Changes in NMR Relaxation of Rice Grains, Kernel Quality and Physicochemical Properties in Response to a High Temperature after Flowering in Heat-Tolerant and Heat-Sensitive Rice Cultivars

Rice productivity is related to the ability of plants to adapt to heat stress. The heat-tolerant: cultivars 'Nikomaru' and 'Chikushi 64' and heat-sensitive cultivar 'Hinohikari' were grown at 30 degrees C and 25 degrees C for 49 days after flowering. At 30 degrees C, only a few white immature kernels were produced in 'Nikomaru' and 'Chikushi 64', but about 22% of grains had immature kernels in 'Hinohikari'. The high temperature(30 degrees C) caused no significant changes in grain dry weight, water content, and the NMR T-1 value during the early ripening stage in 'Nikomaru' and 'Chikushi 64'. It also did not affect grain development, especially with respect to the nucellar epidermis, in `Nikomaru' and 'Chikushi 64', but caused clear cessation of development of the nucellar epidermis at 14 days after flowering in 'Hinohikari'. In addition, high temperature decreased the amylose content: and increased hardness vs. adhesion ratio of cooked rice in both 'Nikomaru' and 'Chikushi 64' resulting a softer, less sticky texture, but not in 'Hinohikari'. The maximum viscosity and breakdown values were increased, and final viscosity decreased at 30 degrees C in all three cultivars. These results suggested that. starch in the endosperm of grains changed from a fluid state to a doughy state more slowly in `Nikomaru' and 'Chikush 64' than in 'Hinohikari', in which the water content and NMR relaxation tune decreased, and transported assimilates accumulated slowly during grain development.