Studies on the Characteristics of Differentiation and Degeneration of Spikelets and Grain-filling in High-yielding Rice Cultivars with Large Panicles

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This study was conducted to clarify the characteristics of the differentiation and degeneration of spikelets and the grain-filling in high-yielding rice cultivars with large panicles (LP-HYC) bred in China, Korea, the Philippines and other countries in recent years. The results were summarized as follows ;1. The number of differentiated and degenerated spikelets, and the percentage of degenerated spikelets varied considerably among LP-HYCs, but the number of degenerated spikelets was much larger in the differentiated spikelets on the secondary rachis-branch (SB) than those on the primary rachis-branch of panicle (PB). The major part of degenerated spikelets on SB was derived from the degenerated SBs on the surviving PB.2. Nitrogen top-dressing at tillering stage to LP-HYCs increased the number of differentiated spikelets per panicle as well as the panicle number, but greater increase in the percentage of degenerated spikelets at the same time decreased the number of surviving spikelets.3. Nitrogen, phosphorous and potassium application at 40-20days before heading increased the number of surviving spikelets due to the increase in differentiation and the decrease in degeneration of spikelets in LP-HYCs. The heavy shading and high-day and/or night temperature during 15 days before heading increased the percentage of degenerated spikelets. The percentage of degenerated spikelets decreased with the increase of shoot dry weight per stem (SDW) and SDW per differentiated spikelets at heading.4. Contribution ratio of the stored carbohydrate in the culm and leaf sheath at heading and the assimilated carbohydrate after heading to the grain-filling varied so much among cultivars belonging to LP-HYC and by the cultivated conditions.