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The effects of drought on seed germination and vigor were determined in soybean grown under field conditions. Drought conditions reduced yield, seed number and seed size by 13 to 64%. However, moisture stress did not affect seed germination and vigor. Defoliation treatments, meant to simulate premature plant death due to moisture stress, reduced seed size but had no effect on seed quality. The results suggest that soybean plants can be subjected to significant levels of moisture stress during seed filling without affecting seed germination or vigor.

Seed number is the primary yield component in soybean [Glycine max (L.) Merrill], but the mechanisms responsible for the regulation of this component are not fully understood. Three soybean cultivars (Elgin 87, Emerald, and Essex) with genetic differences in seed growth rate and seed size were grown in the field in 1995 and 1996 to determine the relationship between photosynthesis, sink characteristics, and seed number at individual nodes. The sixth node from the bottom of the main stem was isolated by heat girdling the stem below the node to disrupt phloem continuity and by removing the part of the plant above the girdled node when flowers first opened at this node. Photosynthesis was varied by defoliating (removing approximately 0, 33, 66, 83, or 100% of the leaf area) the leaf at the girdled node. Carbon dioxide exchange rate (CER) was measured at approximately weekly intervals for up to 40 d after girdling. Girdling temporarily reduced CER in four of six comparisons and defoliation tended to increase CER. Defoliation produced large differences in nodal carbon input (NCI) per node and seed number increased rapidly as average NCI per node increased from 0 to approximately 0.10 µmol [CO.sub.2] [node .sup.-1] [s.sup.-2]. However, there was no further increase in seed number as average NCI continued to increase to 0.5 µmol [CO.sub.2] [node.sup.-1] [s.sup.-2], suggesting that isolated nodes respond differently than soybean communities. Maximum seed number per node was inversely related to cultivar differences in individual seed growth rate and seed size. The girdled node technique should prove useful to investigate seed number-photosynthesis relationships in soybean., THE YIELD OF GRAIN CROPS, including soybean, is a function of the number of seeds produced per unit area and the average weight of the individual seed (seed size) (Shibles [...]

Soybean [Glycine max (L.) Merr.] yield can be limited by the activity of the source (i.e., canopy photosynthesis) or by the ability of the seed (i.e., the sink) to utilize the assimilate produced by the source. An experiment was conducted for 3 yr at Lexington, KY, (38 ° N latitude) to evaluate potential source-sink limitations of early and late maturing cultivars. Two early maturing cultivars (Kasota and Hardin, Maturity Group I) and two late maturing cultivars (Essex and Hutcheson, Maturity Group V) were planted in mid-May in 0.38-m rows and the plots were irrigated to minimize water stress. Shade cloth that reduced irradiance by 63% was placed over half of the plots from approximately growth stage R6 (early seed filling) to maturity. Shade always significantly reduced the individual seed growth rate (9-32%) indicating that the plants were source limited. Leaf starch levels during seed filling were monitored to evaluate potential sink limitations. Starch levels in the late maturing cultivars usually remained constant or declined during seed filling, providing little evidence of a sink limitation. Starch levels in the early cultivars increased approximately 6-fold in the early stages of seed filling in 1 of 3 yr, suggesting that, in that year, the plants were sink limited, i.e., the seeds could not use all of the available assimilate. Source limitations during seed filling are apparently common in soybean, but sink limitations seem to occur infrequently in environments where water is not limiting., Yield of a grain crop is the net result of the production of assimilate by leaves (the source) and translocation of these assimilates to the developing seed (the sink) where [...]