Agricultural and Forest Meteorology 106 2001 317–330
Modeling radiation- and carbon-use efficiencies of maize, sorghum, and rice
Bhaskar J. Choudhury
∗
NASA Goddard Space Flight Center, Hydrological Sciences Branch, Laboratory for Hydrospheric Processes, Greenbelt, MD 20771, USA Received 11 May 2000; received in revised form 1 September 2000; accepted 8 September 2000
Abstract
A previously developed model for radiation-use efficiency RUE for gross photosynthesis and net carbon accumulation by wheat before anthesis [Agric. Forest Meteorol. 101 2000 217], with some improvement, has been applied to maize,
sorghum, and rice during their vegetative period under unstressed conditions. The objective of the present study is to assess the extent to which the model can provide RUE for net carbon accumulation and carbon-use efficiency CUE; the ratio of
daily net carbon accumulation and gross photosynthesis for a range of incident irradiance and leaf area indices of maize, sorghum and rice, recognizing that a these crops are grown in substantially different environmental conditions than those
for wheat, and b while rice is a C
3
crop like wheat, maize and sorghum are C
4
crops. The calculated RUE values for net carbon accumulation differed from observations five for maize, two for sorghum and three for rice by −24 to +19,
with an average n = 10 of −4 underestimation, while the calculated CUE values are found to be within the range of observations. The model parameters have not been calibrated or adjusted for these comparisons. Calculations suggest that
there is much room to increase the RUE of sorghum over the currently available measurements, although it might not attain the potential maximum value for maize. The effects of variations in the maximum rate of leaf photosynthesis among cultivars,
changes in the relationship between maximum rate of photosynthesis by leaves and its nitrogen content, and uncertainties in the input canopy parameters on RUE and CUE are assessed by sensitivity analysis. Published by Elsevier Science B.V.
Keywords: Photosynthesis; Respiration; Radiation-use efficiency; Carbon-use efficiency; Maize; Sorghum; Rice
1. Introduction
A model for radiation-use efficiency RUE of wheat during its vegetative phase under unstressed
conditions has been developed Choudhury, 2000 considering diurnal variations of direct and diffuse
PAR on sunlit and shaded leaves, and variation of the maximum rate of leaf photosynthesis within the
canopy due to changes in leaf nitrogen content to calculate daily total gross photosynthesis. The main-
∗
Tel.: +1-301-614-5767; fax: +1-301-614-5808. E-mail address: bhaskarte.gsfc.nasa.gov B.J. Choudhury.
tenance respiration at a prescribed temperature was determined from nitrogen content per unit ground
area of foliage, stem and roots, while growth res- piration was calculated as being proportional to the
difference of gross photosynthesis and maintenance respiration. Comparison of the calculated RUE with
those determined from observations number of ob- servations; n = 3 gave differences of −7 to +25
without any model calibration.
The objective of the present study is to assess the extent to which the model can provide RUE and
carbon-use efficiency CUE; the ratio of daily carbon accumulation and gross photosynthesis for a range
0168-192301 – see front matter Published by Elsevier Science B.V. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 1 7 - 3
318 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330
of incident irradiance and leaf area indices of maize, sorghum, and rice during their vegetative phase under
unstressed conditions. These crops have been cho- sen because environmental conditions particularly
temperature during their growth differ substantially from those for wheat. Also, while rice has C
3
type photosynthesis like wheat, maize and sorghum have
C
4
type photosynthesis.
2. Description of the model
The components of the model Choudhury, 2000, particularly those relevant to the present study, are
presented below. 2.1. Gross photosynthesis by a leaf
The variation of the maximum rate of photosynthe- sis by a leaf A
m
; Eq. 4 in Choudhury, 2000 within a canopy due to changes in nitrogen content per unit
leaf area, n
l
mmol N m
− 2
, has been prescribed as fol- lows; for maize and sorghum Muchow and Sinclair,
1994: A
m
= A
max
2 1 + exp−0.047n
l
− 14
− 1
1a and, for rice Sinclair and Horie, 1989:
A
m
= 34
2 1 + exp−0.020n
l
− 2
− 1
1b where A
max
is the potential maximum rate of gross photosynthesis by leaves.
Considering field measurements, the mean maxi- mum rate of net photosynthesis for maize leaves ap-
pears to be 55 mmol CO
2
m
− 2
s
− 1
n = 11; range 50–60 mmol CO
2
m
− 2
s
− 1
, while dark respiration has been found to be 2 mmol CO
2
m
− 2
s
− 1
. Thus, A
max
for maize leaves has been taken to be 57 mmol CO
2
m
− 2
s
− 1
, and the effect of variations in A
max
has been addressed by sensitivity analysis Section 4.3.1.
Field observations by Kidambi et al. 1990 gave mean maximum net photosynthesis of sorghum leaves
as 45 mmol CO
2
m
− 2
s
− 1
n = 30; range 30–55 mmol CO
2
m
− 2
s
− 1
. With dark respiration as 2 mmol CO
2
m
− 2
s
− 1
, A
max
for sorghum leaves has been taken to be 47 mmol CO
2
m
− 2
s
− 1
, while the effect of varietal differences in A
max
on RUE and CUE has been as- sessed by sensitivity analysis Section 4.3.1.
