Abstract
Several water yam strains have been introduced from areas at an elevation of 600m to areas at an elevation 1800m in Nepal. These strains were grown in Kagoshima, and their growth and enlargement of the tubers were investigated to evaluate their performance and earliness. All the tested strains were found to belong to early-maturing types. Furthermore, extremely early-maturing strains were detected in this group. In this type, the tubers began to enlarge and mature earlier than extremely early-maturing strains originating from Kagoshima, Japan. These strains were distributed in high-altitude areas, where the growth period is limited because of lower temperature. The difference in the maturity and, growth period appeared to be related to the altitude where the temperature is lower in the tropical and sub-tropical regions. Based on these results, the following conclusion was drawn; Late-maturing strains are characterized by long growth period and high yield, hence these strains are distributed in the lowlands of tropical regions. In contrast, early maturing strains exhibit a relatively short growth period and are consequently, distributed in high-altitude areas in the tropical and sub-tropical regions. It may thus be possible to cultivate extremely early-maturing strains with low sensitivity to day length in temperate regions. The growth pattern of the tubers in the extremely early-maturing strains of water yam was similar to that in the Naga-imo and Icyo-imo groups of Chinese yams (D.opposita) . These findings suggest that further experimental data should be collected to determine whether decreased day length is the main factor that triggers the enlargement of the tubers in the extremely early-maturing strains of water yams.
