Abstract
Breeding for high-yielding rice has been directed toward those types that have a larger number of spikelets per panicle. The objective of this study was to examine the relationship within a cultivar between the size of the apical dome (AD) and the morphological characters of the panicle components such as the number of primary rachisbranches (PBs), secondary rachis-branches (SBs), and spikelets per panicle. Rice plants (cv. Akenohoshi) were subjected to four short-day (10h photoperiod) treatments at various developmental stages to change the duration of the vegetative stage and the AD size at panicle initiation (PI). Shoot apices having ADs were sampled at two-day intervals from the late vegetative stage to the early reproductive stage. The sections of the shoot apices were embedded in paraffin and stained with toluidine blue O. The base diameter and height of the ADs were measured with an ocular micrometer, and at heading, the numbers of PBs, SBs, and spikelets were counted. A delay in the start of the short-day treatment increased the total number of leaves on the main culm, extending the vegetative stage. Extension of the vegetative stage increased the AD diameter at PI. However, the effect of the short-day treatments on the AD height was unclear. Enlargement of ADs was accompanied by the increase in the number of differentiated PBs, but not always by that in the number of SBs and spikelets. The results suggested that spikelet number is influenced by plant nutritional conditions as well as by the AD size at PI.
