Canopy conductance, carbon assimilation and water use in wheat

Agricultural and Forest Meteorology, 53 ( 1990 ) 1-18

1

Elsevier Science Publishers B.V., A m s t e r d a m

Canopy conductance, carbon assimilation and water use in wheat D.M. Whitfield Institute for Irrigation and Salinity Research, Tatura, Fic. 3616 (Australia) (Received October 31, 1989; accepted after revision June 15, 1990)

ABSTRACT Whitfield, D.M., 1990. Canopy conductance, carbon assimilation and water use in wheat. Agric. For. Meteorol., 53:1-18. Chambers were used to investigate changes in assimilation (A) and evaporation ( E ) rates in the field on a diurnal and a daily basis in rainfed and irrigated crops of wheat. Measurements were made in crops at growth stages between ear emergence and physiological maturity. Leaf area index ranged from a maximum of ~ 8 to a m i n i m u m of ~ 0.2. Assimilation and canopy conductance, g o decreased rapidly in rainfed treatments during grain filling, with midday values of gc becoming progressively smaller than morning values as stress progressed. Hysteresis was also evident in well-watered crops, indicating that E was adversely affected by the increase in evaporative demand during the day. Evaporative fluxes were analysed in terms of the sensitivity (c~) of leaf conductance to solar radiation. Changes in a implied both long-term and diurnal effects of stress on E. The estimate of c~ in the middle of the day was ~ 0.02 m m s-~ (W m -2) -~ in well-watered crops and so corresponded with the upper limit of the slope, leaf conductance vs. net radiation, reported elsewhere for wheat. Estimates of c~ were generally larger in the morning than at noon. Diurnal changes in gc and A indicated that the diffusive component in the assimilation pathway was the more sensitive to stress. The mean rate of assimilation during the day, A, was curvilinearly related to gc such that the A:'gc ratio decreased with an increase in gc. These changes suggested that stress increased relative stomatal control over CO2 assimilation and increased water use efficiency. Daily carbon assimilation was poorly associated with daily light interception in these data. However, the strength of the relationship between A and gc was sufficient to postulate that estimates of light use efficiency, derived from measurements of A and light interception or, less directly, growth and light interception, may be used to infer effects of stress on canopy conductance to CO: and H20 under conditions of water stress in wheat. INTRODUCTION

Gas exchange measurements are an invaluable aid to the study of crop growth and its dependence on environmental and agronomic factors. This paper reports the results of an investigation of the effects of irrigation treatment and environment on carbon assimilation and water use in wheat using field gas exchange techniques. The analysis of the canopy H 2 0 and CO2 exchanges used here derived from 0168-1923/90/$03.50

© 1990 - - Elsevier Science Publishers B.V.

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D.M. WHITFIELD

the work of Choudhury and Idso (1958a) and Tanner ( 1981 ). On the basis that water stress negatively affects the linear dependence, a, of leaf stomatal conductance on radiation, Choudhury and Idso (1985a) related canopy H20 exchange ( T ) t o intercepted radiation (Q) and vapour density deficit, p ' - p , using

T=o~(p'-p)Q

(1)

Here, Q, depends on leaf area index (L), solar radiation (Qo), solar elevation (fl) and canopy extinction coefficient (k) according to

O=Qo [1 - e x p ( - k L / s i n , 8 ) ]

(2)

Equation 1 provided the basis for analysis of T. Tanner (1981) investigated the links between assimilation (A) and T. He proposed that A is approximately related to T according to A=0.37 ( 1 - C i / C a ) T / ( p ' - p ) = 0 . 3 7 o ) T / ( p ' - p )

(3)

where O9= ( 1-- Ci/Ca)

C~ is the concentration of CO2 in the substomatal cavities of leaves and Ca is the ambient concentration of CO2. o) is a measure of the control exerted by stomates in CO2 fixation. As co tends to be conservative under a range of conditions (Goudriaan and Van Laar, 1978; Wong et al., 1979), eqn. 3 implies that CO2 assimilation and T are directly linked, depending on the inverse of ( p ' - p ) . The significance of eqn. 3 is seen in its direct long-term analogue i.e.

W=k' T/ (p' - p )

(4)

where k' varies with species (Tanner and Sinclair, 1983). Substitution of o~(p' - p ) Q for T (see eqn. 1 ) in eqn. 3 leads to A=0.37 o) c~ Q

(5)

The long-term analogue to eqn. 5 is the equation commonly used to relate growth (W) and intercepted radiation (Monteith, 1988 )

W= Q

(6)

and a comparison of eqns. 5 and 6 suggests that ~ depends strongly on the factors involved in o) and ~. The joint application ofeqns. 4 and 6 has been reported in wheat (Wilson and Jamieson, 1985 ), and the short-term analogues, eqns. 3 and 5, have been applied to the gas exchange of a number of crops, including sunflower (Connor et al., 1985 ), maize (Jones et al., 1986 ) and potato (Vos and Groenwold, 1989 ). Equations 3 and 5 were used here to explore the dependence of T on

CANOPY CONDUCTANCE, CARBON ASSIMILATION AND WATER USE IN WHEAT

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(p' - p ) and Q, and to investigate the role of 09 in the balance between A and T. MATERIALS AND METHODS

The study was conducted at the Institute for Irrigation and Salinity Research, Tatura, Australia (36°26'S, 145 ° 14'E). The measurements of CO2 exchange and H20 exchange were made in crops grown in an experiment conducted to investigate agronomic responses to irrigation, nitrogen and gypsum (Whitfield et al., 1989 ). Three irrigation treatments, two nitrogen treatments and two rates of gypsum were included in factorial combination. Seed of cultivar Condor was sown at 150 kg ha -1 in drill rows spaced at 0.15 m on 14 June 1984. The measurements in this study were confined to treatments which received gypsum ( 5 t h a - 1) at sowing. Nitrogen was applied as a m m o n i u m nitrate at rates of 0 (Treatment N0) and 150 kg N ha -~ (Treatment N~5o) at sowing. Irrigation treatments included rainfed plots which received no irrigation (Treatment Rv), and plots irrigated at 2-weekly (Treatment Iv) or weekly (Treatment Iw) intervals beginning 120 days after sowing (DAS 120). Irrigation was maintained until physiological maturity. A total of 22 m m rainfall was recorded between DAS 120 and harvest in Treatment Rv. In this period, Treatment IF received 310 m m water in five applications and Treatment Iw received 380 m m in 10 irrigations. Treatments IF and Iw were irrigated at mean deficits of ~ 70 and 40 m m of Class A pan evaporation, respectively.

