Changes in plant–soil hydraulic pressure gradients of soybean in response to soil drying

Plant gas-exchange response to drying soil in many instances tracks a common pattern when expressed as a function of fraction of transpirable soil water (FTSW). There is little decrease in gas exchange until FTSW decreases to a value in the range of about 0.3-0.45, then with further drying gas exchange declines approximately linearly. This unique pattern is hypothesised to reflect mainly changes in the water potential gradient between bulk soil and plant. The primary objective was to directly document the basis of this response by measuring the hydrostatic pressure gradient required in the soil to maintain leaf xylem at zero potential with decreasing FTSW. Pots in which soybean (Glycine max) plants were grown were placed in a pressure chamber and the pressure adjusted to maintain zero water potential in a leaf petiolule. These results showed a small, relatively constant hydrostatic pressure had to be applied to the soil to maintain zero leaf xylem water potential until FTSW decreased to approximately 0.3-0.45. Thereafter, the required hydrostatic pressure gradient increased as FTSW continued to decrease. Hydraulic conductance was calculated to be relatively stable early in the drying cycle, and then decrease as the soil dried to comparatively high FTSW of 0.5-0.7.