Abstract
The study has been carried out to evaluate varietal difference in leaf senescence during the ripening period in relation to sink size and nitrogen application. Leaf senescence was early in the medium-duration lines. Among the short-duration types, there were both early and late aging lines. The medium-duration lines had larger leaf area and higher sink sizes at the anthesis than short-duration types. The pattern of leaf area reduction during the ripening period in the short duration line, with relatively higher sink size, was almost similar with that in the medium-duration lines. However, in the short-duration lines with relatively small sink size, no significant leaf area reduction was observed until two weeks after the anthesis, and then the leaf area began to decrease rapidly in these lines. Decrease in leaf area during the ripening period varied with the rate of basal nitrogen (N). Higher basal N increased the rate of leaf area reduction by increasing the sink size. On the other hand, N-topdressing at the anthesis decreased the rate of leaf area reduction during the ripening period. Likewise, the reduction in sink size as imposed by removal of spikelets decreased the rate of leaf area reduction during the ripening period. Significant correlation coefficients between the sink size and the amount of N remobilized from leaves to panicles, and between the decrease in leaf area and the amount of N remobilized from leaves to panicles were observed. Therefore, the decrease in the leaf area during the ripening period is mainly governed by the amount of N remobilized from leaves to panicles, which is affected by the N requirement of grains for ripening and the amount of N absorbed during this period.
