o f Tropical Crops and Pastures, m a d e valuable c o m m e n t s on an early draft o f the manuscript.
REFERENCES Barraclough, P.B. and Weir, A.H., 1988. Effects of a compacted subsoil layer on root and shoot growth, water use and nutrient uptake of winter wheat. J. Agric. Sci., Camb., 110: 207-216. Chapman, S.C., Meinke, H. and Hammer, G.L., 1990. A crop simulation model for sunflower. In: Proceedings of the International Symposium on Climatic Risk in Crop Production, 2-6 July 1990, Brisbane, Australia, pp. 40-41. Foale, M.A., Davis, R. and Macrae, C.D., 1979. A versatile, low-budget, automatic rain shelter for small field experiments. CSIRO, Australia. Tropical Agronomy Technical Memorandum, 18 Nov. 1979. Hamblin, A., 1985. The influence of soil structure on water movement, crop root growth and water uptake. Adv. Agron., 38: 95-158. Jordan, W.R. and Miller, F.R., 1980. Genetic variability in sorghum root systems: implications for drought tolerance. In: N.C. Turner and P.J. Kramer (Editors), Adaptation of Plants to Water and High Temperature Stresses. Wiley Interscience, New York, NY, pp. 383-399.
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Kaigama, B.K., Teare, I.D., Stone, L.R. and Power, W.L., 1977. Root and top growth of irrigated and non-irrigated grain sorghum. Crop Sci., 17: 555-559. Mayaki, W.C., Stone, L.R. and Teare, I.D., 1976. Irrigated and non-irrigated soybean, corn and grain sorghum root systems. Agron. J., 68: 532-534. Meinke, H., Hammer, G.L. and Want, P.J., 1993. Potential soil water extraction by sunflower on a range of soils. Field Crops Res., 32: 59-81. Meyer, W.S. and Ritchie, J.T., 1980. Resistance to water flow in the sorghum plant. Plant Physiol., 65: 33-37. Meyer, W.S., Dunin, F.X., Smith, R.C.G., Shell, G.S.G. and White, N.S., 1987. Characterising water use by irrigated wheat at Griffith, New South Wales. Aust. J. Soil Res., 25:499-515. Miller, E.C., 1916. Comparative study of the root systems and leaf areas of corn and sorghums. J. Agric. Res., 6:311-347. Monteith, J.L., 1986. How do crops manipulate water supply and demand. Phil. Trans. R. Soc. London A., 316: 245-289. Monteith, J.L., Huda, A.K.S. and Midya, D., 1989. RESCAP: A resource capture model for sorghum and pearl millet. In: S.M. Virmani, H.L.S. Tandon and G. Alagarswarmy (Editors), Modeling the Growth and Development of Sorghum and Pearl Millet. Research Bulletin no. 12, ICRISAT, Patancheru, India, pp. 30-34. Nakayama, F.S. and van Bavel, C.H.M., 1963. Root activity distribution patterns of sorghum and soil moisture conditions. Agron. J., 55:271-274. Passioura, J.B., 1983. Roots and drought resistance. Agric. Water Manag., 7: 265-280. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. London Ser. A., 193: 120-146. Ritchie, J.T., 1985. A user-orientated model of the soil water balance in wheat. In: W. Day and R.K. Arkin (Editors), Wheat Growth and Modelling. Plenum, New York, NY, pp. 293-305. Robertson, M.J., Fukai, S., Ludlow, M.M. and Hammer, G.L., 1993. Water extraction by grain sorghum in a sub-humid environment. II. Extraction in relation to root growth. Field Crops Res., 33:99-112. Rosenthal, W.R., Vanderlip, R.L., Jackson, B.S. and Arkin, G.F., 1989. SORKAM: A grain sorghum crop growth model. Research Center Program and Model Documentation MP- 1669, Texas Agricultural Experimental Station, College Station, TX. Squire, G.R., Ong, C.K. and Monteith, J.L., 1987. Crop growth in semi-arid environments. In: Proceedings of the International Pearl Millet Workshop, ICRISAT Center, Patancheru, India, April 1986. Taylor, H.M., 1984. Modifying root systems of cotton and soybean to increase water absorption. In: H.M. Taylor, W.R. Jordan, and T. R. Sinclair (Editors), Limitations to Efficient Water Use in Crop Production. ASA-CSSA-SSSA, Madison, Wl, pp. 57-64.
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Water extraction by grain sorghum in a subhumid environment. I. Analysis of the water extraction pattern M.J. Robertson a, S. Fukai a, M.M. L u d l o w b a n d G.L. H a m m e r c ~Department of Agriculture, University of Queensland, St. Lucia, Qld., Australia bCSIRO Division of Tropical Crops and Pastures, St. Lucia, Qld., Australia CQueensland Department of Primary Industries, Toowoomba, Qld., Australia (Accepted 1 July 1992)
ABSTRACT Robertson, M.J., Fukai, S., Ludlow, M.M. and Hammer, G.L., 1993. Water extraction by grain sorghum in a sub-humid environment. I. Analysis of the water extraction pattern. Field Crops Res., 33:8197. Under water-limiting conditions, water extraction by a dryland crop is limited by the depth of the root system, and by the rate and degree of water extraction. The water extraction pattern of 6 crops of grain sorghum under continuous soil drying in a sub-humid sub-tropical environment was analysed in terms of two components: the rate of descent of the extraction front down the soil profile (extraction front velocity), and the time required to extract 90% of the extractable water from each depth after the extraction front arrived ( l/klgo). Extractable water content (0a), at each depth, was defined as the difference between the stable water content (0) at the start of extraction and the lower asymptote of the exponential decay curve of 0 versus time (lower limit ). The crops varied in genotype, level of evaporative demand, degree of tillering, and plant population density, and were grown on the same soil type over two seasons. The aim of the study was to test the stability of the extraction front velocity, 0, and of l/klgo under different agronomic and environmental conditions, to assess their usefulness for modelling water extraction of sorghum. Extraction front velocity varied little among the 6 crops with an overall average of 3.43 cm day- I. The extraction front of crops that were grown under long, terminal drying cycles continued to descend until early grain-filling, reaching a maximum depth of 190 cm. The value of 1/klgo in the upper 100 cm of the profile varied considerably across crops. It is shown that this variation can be explained by variation in root length density and level of evaporative demand. For crops exposed to a long, terminal drying cycle, the actual water extracted below 150 cm depth was less than 0a in the soil layers above 150 cm. This was due to both a lower 0a below 150 cm, associated with low root length density, and also insufficient time between the arrival of the extraction front and maturity, for the crop to extract all the water above the lower limit.
