Photoperiodic Response of Water Yam (Dioscorea alata L.), Chinese Yam (D. opposita THUNB.) and Jinen-jo (D. japonica THUNB.)

Abstract

In order to study the effect of the photoperiod on the enlargement of tubers in water yam (D. alata), Chinese yam (D. opposita) and jinen jo (D. japonica), 60-days-old plants were exposed to a 10-hour day length (short-day treatment) twenty times from 1st June and from 1st July, respectively. Short-day treatment in June resulted in the increase of the tuber weight of all the strains in the three species. In contrast, treatment in July resulted in the increased tuber weight in the early and late maturing strains of water yam, but not in the enlargement of the tubers in the extremely early maturing strain of water yam, Tsukune-imo group and jinen-jo. Furthermore, short day treatment inhibited the enlargement of tubers in Nagaimo and Icho-imo groups of Chinese yam. The results suggest that short day treatment mainly leads to the enlargement of tubers but that the response to the short day treatment differed in each growth stage of tuber. Specifically short-day treatment applied at the primary growth stage of tuber promoted the enlargement of tubers, while short-day treatment applied after the rapid growth stage of the tubers tended to inhibit the enlargement of the tubers.
In the extremely early maturing strain of water yam, 14 hour day length (long-day treatment) did not promote the enlargement of the tubers. However, exposure to a day length of 12 hour increased the tuber weight, and temperature was not effective in promoting the enlargement of the tubers.
Thus, the reduction of the day length was the main factor causing the enlargement of the tubers in water yam. In contrast, in Chinese yam and jinen jo, the tubers continued to enlarge slowly even under the long-day condition. Since water yam originates from tropical areas, it may be possible to cultivate extremely early maturing strains, which have a low sensitivity to day length in temperate regions.