Interaction between Effects of Photoperiod and Plant Growth Regulators on the Development of Tubers and Inflorescences in Japanese Yams (Dioscorea japonica)

Abstract

The interaction between 8- and 24-hr photoperiods and plant growth regulators (PGR) (gibberellin A₃ (GA₃) and uniconazole-P (Uni)) on the development of the main shoots, aerial tubers (bulbils) and new tubers (below ground), and inflorescences (spikes) in Japanese yam (Dioscorea japonica Thunb.) plants was examined. Within the same PGR treatment, the main shoots of plants grown under the 24-hr photoperiod (constant light, LD) were longer than those grown under the 8-hr photoperiod (SD) in all season; under LD and SD, the main shoots of the GA₃-treated plants were the same size as shoots sprayed with water (control), whereas the main shoots of the Uni-treated plants were significantly shorter. Under LD, whereas the control and PGR-treated plants produced a negligible amount of aerial tubers, under SD, their development was severely inhibited by GA₃ treatment. During the early growing period, the fresh weight of new tubers and that of the combined fresh weight of aerial and new tubers in plants grown under SD were heavier than those grown under LD. From the middle to the end of the growing period, the development of aerial and new tubers in plants grown under LD accelerated so that the difference between the final fresh weights of the combined fresh weight of aerial and new tubers in plants grown under SD and those grown under LD at harvest became small. Although, there was not such LD promoting effect of those growth as would be observed in D.opposita cv. Ichoimo and Nagaimo. Under SD, the new tubers development were significantly promoted by GA₃ treatment because the inhibition of aerial tubers development by GA₃ treatment promotes the diversion of photosynthates stored in shoots to growth of new tubers. Under SD, plants grown developed one flush of inflorescences in June, whereas those grown under LD developed a flush in June and again in August through October. We conclude that the GA₃ and Uni had no significant effects on the ratio of plants that initiated flowers, the nodal position of the first and final spikes, and the total number of spikes per plant in both season of spikes development in the main shoots were not clear.