The effect of variations in A
m
for rice Peng et al., 1995 and varietal differences in A
max
on RUE and CUE have been addressed by sensitivity analysis Sec-
tion 4.3.2. The quantum efficiency ε; Eq. 5 in Choudhury,
2000 is independent of foliage temperature for C
4
species maize and sorghum, but vary with tempera- ture for C
3
species rice. Choudhury 2000 consid- ered foliage temperature to be equal to air temperature
for calculating RUE of wheat, and found by sensitiv- ity analysis that 5
◦
C change in temperature affected the RUE for gross photosynthesis by 3 for clear sky
condition and 6 for overcast condition. To improve accuracy of determining RUE, the diurnal variation
of canopy temperature has been calculated from an energy balance equation.
2.2. Respiration The daily total respiration by a stand foliage, stem
and roots per unit ground area at temperature T
◦
C {RT; mol CO
2
m
− 2
per day } has been calculated as
the sum of maintenance R
m
and growth R
g
com- ponents Choudhury, 2000:
RT = R
m
T + R
g
T 2a
where R
m
T = R
m
T = 20Q
10 T −
2010
2b R
g
T = 1 − Y
G
{A
g
− R
m
T } 2c
A
g
is the daily total gross photosynthesis by a canopy, Q
10
the temperature response coefficient taken to be 2.0 and Y
G
is the growth conversion efficiency. Choudhury 2000 used mean daily air temperature
T in Eq. 2b to calculate R
m
T for wheat. Such a procedure, however, underestimates R
m
T because of diurnal variation of T and non-linear dependence of
R
m
T on T. As an improvement, the following equa- tion, based on a sinusoidal diurnal variation of tem-
perature, is used: R
m
T = R
m
T = 20Q
10 T −
2010
I
1 2
K1T 2d
B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 319
where I x is the modified Bessel function, 1T the
diurnal temperature range and K =
lnQ
10
10 2e
From elemental and proximal analyses, Lafitte and Loomis 1988 obtained the following equation for Y
G
of sorghum in terms of plant nitrogen concentration N percent of dry matter:
Y
G
= 0.814 − 0.051 N
3 The Y
G
for rice has been taken to be 0.74 n = 4. 2.3. Radiation- and carbon-use efficiencies
From daily total gross photosynthesis A
g
; mol CO
2
m
− 2
per day and respiration R; mol CO
2
m
− 2
per day, carbon accumulation per day C; mol CO
2
m
− 2
per day or CO
2
equivalent of crop growth rate has been determined as
C = A
g
− R 4
Then RUE mmol CO
2
per mol intercepted photon; mmol mol
− 1
has been obtained as the ratio of C and daily total IPAR mol photon m
− 2
per day: RUE =
C IPAR
× 1000
5 The CUE mol CO
2
per mol CO
2
; mol mol
− 1
has been obtained as the ratio of C and A
g
: CUE =
C A
g
6 Using Eqs. 2 and 6, one can also express CUE as
CUE = Y
G
1 − R
m
T A
g
7 Eq. 7 shows that Y
G
sets the upper limit of CUE. It also follows from Eq. 7 that, for a given canopy,
CUE will decrease with decreasing irradiance because A
g
decreases with decreasing irradiance, while Y
G
and R
m
T do not depend upon irradiance. If RUE for gross photosynthesis RUE
g
; mmol CO
2
mol
− 1
photon intercepted is defined as
RUE
g
= A
g
IPAR ×
1000 8
then one can express RUE as RUE = RUE
g
× CUE
9 Unlike the case for CUE, changes in RUE due to
variations of irradiance cannot be stated precisely be- cause, while CUE decreases with decreasing irradi-
ance, RUE
g
tends to increase, as will be shown in Section 4.1 see also Choudhury, 2000.
2.4. Maximum radiation-use efficiency To provide a reference to the calculated and ob-
served RUE presented below, some estimates are ob- tained for a theoretical maximum RUE. Knowledge
of a theoretical maximum RUE can be used to nor- malize actual RUE, and thus provide an aesthetically
pleasing dimensionless efficiency varying between 0 and 1.
As discussed in Section 2.3, the growth conversion efficiency Y
G
provides a theoretical maximum value for CUE Eq. 7, although CUE can only approach,
but not attain, this value because neither maintenance respiration can be zero nor gross photosynthesis can
be infinity. It can be shown that under some condi- tions RUE for gross photosynthesis RUE
g
can attain its maximum value RUE
g,max
equal to the appar- ent quantum efficiency =εα of photosynthesis by a
leaf. Thus, from Eq. 9, a theoretical maximum RUE RUE
max
is obtained as RUE
max
= Y
G
εα 10
and any realizable estimated or observed RUE would have to be less than that given by Eq. 10.
Representative values of Y
G
and α are, re- spectively, 0.74 and 0.85. Since ε of C
4
crops maize and sorghum is about 62 mmol CO
2
mol
− 1
photon, RUE
max
for maize and sorghum is obtained as 39 mmol mol
− 1
, which is equivalent to about 5.4 g CH
2
O MJ
− 1
IPAR. The quantum efficiency of C
3
crops wheat and rice is about 69 and 55 mmol CO
2
mol
− 1
photon at day-time foliage temperature of, respectively, 18 and 28
◦
C. For these values of the quantum efficiency, the values of RUE
max
are, respectively, 43 and 34 mmol mol
− 1
. Eq. 10 can be used to assess relative limitations in RUE
g
and CUE in determining RUE, as will be discussed in
Section 4.1.
320 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330
3. Input data
