Vergara - 1976 - Physiological and morphological adaptability of ri

482 CLIh-LATE AND RICE
A question still remains. however. whether sink size and source actisiity as
controlled by climatic factors before and after tlotivering are just matched, or
whether the source activity has still more capacity to fill more grains.
To answer that question. we conducted a CO, enrichment experiment in a
plastic enclosure in the field (Cock and Yoshida. 1973). Crops that were subjected
to CO, enrichment before flowering were exposed to normal environment
after flowering. The pre-tlovcering enrichment increased spikelet number per
square meter and weight per grain. resulting in a significant increase in yield in
both dry and wet seasons. Filled-grain percentage of these crops remained as
high as that ofconlrol crops. Thus. it is clear from the above experiment that the
climatic environment during the ripening period at Los Bafios has more capacity
in increasing the source activity and hence to meet greater requirement for
carbohydrates by increased sink size. Therefore, we conclude that under Los
Bafios conditions. sink size (spikelet number per square meter) is more limiting
to grain yield than is source activity during the ripening period (grain filling).
This conclusion would imply two things: (l) climatic environment during the
reproductive stage is usually’ more critical in determining grain yield than during
the ripening stage at Los Bafios. and (2) grain yield can be further increased if
sink size per unit of land area is increased by varietal improvement or some
other means,
VARIATIONS IN YIELD COMPONENTS
AT DIFFERENT LOCATIONS IN THE TROPICS
As shown in Table 3. seasonal variations in yield and yield components vary
with location and variety Among yield components, the cv (coefficient of variation)
of spikelet number per square meter is
suggesting that spikclet number is the most important yield component.
At Los Bafios, the cv for spikelet number per square meter is as large as the
cv for yield". the other two yield components are fairly constant. Under such
condition. it is only natural to find a close correlation between spikelet number
and grain yield. Even at Los Baflos. however. unusually bad weather conditions
sometimes cause low filled-grain percentage. In December and January of 1973.
we had very cloudy weather. During the ripening period of the fourth crop in
the annual production experiment (Table 4). average solar radiation was only
205 ca] - sq cm-t - day", compared to 366 cal - sq cm-l - day-l for the same period
in I972. As a result. filled-grain percentage of this crop was 76%, compared to
89% of the comparable crop in I972.
At Bangkhen. Thailand. the cv of spikelet number is rather small. tvhile the
cv of filled-grain percentage and of grain weight is relatively" large. With these
variations in yiield components. Osada (1972) found that grain yield was more
closely correlated with ripening grade than with spikelet number per square
meter.