RESUMEN
This study aimed to examine the compounds increasing or decreasing in concentration in brown rice grains ripened under high air temperature during ripening using a heat-tolerant cultivar Fusaotome, a heat-intolerant cultivar Hatsuboshi, and an intermediate cultivar Koshihikari. 6-O-Feruloylsucrose (1), 3',6-di-O-sinapoylsucrose (2), 3'-O-sinapoyl-6-O-feruloylsucrose (3), 3',6-di-O-feruloylsucrose (4), cycloartenyl ferulate (5), and 24-methylenecycloartanyl ferulate (6) were isolated from the extracts of brown rice grains. The structures of the isolated compounds (1-6) were elucidated on the basis of spectroscopic analyses. The mean concentrations of compounds 2, 3, and 6 in the grains ripened under high air temperature were markedly higher than those ripened under normal air temperature. In contrast, the mean concentration of compound 5 in the grains ripened under high air temperature was markedly lower than those ripened under normal air temperature. Thus, compounds 2, 3, 5, and 6 constitute potential biomarkers of heat stress in the cultivars used. The mean concentrations of compound 4 in the grains of Fusaotome were the highest in all cultivars. In contrast, the mean concentration of compound 5 in the grains of Fusaotome was the lowest. Therefore, the unique composition of heat-tolerant Fusaotome combines a high concentration of compound 4 with a low concentration of compound 5.