Studies of grain production in Sorghum vulgare. II. Sites responsible for grain dry matter production during the post-anthesis period

The relative contributions of different photosynthetic sites to the filling of the grain in grain sorghum (Sorghum vulgare cv. Brolga) were estimated by measuring the 14C in the grain after exposing various leaves and the head to radioactive carbon dioxide. Methods for preventing photosynthesis were also used. Of the grain yield, 93% was due to assimilation by the head and upper four leaves. The head contribution of 18 % was due equally to direct assimilation of atmospheric carbon dioxide and to reassimilation of carbon dioxide released within the grain by respiration of material translocated from the leaves. The remaining 75 % was equally assimilated by the upper four leaves, the flag leaf being the most efficient contributor per unit area and the third uppermost leaf the least efficient. The percentage contributions to the grain by the flag leaf and fourth leaf, estimated from the decrease in grain yield when they were shaded, agreed closely with the estimates obtained by using 14CO2.