Vergara - 1976 - Physiological and morphological adaptability of ri

‘ITELI? AND YIELD (‘Ol\'[PONEN'l'S OF LOWLAND RICE IN THE TROPIIIS 473
where 1' is grain yield. S is incident solar radiation or sunshine hours. r is
average daily mean temperature, and a, b, c are constants. In spite of different
sources of data. the optimum average daily mean temperature for ripening was
found to be about the same, ranging from 21.4“ to 218°C. This optimum temperature
for ripening is in agreement with findings obtained experimentally
(Matsushima et al., 1957'. Matsushima and Tsunoda. 1958'. Aimi et a1. 1959'.
Tanaka. 1962).
Munakata et al. (1967) derived a rather complex formula for a relationship
between grain yield. solar radiation. temperature, and crop parameters such as
grain number and leaf blade weight.
These studies indicate that local differences in rice productivity in Japan can
largely be accounted for by differences in solar radiation and temperature during
the ripening period (Murata, 1972).
At IRRI, our agronomists showed a high eoirelation between grain yield and
solar radiation during the ripening period (Moomam et al.. 1967) or during the
45 days from l5 days before flowering to harvest (De Datta and Zarate. 1970).
In addition. Tanaka et a1. (1966) demonstrated a close association between
grain yield and dry weight increase after flowering. These results seem to indicate
that ripening is the most critical stage in grain production even in the tropics.
Since a major portion of grain carbohydrate comes from current photosynthesis
during the ripening period (Yoshida, 1972), it is obvious that active
photosynthesis during that period is important. However, whether ripening is
more limiting to grain yield than any other stage of growth in a given locality
would be a different matter. ln rice. sink size ofa crop is largely} determined by
spikelet number per square meter. Spikelets function as sink after pollination.
Hence the sink size is determined at and before flowering. Therefore, the relative
importance of climatic influence before and after flowering depends on whether
sink size is limiting to grain yield or not.
CLIMATIC INFLUENCE ON RICE YIELD
AT LOS BAIQOS. PHILIPPINES
Grain yield ofa rice crop can be formulated as :
Yield (tfha) = spikelet nojml >< grain weight (g per LOGO-grain) >< filled
grain % >< 10* (2)
Our previous study" indicates that the filled grain percentage and LOGO-grain
weight of variety 1R8 grown at Los Baitos, Philippines, remain almost constant,
ie. 85 % and 29 g. respectively‘, regardless of spikelet number per square meter
and season (Yoshida et al.._ 1972a). As a result. the grain yield is almost linearly
correlated with spikelet number.
These relationships hold for a yield range from 3 to 9 t/ha. In one example
cited by Murata (1969). the filled grain percentage varied from about 65% to
less than 5t')% for a yield range from 4.5 to 5.5 t/ha. Comparison of these results
indicates that yield and yield components of IRS at Los Bafios are very stable for