Journal of Experimental Botany, Vol. 53, No. 368, pp. 489-503,
March 1, 2002
© 2002 Oxford University Press
Original Papers |
Growth-induced water potentials and the growth of maize leaves
College of Marine Studies and College of Agriculture and Natural Resources, University of Delaware, Lewes, DE 19958, USA
Profiles of water potential (
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
w extending upward along the transpiration path, and growth-induced
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
w were absent but the transpiration-induced
w remained. When leaves were growing, the growth-induced
w moved water into the elongating cells during the day and night, and it shifted with changes in transpiration-induced
w. The shift involved solutes accumulating in the growing region. When water was withheld, the growth-induced
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
w reappeared. If pressure was applied to the soil/root system to cause guttation and re-establish the growth-induced
w, elongation resumed immediately. These findings support the hypothesis that the primary control of growth is the disappearance and reappearance of the growth-induced
w because the potential changed in the xylem and nearby cells, blocking or permitting radial water movement and thus blocking or permitting growth.
Key words: Gradients, leaf elongation, osmotic adjustment, osmotic potential, turgor, Zea mays L.
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