Correspondence to: M.J. Robertson, C S I R O Division o f Tropical Crops and Pastures, Private Bag P.O., Aitkenvale, Qld. 4814, Australia.
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INTRODUCTION
Water extraction is a key component of crop growth simulation models, which are a powerful tool for assessing crop performance particularly in environments where water supply is variable and relatively unpredictable. The aim of this series of two papers is to identify conservative quantities that characterise root growth and the water extraction process, and which can be used in crop growth simulation models. The degree to which water availability limits crop production depends upon the balance between the supply of water from the root system and demand from the atmosphere. When water is in ample supply from the soil, extraction is largely controlled by demand. As soil water is depleted by a crop, extraction becomes limited by water supply from the root system. Extraction of water from the soil is determined by the depth of the root system, and by the rate and degree to which it can extract water. Despite being severely stressed, many droughted crops leave a substantial amount of apparently available water in the subsoil at maturity (Hamblin, 1985). What prevents crops from fully utilising this water? Are the dimensions of the root system inadequate to fully utilise the stored water quickly and thoroughly enough (Taylor, 1984)? Or is it that while crop root systems are extensive enough to be able theoretically to extract all the available water, poor extraction is due to less effective root function at depth in the profile (Passioura, 1983)? The factors that limit water extraction by annual crops in water-limiting environments, need clearer identification. A framework, which can be used to analyse these questions, has been proposed by Monteith (1986). It is basically a combination of a mathematical function describing the descent of the extraction front with time and an exponential equation that describes the decline in water content with time after the arrival of the extraction front at each depth. This framework is being used as a basis for simulating water extraction in crop growth models (Monteith et al., 1989; Chapman et al., 1990). The basic aspects of the framework have been validated for sorghum and millet on two soil types in India during the dry season (Monteith, 1986), and for sunflower on a wide range of soil types in semi-arid Australia (Meinke et al., 1993). Both of these environments had high evaporative demand. The first aim of this paper is to further test the Monteith framework and to establish the robustness of the principal parameters in environments of milder evaporative demand, such as the sub-humid subtropics. Under an environment of milder demand, the extraction front may descend at a slower rate and the exponential decline in water content may be slower. Also, little is known of how the values of the parameters of the framework vary due to agronomic factors such as plant population density, genotype and degree of tiUering. Accordingly, the second objective of this paper is to examine the response of the principal parameters of the framework
WATER EXTRACTION BY GRAIN SORGHUM. I.
83
to variation in agronomic conditions. This paper reports the analysis of extraction patterns by crops of grain sorghum grown in two seasons at a subhumid location, under a variety of agronomic conditions. A related issue, of the relation between water extraction and root growth, is addressed in a companion paper (Robertson et al., 1993 ). MATERIALS AND METHODS
Framework for analysing water extraction The analytical framework, proposed by Monteith (1986), is basically a combination of a function describing the downward movement of an extraction front and a function that describes the extraction behaviour of a static root system. The root system of an annual crop growing in a drying soil effectively descends as a two-dimensional front. As roots grow downward, water is extracted progressively from deeper layers. When roots enter a new layer and begin to extract water, the water content (0) in that layer starts to decline. An abrupt change in 0 identifies the time of arrival of the extraction front. The downward velocity of the extraction front can be determined by plotting the depth of each layer against the time after sowing when extraction starts. Extrapolation of this relationship to the time axis gives the intercept, which represents the lag phase for the descent of the extraction front. One assumption in the framework is that when a crop is growing solely on stored water, the extraction front corresponds closely to the root front with a few leading roots advancing ahead of the extraction front. This assumes that extraction begins in a layer of soil as soon as the root front reaches it. Once extraction begins in a layer, 0 usually declines exponentially with time, so long as demand exceeds supply. The exponential decay follows the relation described by Passioura ( 1983 ): 0= 01+ 0aexp ( - k l ( t - t c ) )
(l)
where 0 is water content (cm 3 cm -3 ), 0~is the lower limit of extractable water content (cm 3 cm-3), t - tc is duration (days) of the exponential decay, which starts in a layer at time tc, I is root length density (cm of root per c m 3 of soil) and k (cm 2 day - ~) is a constant relating to the diffusivity of water flow to and through the roots. In reality, I changes with time, so the value of k represents an "average" diffusivity for the duration of extraction in the layer. The product kl can be regarded as an extraction decay constant, i.e. the fraction of remaining extractable water that is extracted each day. 0a is the maximum amount of water that roots are capable of extracting from surrounding soil, and is usually taken as the volumetric water content above the lower limit when the exponential decay begins ( t = to). 01 is defined as the lower asymptote of the exponential curve of 0 with time. It is not always automatically
84
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equal to the lowest value of 0 achieved by a crop under a drying cycle. Whether or not 0 approaches the extrapolated 0t is dependent on the length of the drying cycle and other constraints such as root length density, or the time available for extraction before maturity. The derivative of eqn. ( 1 ) with respect to time gives an expression for the extraction rate (cm 3 c m - 3 d a y - l ) ,
-dO/dt=klOaexp(-kl( t-tc) ) =kl(O-Ol)
(2) (3)
indicating that the extraction rate decreases linearly with 0 above the lower limit. Dividing the extraction rate by the root length density gives the uptake rate per unit root length, q (cm 3 cm - ~ day - ~) as a function of 0
q=k( O-O~)
(4)
This analytical framework assumes that once the root system arrives at a given depth there is relatively little change in root length and therefore kl can be taken as close to constant for a soil layer. The parameter k will be conservative within a soil type across sites and seasons, for a crop growing on stored water in an environment where evaporative d e m a n d nearly always exceeds the supply from the roots. This is because the roots are extracting water from the soil as fast as physically possible and k assumes its m a x i m u m value. The series of experiments reported here were undertaken to derive the values of the parameters to, kl, Oaand k as a function of soil depth, for grain sorghum (Sorghum bicolor (L.) Moench) growing on stored soil water. Factors such as genotype, tillering pattern, plant population, and level of d e m a n d were varied among the experiments to test the stability of these parameters.
