Aust. J. Agric. Res., 1990, 41, 253-65

Root Growth and Water Uptake by Wheat during Drying of Undisturbed and Repacked Soil in Drainage Lysimeters

W. S. MeyerA, C. S. TanB, H. D. BurrsA and R. C. G. Smith*

*Division of Water Resources, CSIRO, Private Mail Bag No. 3, Griffith, N.S.W. 2680. BVisiting Scientist from Agriculture Canada, Research Station, Harrow, Ontario, NOR IGO, Canada.

Abstract

The availability of water to crops depends on both soil properties and root distribution. The dynamics of root development and water uptake in undisturbed and repacked clay soil were studied during increasing soil water deficit pre- and post-anthesis to find how root distribution, soil type and plant development affect plant available water (PAW). Treatments were imposed in a randomized design with two replications. Volumetric water fraction was measured with a neutron probe, while root dis- tribution was measured non-destructively using a series of horizontal perspex observation tubes.

Soil modification affected the rate of downward root growth which changed little over time for undisturbed (W) soil (6.7 to 13.6 mm day-') compared with the repacked (R) soil (7.5-42.9 mm day-'). In the well-watered treatments root distribution was mostly above the 0.55 m depth in U soil, while there was a reasonably uniform vertical distribution in R soil. The rate of root growth during post-anthesis drying was 61% of that during pre-anthesis drying. The specific root water uptake rate (volume per unit root length per time) was linearly related to the relative root growth rate, indicating the importance of root growth in supplying water during soil drying. Estimated PAW values reflected effects of soil hydraulic properties, root distribution and a propensity to grow new roots during soil drying. Differences in grain yield between treatments were thought to result from the effects of dif- ferent root distributions and the propensity to grow roots during soil drying.

Introduction

Irrigation of agronomic crops generally aims to avoid deficit stress which would adversely affect productivity. The onset of stress has generally been related to a proportion of available water within the soil root-zone, but as pointed out by Gardner (1979) and Ritchie (1 98 I), this proportion is not a static value, being dependent on the extent of root development within the soil profile.

Large areas of irrigated land in south-eastern Australia have soils which are difficult to manage. These predominantly clay soils not only have topsoils which make cultural practices difficult, but also have hydraulic properties which limit water entry and restrict internal drainage (Loveday et al. 1978). It is also known that these soils present a hostile environment for extensive root growth through effects of high soil strength, small pore size, poor chemical fertility and poor aeration. It has been observed that much of the root development occurs in fissures and large pores (Loveday et al. 1978; Barrs and Meyer, unpublished data), which limits the uniformity of root distribution necessary for rapid and complete extraction of available soil water (Passioura 1985). With such potentially limiting properties, it follows that improvement of irrigation scheduling practice on these soils depends largely on understanding how root distribution, soil type and irrigation manage- ment affect plant available water and the onset of deficit stress.

To improve the physical properties of these soils and thus indirectly improve effective root exploration, various forms of soil amelioration have been advocated. The effect of

W. S. Meyer et al.

these treatments on root exploration, amounts of available water and thus optimum irrigation scheduling practice, is not known. The present experiment was designed to improve understanding of factors affecting the availability of soil water to irrigated wheat grown on an undisturbed clay soil. Responses in this soil where restricted root distribution was expected, were compared with the same soil repacked to a lower bulk density and in which more uniform and extensive root development was anticipated. It was hypothesized that in the latter case plant-available water would be greatly increased. To evaluate the effect of crop phasic development on the availability of water, two different periods of soil drying were imposed, one prior to anthesis, the other post-anthesis. This paper deals with the interaction between soil type, root growth and distribution, total available water and yield, while the onset of deficit stress, its detection and relation to soil water depletion is discussed in an accompanying paper (Tan et al. 1990).

Materials and Methods The experiment was carried out during 1987 in the drainage lysimeter facility (Meyer et al. 1985;

Meyer et al. 1987a) located at the CSIRO Division of Water Resources, Griffith, N.S.W. The design was a completely randomized factorial, being made up of undisturbed (U) and repacked (R) soil, three watering treatments (well watered (WW), drying 1 (Dl) , and drying 2 (D2)) and two replications.

