Abstract
In the Hokkaido-bred wheat cultivar Haruyutaka grown in a warm district of western Japan, crop growth rate (CGR) during ripening period was low and a kernel weight was light. The low CGR was caused by a low net assimilation rate (NAR), although the mean green surface area index (GAI) was high. Hypothesizing that an overluxuriant canopy of Haruyutaka caused the low NAR, we released the canopy from the overluxuriant condition by removing the lower leaves or detaching tillers. We removed the lower leaves except for the flag and second leaves at the flag-leaf expanding stage, or detached all tillers as the tillers emerged. The removal of lower leaves increased the kernel weight of Haruyutaka in some seasons, when this defoliation increased NAR and CGR during ripening period. In addition, it increased the water-soluble carbohydrate (WSC) content of culms from anthesis to the milk ripe stage in Haruyutaka, suggesting that the increased WSC was translocated to the grain at the maturation stage. Detaching of tillers decreased the numbers of ears and kernels, although it increased the 1000-grain weight. These results suggest that the removal of lower leaves could increase kernel weight by releasing the canopy from an overluxuriant condition, and thus increasing grain weight, although it largely decreases the source/sink ratio in Haruyutaka.