Experimental Location. All experiments reported in this series of papers were conducted at the University of Queensland Research Farm at Redland Bay (latitude 27 ° 37'S, longitude 153 ° 19'E, 5 m a.s.l. ), south-east Queensland, Australia. The soil was a deep friable red soil (oxisol) with a clay content 60-80% throughout the profile and a pH range of 5.5-6.2. The dry bulk density ranged from 1.03 g c m - 3 at l 0 cm to 1.4 g c m - 3 below 130 cm. No obvious chemical or physical constraints to root growth were present in the profile to the maxi m u m rooting depth of sorghum. The climate at this location during the s u m m e r growing season is characterised by a mean temperature of 23-25 oC, incident solar radiation of 20-25 MJ m -2 d a y - 1, a daytime saturation deficit of 1.0 kPa and wind speed of 14 m s-1. The level of evaporative d e m a n d is moderate for the subtropics. Solar radiation, wind run, m i n i m u m and m a x i m u m air temperature, rainfall,
WATEREXTRACTIONBY GRAIN SORGHUM. I.
85
wet and dry bulb temperature were recorded daily at 0900 h local time, at a location 200 m from the experimental site. The mean daytime saturation deficit was calculated from wet bulb temperature depression at 0900 h, and maximum temperature. Potential evaporation was estimated by the Penman (1948) equation using the algorithms of Meyer et al. (1987), and a wind function for grain sorghum from Rosenthal et al. (1989).
Treatments and design. In four experiments an automatic, mobile rain shelter, of the the type described by Foale et al. (1979), was used to exclude rainfall from the plots in two adjoining areas at different times during each experiment. Experiment l: The aim was to establish the validity of the analytical framework for describing the extraction patterns. The plots were subjected to a preanthesis drying cycle. Experiment 2: Genotypic variation in the extraction parameters was investigated by comparing the extraction of two cultivars, cv. E57 (regarded as highly drought-resistant) and cv. Pride (regarded as moderately drought-resistant), during a pre-anthesis drying cycle. Experiment 3: The effect of tiller removal on water extraction was investigated during a long drying cycle extending from 22 days after sowing (das) TABLE 1
Sowing date, length of the drying cycle, crop growth characteristics, and mean weather conditions during the drying cycle for each experiment Experiment
1
Sowingdate
17 Nov. 1987 20Oct. 1988
2
Treatment
Duration of drying cycle (das)
3 16 Jan. 1989
16 Jan. 1989
E57
Pride Uniculm +Tillers
28-66
21-61
2161
22-102
22-102
22-102
56 90 4.22 1199 532
65 55 59 102 95 102 3.63 2.85 1.52 1247 969 683 663 515 367
61 104 1.55 698 386
60 103
Crop growth Days from sowing to: Anthesis Maturity LAIat anthesis Maturity biomass ( g m -2) Grain yield (g m -z)
1.96 758 373
Weather conditions Solar radiation (MJ m -2 day- ~) Min. temperature (°C) Max. temperature (°C) Saturation deficit (kPa) Penman evaporation ( m m d a y -~)
24 21 28 1.1 4.18
23 19 28 1.2 3.91
23 17 19 19 28 27 1.2 0.8 3.91 2.81
17 19 27 0.8 2.81
17 19 27 0.8 2.81
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to maturity. The two tillering treatments were: ( 1 ) tillers were prevented from developing, by removing them whenever they appeared ("uniculm"), and (2) a freely-tillering treatment where tillers were left intact (" + tillers" ). The crop was grown at a low plant population density ( 10 plants m -2) to stimulate tillering. Experiment 4: This experiment was sown at the same time as Experiment 3 at a higher density (20 plants m - 2 ) to see if there was any impact of plant population density on the extraction parameters. Experiments 1 and 2 were sown early and Experiments 3 and 4 late in the summer growing season resulting a higher level of evaporative demand in Experiments 1 and 2 than Experiments 3 and 4. The sowing date, length of the drying cycles, crop growth characteristics, and environmental conditions in each experiment are presented in Table 1. In all experiments, except where otherwise stated, cultivar Pride was sown. Treatments (e.g. genotype or tiller manipulation) were randomised. There were 4 replicates, except in Experiment 1 where there were 6. Plot size was 8 × 4 m in Experiment 1, and 6 × 3 m in all other experiments.