Soil The soil was a Mundiwa clay loam (van Dijk 1961 ; Dr 2.33 ofNorthcote 197 1; Loveday el al. 1984),

which is described more generally as a transitional red-brown earth. It is used for general irrigated cropping purposes, being well suited to ponded rice culture because of slow water infiltration rates once wet. The sampling site was the Whitton Common experimental farm located 40 km south-east of Griffith. Undisturbed cores of soils were obtained during May 1987 by pushing steel cylinders (0.75 m 0.d.) 1.35 m into the soil using a system described by Meyer et al. (1985). The cylinders were sealed to a sand-filled drainage base and transported to the lysimeter facility.

Soil used for repacking cylinders was excavated next to the undisturbed core sampling site. Four layers (0-0.15 m, 0.15-0.65 m, 0.65-1.05 m, 1.05-1.35 m) were stockpiled and allowed to dry for 7 days. Soil from each layer was then passed through a rotating screen (sieve holes 20 by 70 mm), weighed, mixed thoroughly with gypsum at a rate equivalent to 5 t ha-1 over the layer depth and packed into the cylinders to a bulk density of 1.2 mg mm-3.

Agronomy Eight days before sowing, the topsoil layer of all 12 cylinders was cultivated and fertilizer (50 kg

ha- of N as urea, 50 kg ha-' of P as single superphosphate) was incorporated. Immediately prior to sowing on 24 June 1987 an additional 34 kg ha-' of P and 17 kg ha-1 of N as single superphosphate and compound fertilizer ('Starter 15') were added. Sufficient seed of cv. Yecora was hand-sown in two concentric circles to establish at least 100 plants per cylinder (surface area 0.43 m2). Additional N (33 kg ha- ') as urea was added to all treatments 19 and 55 days from sowing (DFS), to WW and D2 treatments 76 DFS and to D l , 11 1 DFS. Total N supplied was 149 kg ha-1.

The area immediately around the treatment cylinders was sown at the same time. This area was maintained as a well-watered guard crop throughout the experiment.

All irrigation was by hand so that volumetric additions could be accurately recorded. The well- watered treatment (WW) was watered to the upper limit (drained field capacity) when a tensiometer at a depth of 0.125 m showed a matric suction of - 60 kPa. The first drying treatment (D 1) was watered before and after a drying period which occurred prior to anthesis. For the undisturbed (U) soil, no water was applied between days 6 1 and 1 10, while for the repacked (R) soil the drying period was from 53 to 110 DFS. The drying period for R soil started earlier in an attempt to have the onset of deficit stress for the two soils coincide, since it was anticipated that more water would be available in the R soil. The second drying treatment (D2) was well watered until anthesis, with no further water additions after day 96.

A meteorological station was located 80 m from the experimental site. Potential evaporation values for wheat with a complete canopy were calculated from daily observations (Meyer et al. 1987b). Ten- day mean values for the period 50 to 149 DFS were 2.6, 3.6, 3.0, 4.8, 6.0, 5.6, 5.7, 6.9, 8.8, 8.6.

Wheat Growth during Drying of Soil

Measurements Soil water Profile volumetric soil water content was measured every 3-4 days using a neutron probe inserted

in a central vertical aluminium access tube. The probe had been calibrated in undisturbed cores of a similar soil (Jayawardane et al. 1984) and adapted for the Mundiwa clay loam from field measure- ments (N. S. Jayawardane, pers. commun.). Calibration coefficients used are given in Appendix 1. For the repacked soil, calibration equations were obtained by taking four core samples (43 mm O.D.) from a spare repacked cylinder prior to any water addition, back-filling the cored holes, thoroughly wetting the whole cylinder and core sampling again when it was at the drained upper limit of the water content range.

Irrigation frequency was determined from a tensiometer placed at a depth of 0.125 m in each cylinder, while irrigation volume was determined from the volumetric soil water fraction values measured with the neutron probe. An additional check of the total soil water balance was made from changes in mass of each cylinder determined weekly using a gantry hoist with an in-line load cell.

