Vergara - 1976 - Physiological and morphological adaptability of ri

132 euixixna ma) RICE
0.19 for a rice canopy planted densely and for a rice canopy with a standard
planting density. respectively (Iehimura et a1, 1965) The values of extinction
coefficient were smaller than k,,. mainly because the extinction of net radiation
is compensated for by an increase of the storage ratio with LAI. Since the large
amount of heat energy stored in water and soil during daylight hours was
released gradually at night, the minimum water temperature was observed at a
constant level independent of LAI (Fig. 8D). Results presented in Fig. 8C and D
imply that the decrease in the relative amplitude of the temperature u=avc of
water beneath the rice canopy is mainly due to the lowering of the maximum
water temperature with increasing LA].
CARBON DIOXIDE ENVIRONh/[ENT WITHIN RICE CANOPY
Profiles and diurnal change of carbon dioxide level
Because rice plants act as a sink or source of carbon dioxide through the
action of photosynthesis and respiration, the CO, concentration in the air near
crop canopies has a pronounced diurnal change with a minimum during the day
and a maximum at night. The profiles of CO, in the air above and within plant
canopies are affected significantly by the growth ofplants, soil respiration. solar
radiation. and wind. Figure 9A shows that before sunrise and after sunset, a
lapse profile of CO, was present throughout the air layer and the crop foliage.
As the day proceeded, it is interesting to follow the region of minimum CO,
concentration within the canopy. At 0830, 0930, and 1030, the minimum was
located in the layer with the maximum leaf area density. At 1145, when the
solar elevation was highest and solar radiation penetrated deeply into the stand,
the CO, level increased monotonically with height above water surface, indicating
that carbon dioxide was transferred downward throughout the 031101))’.
The position of the minimum rose again at 1325, 1415. and 1620 as the solar
elevation decreased. The CO, profiles xvith a minimum in the middle of the
canopy imply that CO, absorbed by rice is supplied both from the air layer above
the canopy and from the underlying surface. Similar trends in the daily movement
of the CO, profiles within a rice canopy can also be seen in data of Ishibashi
(1970). Tanaka et a1. (1966) reported that the CO, profiles in a rice canopy
became less steep on a windy day
The diurnal variation of CO, concentration observed in the air immediately
above rice canopies is also presented in Fig. 9B. Al night when the canopy was
respiring and the air mixing was weak. the concentration ofCOg remained at a
high level such as 400 and 500 ppm. Immediately after sunrise, the CO, level
decreased drastically as the solar elevation increased, because of faster plant
photosynthesis and air mixing. During the period from 0900 to 1600, the CO,
W33 kept at concentration of about 300 ppm. The CO, level rose rapidly‘ again as
solar radiation weakened.