Crop management. All crops were grown at 20 plants m -2 with 0.5 m row spacing, except in Experiment 3 where the plant population was l0 plants m -2. Before sowing, all crops received a broadcast application of nitrogen, phosphorus and potassium in a commercial fertiliser formulation, at a rate of 8 g (each element) m -2. The crops were thinned to the required plant populations two weeks after sowing, and irrigated frequently to aid establishment. A side dressing of 8 g m - 2 of nitrogen was broadcast as urea at about 20 days after sowing, and the plots were then irrigated to aid incorporation of the fertiliser. Helicoverpa spp., aphids and sorghum midge (Contarinia sorghicola) were controlled by applications, respectively, of 400 g ha -1 dimethoate (75% BASF Perfektion E240), 225 g h a - 1 methomyl ( 1% Du Pont Lannate L) and 1 g ha -1 diazinon 80. Leaf rust (Puccinia purpurea) was controlled by an application of 80% mancozeb (Crop King Dithane M-45 ) at the rate of 0.2 g m -2. Developing panicles were protected from bird damage by enclosing sample areas in 16-mm nylon mesh netting intercepting 10% of solar radiation at anthesis. Weeds were controlled by hand-hoeing. Irrigation was applied from 1-m-high sprinkler lines at a rate of about 15 m m h - t, and applied every week to plots not currently in a drying cycle, at a rate sufficient to replace water lost by evapotranspiration. Measurements. 0 was measured by neutron moderation (CPN, Model 503 DR). The access tubes were sited at the mid-interrow position. Readings were made using a 16-s count at 10- or 20-cm depth intervals, depending on the experiment, every 3-4 days during the drying cycle. A calibration equation to determine 0 from neutron moisture meter counts was determined from field
WATER EXTRACTION BY GRAIN SORGHUM. I.
87
measurements made at the same site during crop growth at various stages of soil water depletion. 0 in the 0-20 cm layer was determined gravimetrically by taking a 36-mm core within 1 m radius of the access tube. Dates of anthesis (exsertion of anthers on more than 50% of the panicle) and maturity (presence of black layer on more than 75% of the grains of the panicle) was determined by regular inspection and scoring 8 adjacent plants per plot. Above-ground biomass at anthesis and maturity was sampled by cutting plants in a 1.0-m 2 quadrat at ground level from the inner rows of each plot. A distance of at least 1.0 m was left between successive quadrats. The number o f # a n t s in each sample was recorded. A subsample of 5 plants was taken and the fresh weight of the subsample and of the remainder of the sample was determined. The subsample was partitioned into green lamina, stem plus sheath plus dead lamina, and panicle and then dried at 80°C. At anthesis the green lamina area was measured with a planimeter and leaf area index was calculated from the weight of the green lamina sub-sample as a proportion of above-ground biomass, and specific leaf area. At maturity, five panicles per plot were threshed and grain yield determined. All above-ground biomass is expressed on an oven-dry basis. Roots were sampled from each layer of 10 or 20 cm thickness to 200 cm depth, with a 36- or 42-ram i.d. coring tube. The cores were obtained at 66, 67, 102 and 102 days after sowing in Experiments l, 2, 3 and 4, respectively. Two cores per plot were taken, one at the mid-interrow position and the other in the row. 'Background' roots were sampled from about four cores taken in an adjacent fallow area. The soil cores were stored at 4°C until they were washed. The washed roots were stored in water at 4 °C until they were read with a digital scanner that had been calibrated with known lengths of cotton thread. The scanner detected only live, white roots with a diameter exceeding 0. l ram. Repeated measurements on the same sample showed that the estimated length had a coefficient of variation of about 5%. The treatment mean of the background roots was subtracted from the measured mean to obtain the value of root length density.
Data analysis.
In each experiment, for each access tube and at each depth of measurement, equations were fitted using an iterative fitting procedure to the time course of 0 during the drying cycle:
O=Ou
t<~tc
(6)
0=0~+((0u-0~) X e x p ( - k l x ( t - t c ) ) )
t>tc
(7)
where 0 is volumetric water content (cm 3 cm-3 ), 0u is the initial soil water content, 0~ is the lower limit of extractable soil water and therefore 0u-01 is
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M.J. ROBERTSON ET AL.
