THE RELATION OF MIDDAY STEM WATER POTENTIAL TO THE GROWTH AND PHYSIOLOGY OF FRUIT TREES UNDER WATER LIMITED CONDITIONS

Reductions in rates of both vegetative and fruit growth, and reductions in other physiological parameters such as leaf conductance and photosynthesis, are all well known responses of plants to water limited conditions. One simple hypothesis to explain the overall integration of these responses at the whole plant level is that a reduction in the availability of soil water to the roots will ultimately lead to a reduction in the water potential of the plant as a whole, and hence that a corresponding reduction in the turgor pressure of cells will occur. A reduction in cell turgor may lead, directly or indirectly, to many of the well known plant physiological responses to water limited conditions. A key observation that does not support this simple hypothesis is that under some water limited conditions, reductions in growth and stomatal conductance can occur in the absence of any detectable reduction in the water potential of fully exposed, transpiring leaves. This observation however, does not take into account the possibility that individual leaf stomatal responses may mask reductions in whole plant water potential. By using midday measurements of water potential made on non-transpiring leaves (midday stem water potential) we have found a clear relation between water potential and a number of physiological responses, such as fruit growth and leaf photosynthesis, across a range of fruit tree species growing under field conditions. In almond, overall tree size after two years of growth was linearly related to the average midday stem water potential measured during the same period. A similar linear relation was observed between fruit size and the seasonal average midday stem water potential measured in pear trees. In prune trees, despite an overall linear relation between water potential and light-saturated photosynthesis at the individual leaf level, evidence was found that whole canopy photosynthesis may be relatively unaffected by moderate levels of water stress (midday stem water potential around -1.5 MPa). We have found that moderate water stress is associated with improvements in fruit quality and economic return in prunes, and are currently using midday stem water potential to guide the management of RDI (regulated deficit irrigation) in this crop.