Vergara - 1976 - Physiological and morphological adaptability of ri

218 CLINIATE AND RICE
could largely‘ be attributed to increased total grain number and increased grain
weight.
The 9- to 10-day enrichment at raehis-branch differentiation stage or spikelet
differentiation stage increased grain y‘ie1d by 10%; the yield increase resulted
largely from increased grain number. On the other hand. 2-week enrichment
from reduction division stage to flowering. the period most sensitive to such
stresses as low temperature and shading [Matsushima 1957'. Satake. 1969),
increased weight per grain but did not have any over-all effects on grain yield.
Thirty-day enrichment after flowering increased grain yield by 10% over the
control. This increase resulted from increased filled grain percentage and
increased grain weight.
Since the number of developed spikelets (grains) is the difference between
number of differentiated spikelets and number of degenerated ones (Matsushima.
1957). the failure of CO, enrichment for 9 weeks before flowering t0
increase grain number suggests that degeneration of spikelets was negligible
under experimental conditions at Los Bafios. As a result. the critical time for
determining number of grains lies within the period between neck-node differentiation
and spikelet differentiation stages, i.e. about 33 to 14 days before flowering.
These results appear to indicate that total grain number per unit of land area
is more limiting to grain yield than grain filling at Los Bafios.
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