Measurement of crop C02 and 1120 exchange Crop gas exchange was measured diurnally using three or four transparent open-system field assimilation chambers (Connor et al., 1985; Whitfield et al., 1986 ). Each chamber covered a ground area, ach, of 1 m 2. Air temperature in the chambers was maintained to within 4 °C of ambient temperature by cooling the influx air stream. Ambient air was drawn through the chambers at a flow rate, F, of ~ 0.08 m 3 s -1 . F w a s measured regularly with orifice-plate flowmeters. Turbulence and mixing were provided by a fan inside each chamber. Solar radiation (Qo) and the temperature (T) of the effiux air stream (TE) were measured in conjunction with rates of gas exchange. Qo was measured with a Kipp solarimeter and TE was measured with LM-35 electronic temperature transducers (National Semiconductor, Santa Clara, CA ).

CO2 exchange The CO2 differential of the influx and effiux air stream (3CO2) of each chamber was measured directly with an infrared gas analyser (Type 225, Mark

4

D.M. WH1TF1ELD

III, Analytical Development Corp., U.K.) over periods of 5 min in each 30 min throughout the day and night. At maximal rates of exchange, 6CO2 was ~ 15 #11- ~. The CO2 analyser was calibrated weekly using Wosthoff pumps (H. Wostoff, Bochum, F.R.G. ) to provide a reference concentration of CO2. The rate of CO2 exchange, A, was calculated as A = 6CO~ F/ach

(mg CO2 m - 2 s-

1)

A positive pressure maintained in each chamber largely excluded effects of soil and root respiration on measured CO2 exchange (Kanemasu et al., 1974; Whitfield et al., 1986; McCullough and Hunt, 1989 ).

Water vapour exchange Relative humidity of the influx (qh) and effiux (OE) air streams of each chamber were measured in conjunction with riCO2.01 and 0E were measured with capacitative humidity sensors (Vaisala, Helsinki, Finland) incorporated in a brass block thermostatically maintained at 39.0°C (G. Henstridge, unpublished work, 1980). Each channel was calibrated weekly using a water vapour generator (Analytical Development Corp., U.K. ). The rate of vapour exchange, E, which included both soil (S) and transpiration (T) components, was calculated as

E=(¢)E--OI)p~T=39)F/ach

(mg H~O m-2 s-I )

Here, P'(T=39) was the saturated vapour density at 39°C ( ~ 50 g H 2 0 m - 3 ) . An exchange rate of 280 mg H 2 0 m -2 s - l ( 1 m m H20 h - 1) was therefore measured as a difference in vapour concentration of ~ 3.5 g H20 m -3 (i.e. ¢E-- ~1 = 0.07 ). Whereas the depression in fiCO2 at maximal A was minor, the concentration of water vapour in the chambers varied more strongly with E. For TE ~ 20°C the vapour pressure deficit of the effiux air stream decreased by ~ 1.7 Pa (mg H20 m -2 s -~ )-1 evaporation rate. Assuming that TE approximates canopy temperature (Tc) in well-stirred chambers (Connor et al., 1985 ), gc was calculated as

gc=E/(p~r=TE)--p)

( m m s -1 )

(7)

Errors are introduced in estimates of gc if Tc differs significantly from TE. According to eq. 7, gc increases by 6-12% per unit increase in the difference, TE-- Tc. Potential errors are greatest at high rates of E under conditions of high relative humidity. Estimates of gc made on the basis of eqn. 7 are also artificially large if the rate of soil evaporation, S, is a significant c o m p o n e n t of E.

Leaf area Samples ( 1 m of row) were taken from each plot at approximately weekly

CANOPY CONDUCTANCE, CARBON ASSIMILATIONAND WATER USE IN WHEAT

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intervals in the period DAS 95 to final harvest (DAS 187, DAS 193 and DAS 200 in Treatments RF, IF and Iw, respectively). Leaf area index, L, was calculated from the area of green leaf tissue (excluding leaf sheaths) measured on a subsample using an electronic planimeter (LI3100 Area Meter, LI-Cor Inc., Lincoln, NE). Estimates of L were interpolated from measures of L in each treatment (Whitfield and Smith, 1989).

Analysis of the diurnal evaporativeflux Following Choudhury and Idso (1985a), diurnal changes in E were described empirically using eqns. 1 and 2. It was assumed that leaf conductance, gL, was proportional to solar radiation, i.e. g L = a Q' and that radiation in the canopy ( Q ' ) was attenuated according to

(8) [

Q' = Qo exp ( - kL/sin//) Canopy conductance was then derived as a function of Q L

gc = | a (k/sin//) Q' dL=a Qo[1 - e x p ( -kL/sin//)] = a Q

(9)

0

Here, Q equates to the solar radiation intercepted by the canopy (eqn. 2 ). k, the canopy extinction coefficient, was taken as 0.45 (cf. Sheedy and Peacock, 1975; Denmead, 1976; Versteeg and Van Keulen, 1986 ). Equation 1 was derived by equating aQ (eqn. 9) with the standard expression for g o gc = T / ( p ' - p ) , and therefore provided a means of describing diurnal changes in T based on measures of Q and ( p ' - p ) . Assuming S = 0 , eqn. 1 leads to the simple model, E = a Q(p' - p ) , which may be used to derive an average estimate of a in a given set of data by regressing observed values of E on the product Q(p'-p). However, a depends on leaf water potential (Choudhury and Idso, 1985a) and so varies during the day. This implies that gc changes simultaneously with stress and radiation, a was therefore considered to vary quadratically with time of day, t. Given O~=O~ 0 -kO~ l t-kO~ 2 t 2

(10a)

it follows that g L = (O~0 -k- O~1 t ' k O~2 t 2 ) Q '

(10b)

go= (ao +al t+az t z) Q

( lOc)

and E = ( a o + a l t + a 2 t z) Q ( p ' - p )

(10d)

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D.M. WH1TFIELD

Diurnal changes in E were therefore estimated as E=Eo+o~o Q(p'-p)+o~l Q(p'-p) t+o~2 Q ( p ' - p ) t 2

(11)

where the constant, Eo, accounted for evapotranspiration at night. The coefficients Eo, O~o, a l and a2 in eqn. 11 were estimated by least squares, t was measured about solar n o o n (t = 0 ).

