Modelling the response of wheat canopy assimilation to CO 2 using two

models of different level of empiricism

D Rodriguez1,2, F Ewert3, J Goudriaan1, JR Porter3, R Manderscheid4, S Burkart4, RAC Mitchell5 & HJ Weigel4

1

Department of Plant Sciences, Wageningen University, The Netherlands, 2 Dept. Suelos, Facultad de Agronoma, Universidad de Buenos

Aires, Argentina, 3 Department of Agricultural Scences, Royal Veterinary & Agricultural University, Denmark,

4

Bundesforschungsanstalt fr Landwirtschft, Germany, 5 IACR-Rothamsted, Harpenden, UK

Objective

Future increase in CO2 concentration will affect wheat growth and yield primarily through increase in assimilation rate per unit leaf area. While many studies have

investigated CO2 effects on leaf photosynthesis, little is known about the integration of responses and up-scaling from the leaf to the canopy. The objective of this

work was to compare observed hourly values of canopy assimilation at two levels of CO2, with simulations from two models with different level of complexity.

Models & Data

The models simulate crop assimilation using either a simple light response

curve equation (AFRCWHEAT2) or detail calculations of leaf energy balances,

and the coupling of photosynthesis with stomatal conductance (LINTULCC2).

LINTULCC2 up-scales leaf gas exchange to canopy as proposed by Leuning

(1995). It uses concepts of the sun/shade model (de Pury & Farquhar, 1997), of

responses of stomata to photosynthesis, external CO2 and water availability

(Wang & Leuning (1998), and a description of the biochemistry of

photosynthesis (Farquhar et al., 1980). Both models allow for the within day

variations in temperature, radiation and vapour pressure deficit.

Hourly values of net assimilation (Pn, mol CO2 m-2 s-1) and

evapotranspiration (ET, mmol H2O m-2 s-1), together with weather inputs were

obtained from an OTC experiment (ambient 380 and high 670 mol mol-1)

with spring wheat (cv. Minaret) at Braunschweig, Germany. Both models

used the same input values for LAI. Observed and simulated values of hourly,

and daily total Pn, and instantaneous ET were compared at 50, 64, 89, 103,

54, 68, 94 and 105 days after emergence of the crop.

Results & conclusions

50

40

30

-1

-2

60

40

20

High = 0.8048x + 6.455

R2 = 0.5916

0

20

40

60

80

-2

100

AFRCWHEAT2

Low = 0.6334x + 4.7298

R2 = 0.8406

80

60

40

20

High = 0.8275x - 1.434

R2 = 0.8396

0

100

0

-1

Observed DTNA [g CO 2 m d ]

20

20

40

60

80

100

-2 -1

Observed DTNA [g CO2 m d ]

Figure 2. Simulated versus observed values of daily total assimilation (DTNA) for

ambient and high CO2 crops calculated by both models.

0

15

y = 0.7841x + 1.3245

R2 = 0.7247

10

5

LINTULCC2

0

5

10

15

Observed ET [mmol H2O m-2 s -1]

y = 0.8216x + 0.9566

R2 = 0.7911

15 Low = 0.9823x

10

R 2 = 0.5088

High =0.9922x

R 2 =0.626

5

LINTULCC2

0

10

Low =0.6652x

R 2 =0.079

15

1:1 Line

High =0.6911x

R 2 =0.0909

10

5

AFRCWHEAT2

0

0

5

10

15

Obs. RUE [g CO2 MJ-1 PAR]

0

5

10

15

Obs. RUE [g CO2 MJ-1 PAR]

Figure 3. Simulated versus observed radiation use efficiency (RUE) for LINTULCC2

and AFRCWHEAT2

5

LINTULCC2

0

0

5

10

15

Observed ET [mmol H2O m-2 s -2]

Figure 4. Simulated versus observed canopy evapotranspiration for ambient (open circles)

and high (closed circles) CO2, simulations are by LINTULCC2.

EU IMPETUS

Irrespective of the CO2 treatment both models reproduced well the

observed values of radiation use efficiency calculated as the ratio between

DTNA and daily intercepted PAR.

Sim. RUE [g CO2 MJ-1 PA R]

10

Simulated ET [mmol 2HO m-2 s -1]

Simulated ET [mmol 2HO m-2 s -1]

Low = 0.7244x + 9.0348

R2 = 0.6439

80

0

Figure 1. Simulated and observed canopy instantaneous Pn for ambient (open circles) and

high (closed circles) CO2. Simulations by LINTULCC2 (continuous lines) and

AFRCWHEAT2 (dashed lines).

0

LINTULCC2

100

-2

-1

2 m d ]

Simulated DTNA [g CO

60 50 DAE

0 2 4 6 8 10 12 14 16 18 20 22 24

15

Predictions of both models had similar errors for hourly and daily total values

of assimilate production.

Sim. RUE [g CO2 MJ-1 PA R]

Pn [umol CO

2

m -2 s -1 ]

..

2 m d ]

Simulated DTNA [g CO

Irrespective of the developmental stage of the crop the models were able to

capture the main signals from the environment

As LINTULCC2 calculates stomatal conductance this model allows us to

study the simulated response of crop evapotranspiration (ET) to the ambient

CO2 which is of particular importance in rain fed crops.

We conclude that for well-irrigated conditions a simple approximation based

on a light response curve avoiding the calculation of the coupling between

photosynthesis and stomatal conductance could be used. When water

supply is not optimal a more detailed approach might be needed to

reproduce the interactive effects between CO2 and water supply on

assimilation and transpiration.