Studies on mechanism of heat-tolerance in rice cultivar 'Genkitsukushi' and improvement of grain quality under high temperature condition

Global warming reduces grain quality of rice in Japan. In particular, the occurrence of white immature kernel has become a serious problem. White immature kernel occurs when average temperature during 20 days after heading (DAH) was 27℃ and over and cultivars varied great deal in the percentage of white immature kernels. Extremely hot summer 2010, it caused severely deterioration grain quality of rice all over Japan. Nevertheless, the rate of the first inspection grade of Oryza sativa L. 'Genkitsukushi' was more than 90%, while that of 'Hinohikari' and 'Koshihikari' that are major cultivars in Japan, and 'Tsukushiroman' was less than 20%, respectively. 'Genkitsukushi' was bred and cultivated in Fukuoka Prefecture is tolerant to high temperature, and 'Tsukushiroman' is a pollen parent of 'Genkitsukushi'. Therefore, I studied on heat-tolerant mechanism of 'Genkitsukushi' as well as improvement method for yield and quality of heat sensitive cultivars under high temperature conditions. The number of spikelet per square meter of 'Hinohikari' was necessary to be suppressed 28,000-30,000 to obtain the perfect kernel rate more than 75% and a yield more than 530 kg/10a under high temperature conditions. Especially, it was effective method to be delayed application timing of top-dressing nitrogen from 18 days before heading (DBH) to 7 DBH to suppress the number of spikelet per unit area and occurrence of white immature kernel. Grain quality exposed to 30℃ before and after the heading stage was inferior to those of 25℃ treatments due to the occurrence of white immature kernels of 'Koshihikari'. At first, a biophysical approach was carried out to investigate the effects of high temperature stress on dynamic states of water in rice grains before and after the heading stage of 'Hinohikari' and 'Koshihikari' by monitoring sup(1)H-nuclear magnetic resonance (NMR) relaxation times (Tsub(1), Tsub(2)). The Tsub(1) of grains showed parallel relationship to the changes in the degree of dehydration, and that exposed to 30℃ before and after the heading stage was lower than those of 25℃ treatments at 14 or 21 DAH. The Tsub(2) of grains exposed to 30℃ before and after the heading stage treatments were also abruptly shortened at 21 or 28 DAH. Furthermore, the next approach was focused on the water channels called aquaporins regulating the dynamic state of water molecules. The expression of aquaporin gene, PIP1;1, was higher in the grains of 'Koshihikari' exposed to 30℃ before and after the heading stage at 14 DAH. These results suggested that the changes on dynamic states of water such as NMR relaxation times and expression of aquaporin genes in rice grain exposed to high temperature before or after the heading stage would reflect a reduction in the quality of rice. During the extremely hot summer 2010, especially late August and early September that is an early ripening period of the cultivars used in this study, the average air temperature was over 29℃. Therefore, it was carried out physiological and molecular approaches such as sugar translocation and expression of sucrose transporter gene, SUT1, during the ripening period under high temperature stress after the heading stage. Non-structural carbohydrate content in the stem of heat tolerant 'Genkitsukushi' at the early maturation was significantly higher than that of heat sensitive 'Tsukushiroman' and it greatly decreased under high temperature. From 14 to 21 DAH, the expression of SUT1 was markedly higher in the stem of 'Genkitsukushi' grown under high temperature than that of 'Tsukushiroman'. In addition, the gene expression of SUT1 in the grains of 'Genkitsukushi' was significantly higher than that in 'Tsukushiroman' during the ripening period. In conclusion, it was revealed that sugar transport functions more effectively in 'Genkitsukushi' than in 'Tsukushiroman', and that the effectiveness of sugar transport contributes to maintain high grain quality in 'Genkitsukushi' even under high-temperature conditions.