Sucrose Concentration at the Apoplastic Interface between Seed Coat and Cotyledons of Developing Soybean Seeds

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Abstract
The apoplastic sucrose concentration at the interface between cotyledons and surrounding seed coats of developing soybeans was by 3 indirect methods in the range of 150-200 mm. This is an order of magnitude higher than was reported for soybean. It was also higher than the overall sucrose concentration in the cotyledons and seed coats, each of which was .apprx. 90 mm. By defoliating plants 24 h before measurement, both the overall sucrose concentration in the cotyledons and the interfacial apoplastic sucrose concentration were reduced by 3/4. There was no day/night difference in overall tissue sucrose concentration of cotyledons or seed coats from intact plants suggesting the existence of a homeostatic mechanism compensating for the diurnal photosynthetic cycle. About 7 h were required for a tritiated polyethylene glycol-900 solution to fully permeate developing cotyledons (from .apprx. 200 mm fresh wt embryo), implying high diffusion resistance through the tissue. A high interfacial sucrose concentration may exist in vivo. The saturable carrier-mediated component of sucrose uptake may be of little physiological significance in the outermost cell layers of the cotyledons.