Previous investigations have shown the feasibility of increasing pod number on legumes by the application of 6-benzylaminopurine (BA) directly to the raceme. These investiptions were designed to determine what reproductive parameter was affected by cytokinin application, and if these applications were overcoming a deficiency in root-produced cytokinins during late flowering. Five individual main stem racemes on greenhouse grown soybeans (Glycine max L. Menf.) were treated with 2 millimolar BA. A single application of BA when pods appeared at 25 to 50% of the proximal floral positions resulted in a 58% increase in pod set due primarily to a 33% reduction in floral abscission. Applications of BA at later intervals also resulted in significant reductions in total abscission. When three applications of BA were imposed on the upper five nodes of field grown soybeans, total pod number and seed weight were significantly increased in this section of the canopy by 27 and 18%, respectively. Throughout the flowering period, root pressure exudate was sampled for the subsequent separation and quantification of zeatin, dihydrozeatin, zeatin riboside, dihydrozeatin riboside, and isopentenyladenine. Total cytokinin flux peaked from 0 to 9 days after flowering began, and then dropped to one-half of this level by 15 days postanthesis. The probability that a flower would initiate a pod was directly related to the concentration of total cytokinins present in the exudate when the flower opened.Yield in soybean is determined by the number of seeds per unit area, and the average weight per seed. The number of seeds per unit area is determined by the number of flowers which initiate pods that attain maturity. Soybeans produce an abundance of flowers, but shed a rather large proportion of them before the seeds begin to fill. Estimates vary, but investigators have shown that the abscission of flowers and small pods ranges from 32 to 82% of the total flowers produced (29,30

The previously reported activity of benzyladenine and selected other cytokinin analogs to increase pod set in soybean (Glycine max [L.I Merr.) was further investigated to define the structure-activity relationship and evaluate the effects of the cytokinins on yield parameters. Enhancement of pod set was found to be greatest with N-6 saturated alkyl substituted analogs, and was only weakly associated with activity in a callus growth bioassay. The response of yield parameters to increasing pod load was evaluated by applying various cytokinin analogs having a range of pod set enhancement activity. The increased pod load at the treated nodes was not compensated by a reduction in pod number on the remainder of the plant. However, there was a compensatory decrease in seed size.Overall, a significant trend to greater total seed weight per plant was associated with the increased pod number. Initial evaluations indicated that foliar applications of select cytokinins could temporarily increase pod number. However, the increases in pod number obtained with foliar treatments were too small to be of practical utility and were not maintained to maturity.In the companion paper (1), we describe the relationship of cytokinin flux to pod set in soybean, and the use of BA as an externally supplied synthetic cytokinin to alter pod set patterns. BA has previously been reported to increase pod number of soybeans under field conditions (4, 10), as well as to increase fruit set in other species (2, 7). The close association between cytokinin flux and pod set in soybean, and the ability to enhance pod set through exogenous applications of BA, make this system especially useful for studying the natural regulation of pod set.Very few cytokinin analogs have been evaluated for pod set enhancement activity, so little is known regarding the optimum chemical structure to obtain maximal response. A diversity of cytokinin analogs have been reported in the literature (9). The activity of these compounds has usually been tested in tissue or organ bioassays, such as callus growth. Whether the activity in those assays is related to the pod set enhancement activity for the same compounds is not known.In this paper we report on the results of evaluations comparing the pod set enhancing activity of naturally occurring cytokinins to that of BA. In addition, we characterized the relative activity of a number of synthetic cytokinins, and defined chemical attributes associated with high pod set enhancing activity. Finally, by utilizing several cytokinin analogs which varied in pod set en- Plants were grown in a greenhouse with a 13.5 h photoperiod maintained with supplemental metal halide lamps, which supplied a photosynthetic photon flux density of 300 ,uE m-2 s-' PAR, and temperature was regulated to 28°C day/22°C night.Seeds were inoculated with Bradyrhizobium japonicum (Nitragin, Nitragin Co., Milwaukee, WI) at the time of planting. Plants were thinned to one plant per pot at the unifoliate leaf stage (VI according to Fehr et al. [5]). All b...

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The growth characteristics of soybean (Glycine max [L.] Merr.) embryos in culture and seeds in situ were found to be similar, but developmental differences were observed. Embryos placed in culture when very small (<2 milligrams dry weight) failed to attain the maximal growth rates attained by embryos which were more mature when placed in culture. When nutrient levels were maintained in the culture medium, embryos continued to grow indefinitely, reaching dry weights far in excess of seeds matured in situ. Apparently, maternal factors were important in early and late development during the determination of maximum growth rate and the cessation of growth. Embryo growth rate was not affected by substituting glucose plus fructose for sucrose in the medium, nor by hormone treatments, including abscisic acid. Glutamine was found to give substantially better growth than glutamate, however. Contrary to prior reports, the response of soybean embryo growth rate to irradiance was found to be primarily an artifact of the effect of irradiance on media temperature. Across seven genotypes the correlation coefficient between seed growth rate in situ and embryo growth rate in vitro was 0.94, indicating essentially all of the variability of in situ seed growth rate between cultivars could be attributed to inherent growth rate differences associated with the embryos. The response to temperature was very similar for both embryos in culture and seeds in situ at temperatures below 30 degrees C. Beyond that temperature, embryo growth rate continued to increase, while seed growth rate did not. The implication is that in situ seed growth rate is determined by the inherent growth potential of the embryo at low to moderate temperatures; however, at higher temperatures, the maternal plant is unable to support the rapid growth rates that the embryo is capable of attaining under conditions of unlimited assimilate supply.

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