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Osmotic Potential and Starch Accumulation in Leaves of Field-Grown Soybean¹

Although osmotic adjustment in response to a variety of factors has been observed in the leaves of numerous species, the mechanism by which it occurs is still unclear. It has been suggested that, in some cases, enlarging starch granules within leaf cells displace part of the symplasm and result in osmotic adjustment of the remaining osmotic volume. This study tested whether such a mechanism operated in the leaves of field-grown soybean [Glycine max (L.) Merr.]. Plants of the cv. UFV-1 were grown in the field on Arredondo fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult) and their uppermost, fully expanded leaves were sampled at dawn during midpodfill. In order to vary the amount of leaf starch, the plants were subjected to four treatments; (i) no shading, (ii) intermittent shade without pods, (iii) intermittent shade with pods, and (iv) continuous shading. Leaf osmotic potential was measured with a thermocouple psychrometer and starch concentration was determined for the same leaves by first hydrolyzing the starch and then measuring the resulting glucose. Naturally occurring light fluctuations were found to result in predawn osmotic adjustment, and increases in incident radiation resulted in an increase in starch and an associated decrease in osmotic potential. The changes in osmotic potential could only be partially explained as the result of a physical system in which the volume of starch varied and changed the osmotic volume.

Key Words: Glycine max L. Merr. • Water relations • Osmotic adjustment • Starch granules • Pod removal • Shade

² Research associate (current address: Res. Inst. Forestry and Landscape Planning, P.O. Box 23, 6700 AA Wageningen, The Netherlands); plant physiologist, USDA-ARS and adjunct professor, Agronomy Dep., Univ. of Florida, Gainesville, FL 32611, respectively.

Received for publication September 3, 1985.