Abstract
A wheat cultivar "Haruyutaka" bred in Hokkaido has a small harvest index, grain weight and yield when it grows in Yamaguchi, Japan. Ethephon, a growth regulator, applied to the surface of wheat canopy is expected to shorten the wheat culm and translocate the resulting surplus assimilate to the grain resulting in the increase of harvest index, grain weight and yield. In this study, we planted two wheat cultivars, Haruyutaka and Daichinominori (bred in Kyushu), at two sowing densities, 400 and 100 seeds m-2 in Yamaguchi, and sprayed 100 ppm ethephon on the canopy once every three days from flag leaf stage to anthesis with the intention of improving the harvest index Ethephon shortened the culms of both cultivars, but gave no effect on harvest index, grain weight or yield. Culm length was reduced not by shortening the culm elongation period but by reducing the culm elongation rate. The shortening of the culm by ethephon treatment resulted in the accumulation of surplus assimilate in the culm, and increased the percentage of water-soluble carbohydrate (WSC) in the culm. However, the WSC was not used for grain growth, because it remained in the culm until maturity.
