Effect of Stem Excision Under Water on Bulk Leaf Water Potential, Leaf Conductance, CO₂ Assimilation and Stemwood Water Storage in Eucalyptus behriana F. Muell

Abstract

Cutting the stem, under water and above the lignotuber, of intact trees of Eucalyptus behriana caused rapid increases in the bulk leaf water potential (Ψ) in the canopy above. In eight separate experiments the mean increase in Ψ of 3.1 MPa in 1 h was associated with rapid and persistent decreases in leaf conductance (g), which have been ascribed to hydropassive changes in leaf turgor.

The water content of the stemwood increased by 1.8-2.0% as the bulk water potential increased by approximately 2.5 MPa. In the first hour after cutting the stems under water, the observed rate of water uptake by the cut stem was approximately six times the estimated transpiration rate of the leaves. This discrepancy is attributed to an increase in stem water content. The specific capacitance of the stemwood was estimated to be 0.008 g g^-1 (dry weight) Mpa^-1. The quantity of water available from storage in the stem was estimated to be less than 0.1 mm.

Continuous measurements of gas exchange on a leaf before, during and after stem-cutting showed that CO₂ assimilation decreased in parallel with g, but in a manner that caused increases in intercellular CO₂ concentration. This indicated that the rapid increases of bulk leaf turgor, induced by stem-cutting, affected photosynthetic capacity.