Significance and limits in the use of predawn leaf water potential for tree irrigation

Research in estimating the water status of crops is increasingly based on p lant responses to water stress. Several indicators can now be used to estim ate this response, the most widely available of which is leaf water potenti al (Psi(LWP)) as measured with a pressure chamber. For many annual crops, t he predawn leaf water potential (Psi(PLWP)), assumed to represent the mean soil water potential next to the roots, is closely correlated to the relati ve transpiration rate, RT. A similar correlation also holds for young fruit trees grown in containers. However, exceptions to this rule are common whe n soil water content is markedly heterogeneous. Two experimental conditions were chosen to assess the validity of this corr elation for heterogeneous soil water content: 1) young walnut trees in spli t-root containers. The heterogeneity was created by two unequal compartment s (20% and 80% of total volume), of which only the smaller was irrigated an d kept at a moisture content higher than field capacity (permanent drainage ). 2) adult walnut trees in an orchard. In this case, soil water heterogene ity was achieved by limiting the amount of localised irrigation (20% of the irrigated control) which was applied every evening. Values of sap flux and of minimum and predawn leaf water potentials with ho mogeneous and heterogeneous soil water content were compared for trees grow n in the orchard and in containers. In spite of intense drought reflected b y very low RT or stem water potential, Psi(PLWP) of trees under heterogeneo us moisture conditions remained high (between -0. 2 and -0.4 MPa) both in t he orchard and in containers. These values were higher than those usually c onsidered critical under homogeneous soil conditions. A semi-quantitative m odel, based on the application of Ohm's analogy to split-root conditions, i s proposed to explain the apparently conflicting results in the literature on the relation between Psi(PLWP) and soil water potential.