Sink development in rice genotypes of different growth durations as affected by nitrogen, nutrition, plant spacing, and shading.

Grain yield and sink size were quadratic functions of GD [growth duration]. On the other hand, potential sink size increased linearly with GD. The filled spikelet decreased while degenerated spikelet increased with increase in GD. Highest sink size and grain yields were obtained in medium duration genotypes with optimum GD of 128 days for sink size and 123 days for grain yield. The N-use efficiency expressed as the contribution of N in plant to sink size and grain yield and nitrogen harvest index (NHI) which the amount of N in grains per total N of above ground biomass followed a quadratic curve with GD. The short and long duration genotypes have relatively low N-use efficiency than medium duration genotypes. The low grain yield of short duration genotypes was attributed to limited sink size due to low total N in plant at panicle initiation. On the other hand, the low grain yield of long duration genotypes was attributed to high spikelet degeneration due to low total N uptake from late stage of spikelet initiation to flowering in spite of high total N in plant at panicle initiation. Differences in N uptake in rice genotypes was observed during tillering stage while little variation was noted after the maximum tiller number stage. The total N in plant at 30 days after transplanting (NE) was correlated with potential sink size and grain yield but not correlated with spikelet degeneration in short duration varieties. Likewise, NE was with the contribution of N in plant at flowering to potential sink size and was correlated with NHI in short and medium GD groups. Closer spacing significantly increased grain yield in short duration genotypes but showed less effect on increasing grain yield of long duration genotypes. The percent filled spikelet was significantly increased in short duration genotypes. However, close spacing generally increased spikelet degeneration with highest in long duration genotypes. Partial shading (50%) during reduction division stage of panicle development caused significant high spikelet degeneration, reduced the total number of spikelets but did not affect the grain filling of short duration genotypes