P Nutrition during Seed Development : Leaf Senescence, Pod Retention, and Seed Weight of Soybean.

Soybean (Glycine max [L.] Merrill) leaf senescence, which may partially result from mineral redistribution, appears to limit grain yield. Two experiments were designed to test the effects of supplemental inorganic phosphate (Pi), K, malate, and methionine (Met) infusions on senescence and yield. A novel stem infusion technique using pediatric intravenous kits was developed to supply these nutrients throughout seed growth. An average of 48.4 milliliters per plant was successfully infused into lower stem internodes during a 4 to 6 week period. Senescence was unaffected by K or malate infusions, but was delayed by Pi infusions (up to 8 days) and by increased nutrient solution Pi levels (up to 21 days) in separate experiments. Treatments which delayed senescence also improved yield as much as 3-fold, due primarily to increased pod retention and secondarily to increased seed size. Met infusions further increased pod retention at the lower, infused nodes, and thus increased total plant yield also. The influence of higher Pi levels during reproductive growth on soybean pod retention and yield may have been the result of sustained sucrose export due to altered C partitioning in leaves. The role of Met in improving yield was not clear. However, these results clearly demonstrate the importance of adequate Pi for delaying senescence and improving pod retention and yield.