Symbiotic nitrogen fixation of two field grown soybean [Glycine max (L.) Merr.] varieties, 'Chippewa 64' and 'Clay,' was measured at 7 to 10 day intervals starting at flowering, using the acetylene reduction assay. At he end of flowering, treatments were applied which were designed to alter the relationship between the photosynthetic source and sink components in the plants. The treatments were supplemental light, 25% shade, 50% depodding, 60% defoliation, and control.Total acetylene reduction activity per plant increased in both varieties during the flowering period, reached a maximum near the end of flowering, and then declined markedly during the early podfilling stage. The activity decline was due to a decline in the specific activity of the nodules, and occurred immediately prior to the time when the growth rate of the pods (including seed) exceeded that of the total plant tops in the respective varieties. Treatments designed to enhance he photosynthetic source/sink ratio (supplemental light and depodding) maintained nodule activity well above the control in both varieties. Conversely, treatments designed to reduce the source/sink ratio (shading and defoliation) decreased nodule activity below the level of the control. Treatment effects on total plant protein recovered at maturity closely reflected the treatment effects on total nodule activity. The results of this study are interpreted as evidence that symbiotic nitrogen fixation in these varieties declined during podfilling as the result of inadequate assimilate supply to the nodules.Additional index words: Glycine max (L.) Merr., Acetylene reduction, Protein yield. P RELIMINARY studies at the University of Minnesota indicated that senescence of soybean [Glycine max (L.) Merr.] nodules commenced during the early podfilling stage, causing a marked decline

The objective of this study was to determine whether the supply of current photosynthate was limiting root nodule activity. Both short-term (36 hours) and long-term (16 days) periods of CO2 enrichment were imposed on vegetative, growth chamber-grown soybean plants (Glycine mar. IL.I Merr. cv. 'Clay') to increase the supply of current photosynthate and to observe the effects on photosynthate partitioning in the plants, plant growth, and root nodule activity.Neither total nor specific nodule activities were increased during exposure to short-term (36 hours) CO2 enrichment. Dry weight of the leaves increased after 12, 24, and 36 hours of CO2 enrichment and dry weight of the stems plus petioles increased after 36 hours of CO2 enrichment. Dry weights of the roots and nodules were not altered by short-term CO2 enrichment. Short-term CO2 enrichment increased the total nonstructural carbohydrates in the leaves and stems plus petioles, but not in the roots and nodules. Analyses of the separate pools of carbohydrate reserves indicated that the majority of the additional carbohydrate provided by short-term CO2 enrichment was stored as leaf starch with relatively little being partitioned to the roots and nodules.Long-term CO2 enrichment (16 days) did not enhance specific nodule activity. Shoot, root, and nodule dry weights were increased 109, 34%, and 56% respectively. Total nodule activity per plant was significantly enhanced only after 16 days of treatment and was related to increased nodule mass. These results indicate that the increased total nodule activity in response to CO2 enrichment is a consequence of a general growth response of the plant.Results of both studies indicate that nodule activity was not directly limited by current photosynthesis but rather by the partitioning and utilization of photosynthate in the plant.nodule activity when photosynthesis is limited.Competition for photosynthate between filling seeds and root nodules during reproductive development is a possible reason for the decline in nitrogen fLxation during podfilling. Hume and Criswell (13) observed that after rapid seed development began, roots and nodules accumulated little radioactive carbon. Depodding, which reduced the competition for assimilates, resulted in greater nitrogenase activity in the nodules (14).Perhaps the strongest evidence for a limitation of nitrogen ftxation by photosynthesis is provided by CO2 enrichment studies. Nodule activity (total and specific) ofsoybean plants was increased by long-term C02 enrichment to 1,200 ,ul I`C02 in both growth chamber and field environments (8, 19). The authors suggested that the increased supply of current photosynthate was directly responsible for the increase in both the total and specific nodule activity of the plants. Phillips et al. (18) observed an increase in specific nodule activity of Alaska peas with short-term CO2 enrichment, and increased total nodule activity with long-term CO2 enrichment. The increase in total nodule activity was associated with increased plant and ro...

