GRAIN FILLING MECHANISMS IN SPRING WHEAT .1. GRAIN FILLING PHASES ACCORDING TO THE DEVELOPMENT OF PLANT ORGANS

To define the growth phases for analyzing the physiological and morpho logical mechanisms of grain filling in a spring wheat canopy, the dry weight of the grains and the water soluble sugar content (WSC) and str uctural weight of the culm were measured for the variety Haruyutaka ev eryday during the grain-filling period. The grain-filling period was d ivided into four phases; (1) Intitial grain filling (from anthesis to cessation of culm elongation) where assimilate is used mainly for culm elongation and the dry weight of the grains increase slightly; (2) Ea rly grain filling (from cessation of culm elongation to the milk ripe stage) where assimilate is used for both grain growth and culm reserve . (3) Late grain filling (from the milk ripe stage to termination of p hotosynthesis) where assimilate is used entirely for grain growth and the culm reserve is also translocated to the grain; and (4) Final grai n filling (from the end of photosynthesis to maturity) where grain gro wth is supported only by translocation of the culm reserve because ass imilate is no longer being actively produced. In the early and late gr ain-filling phases, the increase rate of the WSC in the culm was relat ed linearly to radiation (r = 0.616**), while grain weight increased linearly with time (r = 0.992**), independent of radiation.