Leaf water relations and gas exchange response to water deficit of olive (cv. Cobran‡osa) in field grown conditions in Portugal

To evaluate the effects of changes in water stress and variable evaporative demand on water potential (I), relative water content (RWC) and gas exchange during two consecutive years (2005 and 2006) providing insight into the mechanisms of stomatal control in the little-studied cultivar 'Cobran double dagger osa'. Measurements of water potential, gas exchange in olive trees subjected to three irrigation treatments: rainfed (T0), continuous deficit irrigation (T1) and well irrigated (T2) that received 0, 30 and 100 % of estimated crop evapotranspiration. Both the differences in irrigation amounts and the prolonged summer drought conditions characteristic of the study area (Northeast Portugal) resulted in considerable divergences among treatments, with T2 showing the highest values of both I, RWC and gas exchange rates and T0 the lowest. Stomatal conductance exhibited a strong down-regulation under increasing vapor pressure deficit (VPD), with water stress leading to parallel reductions in stomatal conductance values at any given VPD and sensitivity to changes in VPD. These effects might be associated to the deduced drought-induced declines in leaf specific hydraulic conductance (K (L)). The results indicated that Cobran double dagger osa olive trees have a near-isohydric behavior with a tendency to maintain a constant root-to-leaf water gradient (Delta Psi).