Variation in leaf starch and sink limitations during seed filling in soybean

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 degrees 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.