Effects of irrigation and nitrogen on growth, light interception and efficiency of light conversion in wheat

Field Crops Research, 20 (1989) 279-295

279

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Effects of Irrigation and Nitrogen on Growth, Light Interception and Efficiency of Light Conversion in Wheat D.M. WHITFIELD and C.J. SMITH

Institute for Irrigation and Salinity Research, Tatura, Victoria 4616 (Australia)

ABSTRACT Whitfield, D.M. and Smith, C.J., 1989. Effects of irrigation and nitrogen on growth, light interception and efficiency of light conversion in wheat. Field Crops Res., 20: 279-295. Effects of three irrigation treatments (rainfed, and irrigation at 7-day and 14-day frequencies beginning in spring) and two rates of nitrogen (0 and 150 kg N h a - ' ) on growth, light absorption, and conversion efficiency in wheat were studied. Growth was considered in four phases extending from 95 days after sowing (DAS 95) to the beginning of rapid stem growth (DAS 120), the stem growth-phase lasting to the onset of rapid grain-filling (DAS 148), the grain-filling phase between DAS 148 and DAS 170, and the final period to harvest. The first irrigation treatments were applied at DAS 120. Radiation interception was the major determinant of growth. Rainfed treatments captured ca. 1100 MJ m -2 between DAS 95 and DAS 148, by which time they had achieved maximum aboveground biomass. Irrigated treatments continued to grow until DAS 170. They captured ca. 1300 MJ m -2 to DAS 170 where no nitrogen was applied, and ca. 1500 MJ m -2 where N was applied. In addition to effects on leaf-area duration and radiation absorption, treatments also affected conversion efficiency, ~. In the first phase, e increased from 0.85 g M J - ' to 1.15 g M J - ' where N was applied. After DAS 120, irrigation increased ~ from a mean of 0.8 g M J - 1in rainfed treatments to 1.2 g M J - ' . In the periods of rapid stem-growth and grain-filling, e was a maximum of 1.45 g MJ-1 in the frequently irrigated treatment which received N, resulting in a maximum aboveground biomass of 2100 g m -2. Mean maximum biomass was 1670 g m -2 in the other irrigated treatments, as compared with a mean of 1100 g m -2 in rainfed treatments. Growth rates were compared with predicted potential rates. After accounting for differences in light absorbtion between treatments, rates of growth ranged between 0.4 and 0.65 of potential rates in treatments other than IwNl~o, in which the growth rate between DAS 120 and DAS 170 was almost 0.8 of the potential rate. These proportions were strongly correlated with estimates of e, although the relationship varied between phases as a result of differences in global radiation. Collectively, the data suggest that physiological constraints, associated with both N and water, contributed to differences in rates of growth in addition to those imposed by leaf-area duration and radiation absorption. The yield potential of the frequently irrigated treatment which received N was, however, not realised in the field. Lodging after DAS 162 was estimated to decrease yield from a potential of ca. 900 g m-2 to 650 g m-2.

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© 1989 Elsevier Science Publishers B.V.

280 INTRODUCTION

In the companion paper (Whitfield et al., 1989b, this volume) effects of irrigation and nitrogen treatments on productivity and water-use of irrigated wheat grown in the Goulburn-Murray irrigation region of south-eastern Australia were reported. Yield increased from 4 t h a - 1 under rainfed conditions to 6.5 t ha -1 with irrigation, but there were no significant effects of nitrogen applied at sowing (150 kg N ha -1 ), or of a 7-day as contrasted with a 14-day irrigation frequency in spring and early summer. This paper examines the growth of these crops in terms of radiation interception (RI) and the efficiency of utilisation of intercepted light (radiationuse efficiency, RUE) in biomass production. In addition to accounting for the difference in yield of the rainfed and irrigated treatments, the analysis is used to explore the cause of the conservative yield response to N in the irrigated treatments. Economic yield is a function of growth rate, duration of growth, and the proportion of growth realized in the grain component (Gallagher and Biscoe, 1978). Growth rate (G) depends on the ability of a crop to capture light and the efficiency of conversion of captured light into biomass (Warren-Wilson, 1967; Gallagher and Biscoe, 1978). Thus, growth may be analysed in terms of V = eQi = ef Qi'

(1)

where: e is conversion efficiency (g biomass MJ-1); [, the fraction of light intercepted; and Qi,Qi', the daily intercepted and total solar radiation, respectively, e is a measure of photosynthetic efficiency (Monteith and Elston, 1983 ). while f depends on leaf area index (L) and an extinction coefficient, k, which depends on leaf geometry. Hence, /=l-exp

(-kL)

(2)

Simulation models of crop growth treat the processes of light interception and utilisation in greater detail. For example, the model of De Wit (1965) accounts for factors such as number, size and spatial arrangement of leaves, transmission and reflection of radiation, solar altitude and cloudiness. This model has been reformulated in much-simplified summary models (Goudriaan and van Laar, 1978; Versteeg and van Keulen, 1986), and results have been tabulated as a function of latitude, time of year, and daily radiation flux (De Wit, 1965; Rijtema, 1973); these approaches enable its ready implementation on microcomputers (Versteeg and van Keulen, 1986). De Wit's model calculates the potential growth rate (Gp) of a closed green crop (L= 5) with an optimum supply of nutrients and water, and has been used extensively to compare observed crop performance with theoretical behaviour (e.g., Slabbers et al., 1979). For example, actual crop performance has

281

been related to Gp using a relationship of the form (Rijtema, 1973; Ryhiner and Matsuda, 1978): G= (4.9/Zr) (1-kresp)(1-kroot)fGp

(3a)

Here, Zr is the sum of the exchange, diffusive and mesophyll resistances to CO2 transfer from the atmosphere to the reducing sites which total 4.9 s c m in the standard crop simulated by De Wit (Rijtema and EndrSdi, 1970). The factors kresp and kroot account for respiratory losses and partitioning to roots, respectively (Slabbers et al., 1979). Incomplete soil cover is accounted for by f, defined above (Rijtema, 1973). Similarly, Versteeg and van Keulen (1986) used a relationship of the form G= ( 1 - k r e s p ) ( 1 - koot)/ep

