Effects of Pod Removal on Metabolism and Senescence of Nodulating and Nonnodulating Soybean Isolines: II. Enzymes and Chlorophyll.

The objectives of this work were to determine the effect of sink strength (presence or absence of pods) and nitrogen source (nodulating versus nonnodulating plants) on enzymic activities, chlorophyll concentration, and senescence of soybean (Glycine max [L.] Merr. cv Harosoy) isolines. A 2-year (1981-1982) field study was conducted.For both nodulated and nonnodulated plants, ribulose bisphosphate carboxylase (RuBPCase) activity of upper-canopy leaves was decreased by pod removal in both years, while chlorophyll concentration was decreased in 1981 only. Nonnodulated plants had lower RuBPCase activity in 1981 and lower chlorophyll concentration in both years compared with nodulated plants. In both years, and for all treatments, RuBPCase activity and chlorophyll began to decline at about the same time, but the rate of decline was less for depodded than for podded plants. Leaves in the middle and lower parts of the canopy had similar RuBPCase activity and chlorophyll concentration trends as upper-canopy leaves for all treatments.Profiles of nitrate reductase activity (NRA) were similar for all treatments in both 1981 and 1982. Acetylene reduction profiles were similar for nodulated-podded and nodulated-depodded plants. The peak and decline in NRA profiles preceded the peak and decline in acetylene reduction profiles. The rate of decline in acetylene reduction activity was less for depodded plants, especially in 1982, but activities reached zero by the final sampling time. Thus, nodule senescence was not prevented by pod removal.Based on seasonal profiles of RuBPCase activity, chlorophyll, NRA, and acetylene reduction activity, the initiation of senescence appeared to occur at the same approximate time for all treatments and, thus, did not depend on the presence or absence of pods or nodules. The hypothesis that nodules act as a nitrogen source and carbohydrate sink to delay senescence in the absence of pods was not correct.