Gas Exchanges and Stem Water Potential Define Stress Thresholds for Efficient Irrigation Management in Olive (Olea europea L.)

With climate change and decreased water supplies, interest in irrigation scheduling based on plant water status is increasing. Stem water potential (Ψ_SWP) thresholds for irrigation scheduling in olive have been proposed, however, a physiologically-based evaluation of their reliability is needed. A large dataset collected at variable environmental conditions, growing systems, and genotypes was used to characterize the relation between Ψ_SWP and gas exchanges for olive. Based on the effect of drought stress on the ecophysiological parameters monitored, we described three levels of stress: no stress (Ψ_SWP above about −2 MPa), where the high variability of stomatal conductance (g_s) suggests a tight stomatal control of water loss that limit Ψ_SWP drop, irrigation volumes applied to overcome this threshold had no effect on assimilation but reduced intrinsic water use efficiency (iWUE); moderate-stress (Ψ_SWP between about −2.0 and −3.5 MPa), where iWUE can be increased without damage to the photosynthetic apparatus of leaves; and high-stress (Ψ_SWP below about −3.5 MPa), where g_s dropped below 150 mmol m⁻² s⁻¹ and the intercellular CO₂ concentration increased proportionally, suggesting non-stomatal limitation to photosynthesis was operative. This study confirmed that olive Ψ_SWP should be maintained between −2 and −3.5 MPa for optimal irrigation efficiency and to avoid harmful water stress levels.