the plant-extractable water content, t is time after sowing (days), tc is time after sowing at which the exponential decline begins, k l i s the extraction decay constant ( d a y - l ). The equation was not fitted to the data from the 0-20 cm layer because of soil evaporation and the associated diffusion of water towards the surface as it dries. The function was also not fitted to some of the deep layers because of lack of points in the exponential phase. For ease of comprehension, values of k l were converted to a time constant for extraction of 90% of the extractable water - hereafter called 1/kl9o - with units of days. A linear regression was fitted to tc and depth of each layer from the soil surface to determine the downward progress of the extraction front. In the experiments with long drying cycles extending until maturity, step-wise linear regression was used to determine the final depth of the extraction front. Treatment means were compared in a randomised design using least significant differences (LSD). Where error indices occur in the text and on figures, they refer to the standard error of the mean (sem). RESULTS
The crops in this study all produced biomass at maturity in excess of 650 g m - 2 and grain yield higher than 350 g m -2 (Table 1 ). LAI at anthesis was greater than 1.5. The crops in Experiments 1 and 2, which were droughted only for the pre-anthesis period, produced the highest biomass and grain yield at maturity. A typical example of the water extraction pattern is presented for selected layers from one access tube in the uniculm treatment of Experiment 3 (Fig. 1 ). The pattern followed the general behaviour predicted by the Monteith (1986) framework. At the start of the drying cycle (22 days after sowing), extraction started simultaneously in all layers down to 50 cm (data not shown), presumably because roots were already present to this depth. In deeper layers, 0 remained constant at 0u, until the extraction front arrived. Once extraction started at each depth 0 declined exponentially with time. Extraction was observed to 190 cm, although in this experiment, the exponential curves could only be fitted with confidence to layers at and above 170 cm. The descent of the extraction front was similar across experiments for depths down to 150 cm. The combined regression for all experiments and treatments gave an extraction front velocity of 3.43 c m d a y - I starting 14.8 days after sowing (Fig. 2 ). Most of the variation in the extraction front velocity among experiments was due to variation in tc for layers below 150 cm and values of the extraction front velocity ranged across the experiments and treatments from 3.27 to 4.92 cm day -1 (Table 2). The variation was probably due the errors in estimating tc in deep layers where the decline in 0 was slow. The length of the lag phase varied little, being between 14.8 and 19.3 days (Table 2 ). In Experiment 3, the regression gave a final depth of extraction of 190 cm,
89
WATER EXTRACTION BY GRAIN SORGHUM. I. 0.38
i
Depth (cm) :'o i
lgO 170 150
0.36
B 0.34 © "*~
0,32
O L,
0.30
-~
0 28
"~ 0
0.26
~,
vv~7
110
024 0
90
V V v
50
D D [Z
...
"E].D
I
I
I
I
I
20
40
60
80
1 O0
20
Time after sowing (days) Fig. 1. The water extraction pattern and fitted curves for selected layers measured from one access tube o f the +tillers treatment o f Experiment 3. A curve was not fitted to the data at 190 cm.
reached at 73 das, 14 days after anthesis. In Experiment 4, the maximum depth of extraction was also 190 cm, reached at 64 days after sowing. There was no significant difference in extraction front velocity between E57 and Pride in Experiment 2, and no effect of tiller removal in Experiment 3. There was little effect on the extraction front velocity of the lower plant population density in Experiment 3, compared to the higher density crop in Experiment 4, which was sown at the same time. Values of the extractable water content (0a), are presented in Table 3. In the upper 100 cm of the profile 0a varied little across experiments, being between 0.042 and 0.072 cm a cm -3 (Table 3 ). There was no significant difference in 0, at any depth, between E57 and Pride in Experiment 2 and between the uniculm and + tillers treatments in Experiment 3. In the experiments with long drying cycles (3 and 4), 0a below about 150 cm declined with depth to be negligible at 170 cm. Total extractable water in the 200-cm profile was 81 _+5, 85 _+5 and 90+ 5 mm for the uniculm and +tillers crops in Experiments 3 and 4, respectively. In all three crops, the proportion of 0a that was actually extracted at maturity declined from about 90% at 130 cm to less than 40% at 170 cm. The decline in 0a with depth below 150 cm was associated with sparse root length density at depth. Figure 3 shows the relation between root length density measured during early grain-filling, and 0~ measured in the treatments subject to terminal drying. 0, was 0.04-0.06 cm 3 cm-3 for root length densi-
90
M.J. ROBERTSONET AL.
Time after sowing 20 0
40 p
(days)
60 i
80 i
1 O0 i
Experiment 20
I
O
2 3 unieulm
v D
+Tillers
•
40
60 v
0
J:=
80
oJ 100 0
120 0 0
140
X r-..4
160
A \Dill ~O\D• z~ D \ •
180
A
200 I
I
I
I
I
Fig. 2. Depth o f the extraction front with time using mean values at each depth from each experiment. The fitted line is Y= 3.43 ( + 0.20) X X - 50.7 ( + 10.1 ) (R 2 = 94%, n = 81, P < 0.001 ).
TABLE2 Regression coefficients ( + sem) for the relationship between extraction front depth and time after sowing. The lag period for the descent of the extraction front, is the intercept of the regression with the time axis and is calculated from -c/m, where c and m are the intercept and slope of the regression, respectively; n = number of observations in the regression Experiment
1
Treatment Extraction front velocity (cm day -1 ) Lag period (days) R 2 (%) n
3.88 (0.15) 16.3 (3.05) 94 81
2
3
4
E57
Pride
Unicuim
+Tillers
3.97 (0.41) 17.8 (4.01) 74 34
4.92 (0.25) 14.8 (2.00) 87 45
3.62 (0.19) 19.3 (2.60) 88 50
3.27 (0.17) 15.9 (2.51) 88 51
4.39
(0.18) 18.0
(1.75) 91 72
91
WATER E X T R A C T I O N BY G R A I N S O R G H U M . I.
TABLE 3 Values of the extractable water content ( + sem) (cm 3 cm- 3), in selected layers for each experiment Experiment
1
2
Treatment Depth (cm) 50
0.058
(0.002) 70
0.053 (0.001) 0.052 (0.002) 0.046 (0.002)
90 110
3
4
E57
Pride
Uniculm
+Tillers
0.050 (0.003) 0.063 (0.007) 0.049 (0.010) 0.047 (0.082)
0.062 (0.011) 0.072 (0.006) 0.061 (0.009) 0.051 (0.006)
0.050 (0.005) 0.050 (0.004) 0.050 (0.001) 0.049 (0.004) 0.048
0.043 (0.001) 0.045 (0.005) 0.049 (0.005) 0.049 (0.004) 0.056 (0.002) 0.046 (0.025) 0.010 (0.004)
130
(0.011) 150
0.065 (-)' 0.049 (-)
170
0.042 (0.001) 0.053 (0.004) 0.055 (0.005) 0.051 (0.004) 0.048 (0.005) 0.037 (0.008) 0.037 (O.006)
qndicates only one observation per mean.
co I
E © co
C' 0.06
© 4.J 0.04 O O
[
•
.
