The effects of kinetin and gibberellin on the germination of dehusked seeds of indica and japonica rice (Oryza sativa L.) under anaerobic and aerobic conditions

The effects of kinetin and gibberellin were examined under anaerobic conditions (0 % oxygen) and aerobic conditions (20 % oxygen) on the germination of dehusked seeds of indica and japonica rice cultivars that had been harvested at different times during the formation of seeds. Surjamkhi was used as a representative of deep dormant indica cultivars and Assam IV as a less dormant indica cultivar. Sasanishiki was used as the japonica rice cultivar. Both phytohormones were applied at a concentration of 10(-3) M which proved to have the greatest stimulatory effect in preliminary work at concentrations of 10(-8)-10(-5) M. Under aerobic conditions, inhibition of germination by dehusking of Sasanishiki seeds that had been harvested either 30 or 60 d after anthesis was overcome by kinetin and all seeds germinated. Complete germination induced by kinetin under aerobic conditions was also achieved with the dehusked seeds of the indica rice cultivar Assam IV that had been harvested on two occasions and of Surjamkhi that had been harvested 28 d after anthesis. In contrast, germination of dehusked japonica seeds stimulated by anaerobiosis was inhibited by kinetin. The stimulatory effects of gibberellin on the germination of indica and japonica rice seeds were observed under aerobic and anaerobic conditions. Under anaerobic conditions, the responses of dehusked indica and japonica rice seeds to kinetin and gibberellin differed, being negative with kinetin and positive with gibberellin. Under aerobic conditions, the stimulatory effects of kinetin on germination of dehusked seeds were greater than those of gibberellin. Thus, treatment with kinetin appears to be useful for breaking the considerable dormancy commonly observed in the dehusked seeds of indica rice. Mechanisms are proposed to explain the stimulatory effects of these phytohormones on the germination of dehusked seeds of indica and japonica rice under aerobic and anaerobic conditions. (C) 1997 Annals of Botany Company.