ABSTRACrThe effect of exogenous abscisic acid (ABA) on the rate of sucrose uptake by soybean (Glycine max L. Men.) embryos was evaluated in an in vitro system. In addition, the concentrations of endogenous ABA in seeds of three soybean Plant Introduction (PI) lines, differing in seed size, were commpared to their seed growth rates. ABA (10-molar) stimulated in vitro sucrose uptake in soybean (cv 'Cly') embryos removed from plants grown in a controled environment chamber, but not in embryos removed from field-grown plants of the Sink strength describes the capacity ofan organ to accumulate photoassimilates, and is the product of sink size and sink activity (25). Plant hormones are probably involved in determining both sink size and sink intensity. While cytokinins, gibberellins, and auxins generally promote growth processes, ABA has most often been considered an inhibitory substance (24). However, promotive effects of ABA in sink tissues have now been reported. ABA enhanced sugar content in roots of intact bean plants, possibly by stimulating sugar transport from the shoot to the root (11). In some cases, applications of ABA to ears of wheat (7) and barley (22) have stimulated 14C assimilate transfer from the flag leaf to the ear. However, King and Patrick (12) saw no stimulation of 14C transport to wheat ears after injection of ABA into the grains. Correlations between rate of growth and ABA accumulation in major sinks have been reported for wheat (13,14), grapes (5), pea (3), bean (10), and soybeans (4, 15). The relatively high levels (up to 12.2 #g g-' fresh weight) (15) and timing of ABA accumulation in soybean seeds (1,4,15) suggest a role in seed growth above simply preventing precocious germination. Browning (3) found that ABA accumulated more rapidly in a lare-seeded pea line and suggested a role for ABA in controlling cotyledon enlargement during linear seed fill. It has also been suggested that ABA may stimulate phloem unloading (8, 18) by decreasing the proton motive force across the sieve tube plasmalemma. The embryo-seed coat interface of the soybean seed (21) In addition, the endogenous ABA concentrations of seed coats, cotyledons, and embryonic axes of three field-grown soybean PI lines differing in seed size were compared to their seed growth rates during filling. MATERIALS AND METHODSGrowth Chamber Study. Plant Culture. Variety 'Clay' soybeans (Group 00 maturity) were grown in a controlled environment chamber in 1-L plastic pots in a 1.5:1.5:1 (v/v/v) mixture of vermiculite, peat, and sand. Pots were watered twice daily with automatic drip irrigation and given 250 ml of Peters 20-20-20 (200 ppm) twice weekly. A 14-h photoperiod with a 25/19C day/night temperature regime was provided. The photosynthetic photon flux density at the top of the canopy was 800 jE m-2 s-1.In Vitro Sucrose Uptake. During mid pod fill (seed dry weight 40% of final seed dry weight) pods were removed. Seed coats were surgically removed from a number of seeds and groups of three embryos were placed in 15 ml of sterile 10 ...

The partitioning of recently fixed '4C to setting and abscising flowers within the axilary raceme of 'Clark' isoline Elt soybeans (Glycine max L. Men) was examied as a function of time after anthesis of individual flowers. In such racemes, the first four flowers showed a 17% abscission while the next four flowers showed 47% abscission.Source/sink relations offlowers I-IV (normally setting) were compared to those of flowers V-VIII (normally abscising) by pulse labeling source leaves with '4C02 and determining the radioactivity of individual flowers after a 4-hour chase period. conditions.The physiological mechanisms of soybean flower and pod abscission are not fully understood. Some studies have implied that reproductive abscission in legumes is hormonally mediated (6,8,12,14), while others have implied that it is related to> the availability of photosynthetic assimilates (4, 10, 11). However, since reproductive abscission occurs at a time when plant photosynthetic rate is relatively high and reproductive growth rate is relatively low, it appears not to be limited by the contemporary supply of photoassimilate.The objectives of the current work were to examine the partitioning ofrecently fixed carbon to normally setting and normally abscising flowers within a given soybean raceme. By examining this partitioning as a function oftime after anthesis ofindividual flowers, and in relation to the time of abscission, it was hoped to shed light on the mechanism of abscission.