0 02
T • ±
Expt 3 +tillers Expt 3 umculm Expt 4
.+..)
N f~
0.00 0.0
i
t
0.1
0.2
l
03
• Q •
[] 0
i
0.4
i
05
0.6
Root length density (era e m -3) Fig. 3. Relation between root length density during early grain-filling and the extractable water content. Data are from Experiments 3 and 4, which were subjected to a terminal drying cycle. Points from depths above 150 cm have open symbols, and those from below 150 cm have closed symbols. Standard errors are shown except where they are smaller than the symbol.
92
M.J. ROBERTSON ET AL.
ties above about 0.2 cm cm-3, but below this threshold, extractable water was less. While 0a was fairly stable across experiments, the extraction decay constant, kl, varied markedly, particularly in the upper profile (Table 4). The largest values in the top 120 cm of the profile were obtained in Experiment 1 (0.12-0.19 day - 1 ), followed by Experiment 2 (0.06-0.12 day - ~), and the smallest values were recorded in Experiments 3 and 4 (0.03-0.08 day-~ ). These values are equivalent to values of 1/k19o of 12-19 days for Experiment 1, 20-38 days for Experiment 2, and 28-62 days for Experiments 3 and 4. In the experiments with terminal drying, k/decreased with depth below 110 cm (i.e. the rate of decline in water content was slower) from 0.04-0.08 dayat 110 cm (1/kl9o=30-60 days) to about 0.01-0.02 day -~ at 170 cm (1/ k19o= 115-230 days). There were no significant differences in kl between E57 and Pride in Experiment 2, and neither between the tillering treatments in Experiment 3. The extraction front passed 120 cm about 50 days after sowing (see Fig. 2). As 1/klgo was greater than 50 days for the layers below the 120-cm depth, there was insufficient time to extract 90% of the total extractable water before physiological maturity at about 90-100 days after sowing. For instance, in TABLE 4 Values of the extraction decay constant, k l ( + sem) ( d a y - ' ), in selected layers for each experiment. Also shown are the mean values for layers between 50 and 110 cm Experiment
1
2
3
4
E57
Pride
Uniculm
+Tillers
0.12 (0.013) 0.19 (0.016) 0.14 (0.010) 0.14 (0.008)
0.08 (0.017) 0.07 (0.019) 0.14 (0.044) 0.07 (0.013)
0.09 (0.021) 0.07 (0.023) 0.12 (0.054) 0.06 (0.027)
0.06 (0.009) 0.05 (0.011 ) 0.06 (0.027) 0.04 (0.016) 0.03 (0.004) 0.02 (-)' 0.02 (-)
0.08 (0.008) 0.06 (0.006) 0.06 (0.020) 0.04 (0.008) 0.02 (0.004) 0.02 (0.009) 0.02 (0.001)
0.08 (0.014) 0.06 (0.009) 0.07 (0.021) 0.07 (0.029) 0.04 (0.012) 0.04 (0.008) 0.02 (0.003)
0.14
0.09
0.08
0.05
0.05
0.07
Treatment Depth (cm)
50 70 90 110 130 150 170
Mean
(50-110 cm)
qndicates only one observation per mean.
WATEREXTRACTION BY GRAIN SORGHUM. I.
93
the uniculm treatment in Experiment 3, the residual 0a, increased from 0.00 at 120 cm to 0.03 cm 3 cm -3 at 170 cm. Across the three crops subjected to a terminal drying cycle, the residual water was about 10 m m or 12% of the extractable water in the 200 cm profile. Differences among the experiments in the values of the extraction decay constant (kl) could be related in part to differences in root length density (l). The parameter k, when multiplied by ( 0 - 01) ( see equation 4 ), represents the uptake rate per unit root length, and may be similar among crops if variation in kl is due to variation in/. Taking the mean value for the layers between 50 and 110 cm, kl varied nearly 3-fold across experiments. Variation in I accounts for some of the variation in kl, because k varied only 1.5-fold across experiments (Table 5 ). In Experiments 1 and 2 values of k in the upper profile were similar, indicating that differences in root length density explain the differences in rate of extraction between these experiments. However, root length density can only partially explain the differences when Experiments 1 and 2 are compared to 3 and 4, i.e. the uptake rate per unit root length was higher for Experiment 1 and 2. The difference in k between these two groups of experiments is probably related to the level of evaporative demand. The mean rate of potential evaporation during the drying cycle, as estimated by the Penman (1948) equation was 3.9-4.2 m m day -~ for Experiments 1 and 2, and 2.8-3.0 m m d a y - l for Experiments 3 and 4. In Experiment 3, the value of k declined with depth below 110 cm, whereas in Experiment 4 it was relatively constant with depth. A decline in k with TABLE5 Values of k (cm 2 day -~ ) in selected layers for each experiment. The values of k were derived by dividing kl by the/, the root length density using treatment means. Also shown are the mean values for layers between 50 and 110 cm Experiment
1
Treatment Depth (cm) 50 70 90 110 130 150 170 Mean (50-110cm)
2
Uniculm
+ Tillers
4
E57
Pride
0.21 0.37 0.27 0.28
0.20 0.16 0.30 0.40
0.22 0.14 0.36 0.46
0.20 0.14 0.23 0.18 0.15 0.06 0.05
0.27 0.19 0.31 0.15 0.09 0.05 0.03
0.35 0.12 0.20 0.17 0.28 0.24 0.19
0.28
0.27
0.30
0.19
0.23
0.21
94
M.J. ROBERTSON ETAL.