(35)

In this approach, G, depends on the maximum rate of gross CO2 assimilation of individual leaves, and so varies with genotype and growing conditions. Equations 3a, b therefore provide a strong physiological basis for the analysis of crop growth. Effects of treatment on the proportion of G, realised in the field were estimated in this study. METHODS

Basic experimental details, described elsewhere in this volume (Whitfield et al., 1989b), are given here briefly. The experiment was conducted on a redbrown earth (classification, Dr2.33; Northcote, 1979) at Tatura, Australia (lat. 36 ° 26' S, long. 145 ° 14'E). Two nitrogen treatments, three irrigation treatments and two rates of the soil ameliorant, gypsum, were included in factorial combination. Plot size was 25 m × 3 m. Treatments were randomized in three replicates. Nitrogen treatments were nil (treatment No) and 150 kg N ha-1 (treatment N15o) applied as ammonium nitrate immediately prior to sowing cultivar Condor at 150 kg seed ha -1 on 14 June 1984 in drill rows spaced at 0.15 m. Effects of gypsum on yield were not significant (Whitfield et al., 1989b) and were not considered in this analysis. Irrigation treatments included rainfed plots (treatment RF), and irrigation on a fortnightly (treatment (IF) or weekly (treatment (Iw) basis beginning 120 days after sowing (DAS 120) and maintained until DAS 177 and DAS 183 in treatments IF and Iw, respectively. Between DAS 120 and DAS 200, 22 mm rainfall was recorded. In this period, treatment IF received 310 mm water in 5 irrigations, and treatment Iw 380 mm in 10 irrigations. Treatments I~ and Iv were irrigated at mean deficits of 70 and 40 m m of Class-A pan evaporation, respectively. Irrigated treatments lodged to varying extents after the irrigation at DAS 162. Effects persisted until harvest. Global radiation, rainfall, Class-A pan evaporation and other standard

282 weather conditions were measured daily at a weather station approximately 250 m from the experimental site.

Measurement of growth Samples of 1 m of row were taken from each plot at approximate intervals of 7 days in the period DAS 95 to final harvest (DAS187, DAS 193 and DAS 200 for treatments RF, IF and Iw, respectively). Samples were cut as closely to ground-level as possible and separated into green leaf-blades, stem plus leaf sheaths, dead and senescent leaf blades, and head fractions. A subsample was used if the main sample was large. Leaf area on the green-leaf fraction was measured using an electronic planimeter (LI-3100 Area Meter, Li-Cor Inc., Lincoln, Nebraska) to determine green leaf-area index, L. Weights of the main sample, and of the components of the subsample, were measured after ovendrying at 70 °C for 3 days.

Estimates of light interception Estimates of daily fractional light interception (f) were made using Eq. 2. The extinction coefficient, k, was taken as 0.6 (Versteeg and van Keulen, 1986). Estimates of k generally range from 0.4 to 0.6 in cereals (Versteeg and van Keulen, 1986), although Hipps et al. (1983) estimated a coefficient of 0.93 for wheat in Kansas, and a range of 0.4-0.8 has been reported for grasses (Sheehy and Peacock, 1975 ). Daily estimates of f were interpolated from measures of L in each treatment, and Qi was summed from DAS 95 to calculate cumulative intercepted solar radiation (ZQi) as a function of time for each treatment.

Estimates of potential growth rate Gp Estimates of Gp were made according to relationships published by Versteeg and van Keulen (1986). The maximum rate of CO2 assimilation of individual leaves was assumed to be 4 g C02 m -2 h -1 (Versteeg and van Keulen, 1986; Van Keulen and Seligman, 1987). On this basis, Gp was calculated as Gp = m i n (2.7+2.55RI', 12.7+ 1.35Q( )

(4)

Equation 4 describes a rapid increase in Gp with increase in total daily solar radiation Q~' to ca. 8.5 MJ m -2 day -1 and a less-rapid rate thereafter. No allowances are made for incomplete ground cover (i.e., f < 1 ), respiratory losses or partitioning to roots (cf. Eq. 3b ). Daily estimates of the product fGp account for incomplete cover, and were summed from DAS 95 to calculate cumulative potential growth of each treatment (ZGp) with time.

283

Statistical analysis o/growth data Measurements of L, total biomass (B) and component biomass were subjected to analysis of variance on each date of harvest to identify effects of N treatment prior to DAS 120, and effects of irrigation and nitrogen treatments after DAS 120. Logarithmic transforms, which allow for changes in variance with size, were employed to analyse changes with time within treatments.

Estimation o/growth rates The period of growth considered in this study encompassed four distinct phases: these were tillering, Phase 1, the end of which coincided with the first irrigation applied at DAS 120; Phase 2, DAS 120 to DAS 148, included rapid stemgrowth, heading and anthesis (DAS 140), ending with the onset of rapid grainfilling at DAS 148; Phase 3 encompassed grain-filling, lasting from DAS 148 to DAS 170, after which crops matured prior to harvest, Phase 4. The onset of stem-elongation and rapid stem-growth marks the start of the rapid linear phase of growth in wheat (Green, 1987 ). The delay of 8 days between anthesis (DAS 140) and the beginning of Phase 3 represented the 'lag' phase of graingrowth prior to the start of linear grain-growth (van Keulen and Seligman, 1987). There was no apparent effect of irrigation treatment on duration of growth phases. Growth rate, G, in each phase was estimated as the slope dB/dt of the relationship between biomass B and time t. Average rates in each phase were estimated using splines. Dummy variables were employed assuming known points of intersection (Draper and Smith, 1966). The full relationship between B and t was

