Vergara - 1976 - Physiological and morphological adaptability of ri

472 CLIMATE AND RICE
Reviewing several statistical studies of climatic (weather) effects on crop yield.
Watson (1963) stated that “past experience, therefore, does not encourage us
to expect that knowledge of how yield depends on weather can come from
measurement of yields in naturally varying environments. The fundamental
defect of this approach is that the dependence of yield on climatic factors is
usually far too complex to be described adequately by linear regressions on a
few gross measures of climatic variation, except perhaps when one factor. most
likely to be rainfall or lack of it. dominates over all tithers."
Among climatic factors, rainfall appears to have a paramount influence on
yield of upland crops (Gangopadhy/ayxa and Sarkcr. 1965; Runge. 1968; Russel.
1968). Gangopadhyayra and Sarker. using a polynomial technique developed by
R. A. Fisher. showed that about 75% of yield variation in wheat could be
accounted for by variation in rainfall. The rainfall should have a similar influence
on y/ields of upland and rainfed rice (Allcs, 1969; Kawano et al.. 1972).
In irrigated rice culture. however, soil is kept flooded, and hence soil water
content remains favorable for rice growth throughout the growing period.
Murata (1964) was the first to demonstrate that differences in rice yfields in
different localities in Japan could be accounted for by a simple regression using
temperature and solar radiation. This was made possible perhaps because in
Japan the use of a recently developed superior variety" is coupled with good
soil. ample and timely supply of nutrients as well as water. and with thorough
control of pests and diseases. climatic or weather factors thus becoming more
limiting to grain yield (Murata and Togari. 1972). After the introduction of
scmidwarf varieties. these requirements can be met at experimental fields in the
tropics.
A number of studies made in Japan indicate that yield per spikelet decreases
with increased spikelet number per unit of land area (Matsushima. 1957;
Munakata et a1, 1967; Murata. 1969; Wada, I969). This has led many rice
scientists. regardless of whether they work in the temperate region or in the
tropics. to study ripening as the most critical stage in grain production.
In Japan. Murata (1964) found a close correlation between grain yield and
solar radiation and daily mean temperature for August and September. Hanyu
et al. (1966) also found a high correlation between grain yield and sunshine
hours and daily mean temperature for 40 day-us of the ripening period. These nvo
studies were made with data collected from different localities where soil conditions
as well as climatic conditions differed. Therefore, a question still
remained whether effects of climatic factors on yield were adequately separated
from those of soil factors. Murakami (1973). by growing rice in the same synthetic
medium at different localities. also demonstrated a close correlation
betxveen grain yield and daily mean temperature and sunshine hours for 40 day's
of the ripening period.
These three authors came up with a similar formula;
r = Sp, - an - Cy] (1)