Variation of Rachis Branches, Grain Ripening and Grain Weight by Grain Setting Site in Japonica Rice

This study was carried out to analyze the varietal difference of panicle traits and to select for desirable panicle structure to simultaneously increase sink size and ripened grain ratio. Different nitrogen levels, planting densities and planted seedling numbers in four different panicle types of japonica rices were applied. The number of primary rachis branch (PRB) per panicle was more in Iksan467 and Sindongjinbyeo than in Dongjin#1 and Saegyehwabyeo, while the number of secondary rachis branch(SRB) per panicle and PRB was most in Dongjin#1, medium in Sindongjinbyeo and Saegyehwabyeo and fewest in Iksan467. The ratio of grain number setting on PRB to total grain number per panicle was in order of Iksan467 > Sindongjinbyeo> Saegyehwabyeo > Dongjin#1. The usable criteria for the desirable panicle structure was the number of SRB per PRB which showed the significant varietal difference but did not showed the environmental difference. To maintain over 90% of ripened grain ratio, it was required to keep 130 or fewer grains per panicle, 2 or fewer SRBs per PRB, 5.5 or fewer grains setting on SRB per PRB and 57% or higher ratio of grains setting on PRB to total grains per panicle. Both 1,000-grain weight of brown rice and ripened grain ratio decreased more in grains setting on SRB, especially SRB positioned near the rachis base, than those on PRB.