Scope of measurements Chambers were initially deployed at DAS 106, and measurements o f A and TABLE1 Daily totals of solar radiation (Qo), intercepted radiation (Q), net assimilation (YA) and evaporation ( Z E ) , and change in mean daily light use efficiency (e, g CO2 M J-~ ), mean canopy conductance (go, mm s -~ ) and the ratio (o3) with time (DAS), treatment, leaf area index (L) and days after irrigation (DAI). n describes the number of measurements, taken at half-hourly intervals, made during daylight in each data set DAS Treatment

L

108 RvNiso 108 RvN15o 108 RvNiso 119 RFN15o 119 RvNl~o 119 RvNiso

6.5 7.5 6.3 8.1 7.5 7.1

-

23.6 23.6 23.6 24.1 24.3 25.4

23.0 23.2 22.9 23.7 23.9 24.8

58.1 49.4 56.6 60.5 68.9 66.1

137 145 154 168

RvNlso RvNiso RFNIso RFNlso

3.6 3.3 1.2 0.1

-

28.5 29.1 31.6 32.0

24.6 24.0 15.5 0.9

137 lwNiso 145 lwN15o 154 IwNlso 168 IwN15o 168 IwNo 174 lwNiso 174 IwNo

6.9 7.1 5.6 3.1 1.3 3.2 0.4

3 4 6 6 6 5 5

29.0 28.9 31.5 32.3 31.7 32.4 32.2

137 145 154 168 174 174

5.7 5.7 3.4 2.3 0.6 0.2

3 11 6 6 12 12

28.8 29.1 31,2 32,1 30,4 32.3

IvNiso lFNiso IvNl~o IvN~5o IFNIso IvNo

DAI

Qo Q (MJ m -2 d a y - ~)

~A ~E (g m -2 day - l )

e

go,

09

n

4590 4560 5180 3790 4900 4970

2.43 2.06 2.33 2.41 2.75 2.60

12.1 11.6 12.6 10.4 13.9 13.4

0.31 0.27 0.28 0.38 0.32 0.31

23 23 24 24 24 24

21.3 15.8 21.2 --0.9

3190 2130 2770 490

0.80 0.57 1.33 0.00

4.1 2.7 4.0 0.5

0.33 0.33 0.31 --0.09

23 26 26 27

28.2 28.0 29.7 25.8 16.2 26.2 5.9

89.6 84.2 85.8 54.6 25.0 33.8 2.5

9410 9790 10100 9330 6110 8180 3830

3.14 2.97 2.86 2.11 1.58 1.31 0.48

16.8 18.1 21.1 14.6 9.5 14.5 5.9

0.31 0.28 0.23 0.20 0.14 0.13 0.02

25 25 27 27 27 27 27

27.3 27.5 25.9 22.6 9.2 2.9

73.9 53.6 79.9 30.2 5.0 0.5

9040 5640 9800 6550 2250 2720

2.74 1.89 3.1",0 1.36 0.55 0.21

15.9 8.7 20.0 9.2 3.1 3.6

0.28 0.37 0.23 0.19 0.09 0.01

24 25 26 27 27 27

Rv denotes rainfed treatments, and IF and lw denote treatments irrigated on a fortnightly and weekly basis, respectively. No and N~so denote treatments which received 0 and 150 kg N ha-~ at sowing, respectively.

CANOPY CONDUCTANCE, CARBON ASSIMILATION AND WATER USE IN WHEAT

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E were made 12 days before irrigation treatments were imposed. Measurements were made continuously on individual sections of crop for 3 or 4 days. After DAS 120, chambers were usually deployed 2 or 3 days after irrigation and measurements were again made continuously on the one section of crop for 3 or 4 days. Different areas of crop were used each week (achieved by moving chambers between replicates within treatments and by varying sites within plots). The weekly sequence of measurements was maintained for 7 weeks in Treatment RF, and for 9 weeks in Treatments IF and Iw. The data considered here were confined to seven clear days in the period DAS 108-174. On these days, the mean m a x i m u m temperature of the efflux air stream (TE) ranged from 21 to 30 ° C. Mean m a x i m u m vapour pressure deficit varied from ~ 1.7 to 2.5 kPa. Solar radiation at midday ranged from ~ 9 0 0 t o l l 0 0 W m -2. The data described the activity of three similarly treated crops on each of 2 days before the imposition of irrigation treatments (Treatment RFNd 5o; DAS 108, DAS 119). L in these crops was ~ 7. The response in Treatment RvN150 after DAS 120 was described by data for 4 days in the period DAS 135-168. The remaining data described responses in the irrigated treatments. Data for Treatment IF included measurements made between 3 and 11 days after irrigation (Table 1). RESULTS AND DISCUSSION

Leaf area index In the period DAS 95-120, L increased from ~ 5.3 to a m a x i m u m of ~ 7.7 (Whitfield and Smith, 1989 ). L decreased rapidly in Treatment Rv after DAS 120, whereas there was little change in the irrigated treatments, Iw and Iv, until a rapid decrease after DAS 148. Full cover was maintained in Treatment Iv until anthesis (DAS 140) and in Treatment Iw until ~ DAS 160.

