Effect of water deficits on seed development in soybean : I. Tissue water status.

Water deficits during seed filling often decrease seed size in soybean (Glycine max L.). The physiological basis for this response is not known but may result from direct effects of low seed water potential (Psi(w)) on the seed filling process. To determine whether low Psi(w) occurred in reproductive tissues of soybean, we monitored the water status (Psi(w), Psi(s), and Psi(p)) of leaf, pericarp, and seed (embryo and testa) tissue of greenhouse-grown plants subjected to a brief water deficit during the linear period of seed growth. Water deficits were imposed by withholding water and monitored in the reproductive tissues by thermocouple psychrometry. When water was abundant, leaf, pericarp, and seed Psi(w) were -0.5 to -0.7 megapascal at midday. When water was withheld, leaf Psi(w) decreased to -2.3 megapascals within 6 days. Pericarp Psi(w) also decreased to -1.9 megapascal during this time. Pericarp Psi(s) followed the decline in Psi(w), but osmotic adjustment was not evident as the pericarp lost turgor completely by day 6. However, seed Psi(w), Psi(s), and Psi(p) were not significantly different from the controls. These results indicate that the water status of the developing seeds of soybean is not altered by short-term water deficits severe enough to inhibit the metabolic activity of the maternal plant. Maintenance of a favorable water status may be important for the conservation of seed growth rate exhibited by soybean under dry conditions.