Carbon and Nitrogen Supply during Seed Filling and Leaf Senescence in Soybean

Abstract

Leaf senescence and N redistribution during seed filling may limit soybean [Glycine max (L.) Merr.] yield by restricting the seed filling period. Experiments were conducted in the field and greenhouse to evaluate the relationship between seed N demand and leaf senescence. Plants (cv. McCall) were grown under shade cloth (which reduced insolation by 63%) from R1 to R5 (beginning seed fill), then the shade was removed from some plants with the objective of increasing photosynthesis, seed dry matter, and N accumulation. Experiments included plants grown in soil with no nodules (NO−3 as primary source), plants grown in soil with many nodules (N from NO−3 and N2 fixation), and in a hydroponics system (NO−3 as primary N source) where N was removed from the media at R5. Increasing photosynthesis at R5 increased seed dry matter accumulation and the response was larger when N was available. Seed N accumulation also increased if N was available. Increasing photosynthesis had no effect on seed N accumulation when N was not available to the plant. Temporal changes in chlorophyll and N of an upper leaf were measured to characterize leaf senescence. Increasing photosynthesis did not accelerate leaf senescence in any of the treatments. If N was available, N uptake increased when photosynthesis increased. If N was not available, seed N concentration decreased. The data suggest that leaf senescence does not occur because of seed N demand, but may be regulated by processes in the leaf.