Field experiments were conducted in 1981 and 1982 to study the effects of low-irradiance supplemental light on soybean (Glycine max [L.] Merr. cv Evans) flower and pod abscission. Cool-white and red fluorescent lights illuminated the lower part of the soybean canopy during daylight hours for 3 weeks late in flowering. At the same time, flowers and young pods on half the plants were shaded with aluminum foil. Flowers were tagged at anthesis and monitored through abscission or pod maturity.Responses to red and white lights were similar. Supplemental light tended to reduce abscission and increase seed weight per node compared to natural light. Shading flowers and pods increased abscission and reduced seed weight per node. Number of flowers produced per node, individual seed weight, and seeds per pod were not affected by light or shade treatments.Further studies examined the effects ofshading reproductive structures on their capacity to accumulate '4C-photoassimilates. Individual leaves were pulse labeled with '4CO2 1, 2, and 4 weeks post anthesis. Flowers and pods in the axil of the labeled leaf were covered with aluminum foil 0, 24, 72, and 120 hours before pulsing.Shading flowers and pods resulted in a 30% reduction in the relative amount of radiolabel accumulated from the source leaf. The reduction in '4C accumulation due to shading was evident regardless of the length of the shading period and was most pronounced when the shades were applied early in reproductive development. We conclude that light perceived by soybean flowers and young pods has a role in regulating both their abscission and their capacity to accumulate photoassimilates.Soybeans flower profusely, but only a small fraction of those flowers develop into mature pods with seeds. Combined flower and pod abscission from 32 to 63% for indeterminate (7,23) and from 57 to 82% for determinate genotypes (23, 26) has been reported. Although the cause is unknown, this extensive reproductive abscission does not appear to be due to inviable pollen (23) or lack of fertilization (1, 3). Abscission is not restricted to flowers, as a significant proportion of young pods also drop, usually before the onset of rapid seed growth (24,26). Percent

No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.The roles of cytokinins in plant growth and development are not fully understood, but evidence supports the hypothesis that cytokinins are produced by the root system and transported to the shoot, where they are involved in the regulation of shoot processes (3,22,27). Transport in the xylem sap of materials with cytokininlike activity (determined by bioassay) has been demonstrated in numerous species including sunflower (22), tomato (7), and lupin (8, 9). Cytokinin-like materials have also been reported in the root nodules of Viciafaba (13) nodules, they detected Z2 and a number ofits metabolites throughout the plant.If cytokinins from the root system are involved in regulating shoot growth and function, then their supply to the shoot should fluctuate over the course of development. Seasonal variations of cytokinin-like activity in root pressure exudate have been reported in a number of genera (2, 3, 7-9, 22, 24). In Perilla, there was a 5-fold increase in cytokinin-like activity in root pressure exudate after floral induction (2, 3). Conversely, the root sap of Xanthium plants which had been induced to flower contained less than onethird as much cytokinin-like activity as did the sap of vegetative plants (24).Although cytokinins were not specifically identified as the active regulating component, several studies have shown that roots or factors from the roots are required for maintaining leaf Chl content (18) and photosynthesis (4,5,18,27). In Phaseolus, foliar applications of BA delayed Chl loss and photosynthetic decline associated with senescence (1).In A. glutinosa, cytokinin-like activity in the root nodules increased prior to the onset of nitrogenase activity (acetylene reduction) and declined by the time N2-fixing capacity had been fully established (14). Likewise, Newcomb et al. demonstrated that cytokinin-like activity in pea nodules was greatest early in nodule development and decreased thereafter (20). Henson and Wheeler hypothesized that cytokinins exported from nodules affected N2-fixation through either local action in the nodule or by a remote effect on the shoot (14).Presence of cytokinins in soybean root pressure exudate has not been demonstrated. Therefore, it was not known whether or not these hormones could be involved in the regulation of soybean development. Hence, the objectives of our research were to (a) establish the presence of cytokinins and (b) describe profiles of cytokinin flux in root pressure exudate in relation to the ontogeny of soybean plants grown in the field and in growth chambers. Root pressure exudate was collected and acetylene reduction activity was measured from late in vegetative growth through maturity. Four cytokinin...

Field studies were conducted in 1981 and 1982 to determine patterns of flowering and reproductive abscission for nodes on the main stem of soybean [Glycine max (L.) Merr.] and to relate these patterns to seed yield components at nodes in various levels of the canopy. ‘Evans,’ an indeterminate, Maturity Group 0 cultivar was used. Individual flowers were tagged at anthesis and monitored through abscission or pod maturity. Date of anthesis and date of flower or pod abscission were recorded. Percent abscission was calculated on a per node basis. Number of mature pods, seeds/pod, and seed weight/pod were recorded for each main stem node. Data are reported as averages over nodes within four sections of the stem, each of which included approximately one‐fourth of the flowering nodes. Seed weight/node and seed weight/section were significantly greater in the middle two sections than in the top or bottom sections in both years. Pods at nodes in the middle two sections accounted for at least 75% of the main stem yield. Flowers produced per node generally varied only slightly among the sections, so percent abscission was the main determinant of pods/node. Pods/node varied as much as 300% among sections, whereas individual seed weight varied less than 35%. The primary cause of differences in number of pods/node and therefore in productivity of nodes in various stem sections was differential flower and pod abscission.

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