[~=B95 -G, tl +G2t2 +G3t3 -G4t4

(5a)

where: the regression coefficients, Gi ( i = 1,4 ), estimated the mean growth rate, dB/dt in each phase; Bg~ was the measured mean biomass of all treatments at DAS 95; and ti (i= 1,4) were dummy variables appropriate to phases. These were calculated as: tl = m i n ( t - 9 5 , dl) t2 = m a x (0, min (t-120, d2)) t3 = m a x (0, rain (t-148, d3)) t4 = m a x (0, t - 170) The durations of Phases 1, 2 and 3 (25, 28 and 22 days, respectively) are given by dl, d2 and d3. The GENSTAT 'OPTIMIZE' function (Alvey et al., 1983) was used to minimise 27 (log (/~)-log(B))2. Goodness-of-fit of Eq. 5a was appraised using an F-test for significance of deviations from regression relative

284 to the residual error mean square. The latter was derived from an analysis of variance which accounted for sums of squares attributable to replication and gypsum treatment (a total of 3 degrees of freedom) and the variance attributable to time of harvest (12-15 d.f., depending on treatment). Significance of the coefficient G4 was tested by dropping the term G4t4,from Eq. 5a. This tested the hypothesis that biomass did not change during Phase 4. If G4 was non-significant, significance of the coefficient G3 was tested in an analagous manner. The hypothesis G2 = G3 was tested, where appropriate, by replacing the terms G2t2 and G3t3,by the term G23t23,which encompassed both Phases 2 and 3. Here, t23 and d23 were t23-- t2 + t3 and d23 = d2 + d3, respectively. Partial F-tests were used to test the significance of the hypotheses (Snedecor and Cochran, 1967).

Estimates of efficiency of utilisation of intercepted light, and comparison of observed and potential crop growth rates In analogous procedures, estimates of ZQi and XGp appropriate to Phases 1, 2 and 3 replaced the terms, ti (i= 1,3 ), in Eq. 5a. The respective models were /~ = B95 - el ~'Qn - ~2~V'Qi2- e3 ~Qi3 -b G4 t4

= b95+ B~ ZG,~ + 82XG,2 + 83ZG,3 + G4t4

(5b)

(5c)

Estimates of maximum intercepted radiation, or maximum potential growth rates, were substituted for the terms di, (i= 1,3 ) in each phase. The regression coefficients, 8i, ( i = 1,3 ), directly estimated conversion efficiency ( i.e., 81 = dB/ dXQ and the coefficients 8i, (i= 1,3) estimated the proportion of potential growth achieved in each phase (8i = dB/d_ZG,. The term G4t4,which accounted for changes in biomass during Phase 4, was retained without change from Eq. 5a. RESULTS

Leaf-area index (L) Leaf area index of treatment No was a maximum of 5.3 at DAS 95, and there was no significant change to DAS 117 (Fig. 1 ). By contrast, L increased after DAS 95 in treatments N15o to reach a maximum of 7.7 at DAS 117. Differences in L were therefore significant at DAS 117 (SED =0.46, P < 0.01 ). Irrigation largely maintained leaf area after DAS 120. Leaf-area index decreased rapidly in treatments RF whereas there was little change in treatments IFN15o and IwN15o in Phase 2. Treatments IFNo and IwNo showed an intermediate response. Effects of irrigation and N treatment, measured at DAS 144, were additive: the mean difference between irrigated and rainfed treatments

285 was 2.3 (SED ----0.4) and that between N treatments was 1.8 (SED = 0.4). Leafarea index fell rapidly in all treatments during grain-filling; by DAS 160, for example, L was less than 2.0 in all treatments other than I~Nl~o and IwNl~o. In these, green leaf tissue persisted until DAS 180.

Light interception Despite differences in maximum L due to N treatment (Fig. 1 ), L was sufficient in practically all treatments to maximize funtil anthesis (DAS 140, Fig. 2). The greater L of treatments IFN15o and IwNiso, in particular, contributed little in terms of light interception; however, [ decreased rapidly once L had fallen below 3. Treatment effects on leaf-area duration therefore had major effects on estimates of ~Qi after DAS 148 (Fig. 3). Light capture between DAS 95 and the start of rapid grain-filling (DAS 148) ranged from 920 MJ m -2 to 1020 MJ m -2. Total interception was ca. 1050 MJ m -2 in treatments RF, increasing to ca. 1300 MJ m -2 in treatments IwNo and IFNo, to ca. 1500 MJ m -2 in treatment IFNl~o, and to a maximum of 1770 MJ m -2 in treatment IwNiso. 10 •I

II

~ I I

II

i

m

8

i III I II II m z .c

-

NO

o

_.1 J

100

120

140

160

100 120 180 200 Days after sowing

140

160

180

200

Fig. 1. Effects of nitrogen treatments No and Nl~o on leaf area index. Symbols distinguish irrigation treatments RE (O), IF (qD)and Iw (Q). Vertical bars representthe standard errorsof differences between means on each date of measurement. oc u

10

I.C N0

o.~

0.8

0.6 JE m O.Z,

0.6

a

0.2

0.2 ~ I

o

0

•£

b-

0.4

100

t20

140

160-

, 5i

180 200 100 120 Days after sowing

, °i 140

,

i u. 160

~ i, 180

200

Fig. 2. Effects of nitrogen treatment on estimates of fractional light interception, [. Symbols as in Fig. 1.

286 2000 A "7', 1500

150(

I

100(

1000

8 u

50(

NO

U, .~_

OU[~I , 100

I , I m I I 120 140 160

! i 180 200

C 100

120

140

160

180

200

Days after sowing

Fig. 3. Effects of nitrogen treatment on estimates of cumulative light interception. Symbols as in Fig. 1. 2500 N150

2000

~' 2000,

:F ,sooF

o~ 1000

6

o

1500

:'"

1000

©

500[-~-~

.

ot

.

.

100

120

.

140

500

T T T

.

.:

:.

160

I.l I 180

,

0 200

,i~.; ,j.z I, I7 ~ I.I ~ . 100

120

1/~0

160

180

200

Days after sowing

Fig. 4. Effects of nitrogen treatment on changes in total biomass with time. Curves represent estimates of biomass made according to the e model, Eq. 5b. Vertical bars represent the standard errors of differences between means on each date of measurement. Symbols as for Fig. 1.

