Water Stress Gradients in Plants and Soil-Root Systems1
- H. C. De Roo2
The plant moisture stresses (PMS) in leaves, shoots, and roots of tobacco were determined as the negative hydrostatic xylem sap pressure in a pressure bomb.
In the field, PMS in the midribs of leaves varied with the atmospheric stress and showed a reasonable diurnal course, while PMS of the adventitious roots did not change diurnally. Thus on a hot, sunny day during a dry period a steep PMS gradient between leaves and roots developed, which throughout the night gradually diminished. During the cool, humid early morning hours, leaf and root PMS were about the same. A comparison of root PMS with soil moisture stress, however, revealed an unexpected, steep, inverted gradient.
In the greenhouse, small pot-grown plants were observed to verify the field observations. The differences in equilibrated sap pressures or PMS of complete shoots and intact soil-root systems clearly depended upon the relative rates of water absorption and water loss. In dry Merrimac sandy loam with soil moisture stresses (SMS) higher than about 12 bars, root PMS were lower than SMS, as had been observed in adventitious roots in the field. In moist soil with SMS lower than 12 bars, this situation reversed and a normal moisture stress gradient from soil to root existed.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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