depth suggests that the extraction rate per unit root length at depth was lower than at the surface. DISCUSSION
Extraction front The results of Experiments 3 and 4 show that when a sorghum crop is grown only on stored water, the extraction front descends at a constant rate until about 10 days after anthesis to a maximum depth of about 190 cm. Other studies have shown that if soil conditions do not seriously limit root growth, sorghum roots reach a maximum depth of 160-200 cm (Miller, 1916; Mayaki et al., 1976; Kaigama et al., 1977), although none of these studies showed at what stage of crop growth roots reach the maximum depth of penetration. The extraction front descended at rates that varied between 3.3 and 4.9 cm day-~, and was remarkably stable across experiments despite variation in genotype, tillering pattern, plant population density, and level of evaporative demand. Nakayama and van Bavel (1963) recorded a range in root front penetration rates in the field of 1.9-4.9 cm day-~ using a radiotracer technique. The values found in this study also agree with those measured for tropical cereals in India on vertisol and alfisol soil types (Monteith, 1986 ). This suggests that the extraction front velocity of sorghum is relatively insensitive to soil properties, as also found by Meinke et al. (1993) for sunflower. The extraction front velocity was also stable across experiments despite differences in evaporative demand in agreement with other studies (Squire et al., 1987). There was little effect of the lower plant population density on the extraction front velocity in Experiment 3. Extraction decay constant The variation in extraction rate (k/) among experiments shows that it is not a conservative parameter for describing water extraction for a particular combination of species and soil type. The value of kl varied between 0.04 and 0.19 d a y - i in the top 100 cm of the profile, became smaller with increasing depth below 100 cm, and varied among experiments due to differences in root length density and evaporative demand. The difference between Experiments 1 and 2 on one hand, and 3 and 4 on the other, is probably due to later sowing of the latter, when radiation, temperature and saturation deficit are lower (Table 1 ). This had the combined effect of reducing the mean rate of potential evaporation during the drying cycle from 3.9 (Experiment 1 ) and 4.2 m m day -~ (Experiment 2) to 2.8 (Experiment 3) and 3.0 m m day -~ (Experiment 4). This lower demand was probably associated with a lower extraction rate over a prolonged period of time. This suggests that the value of the extraction decay constant is determined by conditions over the length of the
WATER EXTRACTION BY GRAIN SORGHUM. I.
95
drying cycle, which makes it difficult to determine values of kl for supplylimiting conditions. Across experiments the m a x i m u m value of k, the root-soil diffusivity, in the upper profile was about 0.4 c m 2 day - 1. From equation 4, it can be shown that if the m a x i m u m value of k is 0.4 cm 2 day - 1, and 0 above the lower limit is 0.05 c m 3 c m - 3 , then q has a value of 0.02 cm 3 c m - i d a y - ~ when 0 is at the upper limit. This value is close to the m a x i m u m potential uptake rate per unit root length of 0.03 cm 3 cm - ~ day - l quoted by Ritchie ( 1985 ) for wheat and maize. M a x i m u m instantaneous rates of 0.19 cm 3 c m - 1 d a y - t for sorghum growing in solution culture have been recorded (Meyer and Ritchie, 1980). However, as Ritchie ( 1985 ) points out, m a x i m u m uptake rates in the field will be lower, because flow resistance in unsaturated soil near the roots is always higher than in solution culture, and uptake rates in the field are an average for a day rather than instantaneous. Limitations to poor extraction at depth Extrapolation of the exponential decay in 0 to the lower limit allows one to distinguish between extractable water (0a) and water actually extracted by the end of the growth period: Under long drying cycles, most of the extractable water was extracted above the 150-cm depth in the profile. However, below 150 cm the residual water content increased with depth. This lack of extraction was due to two limitations. Firstly, below 150 cm, 0a declined with depth, which was correlated with a decline in root length density. This type of response is a similar to that observed by Barraclough and Weir ( 1988 ), who found for wheat on a clay loam that the a m o u n t of water extracted was limited by root uptake capacity below a threshold root length density of 1.0 cm c m - 3. Secondly, the actual water extracted below 150 cm was considerably less than the potentially extractable because of lack of time before maturity. For example, in those experiments with terminal drying cycles, 70% of the extractable water at 150 cm was actually extracted by maturity. Another 10 days past maturity was needed for the roots at this depth to take up 90% of the extractable water. Other studies have also reported incomplete extraction at depth despite crops being severely stressed (e.g. Jordan and Miller, 1980; Barraclough and Weir, 1988 ), but did not attempt to define quantitatively the limitations to poor extraction. Application of the Monteith framework allows analysis of the reasons for poor extraction at depth in terms of a poorly developed root system or insufficient time to extract all the water before maturity. At depths below about 110 cm in Experiment 3, k decreased with depth, implying that the potential uptake rate per unit root length (q) declined with depth, presumably because some component of the root-soil hydraulic resistance increased with depth. However, in Experiment 4, k and hence q was
96
M.J. ROBERTSON ET AL.
relatively stable with depth. The variation o f k with depth shows that q cannot be assumed to be always constant for all parts o f the root system.
