Response of olive trees subjected to various levels of water stress

This paper reports the results of trials involving the cultivar "Coratina" grown in pots and subjected to various levels of stress. When predawn leaf water potentials reached -0.9 MPa water consumption per unit of leaf area was reduced by around 70% compared to controls but, unlike in other species, leaves continued to function even at predawn leaf water potentials of -6.0 MPa. Reduction of soil water content from field capacity to 40% of available water induced only a slight drop in leaf water potential. Leaf water potentials measured at midday are not a reliable index of plant water status in some species, particulary in those which conserve tissue water reserves by rapid stomatal closure. In olive however, since leaves continue to transpire even at very low soil water content, pre-dawn and at midday leaf water potentials are correlated. Tissue capacitance in olive is therefore much higher than in other species. High gradients of potentials between tissues, roots and soil are formed, facilitating use of moisture in the soil even at potentials below the permanent wilting point (-1.5 MPa).