Growth

At DAS 95, there were no differences in total b i o m a s s attributable to N treatm e n t (Fig. 4). However, effects were evident by DAS 117, w h e n total biomass of t r e a t m e n t s No and N:5o was 500 and 600 g m -2, respectively (SED--39 g m - 2 ; P < 0.01 ). T h e difference was primarily due to the greater leaf-mass of t r e a t m e n t N:5o: leaf biomass increased from a m e a n of 140 g m -2 at DAS 95 to m a x i m a of 220 g m -2 and 305 g m -2 at DAS 117 in t r e a t m e n t s No and N15o, respectively (SED = 18 g m - 2; p < 0.01 ). Effects on stem b i o m a s s were n o t significant despite an increase from 70 g m -2 to 300 g m -2 in t h e phase b e t w e e n DAS 95 and DAS 117. N i t r o g e n application had positive effects o n h e a d and leaf b i o m a s s b e t w e e n DAS 120 and DAS 148. However, net effects o n total biomass were n o t significant. At DAS 144, for example, m e a n b i o m a s s of t r e a t m e n t s No and Nlso was 1000 and 1300 g m -2, respectively (SED = 117 g m - 2 ; P = 0.08). Irrigation increased b o t h stem a n d leaf biomass, with the result t h a t total biomass in-

287 c r e a s e d f r o m 1010 g m -2 in t r e a t m e n t RF to a m e a n of 1280 g m -2 in t r e a t m e n t s Iw a n d IF at DAS 144 (SED---- 120 g m - 2 ; P < 0 . 0 5 ) . In a d d i t i o n to a decrease in leaf b i o m a s s w h i c h p a r a l l e l e d t h e decrease in L d u r i n g t h e grain-filling p h a s e , s t e m b i o m a s s also decreased. S t e m b i o m a s s was at a m a x i m u m in all t r e a t m e n t s at anthesis, b u t a n i m m e d i a t e decrease in L in r a i n f e d t r e a t m e n t s p r e c e d e d t h a t in irrigated t r e a t m e n t s b y 2-3 weeks. T r e a t m e n t s h a d a significant effect o n m a x i m u m s t e m b i o m a s s ( T a b l e 1 ). H e a d growth c o n t i n u e d u n t i l a p p r o x i m a t e l y DAS 170 in all t r e a t m e n t s . An analysis of t h e d a t a for t h e p e r i o d DAS 173 to DAS 180 s h o w e d significant m a i n effects of b o t h irrigation t r e a t m e n t a n d N application on m a x i m u m h e a d biomass ( T a b l e 1). N i t r o g e n a p p l i c a t i o n i n c r e a s e d m a x i m u m h e a d biomass b y a p p r o x i m a t e l y 160 g m -2 (SED----56 g m - 2 ; P < 0.01 ) a n d irrigation effected a m e a n increase of 3 70 g m - 2 over r a i n f e d t r e a t m e n t s ( SED ----60 g m - 2; p < 0.01 ). Although m a x i m u m total biomass was achieved before m a x i m u m head weight in t h e r a i n f e d t r e a t m e n t s , t h e m a x i m a in t o t a l biomass were m a i n t a i n e d until h a r v e s t (Fig. 4 ). In t h e irrigated t r e a t m e n t s , m a x i m u m biomass coincided with m a x i m u m h e a d biomass. D a t a for t h e p e r i o d DAS 166 to DAS 180 s h o w e d a significant i n t e r a c t i o n b e t w e e n N a n d irrigation t r e a t m e n t s on m a x i m u m biomass ( P < 0.05 ). T h e difference w i t h i n r a i n f e d t r e a t m e n t s was n o t significant, b u t m a x i m u m t o t a l b i o m a s s i n c r e a s e d f r o m a m e a n of 1580 g m -2 in t r e a t m e n t s IFNo a n d IwNo to 1870 g m -2 in t r e a t m e n t IFNI~0, a n d to 2120 g m -2 in t r e a t m e n t IwN15o (SED---- 145 g m - 2 ; see T a b l e 1 ). TABLE 1 Effects of irrigation and nitrogen treatments on maximum stem biomass, maximum head biomass and maximum total biomass a) Stem biomass (g m-2; means for 3 harvests in the period, DAS 144 to DAS 159; SED----58) Treatment No Nlso

RF 520 490

I~ 735 750

Iw 690 950

b) Head biomass (g m-2; means of 2 harvests in the period, DAS 173 to DAS 180; SED=98) Treatment No Nl~o

RF 645 700

I~

Iw

1000 1125

865 1180

c) Total biomass (g m-2; means for 3 harvests in the period, DAS 166 to DAS 180; SED----145) Treatment R~ I~ Iw No N15o

1050 1110

1650 1870

1510 2120

288

Growth rate, conversion efficiency and proportion of potential growth Estimates of G ranged from a mean of 12.3 g m - 2 d a y - 1 in treatments No to 16.9 g m -2 day -1 in treatments Nlso in the period DAS 95-DAS 120 (mean SE = 1.5 g m - 2 d a y - 1, see Table 2 ). Respective estimates of e were 0.85 g M J - ' and 1.13 g M J - 1, and 0 increased from a mean of 0.39 to 0.52. These data reflect the effect of N t r e a t m e n t on leaf growth, described above. In the stage of rapid stem-growth after DAS 120, growth rate of the rain-fed treatments was ca. 17 g m - 2 day-1. Mean conversion efficiency (0.8 g M J - 1 ) was slightly less than that in the previous phase, but 0 was maintained at ca. 0.43 (see Table 2). Estimates of G were not significantly different from zero after DAS 148, implying that the rate of grain-filling in the rainfed treatments was commensurate with the rate of loss of biomass in the non-grain tissues. High growth rates were maintained until DAS 170 in the irrigated treatments. Furthermore, growth rates, conversion efficiencies and estimates of 0 did not TABLE2 Effects of treatment on estimates of (a) growth rate, (b) light-conversion efficiency and (c) the proportion of potential growth made according to Eq. 5 (standard errors of the estimates in parentheses) Treatment

Period (DAS) 95-120

120-148

148-170

170--.