Patterns of C02 and 1-120exchange during the day Examples of the daily patterns of CO2 and H 2 0 exchange are shown in Fig. 1. Treatment RFN~5o, DAS 119 (Row 6, Table 1 ), typified responses measured before DAS 120. The m a x i m u m in A and E were ~ 2.1 mg CO2 m - 2 sand 170 mg H20 m -2 s - l , respectively (Fig. 1 ). Daytime totals of CO2 exchange (~A) ranged between 50 and 70 g CO2 m -2 (Table 1 ). Totals of H20 exchange ( Z E ) ranged from 3800 to 5200 g H 2 0 m - 2 d a y - ~ (Table 1 ). Marked effects of water stress were seen in rainfed crops after DAS 120, represented in Fig. 1. by Treatment RF, DAS 145. The m a x i m u m rate of CO2 exchange was only ~ 0.7 mg m - 2 s - l and m a x i m u m E was 60 mg H20 m -2

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D.M. WH1TFIELD

I

I

I

I

I

I

I

I

(a)

I

I

I

I

I

1

(b)

3

300

2

200

7 ?

%

g

~

0

100

0 400

800

1200 time

1600

2000

400

(h)

800

1200 time

1600

2000

(h)

Fig. 1. Diurnal changes in (a) assimilation rate, A, and (b) rate of evaporation, E, in Treatment RFN j 50 at DAS 1 19 ( O ), IwN 150 at DAS 13 7 ( • ), I FN 150 at DAS 14 5 ( [] ), RFN ~so at DAS 14 5 ( • ) and Treatment IwNo at DAS 174 ( A ).

s - '. By DAS 168, Y~Awas negative and Y~E was only 500 g H 2 0 m - 2 d a y - ~in the rainfed t r e a t m e n t (Table 1 ). The data for T r e a t m e n t IwN~50, DAS 145, were typical of those found in the irrigated treatments (IwN~5o and IFN~so) in the period DAS 120-154, when measurements were m a d e within a week of irrigation. Data for Treatm e n t IFN~5o, DAS 145, which were measured in the second week after irrigation, showed that the extended irrigation interval significantly affected maxi m u m A and E. In general, daily trends and responses to stress were similar to those reported in other crops ( C o n n o r et al., 1985; Jones et al., 1986; Whirfield et al., 1986; Puech-Suanzes et al., 1989). Data for T r e a t m e n t IwN,5o, DAS 174, demonstrated the response of irrigated crops with low L measured late in the growing season. M a x i m u m A was only ~ 0.15 mg COz m - 2 s - ', whereas m a x i m u m E was ~ 100 mg H20 m - 2 s - l (see Fig. 1 ).

Analysis of diurnal changes in 1120 exchange Estimates of the regression coefficients, Eo and ao, appropriate to eqn. 11 are shown in Table 2. With the exception of T r e a t m e n t RFNlso, DAS 168, when •E was only 490 g H 2 0 m -2 d a y - ', coefficients o f determination (R 2 ) generally exceeded 0.95. The m e a n rate o f vapour exchange at night, Eo, achieved a m a x i m u m o f ~ 29 mg H 2 0 m -2 s - ' ( ~ 0.1 m m h - ~). Large values

CANOPYCONDUCTANCE,CARBONASSIMILATIONAND WATERUSE IN WHEAT

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TABLE 2 Estimates of the regression coefficients, Eo (mg H20 m - 2 s - ~) and ao (mm s - ~ (W m - 2 ) - ~), of the coefficient of determination (R 2) appropriate to eqn. 11, and of derived estimates of a ( m m s-~ (W m - 2 ) - 1, see eqn. 10a) at 08:00 and 16:00 h

DAS Treatment

L

108 RFN~5o 108 RFN~so 108 R~N~5o 119 RvN~5o 119 RvNlso 119 RFN~5o

6.5 7.5 6.3 8.1 7.5 7.1

1.9 1.6 2.0 5.1 2.5 0.3

'137 RvN~so 145 RvNiso 154 RvNiso 168 RvNlso

3.6 3.3 1.2 0.1

IwN15o lwNjso lwNlso IwN15o IwNo IwNl5o lwNo

137 1vNis0 145 IFN150 154 IFNLso 168 IvNls0 174 IvN15o 174 IvNo

137 145 154 168 168 174 174

Eo (X103)

c% (X103)

R2

c~ 08:00 h (X103)

a 16:00 h (XI03)

(1.9) (2.5) (2.1) (2.5) (2.3) (1.6)

16 15 17 13 18 18

(0.5) (0.6) (0.5) (0.6) (0.6) (0.4)

0.976 0.958 0.976 0.944 0.970 0.987

33 35 36 21 32 33

(1.5) (1.9) (1.4) (2.0) (1.9) (1.1)

23 22 26 18 28 25

(1.0) (1.3) (1.1) (1.2) (1.4) (0.9)

5.9 1.7 3.9 -2.2

(1.2) (0.7) (0.4) (0.7)

5 4 9 26

(0.2) (0.1) (0.1) (2.4)

0.974 0.982 0.996 0.783

7 8 14 48

(0.2) (0.2) (0.2) (4.0)

10 5 9 19

(0.7) (0.2) (0.1) (2.3)

6.9 7.1 5.6 3.1 1.3 3.2 0.4

21.9 18.6 29.3 27.6 20.2 27.1 15.6

(1.3) (1.7) (1.7) (3.5) (1.9) (2.4) (1.3)

22 22 24 18 20 17 36

(0.2 (0.3 (0.3 (0.6 (0.5 (0.4 (0.9

0.997 0.995 0.994 0.970 0.980 0.983 0.978

32 32 29 26 24 24 35

(0.6) (0.8) (0.7) (1.4) (1.0) (1.0) (1.8)

21 24 28 22 19 20 30

(0.3) (0.4) (0.5) (0.7) (0.6) (0.6) (0.8)

5.7 5.7 3.4 2.3 0.6 0.2

23.4(2.1) 11.6 (0.8) 23.9 (1.3) 11.5 (2.6) 7.9 (1.1) 9.2 (0.8)

0.991 0.996 0.997 0.975 0.953 0.981

27 18 32 21 15 50

(0.7) (0.3) (0.6) (1.0) (0.9) (1.9)

21 12 27 15 11 38

(0.7) (0.2) (0.4) (0.5) (0.5) (1.1)

20(0.3 10 (0.1 28 (0.3 14 (0.4 10 (0.4 45 (1.0)

Standard errors ( P < 0.05 ) are shown in parentheses.

were associated with recent irrigation. The m a x i m u m estimate found here was only ~ 60% of the nightly rate measured by Meyer et al. ( 1 9 8 7 ) , but was considerably larger than the estimate of nocturnal transpiration (500 g m - 2 per night) reported by R a w s o n and Clarke ( 1988 ).

