Influence of the Soil Water Content and Distribution on Both the Hydraulic and Transpiration Performance of 'Manzanilla' Olive Trees

This work was made with mature 'Manzanilla' olive trees in an orchard of a semi-arid area in southern Spain. Three water treatments were considered: Rainfed, in which the trees had rainfall as the only source of water supply; FAO, in which the trees were under localized irrigation to replace the crop water demand, with some roots left in drying soil; Pond, in which the whole rootzones of the trees were maintained under non-limiting soil water conditions for the whole dry season. Our aim was to obtain information on the mechanisms behind the reduction of transpiration (E-p) in the FAO trees, as compared to the Pond trees. Our results show a near-isohydric behaviour of the FAO trees, i.e. those trees under localized irrigation in which some roots are left in drying showed lower stomatal conductance than the Pond trees in which all roots were in wetted soil. This helped the FAO trees to maintain similar leaf water potentials than the Pond trees. In addition, the FAO trees maintained a constant difference between the water potential of the canopy and that in the soil. This has been described as an isohydrodynamic behaviour, and it is thought to be an improvement over a typically anisohydric behaviour. These mechanisms were behind the similar values of tree hydraulic conductance (K-p) found in the FAO and Pond treatments. The Rainfed trees showed lower K-p values because of the low E-p values of those trees, due to the low soil water availability in that treatment. Our results show, however, that the Rainfed trees were able to maintain similar values of K-p all throughout the dry season, which shows that the hydraulic efficiency of the xylem of those trees was little affected by embolism, despite of the high demanding conditions in the area.