(a) Growth rate, G (g m -2 d a y -1) RFNo RFNlso IFNo IFN15o IwNo IwN15o

12.8 15.1 11.6 16.5 12.8 18.0

(1.46) (1.64) (1.35) (1.52) (1.24) (1.52)

17.7 (2.21) 16.3 (2.39)

(b) L i g h t c o n v e r s i o n e f f i c i e n c y , ~ (g M J - 1 ) RFNo 0.88 (0.10) 0.87 (0.12) RFN15o 1.03 (0.11) 0.74 (0.12) IFNo 0.80 (0.09) IFN,so 1.12 (0.10) IwNo 0.87 (0.08) L,N,so 1.24 (0.10) (c) P r o p o r t i o n RFNo RFN,so IvNo I~Nl~o IwNo IwN15o

of potential growth, O 0.40 (0.05) 0.47 (0.06) 0.47 (0.05) 0.40 (0.06) 0.36 (0.04) 0.51 (0.05) 0.40 (0.04) 0.57 (0.05)

23.5 24.8 18.4 33.9

1.21 1.10 0.94 1.45

n.s,

n,s.

n.s.

n.s. n.s. --14 (7.4)

(1.67) (2.20) (2.20) (2.34)

n.s.

- 2 7 (5.7) n.s.

n,s,

n.s.

n.s.

(0.09) (0.10) (0.07) (0.11)

n.s.

--12 (7.2) n.s.

- 2 7 (5.7) n.s. n,s,

0.65 0.59 0.50 0.78

(0.05) (0.05) (0.04) (0.05)

n.s.

- 1 2 (7.3) n.s.

--27 (5.6)

289 TABLE3 Analyses of variance appropriate to models describing changes in biomass as a function of time (G model, Eq. 5a), cumulative intercepted solar radiation (e model, Eq. 5b), and cumulative potential growth (0 model, Eq. 5c ) Treatment

Source of v a r i a t i o n

RFNo ss

df

MS

Total Rep+ Gyp Harvests Residual

26.959 0.218 22.296 4.445

77 3 12 62

1.051 0.762 0.845

10 10 10

IFNo SS

MS

IwNo SS

df

MS

50.592 89 0.351 3 45.748 14 0 . 0 7 2 4.493 72

0.062

44.393 0.162 39.771 4.460

95 3 15 77

0.058

0.105 0.076 0.085

0 . 1 5 1 " 1.159 13 0.107 0.822 13 0.117 0.891 13

0.089 0.063 0.068

df

Deviations

Gmodel model 0model

1.818 12 1.281 12 1.406 12

Treatment

Source of v a r i a t i o n

RFN15o ss df

MS

Total Rep + Gyp Harvests Residual

26.988 0.049 22.156 4.783

77 3 12 62

1.271 0.962 1.054

10 10 10

IFN15o SS df

MS

IwN15o SS df

MS

44.410 89 0.078 3 39.967 14 0 . 0 7 7 4.365 72

0.061

55.308 0.648 50.501 4.159

95 3 15 77

0.054

0 . 1 2 7 1.205 11 0 . 0 9 6 0.987 11 0 . 1 0 5 1.061 11

0.109 0.090 0.096

0.460 0.339 0.366

12 12 12

0.038 0,028 0.031

Deviations

Gmodel e model 0model

Rep + Gyp reference the sum of squares (ss) and degrees of freedom (dr) associated with replication (df= 2) and gypsum treatment (df= 1). differ significantly b e t w e e n P h a s e s 2 a n d 3. E s t i m a t e s of G were h i g h e r t h a n t h o s e in P h a s e 1 (Table 2). T h i s was initially a s s o c i a t e d w i t h t h e r a p i d stemg r o w t h of irrigated t r e a t m e n t s in t h e p e r i o d to DAS 148. A p a r t f r o m t r e a t m e n t IwNo, e s t i m a t e s of G, e a n d 0 ere also h i g h e r in irrigated t r e a t m e n t s t h a n in t h e r a i n f e d t r e a t m e n t s . E s t i m a t e s of G r a n g e d f r o m 18.4 g m -2 d a y - 1 in t r e a t m e n t IwNo (SE----1.3 g m -2 day - 1 ) to a m e a n of 24 g m -2 d a y -1 in t r e a t m e n t s IeNo a n d IFN 150 ( m e a n SE = 1.9 g m - 2 d a y - 1 ), a n d to a m a x i m u m of 34 g m - 2 d a y - 1 in t r e a t m e n t IwN15o ( s E = 2 . 3 g m -2 d a y - l ) . E s t i m a t e s of e r a n g e d f r o m 0.9 g M J -~ in t r e a t m e n t IwNo to a m e a n of 1.15 g M J -~ in t r e a t m e n t s IFNo a n d IFN~5o, a n d to a m a x i m u m of 1.45 g M J - 1 in t r e a t m e n t IwN15o ( m e a n SE = 0.09 g M J -~ ). R e s p e c t i v e e s t i m a t e s of 0 were 0.50, 0.62 a n d 0.78 ( m e a n S E = 0 . 0 5 ) . T o t a l b i o m a s s of t r e a t m e n t IwNl~o d e c r e a s e d s i g n i f i c a n t l y a f t e r DAS 170

290 2000

1500 E ~

10017

~

500

0

I

I

I

500

1000

1500

2000

Cumulative light interception (MJ m -2)

Fig. 5. Relationship between crop growth between DAS 95 and DAS 170 and total intercepted radiation. The fittedline was forced through the origin following Gallagher and Biscoe (1978), and is described by the equation, e= 1.07 (SE=0.06; rt=6, R2=0.70). Symbols as in Fig. 1.