Changes in canopy conductance during the day Estimates o f g c made on the basis of eqn. 10c are shown in Fig. 2a. Before DAS 120, gc was a m a x i m u m of ~ 18 m m s - ~. gc increased to ~ 25 m m s in irrigated treatments after D A S 120 in measurements made within a week of irrigation. The smallest m a x i m a in gc were seen in Treatment Rv after DAS

10

D.M. WHITFIELD 30

I

I

I

I

I

t

5O

I

I (b)

(a)

?

i

|

I

I

I

I

I

I

40

20

8 e, o u

10

o 10

I

0 400

0

800

1200 time

1600

2000

(h)

400

800

1200 time

1600

2000

(h)

Fig. 2. Examples of diurnal changes in (a) canopy conductance and (b) leaf conductance. Estimates were made using eqn. 10. Standard errors of the estimate (P<0.05) for noon values ranged between 0.06 and 0.29 mm s-~ for the range of treatments in (a). In (b), the standard error of the estimate was 0.32 mm s-~ in Treatment IwNo, DAS 174. Otherwise, errors appropriate to noon values (P< 0.05 ) ranged from 0.05 to 0.16 mm s- 1. Symbols are as in Fig. 1. 120. During the day, m a x i m u m values o f gc were found between 08:00 and 10:30 h, i.e. before noon.

Estimates of sunlit leaf conductance In general terms, senescence and a reduction in plant water status affecting leaf area and leaf activity per unit area cause the long-term decline in gc during grain filling. Estimates ofgL in the upper canopy provide a measure o f leaf activity which, depending on the validity o f assumptions made in the analysis, may be taken to be independent o f the major changes in L. Estimates o f sunlit leaf conductance were derived from (cf. eqn. 10b )

gL= (OLod-O~l t+O~2 t 2) Qo Figure 2b shows that, apart from Treatment IwNo, DAS 174, daytime trends in gL were similar both qualitatively and quantitatively to those seen in gc (Fig. 2a), and that the estimates made here generally fell in the published range for wheat, 0 - 2 0 m m s-1 (Choudhury and Idso, 1985a,b ). In Treatment IwNo at DAS 174, however, estimates of gL were m u c h greater than 20 m m s - ~ for most o f the day, and were also much larger than those seen in the other examples. This suggested that soil evaporation, S, made a significant contribution to E in those data.

CANOPY CONDUCTANCE, CARBON ASSIMILATION AND WATER USE IN WHEAT

| 1

Treatment effects on the response of leaf conductance to radiation It was assumed here that leaf conductance varies with Q dependent on leaf water status. C h o u d h u r y and Idso (1985a), for example, found that the slope, ~, of the gc/Q relationship decreased almost linearly with a decrease in leaf water potential below ~ - 1.2 MPa. In the present study, o~ was calculated using the regression estimates o f the coefficients C~o,c~ and c~2 in eqn. 11. C~oprovided a direct estimate of o~ at noon (t = 0 ). Given the general similarities in estimates o f gL and gc for most treatments (Fig. 2 ), relative values of O~o (see Table 2 ) followed similar trends to the m a x i m a seen in go. In Treatm e n t RF, % decreased from a m e a n o f ~0.017 m m s -~ (W m - 2 ) -1 before DAS 120 to ~ 0.005 m m s - ~ (W m -2) - 1 in the period DAS 137-154. At DAS 168, O~oincreased markedly to ~ 0.03 m m s- ~ (W m -2 ) - 1. This estimate was subject to errors associated with estimates of L at low L, and m a y further have .been affected by a significant ear a n d / o r stem contribution in E. In T r e a t m e n t Iw, O~owas ~ 0.021 m m s-1 (W m - z ) - t in the period DAS 135-174 (Table 2). Taking net radiation as ~ 0.7 Qo, this corresponds to the upper limit of the slope, gc VS. net radiation, found by C h o u d h u r y and Idso (1985a), and also with estimates m a d e by D e n m e a d and Millar ( 1976 ). The large estimate, C~o=0.036 m m s -~ (W m - Z ) -1, at DAS 174, was again associated with low L and recent irrigation. Similar estimates to those found in T r e a t m e n t Iw, DAS 135-174, were observed in T r e a t m e n t IF at DAS 137 and 154 (Table 2). On these occasions, measurements were m a d e within a week of irrigation. Otherwise, Olo was less in T r e a t m e n t IF than in T r e a t m e n t Iw. ~o decreased late in the grain-filling period, although the final estimate for T r e a t m e n t IvNo, DAS 174, was again large. In addition to errors in m e a s u r e m e n t of L at low L, and the possibility o f a significant water loss from non-leaf organs, later estimates of O~o in the irrigated treatments appear to have been affected by a significant S component in E.