Root Growth and Distribution Roots were observed non-destructively using a series of horizontal or near horizontal acrylic tubes

(25 mm 0.d.) inserted through the side of the cylinder into the soil. Tubes were located at depths of 0.05-0.25,0.4,0-55,0.85 and 1.15 m depths, and were scanned using a fibrescope to count the number of roots crossing a scribed line on each side of the tube (Meyer and Barrs 1985). The number of root interceptions per unit length of observation tube was converted to root length density (L,, m m-3) using the theoretical equation of Melhuish and Lang (1 968). Compilation of L, for each soil layer was used to obtain values of root length per unit ground surface area (L,, m m-2). Rates of root growth were calculated from L, values between observation times and a relative root growth rate (RRGR, m m-3 (m m-9-1 day-') was estimated as

RRGR=(lnL,2 -1nL,l)l(t2 - [I), (1 where t was DFS.

Leaf Area Development From day 75, at approximately weekly intervals light interception readings were made from which

leaf area index (LAI) was estimated. Readings were made around solar noon on clear days with a Li-Cor LI 19 1SB line quantum sensor which was masked at each end to give a sensing length of 0.7 m. The sensor was placed under the plants in each treatment and flux density (PAR,) was expressed as a ratio of that above the plants (PAR,). The ratios were converted to LA1 using an extinction coefficient determined by destructively sampling two areas of well-watered plants adjacent to the treatment cylinders. The conversion equation was

Yield Because of different treatment effects, harvesting of plants for yield analysis occurred on several

days. Plants were removed from the cylinders, tillers and heads counted, oven dried for 48 h at 70"C, and total above-ground dry mass determined. Spikelets were counted on 20 randomly chosen heads and grain number and mass determined. Total grain mass was also measured.

Results and Discussion

Crop Response Seedling vigour was initially better in the undisturbed (U) treatments as indicated by the

lamina length of the first four leaves each being 23 mm longer on average than that of the repacked (R) treatment plants. Subsequently though, R treatment plants tillered more and appeared to be more vigorous than U treatment plants.

Midway through the first drying (Dl) treatment (85 days from sowing, DFS), U plants had leaves which appeared to become more erect with lamina edges slightly upturned, indicating the onset of deficit stress. The decrease in intercepted irradiance caused by the changed leaf geometry is shown in the inferred leaf area index (LAI) value (Fig. 1 a), which decreased below that of the well-watered plants (UWW). Despite the differences in LAI,

W. S. Meyer et al.

Fig. 1. Development of leaf area index (LAI) over the season calculated from intercepted PAR readings taken during the midday period. Treatments: 0 UWW, rn UD1, A UD2, o RWW,

RDl , A RD2. Day 0: 24 June. (a) Undisturbed soil. (6) Repacked soil.

Time from sowing (days, 0 = 24 June 1

measurements of leaf and stem extension (Tan et al. 1990) on UD1 plants differed only slightly from those on UWW plants, indicating that water deficit stress was not severe. By 99 DFS visible leaf curling on U D l occurred during the midday period, but in general, plant appearance, although indicating some deficit stress, did not show a severely increasing level of stress.

Plants in RD1 did not differ in appearance from those in RWW, except after 95 DFS when some minor signs of deficit stress during midday were visible. This effect is again reflected in the inferred LA1 values (Fig. 1 b).

Upon rewatering, plants in D l treatments, and particularly those in UD1 recovered, with leaves becoming less erect and rolled so that inferred LA1 recovered to values very near those of UWW. Since this recovery occurred around anthesis, it was not due to new leaf growth.

Within a few days of irrigation cessation (96 DFS) at the start of the second drying treatment UD2, plants began to wither and turn yellow. This was indicated by the decreased inferred LA1 (Fig. la). WHile some signs of deficit stress were observed in RD2, these were never acute.

Soil Water Total profile water content during the season is shown in Fig. 2. Frequent irrigation

ensured that the profile was within 30 mm of the drained upper limit (Appendix 1) for UWW and within 40 mm for RWW. During D l , the U treatment removed 154 mm in 49 days (3.1 mm day-'), while R removed 174 mm in 57 days (3.1 mm day-1). During the first 20 days of drying (6 1-80 DFS) the rate of water use for UD 1 was 1 a8 mm day- 1, which was slightly less than that of RD1, 2-2 mm day- l. These values compare with a potential evaporation value of 3.0 mm day-1 for the same period.

