Osmotic adjustment and the inhibition of leaf, root, stem and silk growth at low water potentials in maize

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Abstract
The expansion growth of plant organs is inhibited at low water potentials (Ψ w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The Ψ w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the Ψ w in the region of cell expansion decreased in each organ. The Ψ w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these Ψ w and ceased only when Ψ w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the Ψ w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the Ψ w was lower in the expanding regions than in the mature tissues, indicating that there was a Ψ w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a Ψ w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because Ψ w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular Ψ w changed about the same amount as the soil Ψ w. Therefore, the gradient in Ψ w favored water uptake by roots but not leaves at low Ψ w. The data show that expansion growth responds to low Ψ w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of Ψ w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the Ψ w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and Ψ w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.