Diurnal response of leaf conductance, gL, tO radiation The coefficients o~0, c~ and o~2 (eqn. 11 ) generally implied that o~ achieved a m i n i m u m near noon. This suggests, as noted by C h o u d h u r y and Idso (1985a), that the gL/Q' relationship was non-linear (cf. eq. 8) and that decreased with an increase in Q. However, estimates of o~ were asymmetric about noon and implied a significant hysteresis in the ge/Q relationship. Estimates of o~, m a d e according to eqn. 10a, were generally greater in the morning than estimates of O~o (see Table 2 ), whereas estimates o f o~ appropriate to 16:00 h were usually greater than or similar to those m a d e at noon. These data therefore suggest that the transient low leaf water potentials that develop during periods o f high evaporative d e m a n d counter the continued increase in leaf conductance with increasing irradiance and limit the value of

12

D.M. WHITFIELD

gL even in well-watered crops ( D e n m e a d and Millar, 1976; Ehrler et al., 1978; Choudhury and Idso, 1985b). The observation that the maxima in a, gL and gc generally occurred before noon supports the argument that E was adversely affected by the increasing radiation load and vapour pressure deficit during the day. The general hysteresis seen here in o~ is therefore attributed to an increasingly rapid rate of decrease in plant water potential in the morning, and its slower recovery in the afternoon as the soil dries (Denmead and Millar, 1976; Ehrler et al., 1978).

Interrelationships between water use and carbon assimilation Diurnal change in the ratio (1 - C/Ca) Based on eqn. 3, estimates were made of the ratio, co= ( 1 -

Ci/Ca) ,

using

co=A~ [E/ (p' - p ) ] Estimates were made assuming T = E (i.e. S = 0). Examples of daily patterns in 09 are shown in Fig. 3. At DAS 119, and in Treatment IwN~5o measured at DAS 137, 09 increased from ~ 0.25 to 0.36 in the period 08:00-16:00 h. These data compare with the accepted proportion of stomatal control over A, co ~ 0.3, in C3 species (Goudriaan and Van Laar, 1978; Wong et al., 1979; Ramos and Hall, 1982; Farquhar and Richards, 1984). The trends in co suggest that effects of water stress were evident in gc before A, and that the diffusive com0.5

T

I

I

I

I

I

I

l

0.4

0.3

d=

, ,-t

0.2

0.1

0.0

I 400

800

I

1200 time

I 1600

2000

(h)

Fig. 3. Diurnal changes in the ratio o9= ( 1 -C~/Ca) in Treatment RvNtso at DAS 119, IwNlso at DAS 137, IvN~5o at DAS 145, RFN,5o at DAS 145 and Treatment IwNo at DAS 174. Symbols are as in Fig. 1.

CANOPY CONDUCTANCE, CARBON ASSIMILATION AND WATER USE IN WHEAT

|3

ponent in the assimilatory pathway was the more sensitive to stress. Thus, 09 was >0.3 in the period after 10:00 h. A similar finding was reported by Zur and Jones (1984) for soybean. Choudhury (1986) found that o9 increased with an increase in canopy-air vapour deficit in cotton and with crop water stress. Vos and Groenwold ( 1989 ) reported an increase in 09 with water stress in potato. 09 was larger in Treatment I vN15o, DAS 145, increasing from ~ 0.35 to 0.42 during the day. Values of 09 were heavily dependent on the relatively large measures of A in the morning in Treatment RFNIso, DAS 145, and showed wide variation during the day. These findings were associated with low values for gc on that occasion (see Fig. 2a). Estimates of co in Treatment IwNo, DAS 174, were distinct from those in other examples in Fig. 3. The consistently low values of 09 again implied that soil evaporation made a substantial contribution to measures of E in this and other irrigated crops at DAS 168 and 174. Mean daytime estimates o f the ratio (1 - Ci/C.) Mean daytime estimates of m were made for each data set by calculating. ca=A/0.37 gc where A= ~A/n

(mg CO2 m -2 s -~ )

and gc = ~ E / Z (p' - p ) Here, n was the n u m b e r of measurements of A and E made during the day (see Table 1 ). Estimates of ca generally fell in the range 0.24-0.36 for L ~ 3 (Table 1 ). However, ca also fell in this range in Treatment RF, DAS 154, when L was only 1.2. EA was negative in Treatment RF, DAS 168 (see Table 1 ), resulting in the negative value for ca on that occasion. A linear relationship was found between ~A and gc for cases where ca > 0.2 (Fig. 4). The mean ratio was 0.104 (s.e.=0.004, R2=0.83, n = 15), implying that ca was 0.104/0.37, i.e. ~0.28 (s.e.=0.01). Note, however, that gc, in contrast to .4, is numerically independent of daylength ( n ). The ~A-gc relationship therefore depends on both .4 and n. In fact, .~ was found to be curvilinearly related to gc. The relationship was described by A = 0 . 0 0 2 6 6 ( + 0 . 0 0 0 4 7 ) {1 --exp[ --60.9( + 17.6) gc]}, n = 15, R 2 = 0 . 9 3 (12) Similar curvilinear responses between A and gL have been reported for soybean crops (Zur and Jones, 1984) and cotton leaves (Puech-Suanzes et al.,

14

D.M. WHITFIELD i00

,

t

i

i

,

~

i

i

°

~

8o

g u

6O

e, o .,d

4O

,~

20

//-"

.,-4 i~

1411

I

5

gC

I

~

I

i0

15

(ram

s -1)

I

L

20

Fig. 4. Relationship between total daily assimilation ( ZA ) and mean canopy conductance (gc). Open symbols reference crops where the ratio ( 1 - Ci/Ca) was > 0.2.

1989). The lesser sensitivity of.~ to gc at larger values o f g c implies that the A : T ratio, i.e. water use efficiency, increased with a decrease in gc (see Zur and Jones, 1984). It should be noted, however, that errors in gc caused by the assumption of equality of leaf and air temperature in the chamber system possibly contributed to the curvilinearity in the response of.~ to gc; estimates of gc are artificially inflated according to eqn. 7 if T¢ < TE, i.e. conditions that are most probable when gc is large. On the basis of eqns. 3 and 12, 03 may be estimated as 03=A/(0.37~c) =0.0072 [ 1 - e x p ( - 6 0 . 9

gc) ]/~c

03>0.2

These estimates provided a good description of changes in 03 for 6J> 0.2 and data in the range g o > ~ 7 m m s-1 (Fig. 5b). In this range, 03 decreased from ~ 0.34 to 0.24 with an increase in gc to ~ 20 m m s- 1. The smaller estimate, 03~0.24, found in well-developed unstressed crops, corresponded with the values of co used by Choudhury and Idso (1985a) to simulate the wheat canopy assimilation data of D e n m e a d (1976), which described a well-watered crop with a leaf area index of ~ 3.2.03 was ~ 0.34 in the range gc less than ~ 7 m m s --1 Figure 5a demonstrates that there was no apparent relationship between 03 and L for 03> 0.2. In comparison with well-watered crops with large L, large gc and small cO, the increase in 03 with a decrease in gc found here (Fig. 5b ) involved both major (e.g., treatment RvN15o, DAS 154) and minor (e.g. Treatment RFN~5o, DAS 1 19; Treatment IvN15o, DAS 145) differences in L. It therefore appears that stomata exerted greater control over assimilation in stressed crops, with a consequent increase in water use efficiency. A relatively greater proportion of foliage active in assimilation may also have been effective at small L (Tanner and Sinclair, 1983). A combination of both effects

