Vergara - 1976 - Physiological and morphological adaptability of ri

YIELD AND YIELD COMPONENTS 0F LOVtLAND RICE IN THE TROPICS 481
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‘too zoo 30o 40o 50o aoo 70o
Sotor radiation ta: . care. day-ll
T. Effect of solar radiation at different growth stages on
grain yield of IR747-B2-6.
was largely’ due to increased percentage of partially" filled grains, not to unfcrtile
grains.
Table 2 does not allow us to examine the relative importance of solar radiation
at different stages of growth since the amount of incident solar radiation
differed from one stage to another. Figure 6 compares the effect of shading at
different groutth stages on grain yield. The effect of solar radiation on grain
yield was most remarkable at the reproductive stage, followed by the ripening
stage. The overall effect of solar radiation during the vegetative stage on grain
yield was extremely small.
Thus. the shading experiment has indicated the importance of solar radiation
during the reproductive as xvcll as ripening stage. During the reproductive
stage, solar radiation affects spikelet number per square meter. and during ripening
it affects filled-grain percentage.
Figure 7 also shows that a yield of 4 t/ha can be obtained with 200 cal- sq
cm~'- day“ during the reproductive stage. Thus. it is unlikely that incident solar
radiation limits rice yield in farmers" fields in most tropical countries where a
national average yield is about 2 t/ha. Climatic influence will become apparent
only when agronomic management is improved to attain a LAI of 5 to 6, as
shown in Figure I.
Combining correlation studies with the shading experiment. we can conclude
that formation of spikelcts during the reproductive stage is well geared to grain
filling at Los Bafios. In fact. solar radiation during the ripening period is highly
correlated with N = S- f(t) in our experiment (r = (l.83l**). In other words.
formation of larger sink size before flowering is followed by higher solar radiation.
As a result. filled-grain percentage remains unaffected regardless of crop
season.