Nonstructural Carbohydrate Content in the Stem at Full Heading Contributes to High Performance of Ripening in Heat-Tolerant Rice Cultivar Nikomaru

Rice (Oryza sativa L.) grain yields in western Japan have recently decreased as a result of higher temperatures during ripening. The improvement of rice cultivars for stable production is a pressing need because of climate change. In this study, the features of good ripening of the new rice cultivar Nikomaru were analyzed through a 3-yr field experiment. Nikomaru showed a yield increase of 7 to 8% compared with the leading cultivar in western Japan, Hinohikari, even though these cultivars have similar source size (available carbohydrate during ripening) and sink size (total spikelet number and rough grain size). The greater ripening index (percentage of ripened grains x 1000-grain weight) in Nikomaru, which determined the varietal yield differences in this study, was attributed to the translocated dry matter for ripening (Delta T) but not to the newly assimilated dry matter during ripening (Delta W). In addition, Delta T was positively correlated with the nonstructural carbohydrate content in the stem at full heading (NSCh). There was positive correlation between the NSCh and the percentage of ripened grains in the hotter years (2005 and 2007). It was suggested that a large NSCh as the main source of the Delta T contributes to the maintenance of a higher ripening rate especially under high temperatures.