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.

A comprehensive characterization of water stress is needed for the development of automated irrigation protocols aiming to increase olive orchard environmental and economical sustainability. The main aim of this study is to determine whether a combination of continuous leaf turgor, fruit growth, and sap flow responses improves the detection of mild water stress in two olive cultivars characterized by different responses to water stress. The sensitivity of the tested indicators to mild stress depended on the main mechanisms that each cultivar uses to cope with water deficit. One cultivar showed pronounced day to day changes in leaf turgor and fruit relative growth rate in response to water withholding. The other cultivar reduced daily sap flows and showed a pronounced tendency to reach very low values of leaf turgor. Based on these responses, the sensitivity of the selected indicators is discussed in relation to drought response mechanisms, such as stomatal closure, osmotic adjustment, and tissue elasticity. The analysis of the daily dynamics of the monitored parameters highlights the limitation of using non-continuous measurements in drought stress studies, suggesting that the time of the day when data is collected has a great influence on the results and consequent interpretations, particularly when different genotypes are compared. Overall, the results highlight the need to tailor plant-based water management protocols on genotype-specific physiological responses to water deficit and encourage the use of combinations of plant-based continuously monitoring sensors to establish a solid base for irrigation management.