Drought stress and plant hormonal impact on rice peduncle elongation

Drought stress is a major constraint to rice (Oryza sativa) production for which the reproductive period is the most sensitive stage. One of the most important reasons for the impact of drought is the inhibition of peduncle elongation, resulting in panicle retention inside the flag leaf sheath so that the unexserted spikelets remain sterile. This research was focused on studying the peduncle elongation pattern in rice plants under well-watered and drought-stress conditions as well as analyzing the effects of phytohormones (gibberellic acid, abscisic acid and auxin) on elongation of detached peduncles. The inhibitory effect of drought stress on peduncle elongation was observed since leaf rolling up to the end of stress time. After re-watering, the peduncles did not achieve the same length as well-watered plants, since there is only a limited opportunity for peduncle elongation. Peduncle elongation in an elongated uppermost internode IR64 mutant, eui-10, is also inhibited by drought stress. However, after re-watering, its peduncle grew even longer than well-watered IR64 plants in the limited time left. As the peduncle elongation rate of this mutant is much higher than IR64, panicle is fully exserted. In order to study the effect of candidate hormones on the peduncle elongation, a simple and efficient experimental design was employed. The peduncles were detached from the tillers at heading stage, and floated in the desired solutions. The presence of auxin (IAA) had no significant effect on the elongation of detached peduncles while gibberellic acid (GA3) activated peduncle elongation in a concentration dependent manner. Abscisic acid (ABA) antagonized GA effect leading to complete inhibition of peduncle elongation. The effect of GA, as an inducer, and ABA, as an inhibitor, on peduncle elongation was highly significant in Moroberekan (japonica rice), IR64 (indica rice) and eui-10. These data indicate the possible role of hormonal control on peduncle elongation in response to drought stress.