Correlation between Viability of Pollination and Length of Basal Dehiscence of the Theca in Rice undera Hot-and-Humid Condition

Anticipated global warming may increase the floret sterility of rice (Oryza sativa L.). For selection of genotypes tolerant to high temperatures during the flowering period, it is important to identify morphological traits associated with tolerance to temperature stress. This study investigated the relationship between the length of dehiscence at the basal part of thecae and the viability of pollination in 18 cultivars of rice subjected to a hot-and-humid condition (37/25 ˚C, day/night, >90% relative humidity) for three days at flowering. Control plants were left under the ambient conditions in a semi-cylindrical house covered with cheesecloth (30% shading; temperature range: 24-35 ˚C). The length of basal dehiscence of thecae and the number of pollen grains on the stigmata were examined with a light microscope after flowering. The length strongly correlated with the percentage of florets having more than 80 pollen grains on the stigmata under the ambient condition (r = 0.72, P < 0.001), and with the percentage of florets having more than 20 pollen grains on the stigmata under the hot condition (r = 0.93, P < 0.001). In other words, the length correlated with pollination viability or reliability under both conditions. In addition, basal dehiscence was shorter in the non-japonica-type cultivars than in many of the japonica-type cultivars under both conditions. We concluded that the low pollination viability in the non-japonica-type cultivars is associated with their small basal dehiscence on the thecae, and the length of basal dehiscence can be used as a selection marker of high temperature tolerance.