Abstract
We attempted to increase the kernel weight of Hokkaido-bred wheat cultivar “Haruyutaka” by increasing the net assimilation rate (NAR) without inducing an over-luxuriant canopy during the ripening period. For this purpose we applied nitrogen as topdressing at jointing in 1999/2000 and at anthesis in 2000/2001, using the Kyushu-bred cultivar “Daichinominori” as a control. However, kernel weight of either cultivar was not increased by the topdressing in either season. In addition, the topdressing in Haruyutaka increased neither the NAR during the ripening period nor the crop growth rate (CGR). On the other hand, the amount of nitrogen in the whole plant increased from anthesis to maturity only in the plants given nitrogen topdressing. The amount of nitrogen in the kernels increased rapidly from the milk ripe stage to maturity in both plots with and without topdressing in both cultivars, and that at maturity accounted for most of the nitrogen in the whole plant. However, the nitrogen topdressing significantly increased the nitrogen content in the leaf blades at the milk ripe stage and that in the kernels at maturity in both cultivars. These results suggested that nitrogen topdressing increased the nitrogen contents in the whole plant, and increased the translocation of nitrogen into the kernel resulting in an increased nitrogen content of kernel. However, kernel weight was not increased by the topdressing; neither was NAR nor CGR. We suppose that this is because the respiration rate in leaves increased due to the over-luxuriant canopy caused by the topdressing, especially in Haruyutaka.
