Effects of Pod Removal on Metabolism and Senescence of Nodulating and Nonnodulating Soybean Isolines

The objectives of this work were to determine the effect ofsink sBtngth (presce or absence of pods) ad nitrogen source (nodulatng nw nnoting pnnts) on enymk activ chlorophyll co tion, and sescence of soybean (GIycl max [L.] Merr. cv Harosoy) isolnes. A 2-year (1981-1962) feld sdy was ducted. For both nodlatesd ando lated p ts, ribe bisphosphate carboxybse (RuBPCe) acivity of upper-canopy leaves was deead by pod removal in both yeas, while chlorophyll ncentration was decreased in 1981 only. Nonodubted plats had lower RuBPCae activity in 1981 and lower chlophyll conetratn in both yean comp with w odulbted plat. In both years, and for all treatm , RuBPCase activity and chl bhyll bega to dedine at about the same ti b,ut the rate ofdecine wa km for depodded tha for podded plats. Leaves In the middle and lowerpab ofthecaopy had similar RuBPCase activity and chbrophyll concentration trends as upper-canopy leaves for all treatments. Profies of nitrate reductse activity (NRA) were similar for all treatmts in both 1981 and 192. Acetylene reduction profiles were simr for nodulted-podded and nodulated-depodded plats. The peak and dedine In NRA profiles ed the peak and decline in acetylene reductin profiles. The te of decine in acetylene reduction activity was less for depodded p t, es ally in 1982, bet activities reached zero by the finl spi tie. Th, odule seesecewas not prevented by pod removl. Based on seasonal profils of RuBPCae activity, chbrophyll NRA, and acetyle reduction acivt, the o of se ce appered to occur at the same approximate tie for all tretments and, thus, did not depend on the prsence or aboen of pods or nodules. The hypotbeis that oles act as a nitroge source and carbohydrate sink to delay seescence in the absence of pods was not correct. assimilation declined at a faster rate in leaves of depodded soybean plants, relative to podded controls. More recently, Wittenbach (20, 21) found that pod removal resulted in Chl and protein retention by leaves, relative to control plants with pods, but that RuBPCase2 activity and content declined more rapidly within leaves of depodded plants. He initially found three (20) and subsequently four (22) new polypeptides accumulating within leaves of depodded plants, which appeared to be due to de novo protein synthesis and not breakdown products of RuBPCase. Additional work (5) showed that two ofthe polypeptides accumulated in the vacuoles of paraveinal mesophyll cells and associated bundle sheath cells. Wittenbach (20) sugd that leaves of depodded plants changed from photosynthesizing source organs to sink orgns. Seemingly, pod removal should enhance the supply of photosynthate to nodules and result in stimulated and prolonged nodule activity. This result was reported by Lawn and Brun (8) for partially depodded soybean plants. However Wilson et al. (19), working with male-sterile soybean, reported that while abundant starch accumulated in roots of male sterile compared with fertile plants, nodule activity profiles were similar. In a preceding paper (3), we reported changes in metabolic constituent contents and in senescence patterns of whole plants following removal of pods from nodulating and nonnodulating isolines ofsoybean. The objectives ofthis study were to determine the effect of pod removal from nodulating and nonnodulating soybean isolines on senescence as determined by chang in various enzyme and Chl profiles during the seed-filling (normal senescence) period. Specifically, RuBPCase activity, NRA, and Chl concentration in upper-canopy leaves, and acetylene reduction activity in nodules, were determined in 1981 and 1982. In 1982, a separate experiment examined the changes in RuBPCase activity and Chl at different positions within the leaf canopy in response to pod removal.