Abstract
The factors relating to yearly and regional differences in the percentage of white immature kernels (WIK) were analyzed in Koshihikari and Natsuhikari cultured in the early-season in Kochi Prefecture. The percentage of WIK was consistently higher in Koshihikari than in Natsuhikari regardless of the year. The yearly difference in the percentage of WIK was larger in Koshihikari than in Natsuhikari. In Koshihikari, the percentage of white-based kernels (WBK) was high in the district where the daily minimum temperature at the early and middle stages of the ripening period (RP) was 23°C or higher, and the percentage of milky-white kernels (MWK) was high in the district where the daily sunshine hours at the middle stage of the RP were shorter than 6 hours. It was suggested that the yearly difference in the percentage of WIK was caused by the frequent occurrence of WBK in the year of higher daily minimum temperature at the early and middle stages of the RP and by the frequent occurrence of MWK in the year with shorter sunshine hours at the middle stage of the RP. In addition, marked regional differences in the percentage of WIK were observed regardless of the variety and the year, and it was closely associated with the growth state at heading as well as the meteorological conditions during the RP. In Koshihikari, it was suggested that the light manure application in the year of higher daily minimum temperature at the early and middle stages of the RP and heavy manure application in the year with lower sunshine hours at the middle stage of the RP tended to increase the percentages of WBK and MWK, respectively. On the other hand, in Natsuhikari the percentage of WIK tended to increase with increasing panicle length and the number of spikelets per m2 regardless of the meteorological condition during the RP.