CANOPY

CONDUCTANCE,

CARBON

ASSIMILATION

AND WATER USE IN WHEAT

0.4

0.4

i

t

O

oo 0

0.3

qD j t /"

0//

b

0

0.2

•

@

0.i

O/ /

(a)

,

i

0

,

i

/

0.0

l

m £)

qD

/ 0.i

,

0

0.3

,/~

0.2

i

l O O

°°

0 qlDO

m u

m

O

15

i

2 Leaf

i

i

i

i

4

6

Area

Index

i

(b) i

8

I al

0.0 0

t 5

i i0

gc

i

i

~

15

(wan s -1

J 20

)

Fig. 5. Relationship between (a) the ratio co= (1 -Ci/Ca) and leaf area index and ( b ) ~o and mean canopy conductance (gc). Pied symbols reference crops measured within a week of irrigation and o3> 0.2. Otherwise, symbols are as in Fig. 4. The curve in ( a ) was fitted by eye.

should lead to a progressive improvement in water use efficiency during grain filling.

Soil evaporation as a component of the evaporative flux As shown in Table 1, o~ was < 0.2 in measurements made in the irrigated treatments, Iw and IF, at DAS 168 and 174. ~0 increased curvilinearly with an increase in L in these examples and in those data where measurements were made within a week of irrigation (Fig. 5a). The trend with L was therefore similar to the trends in TIE reported by Ritchie and Burnett ( 1971 ) and Denmead ( 1973 ) for a wet soil surface. These data strongly suggest that soil evaporation caused a progressive decline in the A: E ratio with decreasing L and a wet soil surface.

Relationships between canopy conductance, light use efficiency and estimates of(1--C/Co) There was little relationship between Y~A and Y~Q in these data (Fig. 6a). This implies that ot and 09 (eqn. 5 ) played major roles in A. Maximum light use efficiency (A/Q) was ~ 3 g CO2 M J - 1 as shown by the solid line in Fig. 6a. 6, calculated as ~A/YQ, was curvilinearly related to gc for data in the range o~> 0.2, i.e. under conditions where Tlargely accounted for E (Fig. 6b). 6 decreased relatively slowly with a decrease in gc for gc in the range 20-10 mm s - ', whereas the rate of decline was greater in the region ~c < 10 mm s As in the response of A to gc (eqn. 12 ), the curvilinearity in the relationship may have derived, in part, from errors in gc caused by the assumption of equality of leaf and air temperature in the chamber system (eqn. 7 ). 6, however, was clearly not conservative with stress. According to eqn. 5,

16

D.M.WHITFIELD

i00

X

~

3.0

U e,

2.0

~

1.0

.~ -,..i

0.0

o ~ o

(b)

g U

6O O

e~ 0 ,~

40

"

0

20

ul

I~

5

I

I

10

I

I

15

.

0 0 ]

[

20

I

I

25

[

30

Intercepted Radiation (MJ m -2 d -1)

I

0

I

I

5

I

10 gc

I

I

15

I

I

20

(tam s - l )

Fig. 6. Relationship between (a) total daily assimilation ( ~ A ) and intercepted solar radiation ( ~ Q), and (b) light use efficiency and mean canopy conductance (gc)- The solid line in (a) represents a light use efficiency (e) of 3 g CO2 MJ- i. The curve in (b) was described by

6=3.67 ( 1 - e x p [ - 9 1 . 7 ¢c])

R2=0.92, n= 15

Symbols are as in Fig. 4.

depends on both co and a. Given that co was restricted to the approximate limits 0.24<09<0.36 (Fig. 5b) and that cOtends to increase with stress or a decrease in g o it appears that variation in a caused most of the variation in e. The application o f eqn. 6 as a predictive tool for W therefore demands an appreciation o f the effects of water stress on a, gL or g o In fact, the strength o f the relationship between ~ and gc in the range o3>0.2 was sufficient to suggest that variations in e may justifiably be interpreted in terms of effects of stress on leaf activity under conditions o f varying water stress in wheat (see Whitfield and Smith, 1989 ). CONCLUSIONS

Daily trends and responses to water stress in E, A, gc and gL were similar to those reported in other crops. All decreased rapidly in rainfed treatments during grain filling, with midday values of gc and gL becoming progressively smaller than morning values as stress progressed. Diurnal hysteresis was also evident in gc and gL in well-watered crops, indicating that E was adversely affected by the increase in evaporative demand during the day. Changes in the sensitivity of gL to solar radiation implied both long-term and diurnal effects o f stress on E. The estimate o f a in the middle o f the day was ~ 0.02 mm s-~ (W m -2) - ] in well-watered crops and so corresponded with the upper limit o f the slope, gL VS. net radiation, reported elsewhere for wheat. Diurnal changes in gc and A showed that water stress decreased gc sooner

CANOPYCONDUCTANCE,CARBONASSIMILATIONAND WATERUSE IN WHEAT

17

than A, and indicated that the diffusive component in the assimilation pathway was the more sensitive to stress. On a daytime basis, the mean rate of assimilation, A, was curvilinearly related to gc. The stomates assumed greater control over assimilation, with a consequent increase in water use efficiency, at low values of go. By contrast, low values of the.4: gc ratio, found in recently irrigated crops with small L late in the growing period, were attributed to a significant contribution of soil evaporation in measures of E. Light use efficiency was strongly related to gc in these data. It appears that estimates of light use efficiency, derived from measurements of A and light interception or, less directly, growth and light interception, may be used to infer effects of stress on canopy conductance to CO2 and H20 under conditions of varying water stress in wheat. ACKNOWLEDGEMENTS

Ros Runciman is thanked for excellent technical assistance throughout this study. The support of the Wheat Industry Research Council of Australia is gratefully acknowledged.

