The Relationship between Stem Diameter and Water Potentials in Stems of Young Cabbage Plants

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Abstract
Continuous measurements were made of stem shrinkage, stem water potential (ψ3) and transpiration rate (T) in young, pot-grown cabbage plants subjected to cycling evaporative demands. Sequences of increasing evaporative demands induced increases in T and decreases in both ψ3 and stem diameter and conversely, whenever evaporative demand decreased, T declined and ψ3 and stem diameter rose. Over short periods, stem water potentials and stem shrinkage were virtually parallel even when rapid oscillations were induced. Over longer periods the effects of growth were important compared with those of water stress on stem diameter when the moisture content of the soil was high. Growth, however, ceased when the plant was subjected to relatively mild water stress (ψ3 = −0.4 MPa). Stem diameters, after correction for growth, were linearly related to plant water potential. The results suggest that stem shrinkage and only a few calibration measurements might be used to provide continuous estimates of water potentials in field crops.

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