During D2, large differences in amounts of water extracted were apparent. UD2 removed only 57 mm at a rate of only 2.2 mm day- l during the first 20 days. For RD2,

Wheat Growth during Drying of Soil

Fig. 2. Total profile soil water contents (mean of two replications) with the pre-anthesis drying period and the post-anthesis drying period indicated as D l and D2 respectively. Treatments: UWW, UD 1, A UD2, o RWW, RD1, A RD2. Day 0: 24 June. (a) Undisturbed soil. (b) Repacked soil.

60 80 100 120 140

Time from sowing (days, 0=24 June)

Volumetric soil water fraction (m3 m-3)

Fig. 3. Volumetric soil water fractions in the profile at the beginning and end of the drying periods. Treatments: UWW, w UD1, A UD2,o RWW,

RDl , A RD2. Bars associated with each point are half the s.e. of the mean of the two replications. (a) Undisturbed soil. (b) Repacked soil.

Table 1. Measured values of volumetric water fraction and root length density together with the derived specific root uptake rates during the two drying periods

Trmt Volumetric water fraction (&, m3 m-3) Root length density (L X l op4 m m-3) Water uptake rate (U,, mm3 m-I day-')* Depth (m): Depth (m): Depth (m):

DFS 0.125 0.25 0.55 0-85 1-15 DFS 0.125 0.25 0.55 0.85 1.15 Period 0.125 0.25 0-55 0.85 1.15

*Water uptake rate was calculated as (8, fl - 8,Q) X 1051[(LvlJ +Lvt2)/2 X (t2 - tl)], where t is DFS.

Wheat Growth during Drying of Soil

202 mm of water were extracted at the initial rate of 4.2 mm day- l . Potential evaporation during this time was 5.4 mm day-1.

Curvilinear extrapolation of the total water content during drying for U D 1, RD I and RD2 was used to obtain an estimate of the time when the water use rate would approach zero, since plants in these treatments did not die from water deficit stress. The difference between the estimated water content at this time and that ofthe drained upper limit was the plant available water (PAW) as defined by Ritchie (1981). Values thus obtained were 170 mm for UD 1,190 mm for RD 1 and 220 mm for RD2. Plants in UD2 died from deficit stress after using only 57 mm.

The distribution of water extraction from the soil profiles is shown in Fig. 3. For the undisturbed soil most water was extracted from the upper soil layers; all above 0-5 m depth in UD2. For repacked soil, uptake was very evenly distributed through the whole profile. Differences in uptake patterns were related to root distribution as will be described later. Water uptake from the top two layers of UD1 and RD 1 occurred at a similar rate (see 9, values in Table 1). When the rate of uptake in these layers began to slow, that in the next depth, 0.55 m, was at a maximum. This pattern was repeated in the 0.55 m to 0.85 m layers. Thus there was a sequential pattern of uptake such that the maximum decrease in soil water content moved progressively down the profile with time. In RD2 uptake occurred almost simultaneously from all layers.

Root Growth, Distribution and Water Uptake The downward rate of root extension (Table 2) remained reasonably constant in the

undisturbed soil, but generally increased with time in the repacked soil. These rates are within the range reported in the literature (Hamblin 1985) and for the undisturbed soil are similar to that reported by Meyer and Barrs (1985). However, in the repacked soil, the increasing rate with time contrasts with a fairly constant rate (between 60 and 110 DFS) reported previously (Meyer and Barrs 1985) in a similar repacked soil. Recalculation of the Meyer and Barrs (1985) data using the same criteria as used here did indicate a slight increase in downward growth rate from 7.9 mm day-1 at 30 DFS to 12.2 mm day- at 1 13 DFS. The cause of the more rapid downward root growth in the present repacked soil is unknown. There is no evidence in the present data that the onset of deficit stress increased the rate of downward extension growth. Deficit stress did, however, cause L, to increase more rapidly in the lower soil layers as will be discussed below.

