Vergara - 1976 - Physiological and morphological adaptability of ri

iyucitocusmris or THE RICE CROP 135
obtained upward CO, fluxes in the range between l.l and 2.1 CO, mgdrn 1.
h-I. When the rate of CO1 release from underlying surfaces was relatively‘ large.
the determination of CO, fltlX by the aerodynamical method will lead to the
underestimation of canopy photosynthesis rates.
Simulation ofcarbon dioxide environment
In order to study CO, environment and photosynthesis of the rice crop in relation
to wind velocity, radiation intensity’. air temperature, and plant growth, an
equation describing the exchange of CO: between vegetation and the air was
numerically" solved by the Runge-Kutta method on an electronic computer
[Uehijima et al. 1976). The simulated CO2 environment of a rice crop is reproduced
in Fig. 9C. The simulated pattern of diurnal change of CO, level in the air
above a model rice canopy was in good agreement with observation (EIA Fig.
9B and C). The CO; concentration at sunrise is between 400 and 500 ppm, falls
rapidly until 0700. and then declines more slowly and reaches a minimum around
solar noon. The concentration then begins rising with decreasing solar radiation.
Figure lOA denotes the daily net photosynthesis of a model rice canopy (L =
4.0. kd = 0.65, P, = 8.64g C02 ~m 1 - day 1) as influenced by air mixing and
shortwave radiation. When the wind was light (Dun, = 0.3 crn/sec), the rice
canopy showed a plateau-type response of daily net photosynthesis to radiation.
with low photosynthesis. Under windy conditions. the rice canopy showed a
higher net photosynthesis with less decrease of the increasing rate of photosynthesis
with radiation. The photosynthesis in the case ol’ Dim, = 0.3 cm/sec
and S, = l8 MJ/m day‘ was about 50 percent", that in the case of D,_,,,_, = l0.2
cm/sec and S, = l8 Mllm day’. Simulated radiation and photosynthesis relationship
agreed well with those measured with a rice canopy by Tanaka (1972). The
loti-"ering of the canopy photosynthesis in light wind is mainly due to the considerable
depression of the CO; level near the canopy and the increase of plant
respiration during the period of daylight hours.