Seasonal Changes of Hydraulic Conductance of Mature Olive Trees under Different Water Regimes

The well known stomatal control of olive trees under stressing conditions proved to be an efficient mechanism to prevent the loss of hydraulic efficiency during the dry season. Trees with a localized irrigation system that replaced 100% of the crop water demand (FAO trees), as well as Dryfarming trees, were able to keep similar differences between the water potential in the soil and that in the leaves, all throughout the dry season. The differences were of the same order than those in trees with non-limiting soil water conditions in the whole rootzone (Pond trees). As a consequence of stomatal closure, the tree transpiration late in the season was reduced in the FAO trees, but the hydraulic conductance remained unaffected. Results suggest that a root-to-shoot signalling mechanism could have been responsible for stomata control in the FAO trees. At the end of the dry season, values of hydraulic conductance were lower in Dryfarming than in the irrigated treatments, but, once again, the recorded leaf water potential values, when compared to vulnerability curves, suggest that the low hydraulic conductance in those trees was mainly due to a reduced tree transpiration caused by stomatal closure, rather than to a severe loss of the hydraulic efficiency.