Table 2. Time from sowing for root length density to equal 0.05 X 10-4 m m-3 at each observation depth and the calculated downward extension rate (DER) of roots

Soil Treatment depth UWW UD1 RWW RD 1 (m) DFS DER DFS DER DFS DER DFS DER

(mm day-') (mm day-') (mm day-') (mm day-l)

While differences in downward extension growth were apparent well before 70 DFS (Table 2), it was not until after this time that differences between treatments in L, (and thus root length per unit ground surface area La) became obvious (Fig. 4). Root growth in terms

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O ' Qo I I I 1 I I I I

80 100 120 140

Time from sowing (days, 0=24 June)

Fig. 4. Change in root length per unit ground surface area, La for each treatment during the season. Values for each treatment are the mean of the two replications with half the s.e. of the mean for the final observations shown as vertical bars. Day 0: 24 June.

of total root system length was faster in the repacked soil such that RWW had nearly twice the La of UWW. Pre-anthesis drying stimulated UD1 to produce up to 63% more and R D l up to 17% more root length than their respective well-watered counterparts by 106 DFS. Post-anthesis drying also stimulated additional root growth, although the response of plants in UD2 was smaller (0.14 km m-2 day-') than that in RD2 (0.38 km m-2 day-'). This latter growth rate was 61% of that in RWW during pre-anthesis (71-92 DFS) and probably indicates a decreased capacity for root growth in the post-anthesis stage.

In addition to differences in total root length between treatments there were also considerable differences in root distribution (Fig. 5). In UWW there were no roots at

Time from sowing (days, 0=24 June)

Fig. 5. Root length density L, for each layer. Soil depths: 00.05-0.15 m, + 0- 15-0.25 m, 0 0.4 m, A 0.55 m, X 0.85 m, v 1.1 5 m, Values for each treatment are the mean of two replications with half the s.e. of the mean for the final observations shown as vertical bars. Day 0: 24 June.

Wheat Growth during Drying of Soil

0-85 m, while RWW had a much more vertically uniform distribution throughout the profile. During D l , root growth ceased in the topsoil layer (0-05-0.15 m) once 0, was between 0.1 5 and 0.14 m3 mP3, although 0, continued to decline after this time. In U D l the root growth rate in lower layers remained fairly constant after day 85, while in UWW it declined. A similar response occurred in RD1 at the 0.55 m depth and below. Total root length per unit ground surface area was not greatly different between UDl and RD1, but the latter treatment had a greater proportion of its roots deeper in the profile. The contrast in root distribution was even greater in the D2 treatments. During drying both undisturbed and repacked treatments showed renewed root growth when compared with the well-watered controls. In RD2, growth occurred principally in layers other than the dry topsoil layer.

Volumetric water fractions, root length density values and derived specific water uptake rates (U,, mm3 m- day-') for the drying treatments are given in Table 1. In calculating U, no correction for drainage within the soil was made which may lead to underestimates (and even negative values) in lower layers. However, this error is likely to be small during drying, and in this particular soil where hydraulic conductivities are low. Standard errors of the means for the volumetric water fractions were generally less than k 0.02 m3 m-3, while for root length density they were generally less than +- 0-4 X loa4 mm-3. The range of U, values calculated are consistent with those published previously (Walter and Barley 1974; Meyer and Barrs 1985; Hamblin 1985). There is a consistent trend for U, to increase with depth, so that maximum values occur at 0.55 m in UD1 and at 0.85 m in RD1 (ignoring unrealistically large values in deeper layers where water content changes and/or root length density values are very small). This trend with depth is evident in other data discussed by Hamblin (1985). A significant linear relation existed between U, and mean volumetric water fraction (8,, m3 m-3) within a 7-day time period with a slope (dUr/d8,) of 2500 mm3 m-1 day-1 per unit change in 8,. This relation is similar to that (2800 mm3 m-1 day-' (m3 m-9-1 estimated from data shown by Taylor and Klepper (1 975; Fig. 1) for cotton grown on a loamy fine sand. This indicates that not only are peak rates of U, similar for cotton and wheat (Hamblin 1985), but also that dU,/d& is similar across soil types, despite likely differences in the soil water characteristic ( d 8 / d ~ ) .

