Effects of pod removal on metabolism and senescence of nodulating and nonnodulating soybean isolines: I. Metabolic constituents.

Field studies were conducted in 1981 and 1982 to ascertain the effects of pod removal on senescence of nodulating and nonnodulating isolines of soybean (Glycine max [L.] Merr. cv Harosoy) plants. Specifically, the test hypothesis was that nodules act as a nitrogen source and a carbohydrate sink which would in turn prevent or delay senescence in the absence of pods. Senescence was judged by changes in metabolite levels, in dry matter accumulation, and by visual observation.For both nodulated and nonnodulated plants, pod removal had no effect on the magnitude or rate of dry matter and reduced-N accumulation by whole plants. Phosphorus accumulation was significantly less in both nodulated- and nonnodulated-depodded plants, compared with respective control plants with pods. These data suggested a role for pods in phosphorus uptake. Accumulation of dry matter, reduced N, and phosphorus ceased at approximately the same time for all treatments.Pod removal did affect partitioning of plant constitments, with leaves and stems of depodded plants serving as a major alternate sink for accumulation of dry matter, reduced N, phosphorus, and nonstructural carbohydrates (primarily starch). While depodded plants eventually lost a significant amount of leaves, leaf drop was delayed relative to plants with pods; and depodded plants still retained some green leaves at 2 weeks past grain maturity of control (podded) plants.The results indicated that senescence patterns of soybean plants were the same for nodulated and nonnodulated plants, and that pods did not control the initiation of senescence, but rather altered the partitioning of plant constituents and the visual manifestations of senescence.