REFERENCES Choudhury, B.J., 1986. An analysis of observed linear correlations between net photosynthesis and a canopy-temperature-based plant water stress index. Agric. For. Meteorol., 36: 323333. Choudhury, B.J. and Idso, S.B., 1985a. An empirical model for stomatal resistance of fieldgrown wheat. Agric. For. Meteorol., 36: 65-82. Choudhury, B.J. and Idso, S.B., 1985b. Evaluating plant and canopy resistances of field-grown wheat from concurrent diurnal observations of leaf water potential, stomatal resistance, canopy temperature, and evapotranspiration flux. Agric. For. Meteoroi., 34: 67-76. Connor, D.J., Palta, J.A. and Jones, T.R., 1985. Response of sunflower to strategies of irrigation. III. Crop photosynthesis and transpiration. Field Crops Res., 12: 281-293. Denmead, O.T., 1973. Relative significance of soil and plant evaporation in estimating evapotranspiration. In: R.O. Slatyer (Editor), Plant Response to Climatic Factors. UNESCO, Paris, pp. 505-511. Denmead, O.T., 1976. 'Temperate Cereals'. In: J.L. Monteith (Editor), Vegetation and the Atmosphere. Vol. 2. Case Studies. Academic Press, London, pp. 1-31. Denmead, O.T. and Millar, B.D., 1976. Field studies of the conductance of wheat leaves and transpiration. Agron. J., 68:307-311. Ehrler, W.L., Idso, S.B., Jackson, R.D. and Reginato, R.J., 1978. Diurnal changes in plant water potential and canopy temperature of wheat as affected by drought. Agron. J., 70: 999-1004. Farquhar, G.D. and Richards, R.A., 1984. Isotopic composition of plant carbon correlates with water use efficiency of wheat genotypes. Aust. J. Plant Physiol., 11: 539-552. Goudriaan, J. and van Laar, H.H., 1978. Relations between leaf resistance and CO2 assimilation in maize, beans, lalang grass and sunflower. Photosynthetica, 12:241-249. Jones, J.W., Zur, B. and Bennett, J.M., 1986. Interactive effects of water and nitrogen stresses on carbon and water vapor exchange of corn canopies. Agric. For. Meteorol., 38:113-126.

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D.M. WH1TFIELD

Kanemasu, E.T., Powers, W.L. and Sij, V.W., 1974. Field chamber measurements of CO2 flux from soil surface. Soil Sci., 118: 233-237. McCullough, D.E. and Hunt, L.A., 1989. Respiration and dry matter accumulation around the time of anthesis in field stands of winter wheat (Triticum aestivum). Ann. Bot., 6 3 : 3 2 1 329. Meyer, W.S., Dunin, F.X., Smith, R.C.G., Shell, G.S.G. and White, N.S., 1987. Characterizing water use by irrigated wheat at Griffith, New South Wales. Aust. J. Soil Res., 25:499-515. Monteith, J.L., 1988. Does transpiration limit the growth of vegetation or vice versa? J. Hydrol., 100: 57-68. Puech-Suanzes, I., Hsiao, T.C., Fereres, E. and Henderson, D.W., 1989. Water stress effects on the carbon exchange rates of three upland cotton (Gossypium hirsutum) cultivars in the field. Field Crops Res., 21: 239-256. Ramos, C. and Hall, A.E., 1982. Relationship between leaf conductance, intercellular CO2 partial pressure and CO2 uptake rate in two C3 and C4 plant species. Photosynthetica, 16: 343355. Rawson, H.M. and Clarke, J.M., 1988. Nocturnal transpiration in wheat. Aust. J. Plant. Physiol., 15: 397-406. Ritchie, J.T. and Burnett, E., 1971. Dryland evaporative flux in a subhumid climate. II. Plant influences. Agron. J., 63: 56-62. Sheehy, T.T. and Peacock, J.M., 1975. Canopy photosynthesis and crop growth rate of eight temperate forage grasses. J. Exp. Bot., 26: 679-691. Tanner, C.B., 1981. Transpiration efficiency of potato. Agron. J., 73: 59-64. Tanner, C.B. and Sinclair, T.R., 1983. Efficient water use in crop production: Research or research? In: H.M. Taylor, W.R. Jordan and T.R. Sinclair (Editors), Limitations to Efficient Water Use in Crop Production. American Society of Agronomy, Madison, WI, pp. 1-28. 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. Vos, J. and Groenwold, J., 1989. Characteristics of photosynthesis and conductance of potato canopies and the effects of cultivar and transient drought. Field Crops Res., 20:237-250. 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. 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 CO: exchange of lucerne (Medicago sativa L. ) grown on a heavy clay soil. Irrig. Sci., 7:169-181. Whitfield, D.M., Smith, C.J., Gyles, O.A. and Wright, G.C., 1989. Effects of irrigation, nitrogen and gypsum on yield, nitrogen accumulation and water use of wheat. Field Crops Res., 20: 261-277. Wilson, D.R. and Jamieson, P.D., 1985. Models of growth and water use in wheat in New Zealand. In: W. Day and R.K. Atkin (Editors), Wheat Growth and Modelling. Plenum Press, New York, pp. 211-216. Wong, S.C., Cowan, I.R. and Farquhar, G.D., 1979. Stomatal conductance correlates with photosynthetic capacity. Nature (London), 282: 424-426. Zur, B. and Jones, J.W., 1984. Diurnal changes in the instantaneous water use efficiency of a soybean crop. Agric. For. Meteorol., 33:41-51.