The lines drawn on Table 1 indicate the time period during D l when the rates of decrease in 8, and increase in L, were greatest for the particular layer. Maximum U, mostly occur at or shortly after the time of maximum increase in L,. This is not surprising, since detailed work by Graham et al. (1974) indicated that the apical (younger) portions (other than the meristem region) of roots take up water much more readily than do the older, basal portions. To further illustrate this, Ur was plotted against relative root growth rate (RRGR, m m-3 (m m-3)-1 day-'; Fig. 6). This shows a significant (P<0-05) linear relationship with a positive intercept. We interpret from this that a crop in a drying soil profile with a non-growing root system would have a small uptake rate (75 mm3 m-I day-'), so that to meet an evaporation rate of 3 mm day-' it would need a total root length of 40 km m-2 (assuming uniform water uptake over the whole root system). Thus the response of UD2 once watering ceased was indicative that both its initial root length (12.3 km m-2, 112 DFS) and its subsequent rate of root growth (0.14 km m-2 day-') were grossly inadequate to obtain sufficient water from soil in which it was no longer freely available.

We therefore suggest that root growth is critical if plants are to match evaporative demand and adapt to decreasing availability of water during soil drying. The production of new roots enables the exploration of new soil volume, a greater density of roots decreases the path length of water movement from soil to roots, while new roots have higher specific uptake rates. While the positive relationship between U, and 8, is generally interpreted as an effect of decreasing availability of soil water, it may also be caused, in part, by a decreasing proportion of new roots as growth is limited by increasing soil strength. Similarly, the observation of increased U, with depth may be due to the greater proportion of younger roots at depth. In addition, the contradiction spelled out by Hamblin (1985), where in some field studies apparently available water is not taken up, even though L,

W. S. Meyer et al.

RRG R (m m-3 (m rn3 ) - I day-')

Fig. 6. Calculated water uptake rate, U, as a function of the relative root growth rate, RRGR. Soil depths: 0.125 m, + 0.25 m, 0 0.55 m, v 0.85 m, X 1.15 m. y=75.2 + 3051x: r2=0-47.

seems adequate (as was the case in UD2) may be partly caused by the mis-match between evaporative demand, insufficient total Lv and a low propensity for root growth caused by plant or soil constraints. The discussion above does not negate the limiting effect that a horizontally non-uniform root distribution has on effectively extracting all available soii water (Hamblin 1985; Passioura 1985). Indeed, if new root growth is confined to inter-ped preferred pathways, its effectiveness in exploiting available water will be much less than if new root growth explores the soil more uniformly.

With pre-anthesis soil drying (Dl) the onset of water deficit was gradual and plants adapted by growing more roots; in RDl growth was mainly in the deeper layers, while in U D l , growth was mainly in the upper layers. The more vertically uniform distribution of roots in RD1 allowed more water to be extracted from the profile. With post-anthesis soil drying the onset of water deficit was rapid. The UD2 treatment plants, with a relatively short root system confined to the upper layers, also had a low propensity to grow roots and failed to adapt to the declining water status. The large and more vertically uniform root distribution of RD2 had a high propensity to grow roots (although at a rate less than seen in R D l when plants were younger), which together allowed water to be extracted quite uniformly down the profile and thus adapt well to declining water availability.

Yield Grain yield and its components are given in Table 3. The yield from UWW (5.7 t ha-')

is comparable to that from field sites of the same soil type where good management practices are used (W. T. Muirhead, pers. commun.). Repacking the soil, with associated improved root distribution resulted in a 69% increase in yield. The imposition of deficit stress in the repacked soil had a slightly greater effect pre-anthesis (2 1% reduction, RWW to RDl ) compared with post-anthesis (1 7% reduction, RWW to RD2). All components up to anthesis (tiller number, heads per tiller and grains per spikelet) were affected in RD 1, while in RD2 reduced grain size was the main cause of the yield reduction. In the undisturbed

Wheat Growth during Drying of Soil

Table 3. Yield and yield components from repacked and undisturbed soils together with the ratio of grain yield to root length per unit surface area (YIL,)

UWW UD 1 UD2 RWW R D 1 RD2

Plantslm2 306aB 272b 249b 252b 278ab 269b Graindm2 18 690b 12 OOOc 13710d 20590a 1 5 7 1 5 ~ 19830b Masdgrain (mg) 2 8 . 6 ~ 43.4ab 17.0d 43.3ab 45.3a 38.4b Above-ground

dry matter (g/m2) 1 361c 1 157d 948e 2041a 1603b 1712b Grain yield (g/m2)A 568c 534c 248d 960a 758b 798b Harvest index 0.39 0.43 0.25 0.44 0.44 0.44