(G4= - 2 7 g m -2 day -1, SE----5.7 g m -2 d a y - l ) . A similar trend was noted in t r e a t m e n t IFNlso (G4= - 1 4 . 5 g m -2 day -1, SE=7.4 g m -2 day-~). Decreases were evident in the leaf, stem and head components. The coefficient G4 of Eqs. 5a,b and c was not significantly different from zero in treatments IFNo and I~Nlso. Goodness-of-fitof statistical models The data presented in Table significant in only one instance a function of XQi (Eq. 5b) are regression were associated with

3 show that deviations from regression were (G model, t r e a t m e n t IFNo). Estimates of B as shown in Fig. 5. The largest deviations from the G model (Eq. 5a).

DISCUSSION The major effects of t r e a t m e n t were seen in L, f and XQi after DAS 148 (Figs. 1, 2 and 3 ), and resulted in irrigated treatments achieving XQi of approximately 300 M J m -2 higher t h a n rainfed treatments in the period to DAS 170 (Fig. 3 ). Water stress hastened leaf-death in both rainfed treatments {Fig. 1 ), and net growth ceased by DAS 148 (Fig. 4 ). In the irrigated treatments, significant rates of growth were maintained until DAS 170. Growth before DAS 170 correlated with XQi (Fig. 5). On one hand, the difference between B9s and m a x i m u m total B correlated with light interception between DAS 95 and DAS 170 (R2=0.70, n=6), and the associated estimate of

291 mean conversion efficiency ( ~= 1.07 g MJ - 1, SE ----0.07 ) corresponded with that found by Gallagher and Biscoe (1978) in a similar analysis (2.17 g M J - 1PAR). Growth in individual phases was also well correlated with ZQi in each period (R 2_ 0.87, n-- 12 ). These data provide support for the proposal that XQi is the major determinant of growth (Monteith, 1981; Biscoe and Gallagher, 1977 ). Notwithstanding, treatment effects on e were also important. Nitrogen application increased e by ca. 35% in the initial period before irrigation treatments were imposed (Table 2). At this stage f was maximal, (Fig. 2) and the greater e increased total biomass at DAS 120 by approximately 100 g m-2, largely due to an increase of ca. 85 g m-2 in leaf biomass. Previous reports have shown that N application increases ~ (Gallagher and Biscoe, 1978; Green, 1987). The increase may be ascribed to a greater assimilate production or a decreased partitioning of current assimilate to the root system (Eqs. 3a,b). Given that f w a s at a maximum in Phase 1 (Fig. 2), the latter is more probable on the basis that Puckridge (1973) found no effects of N application on the photosynthesis of wheat canopies other than those attributable to differences in L. Furthermore, root demands for assimilate are strongest in the period before the start of rapid stem-growth (Van Keulen and Seligman, 1987). However, other studies have shown that individual leaf photosynthesis increases directly with leaf N concentration (Van Keulen and Seligman, 1987 ). In the period DAS 120 to DAS 148, ~ increased from a mean of 0.8 g M J - 1 in the rainfed treatments to ca. 1.2.g MJ-1 under irrigation (Table 2). Assuming full cover (f~ 0.95, cf. Fig. 2 ), this represents an increase in biomass of ca. 240 g m-2. Whilst the lower E of treatments RF c a n be ascribed in part to morerapid leaf death {Fig. 1 ), stems accounted of the greater part of growth in this phase, and differences in maximum stem biomass were similar to those expected on account of ~: maximum stem biomass was 500 g m -2 in the rainfed treatments and 780 g m -2 in the irrigated treatments (Table 1 ). The lower e in the rainfed treatments was presumably caused by water-stress acting on stomatal aperture, although direct effects of stress on the photosynthetic apparatus, and greater partitioning to roots, cannot be excluded. Data for the irrigated treatments demonstrated effects of both N and irrigation frequency on leaf-area duration and e. Nitrogen stress limited L, f and ZQi of treatments IFNo and IwNo (Figs. 1, 2 and 3), in accordance with the proposal that grain demands on leaf N hasten leaf senescence when N is limited (Spiertz and de Vos, 1983; Green, 1987). In treatments N15o, the more frequent irrigation treatment prolonged leaf-area duration (Fig. 1 ) but there was little effect on f and ~Qi until after DAS 170 (Figs. 1 and 2 ). In the irrigated treatments, e increased in the sequence IwNo < I~No~ IFN15o < IwNl~o. Here, water-stress in treatments I F decreased e in comparison with treatment IwN15o.The low estimate of e in treatment IwNo suggests that photosynthetic efficiency was impaired by the combination of

292

frequent irrigation and N stress. This may have been associated with a more rapid depletion of N from the leaves. Concentrations of N in the leaves of treatments No were consistently ca. 1% less than those in treatments N15o in the period DAS 95 to DAS 163 (data not presented). The depletion of leaf N directly affects wheat-leaf photosynthesis (Van Keulen and Seligman, 1987) as well as hastening senescence. In general, the N demand of the grain, the effect on the N economy of the leaves, and the consequences for canopy photosynthesis and canopy longevity, bear a strong resemblance to those described by the 'self-destruction' hypothesis of Sinclair and de Wit (1976) for soybean. Treatment effects on 0 were strongly associated with effects on e (see Fig. 6), implying that e and 0 were generally subject to the same influences. However, the ratio E : 0 decreased from 2.19 g MJ -1 in the period DAS 95-DAS 120, to 1.86 g MJ -~ in the period DAS 120-DAS 170 (Fig. 6). If OfGp equated to efQ( (Eqs. 1, 3 and 4), then e=O (Gp/Qi'). Gp/Qi' m a y be regarded as the upper limit to e, and varies as (12.7+1.35 Q~' )/Qi' for Qi' >- 8.5 MJ m -2 day -1 (see Eq. 4). This limit changed in the same manner as e/0 between the two periods as a result of an increase in mean solar radiation from 15.6 to 25 MJ m -2 d a y - ' , and suggests that ~ decreased in relation to 0 simply as a result of the greater radiation input. Similar effects have been simulated by Monteith (1981), and they have also been measured directly in the field (Biscoe et al., 1975; Whitfield et al., 1989a). Estimates of 0 ranged from 0.4 to 0.8 (Table 2 ). Ryhiner and Matsuda (1978) found that respiratory losses and root requirements accounted for only 0.7 of Gp in spring and winter wheat in the Netherlands, but estimates in the range 0.5-0.6 may generally be expected on the basis that respiratory losses account for approximately 0.3 of Gp (Rijtema, 1973; Connor, 1975; Slabbers et al., 1979), and additional assimilate is required for root growth (Eqs. 3a,b). Respiratory 1.5 "T o~ f: "~

1.0

u

c o ¢.)

