Vergara - 1976 - Physiological and morphological adaptability of ri

486 (JLIMATE AND RICE
obvious cause ofdeereasc in filled grain percentage. Lodging reduces the crosssectional
area of vascular bundles; the reduction in cross-sectional area in tum
disturbs the movement of photosynthetic assimilates and absorbed nutrients via
the roots. In addition. lodging disturbs leaf display which results in increased
shading, and eventually increases the percentage of unfilled grains (Hitaka, 1968).
The percentage of filled grains may decrease when solar radiation during the
ripening period is low or when some adverse conditions. such as nitrogen deficiency’.
set in (Matsushima. 1957; Wada. 1969'. Table 2). The percentage of filled
grains often decreases with an increasing number of spikelets per unit of land
area (Wada. 1969).
As we discussed earlier, increasing number ofspikelets at Los Bafios, does not
cause a decrease in filled-grain percentage of 1R8 and line IR747-B2-6.Recently’.
however. we found that while 1R8 maintained about the same filled-grain percentage
(about 85%) with increasing spikelet number. Tongil decreased filledgrain
percentage from 90 to 60% as total spikelet number increased. In this
example. it is unlikely’ that excessive vegetative growth of Tongil is a cause of
decreased filled grains. because both 1R8 and Tongil have erect leaves and high
lodging resistance. and TRS usually develops a larger leaf area index by flowering
time. 1t appears. therefore. that the tendency for filled-grain percentage to
decrease with increasing spikelet number is at least an intemal varietal character
(Yoshida, 1973a).
Filled-grain percentage appears to be determined by (a) source activity relativc
to sink size (spikelet number), (b) ability of grains to accept carbohydrates.
and (c) translocation of assimilates from leaves to grains. Climatic factors affect
each of these three in different Ways. Toward maturity’. senescence of grains
comes earlier than that of any other part of the plant such as leaf blades. leaf
sheaths. and neck-node. Nakaywima (1969) demonstrated that the senescence of
grains starts with the conductive tissue of the rachilla. suggesting that translocation
through thc rachilla may determine the ability of grains to accept carbohy'-
drates.
Solar radiation appears to affect grain filling and hence filled-grain percentage
mainly by controlling source activity Under a given scalar radiation, the sink
size relative to source activity affects filled-grain percentage. This is SlIOWII by
increased filled-grain percentage with partial removal of spikelets (Matsushirna.
1957; Wada. 1969).
Within a moderate range. temperature appears to affect filled-grain percentage
mainly by controlling the capability of grains to accept carbohydrates. or the
length of the ripening period. Length of ripening period is inversely’ correlated
with daily mean temperature (Yarnakawa. 1962). Thus. persistence of cloudy
weather conditions will be more detrimental to grain filling under higher temperature
regimes because of a shorter period of ripening.
Nakayama (1974) stated that high temperature has become an adverse
weather factor even in cool regions in Japan. Because early spring planting has
become a common practice to avoid cold damage during the ripening period.