Stay-green trait assessment using the leaf incubation method to examine the maintenance of assimilation rates under high temperature conditions during the grain-filling period in rice plant.

Rice yield and quality decrease under high temperature conditions during the grain-filling period particularly in early-ripening cultivars that are directly subjected to midsummer heat. As the grain growth rate increases under high temperature conditions, the grain requires greater amounts of assimilates for the shortened grain-filling period and occasionally experiences a lack of assimilates. The stay-green trait that can maintain assimilation during the active grain-filling period is expected to mitigate the negative impact on grains due to the lack of assimilates. Our objectives were to evaluate the stay-green trait of rice maintaining assimilation rates under high temperature conditions during the active grain-filling period using the leaf incubation method. When whole leaves or leaf segments were floated on water at 20 - 40℃ under dark conditions, a leaf color reading (SPAD) showed steady genotypic differences at 25 - 35℃. When flag leaf segments of early-ripening cultivars from the Japanese Rice Collection (JRC) were incubated at 35℃ under dark conditions, the SPAD values of incubated leaves well reflected those in the top three-leaves in standing plants during the active grain-filling period (r = 0.774, p < 0.005). Under ambient and elevated temperature conditions (+3 - 4℃), the SPAD values and photosynthetic rates of genotypes that ranked higher for the trait by the leaf-incubation test, tended to be higher during the active grain-filling period. The results suggest that the leaf incubation method is suitable for the first-step selection of the stay-green trait of rice associated with the maintenance of assimilation rates under high temperature conditions during the grain-filling period.