0.5 0

I

1

I

I

0.2

0.4

0,6

0.8

Fraction

of p o t e n t i a l

1.0

growth

Fig. 6. Relationships between light-conversion efficiency and estimated proportion of potential crop growth in the periods DAS 95 to DAS 120 (C)), and DAS 120 to DAS 170 ( 0 ) . The fitted lines were forced through the origin and depict ~/~ ratios of 2.19 g M J - 1 (SE ----0.006 g M J - ' ) and 1.86 g M J -1 (SE=0.004 g M J -~) for the respective periods.

293

losses of 25-30% of gross C02 assimilation have been measured in lucerne and sunflower in this environment (Whitfield et al., 1986, 1989a}. Lucerne was also estimated to transfer ca. 30% of its gross assimilate to the roots, but lower estimates were found under more frequent irrigation (Whitfield et al., 1986). This agrees with the suggestion that root demand is less under optimal conditions (Slabbers et al., 1979). Root demand also decreases after anthesis (Gallagher and Biscoe, 1978) and little or no assimilate is partitioned to the roots during grain-filling (Van Keulen and Seligman, 1987). These effects, coupled with the better water and N status of the canopy, appear to have contributed to the relatively high estimate of 0 in treatment IwNl~o. The high growth rate of treatment IwN15obetween DAS 120 and DAS 170 was also attributable to a consistently high f (Fig. 2 ), and to the high mean daily solar radiation (25 MJ m -2 d a y - l ) . Gallagher and Biscoe (1978) reported growth rates of the order of 30 g m -2 day -1 in Britain under high radiation. Spiertz and van der Haar (1978) measured rates of up to 32 g m -2 day -1 in the linear phase of grain growth and Spiertz and Ellen (1978) reported similar rates in the Netherlands under conditions of high solar radiation, warmth, ample nutrient supply and freedom from disease. Conversion efficiencies of 3 g M J - 1PAR (equivalent to ca. 1.45 g M J - 1on the basis of total solar radiation ) have commonly been reported for cereals (Gallagher and Biscoe, 1978; Monteith and Elston; 1983; Doyle and Fisher, 1979; Green, 1987 ) but have usually been found around anthesis. Green (1987) reported a conversion efficiency of 2 g M J - 1 on the basis of absorbed solar radiation. As a result of their sustained rapid growth, treatments IFN~5o and IwNl~o lodged after DAS 162. Light interception after DAS 170 was apparently ineffective, and the delayed leaf-death made a significant contribution to the loss of biomass after DAS 170 (ca. 150 g m -2 in treatment IwN~5o;see Figs. 1, 4 ). Yield appeared to be adversely affected (Whitfield et al., 1989b). Assuming 10% moisture in the grain and a harvest index of 0.4 (cf. Whitfield et al., 1989b), potential yields of treatments IFN~5o and IwN~aowere ca 800 and 900 g m -2, respectively, on the basis of their maximum biomass (Table 1 ). Yields of this order have been achieved experimentally in the region (Steiner et al., 1985), and, compared with measured yields of 650 g m -2 (Whitfield et al., 1989b ), it appears that lodging decreased yields by ca. 150 and 250 g m -2 in treatments IrN15o and IwN15o, respectively. Fischer and Stapper (1987) recently reported that yield of cv. Yecora decreased from ca. 870 g m -2 ( 10% dry-matter) in this region to 630 g m -2 in a lodging treatment imposed 3 weeks after anthesis. Growth rates in excess of 20 g m-2 day-1 were confined to periods when the crop was irrigated (Table 2 ), and rates were otherwise consistent with a maximum of ca. 20 g m -2 day-1 for crops grown under rainfed conditions (French and Schultz, 1984). In these circumstances, where both nitrogen and water stress may be operating at the same time, the functional balance between the root and shoot system was primarily maintained by decreased rates of leaf

294 e x p a n s i o n , a n d s u b s e q u e n t l y g r e a t e r r a t e s of leaf s e n e s c e n c e , as s h o w n b y t r e a t m e n t effects on L, f a n d XQi. H o w e v e r , t h e b a l a n c e also d e p e n d e d on t h e r a t e o f CO2 a s s i m i l a t i o n a n d p a r t i t i o n i n g of a s s i m i l a t e to t h e r o o t s y s t e m ( W h i t f i e l d et al., 1986; V a n K e u l e n a n d S e l i g m a n , 1987) as d e m o n s t r a t e d h e r e b y t r e a t m e n t effects on 0 a n d e. ACKNOWLEDGEMENTS We gratefully acknowledge the excellent technical assistance provided by R o s R u n c i m a n t h r o u g h o u t t h i s study, a n d t h e f i n a n c i a l s u p p o r t of t h e W h e a t I n d u s t r y R e s e a r c h Council of A u s t r a l i a .

