Abstract
In order to develop a simplified method of selection for ripening ability in paddy rice, the relationship between the ratoon volume at the ripening stage and the percentage of ripened grains was investigated for certain levels of fertilizer application and at some ripening stages using a large number of cultivars. Cultivar differences in the percentage of ripened grains varied considerably depending on the year and level of fertilizer and a large number of years and fertilizer levels were required to estimate the ripening ability. Cultivar differences in the ratoon volume varied largely depending on the ripening stages and appeared clearly at the early and late stages every year or at the intermediate stage in certain years. Cultivar differences in the ratoon volume at these ripening stages appeared clearly for every year and every level of fertilizer application, unlike the percentage of ripened grains. In each experiment at the early, late and intermediate ripening stages, correlation coefficients between the ratoon volume and the percentage of ripened grains for different fertilizer levels were generally low or high without clear or with clear cultivar differences in the percentage of ripened grains and the correlation coefficient for the level at which cultivar differences were most clearly revealed, was higher than +0.7 in each experiment. Furthermore, the correlation coefficients between the ratoon volume for fertilizer levels at which cultivar differences in the percentage of ripened grains could not be revealed and the percentage of ripened grains for fertilizer levels at which cultivar differrences were most clearly revealed, were higher than +0.7 in most of the cases. Based on these results, it was considered that the ripening ability could be easily selected based on ratoon volume at the ripening stage even for low fertilizer levels at which cultivar differences in the percentage of ripened grains did not appear clearly. Secondly, using F3 pedigree lines derived from the crossing of two cultivars, a selection experiment for ripening ability based on the ratoon volume at the early ripening stage was conducted for 2 generations at a low fertilizer level. Inter-line differences in the ratoon volume and percentage of ripened grains were both significantly large and the correlation coefficient between the two characteristics was significantly high in the F5 selected lines. Based on these results, it was considered that the simplified method of selection for ripening ability based on the ratoon volume was effective in rice breeding.