AGrain yield =air dry grain mass X 1.07 to obtain grain yield at 10% water content (R. A. Fischer, pers. comm.). BValues followed by the same letter within a row were not significantly different (P=0.05) using Duncan's multiple range test.

soil, pre-anthesis deficit stress, although reducing heads per tiller, spikelet number and grains per spikelet, was almost offset by large grain size. Yield of UD 1 was only 6% less than UWW. Thus pre-anthesis deficit stress, while encouraging a more uniform root distribution in the undisturbed soil, caused little yield penalty. Indeed, a crop treated in this manner would be well placed to cope with post-anthesis deficits should they occur.

The ratios between grain yield and La (Table 3) indicate that with soil drying, La needs to increase to maintain the same yield. This increase needs to be greater in undisturbed soil than in repacked soil, i.e. the effectiveness of roots in undisturbed soil is less than in repacked, a not-unexpected result if roots are less uniformly distributed in the undisturbed soil. The comparison between U D 1 and RD 1 indicates that root growth in undisturbed soil during drying was only about 80% as effective as that in repacked soil in maintaining yield. Similarly, there is an indication that root length is less effective at maintaining yield during post-anthesis drying than during pre-anthesis.

When root distribution is limited to the topsoil layers by soil type and irrigation practice, the onset of deficit stress, especially under high evaporative conditions, will result in large yield decreases as was the case in UD2. In this case, yield was limited almost entirely by the failure to fill grains.

Conclusions

The amount of PAW varied, and was influenced by soil hydraulic properties and water management which influenced root distribution. Repacking the soil allowed more extensive root growth in the deeper soil layers, while the undisturbed soil was less hospitable for root growth. Increasing soil water deficit stimulated root growth. The development of pre-anthesis deficit stress was gradual owing to the low evaporative demand so that root growth, even in the undisturbed soil, was able to continue supplying sufficient water to match evaporative demand for a considerable period. Where root distribution was limited by poor subsoil conditions and frequent irrigations, a combination of reduced propensity to grow roots post-anthesis and high evaporative demand did not allow adaptation to increasing soil water deficit through the rapid growth of roots.

The results strongly suggest that high wheat yields will be attained even in the presence of transient deficit stress when soil conditions and irrigation are managed so as to produce a uniformly distributed root system which continues to grow throughout the season. In addition, the occurrence of a gradually imposed water deficit prior to anthesis may result in very small yield losses, while stimulating root growth. This will have an insurance benefit against large yield decreases caused by post-anthesis water deficits.

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Acknowledgments

We gratefully acknowledge the valuable technical assistance given by R. Sides, M. Kratochvil and G. Mann.

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Manuscript received 10 October 1988, accepted 17 October 1989

Wheat Growth during Drying of Soil

Appendix 1. Calibration coefficients for the neutron soil water meter, soil bulk density (pd) and drained upper limit volumetric water fraction (UL 8,) in undisturbed and repacked Mundiwa clay loam

The calibration equation is of the form 8, = a + b(clc,), where c is the observed count and c, is the standard water drum count

Depth Undisturbed soil Repacked soil (m) a b P d UL 8, a b Pd UL 8,

(mg mm-3) (m3 m-3) (mg mm-3) (m3 m-3)

0.125 -0.037 0-722 1.4 0.30 -0.025 0.737 1.2 0.30 0.250 -0.192 0.982 1.4 0.45 -0.1 13 0.923 1.2 0.40 0.550 -0.130 0.935 1.5 0.50 -0.1 12 0.868 1.2 0.37 0.850 -0.180 0.901 1.6 0.40 -0.050 0.786 1.2 0.44 1.150 -0.180 0.905 1.6 0.39 -0.041 0.734 1.2 0.44

Total profile void ratioA 0-84 1.27

UL(mm) 565 538

*Void ratio is the volume of voids to the volume of solids defined as pdpd - 1, where pp is particle density taken as 2.72.