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 (Ψ_w) on the seed filling process. To determine whether low Ψ_w occurred in reproductive tissues of soybean, we monitored the water status (Ψ_w, Ψ_s, and Ψ_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 Ψ_w were −0.5 to −0.7 megapascal at midday. When water was withheld, leaf Ψ_w decreased to −2.3 megapascals within 6 days. Pericarp Ψ_w also decreased to −1.9 megapascal during this time. Pericarp Ψ_s followed the decline in Ψ_w, but osmotic adjustment was not evident as the pericarp lost turgor completely by day 6. However, seed Ψ_w, Ψ_s, and Ψ_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.