Usefulness of the Monteith (1986)framework The results o f this study has shown that the framework o f Monteith (1986) can be used to analyse the extraction pattern o f sorghum growing on stored soil water in a sub-humid environment, and add to the evidence that the extraction front velocity is a conservative quantity for a crop species across a range o f soil types, environments, genotypes and agronomic conditions. However, variation in root length density and evaporative d e m a n d among crops means that the extraction decay constant determined under continuous drying, is not necessarily a stable quantity that describes the potential rate o f uptake. This is especially the case where d e m a n d is can be low, such as in the subh u m i d tropics. A strength o f the Monteith ( 1986 ) approach to analysing water extraction is that the possible reasons for limited extraction at depth can be more easily identified as being due either to inadequate time for extraction or lack o f roots. In this study, poor extraction below 150 cm was due both to lack o f effective roots and insufficient time before maturity. Despite the growing use o f the Monteith ( 1 9 8 6 ) framework to analyse the extraction pattern (Meinke et al., 1993), there is little understanding of the relationship between the growth o f the root system and the framework parameters. This topic is examined in a c o m p a n i o n paper ( R o b e r t s o n et al., 1993). ACKNOWLEDGMENT M.A. Foale, o f C S I R O Division o f Tropical Crops and Pastures, m a d e valuable c o m m e n t s on an early draft o f the manuscript.
REFERENCES Barraclough, P.B. and Weir, A.H., 1988. Effects of a compacted subsoil layer on root and shoot growth, water use and nutrient uptake of winter wheat. J. Agric. Sci., Camb., 110: 207-216. Chapman, S.C., Meinke, H. and Hammer, G.L., 1990. A crop simulation model for sunflower. In: Proceedings of the International Symposium on Climatic Risk in Crop Production, 2-6 July 1990, Brisbane, Australia, pp. 40-41. Foale, M.A., Davis, R. and Macrae, C.D., 1979. A versatile, low-budget, automatic rain shelter for small field experiments. CSIRO, Australia. Tropical Agronomy Technical Memorandum, 18 Nov. 1979. Hamblin, A., 1985. The influence of soil structure on water movement, crop root growth and water uptake. Adv. Agron., 38: 95-158. Jordan, W.R. and Miller, F.R., 1980. Genetic variability in sorghum root systems: implications for drought tolerance. In: N.C. Turner and P.J. Kramer (Editors), Adaptation of Plants to Water and High Temperature Stresses. Wiley Interscience, New York, NY, pp. 383-399.
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Kaigama, B.K., Teare, I.D., Stone, L.R. and Power, W.L., 1977. Root and top growth of irrigated and non-irrigated grain sorghum. Crop Sci., 17: 555-559. Mayaki, W.C., Stone, L.R. and Teare, I.D., 1976. Irrigated and non-irrigated soybean, corn and grain sorghum root systems. Agron. J., 68: 532-534. Meinke, H., Hammer, G.L. and Want, P.J., 1993. Potential soil water extraction by sunflower on a range of soils. Field Crops Res., 32: 59-81. Meyer, W.S. and Ritchie, J.T., 1980. Resistance to water flow in the sorghum plant. Plant Physiol., 65: 33-37. Meyer, W.S., Dunin, F.X., Smith, R.C.G., Shell, G.S.G. and White, N.S., 1987. Characterising water use by irrigated wheat at Griffith, New South Wales. Aust. J. Soil Res., 25:499-515. Miller, E.C., 1916. Comparative study of the root systems and leaf areas of corn and sorghums. J. Agric. Res., 6:311-347. Monteith, J.L., 1986. How do crops manipulate water supply and demand. Phil. Trans. R. Soc. London A., 316: 245-289. Monteith, J.L., Huda, A.K.S. and Midya, D., 1989. RESCAP: A resource capture model for sorghum and pearl millet. In: S.M. Virmani, H.L.S. Tandon and G. Alagarswarmy (Editors), Modeling the Growth and Development of Sorghum and Pearl Millet. Research Bulletin no. 12, ICRISAT, Patancheru, India, pp. 30-34. Nakayama, F.S. and van Bavel, C.H.M., 1963. Root activity distribution patterns of sorghum and soil moisture conditions. Agron. J., 55:271-274. Passioura, J.B., 1983. Roots and drought resistance. Agric. Water Manag., 7: 265-280. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. London Ser. A., 193: 120-146. Ritchie, J.T., 1985. A user-orientated model of the soil water balance in wheat. In: W. Day and R.K. Arkin (Editors), Wheat Growth and Modelling. Plenum, New York, NY, pp. 293-305. Robertson, M.J., Fukai, S., Ludlow, M.M. and Hammer, G.L., 1993. Water extraction by grain sorghum in a sub-humid environment. II. Extraction in relation to root growth. Field Crops Res., 33:99-112. Rosenthal, W.R., Vanderlip, R.L., Jackson, B.S. and Arkin, G.F., 1989. SORKAM: A grain sorghum crop growth model. Research Center Program and Model Documentation MP- 1669, Texas Agricultural Experimental Station, College Station, TX. Squire, G.R., Ong, C.K. and Monteith, J.L., 1987. Crop growth in semi-arid environments. In: Proceedings of the International Pearl Millet Workshop, ICRISAT Center, Patancheru, India, April 1986. Taylor, H.M., 1984. Modifying root systems of cotton and soybean to increase water absorption. In: H.M. Taylor, W.R. Jordan, and T. R. Sinclair (Editors), Limitations to Efficient Water Use in Crop Production. ASA-CSSA-SSSA, Madison, Wl, pp. 57-64.