Water status and gas exchange of pistachio trees under different irrigation levels

The aim of this study was to investigate the physiological responses of pistachio trees to varying water supply in the attempt to identify reliable parameters to manage irrigation. Three different irrigation regimes were applied to 25-year-old pistachio trees of cultivar 'Bianca' on P. terebinthus rootstock. Midday stem water potential (ΨSWP), leaf relative water content (RWC), light-saturated net CO2assimilation (An), stomatal conductance (gs), and ambient conditions were monitored throughout the season, and light response curves of net photosynthesis were constructed. To reflect tree water status, trees were separated into three classes of ΨSWP, no deficit (ΨSWP> -1.5 MPa), mild deficit (-2 ≤ ΨSWP≤ -1.5), and severe deficit (ΨSWP< -2 MPa). A weak correlation existed between ΨSWPand RWC due to high variation among RWC measurements. A relationship between ΨSWPand Anor gswas detected only during stage III of kernel filling, which was the most sensitive growth stage to water deficit. Well-watered trees had higher photosynthetic performance compared to mild and severe deficit trees. The results suggest that ΨSWPcan be regarded as a useful physiological indicator of tree water status to manage water supply, especially during critical periods of high atmospheric water demand and greatest sink strength in pistachio.