Growth‐induced water potentials and the growth of maize leaves

Abstract

Profiles of water potential (Ψ_w) were measured from the soil through the plant to the tip of growing leaves of fully established maize (Zea mays L.). The profiles revealed gradients in transpiration‐induced Ψ_w extending upward along the transpiration path, and growth‐induced Ψ_w extending radially between the veins in the elongating region of the leaf base. Water moving upward required a small gradient while that moving radially required a much larger gradient primarily because the protoxylem vessels were encased in many small, undifferentiated cells that were likely to act as a barrier to radial flow. Upon maturation, these small cells enlarged and some began to conduct water, probably decreasing the barrier. In the mature leaf, the growth‐induced Ψ_w were absent but the transpiration‐induced Ψ_w remained. When leaves were growing, the growth‐induced Ψ_w moved water into the elongating cells during the day and night, and it shifted with changes in transpiration‐induced Ψ_w. The shift involved solutes accumulating in the growing region. When water was withheld, the growth‐induced Ψ_w disappeared and leaf elongation ceased even though turgor pressure was at its highest. Turgor was maintained by osmotic adjustment that doubled the osmotic potential of the elongating cells. If elongation resumed at night or with rewatering, the growth‐induced Ψ_w reappeared. If pressure was applied to the soil/root system to cause guttation and re‐establish the growth‐induced Ψ_w, elongation resumed immediately. These findings support the hypothesis that the primary control of growth is the disappearance and reappearance of the growth‐induced Ψ_w because the potential changed in the xylem and nearby cells, blocking or permitting radial water movement and thus blocking or permitting growth.