REFERENCES Alvey, N.G. et al., 1983. Genstat - A General Statistical Program. Rothamsted Experimental Station, Great Britain. Biscoe, P.V. and Gallagher, J.N., 1977. Weather, dry matter production and yield. In: J.J. Landsberg and C.V. Cutting (Editors), Environmental Effects on Crop Physiology. Academic Press, London, pp. 75-100. Biscoe, P.V., Scott, R.K. and Monteith, J.L., 1975. Barley and its environment. III. Carbon budget of the stand. J. Appl. Ecol., 12: 269-293. Connor, D.J., 1975. Growth, water relations and yield of wheat. Aust. J. Plant Physiol., 2: 353366. De Wit, C.T., 1965. Photosynthesis of leaf canopies. Pudoc, Wageningen, Agric. Res. Rep. 63, 57 pp. Doyle, A.D. and Fischer, R.A., 1979. Dry matter accumulation and water use relationships in wheat crops. Aust. J. Agric. Res., 30: 815-829. Draper, N.R. and Smith, H., 1966. Applied Regression Analysis. Wiley, London, 407 pp. Fischer, R.A. and Stapper, M., 1987. Lodging effects on high-yielding crops of irrigated semidwarf wheat. Field Crops Res., 17: 245-258. French, R.J. and Schultz, J.E., 1984. Water use efficiency of wheat in a Mediterranean-type environment. II. Some limitations to efficiency. Aust. J. Agric. Res., 35: 765-775. Gallagher, J.N. and Biscoe, P.V., 1978. Radiation absorption, growth and yield of cereals. J. Agric. Sci., Camb., 91: 47-60. Goudriaan, J. and van Laar, H.H., 1978. Calculations of the daily totals of the gross C02 assimilation of leaf canopies. Neth. J. Agric. Sci., 26: 416-425. Green, C.F., 1987. Nitrogen nutrition and wheat growth in relation to absorbed solar radiation. Agric. For. Meteorol., 41: 207-248. Hipps, L.E., Asrar, G. and Kanemasu, E.T., 1983. Assessing the interception of photosynthetically-active radiation in winter wheat. Agric. Meteorol., 28: 253-259. Monteith, J.L., 1981. Does light limit crop production? In: C.B. Johnson (Editor), Physiological Processes Limiting Plant Productivity. Butterworths, London, pp. 23-38. Monteith, J.L. and Elston, J., 1983. Performance and productivity of foliage in the field. In: J.E. Dale and F.L. Milthorpe (Editors), The Growth and Functioning of Leaves. Cambridge University Press, London, pp. 499-518.

295 Northcote, K.H., 1979. A Factual Key for the Recognition of Australian Soils. Rellim Technical Publications, Glenside, South Australia, 123 pp. Puckridge, D.W., 1973. A quantitative account of the influence of solar radiation, water and nitrogen supply on the photosynthesis of wheat communities in the field. In: R.O. Slatyer (Editor), Plant Response to Climatic Factors. UNESCO, Paris, pp. 519-525. Rijtema, P.E., 1973. The effect of light and water potential on dry matter production of field crops. In: R.O. Slatyer (Editor), Plant Response to Climatic Factors. UNESCO, Paris, pp. 513-518. Rijtema, P.E. and EndrSdi, G., 1970. Calculation of production of potatoes. Neth. J. Agric. Sci., 18: 26-36. Ryhiner, A.H. and Matsuda, M., 1978. Effect of plant density and water supply on wheat production. Neth. J. Agric. Sci., 26: 200-209. Shhehy, T.R. and Peacock, J.M., 1975. Canopy photosynthesis and crop growth rate of eight temperate forage grasses. J. Exp. Bot., 26: 679-691. Sinclair, T.R. and de Wit, C.T., 1976. Analysis of the carbon and nitrogen limitation to soybean yield. Agron. J., 68: 319-324. Slabbers, P.J., Sorbello Herrondorf, V. and Stapper, M., 1979. Evaluation of simplified water-crop yield models. Agric. Water Manage., 2: 95-129. Snedecor, G.W. and Cochran, W.G., 1968. Statistical Methods (6th edition). Iowa State Univ. Press, Ames, IA, 593 pp. Spiertz, J.H.J. and de Vos, N.M., 1983. Agronomical and physiological aspects of the role of nitrogen in field formation in cereals. Plant Soil, 75: 379-391. Spiertz, J.H.J. and Ellen, J., 1978. Effects of nitrogen on crop development and grain growth of winter wheat in relation to assimilation and utilisation of assimilates and nutrients. Neth. J. Agric. Sci., 26: 210-231. Spiertz, J.H.J. and van der Haar, H., 1978. Differences in grain growth, crop photosynthesis and distribution of assimilates between a semi-dwarf and a standard cultivar of winter wheat. Neth. J. Agric. Sci., 26: 233-249. Steiner, J.L., Smith, R.C.G., Meyer, W.S. and Adeney, J.A., 1985. Water use, foliage temperature and yield of irrigated wheat in south-eastern Australia. Aust. J. Agric. Res., 36: 1-11. Van Keulen, H. and Seligman, N.G., 1987. Simulation of Water Use, Nitrogen Nutrition and Growth of a Spring Wheat Crop. Pudoc, Wageningen, 310 pp. Versteeg, M.N. and van Keulen, H., 1986. Potential crop production prediction by some simple calculation methods, as compared with computer simulation. Agric. Syst., 19: 249-272. Warren-Wilson, J., 1967. Ecological data on dry-matter production by plants and plant communities. In: E.F. Bradley and O.T. Denmead (Editors), The Collection and Processing of Field Data. Wiley, New York, pp. 77-123. Whitfield, D.M., Wright, G.C., Gyles, O.A. and Taylor, A.J., 1986. Effects of stage of growth, irrigation frequency and gypsum treatment on CO2 exchange of lucerne (Medicago sativa L. ) grown on a heavy clay soil. Irrig. Sci., 7: 169-181. Whitfield, D.M., Connor, D.J. and Hall, A.J., 1989a. Carbon dioxide balance of sunflower (Helianthus annuus) subjected to water stress during grain filling. Field Crops Res., 20: 65-80. Whitfield, D.M., Smith, C.J., Gyles, O.A. and Wright, G.C., 1989b. Effects of irrigation, nitrogen and gypsum on yield, nitrogen accumulation and water use of wheat. Field Crops Res., 20:261277.