Relationship between apical dome diameter at panicle initiation and the size of panicle components in rice grown under different nitrogen conditions during the vegetative stage

A pot experiment was conducted to analyze the relationship between the size of each apical dome (AD) and the numbers of differentiated primary rachis-branches (PBs) and spikelets. Two rice cultivars were used : one was a heavy-panicle type 'Akenohoshi' and the other was a many-tillering type 'Nipponbare'. Rice plants were applied nitrogen (N) at various rates (75-600mg N pot(-1) week(-1)) during the vegetative stage. The base diameter and the height of ADs were measured at the panicle initiation (PI) stage. At heading, the numbers of differentiated PBs, secondary rachis-branches (SBs), and spikelets were counted. The N treatment increased shoot N concentration in both cultivars. The N treatment increased the base diameter of AD in Akenohoshi at the PI stage, but not in Nipponbare. The ADs in Akenohoshi had a base diameter about 6.2% larger on average than that in Nipponbare. The N treatment did not affect AD height. In Akenohoshi, the plants with a larger AD base diameter differentiated more PBs per panicle and then differentiated more SBs and spikelets than did those plants with a smaller AD diameter. However, in Nipponbare, the N treatments did not affect the number of differentiated PBs per panicle. These results suggest that the AD size at the PI stage, which is enlarged by higher N nutrition in the vegetative stage, is a determinant of the number of differentiated PBs and spikelets and that a larger AD size is responsible for a higher number of PBs and spikelets in Akenohoshi.