Panicle size is positively related to plant height
(Yoshida, 1981). If the large panicle size is caused by tall plants, lodging could become a problem to
limit rice yield. A new parameter, panicle size
per unit plant height at flowering, was pro-posed to identify genotypes with large panicle size without an increase in plant height.
Panicle size per unit plant height at flowering
was higher in hybrids than in indica inbreds
and NPTs in all four seasons (Table 5). This
suggests that the large panicle size of hybrids
was not due to taller plants than indica inbreds
and NPTs. Spikelet production efficiency has
a large influence on sink size (Akita, 1989).
Spikelet production efficiency per unit biomass
at physiological maturity was generally higher
in hybrids than indica inbreds and NPTs
(Table 5). Spikelet production effi ciency per
unit N uptake at physiological maturity was
also higher in hybrids than in indica inbreds
and NPTs, but the difference was much
smaller than spikelet production efficiency per
unit biomass at physiological maturity. Spikelet production efficiency per unit N uptake is
associated with N use efficiency for grain production if the difference in grain-filling percentage is relatively small.