Effect of Defoliation On Leaf Senescence and Nitrogen Redistribution During the Seed-Filling Period In Soybean.

Defoliation of a soybean canopy may affect leaf senescence and nitrogen remobilization during the seed-filling period. Two defoliation treatments- sequential removal of leaf area between growth stage R5 and R7 and single defoliation at growth stage R6 that did not reduced light interception below 90%- were applied in the field. All leaf removal was from the bottom of the canopy. The rate of senescence of an upper leaf on the defoliation treatments, as estimated by the change in leaf carbon exchange rate (CER) and chlorophyll concentration, during seed filling did not differ from the control plants. Thus there was no evidence that defoliation accelerated leaf senescence. The single defoliation treatment did not reduce yield or the total nitrogen redistributed to the seed. Sequential defoliation reduced light interception, seed number, seed size, yield, and shortened the seed-fill period by 10%. Sequential defoliation reduced the nitrogen redistributed to the seed (not significantly different from the control). The nitrogen redistribution from stem, petioles, and pod wall in the sequential defoliation treatment was slightly higher than in the control, which, coupled with the reduction in yield and seed nitrogen content, sustained the same proportion of nitrogen that came from redistribution as the control.  The primary effect of defoliation, such as that caused by soybean rust, was to reduce the assimilate supply as result of reduction in the leaf area and light interception, which ultimately shortened the seed-filling duration and reduced yield. The effects on leaf senescence and nitrogen redistribution were minimal.