Source-sink relationships, seed sucrose levels and seed growth rates in soybean

Field experiments using two soybean (Glycine max L. Merrill) cultivars ('Elgin 87' and 'Essex') were conducted for 2 years near Lexington, KY, USA to evaluate the effect of source-sink alterations on seed carbohydrate status and growth. Sucrose concentrations in developing cotyledons of control plants were consistently low ( < 50 mM) early in seed development, but they increased to 100-150 mM by physiological maturity. The concentrations increased in both years by 47 to 59 % when 90 % of the pods were removed from 'Elgin 87', but the increase had no effect on individual seed growth rate (SGR), Shading (80 %) reduced cotyledon sucrose levels and SGR in both years. The critical cotyledon sucrose concentration (the concentration providing 80 % of the maximum cotyledon growth rate) was estimated from in vitro cotyledon growth at sucrose concentrations of 0-200 mM. These critical concentrations varied from 72-124 mM: in planta control cotyledon sucrose concentrations were below this critical level during the first half of seed growth but exceeded it in the later stages of growth in all experiments. The estimated critical concentration was consistent with the failure of in planta SGR to respond to an increase in assimilate supply and with the reduction in SGR associated with a decrease in assimilate supply. The results suggest that soybean SGR is generally sink limited if photosynthesis increases during seed filling, but source limited if photosynthesis is reduced. (C) 2001 Annals of Botany Company.