To determine whether irrigation strategy altered the sensitivity of Citrus leaf gas exchange to soil, plant and atmospheric variables, mature (16-year-old) Fino 49 lemon trees ( Citrus limon (L.) Burm. fil. grafted on Citrus macrophylla Wester) were exposed to three irrigation treatments: control (irrigated with 100% of crop potential evapotranspiration, ET c), deficit irrigation (DI) and partial rootzone drying (PRD) treatments,which received 75% ET c during the period of highest evaporative demand and 50% ET c otherwise. Furthermore, to assess the physiological significance of root-to-shoot ABA signalling, the seasonal dynamics of leaf xylem ABA concentration ([X-ABA] leaf) were evaluated over two soil wetting-drying cycles during a 2-week period in summer. Although stomatal conductance ( gs) declined with increased leaf-to-air vapour pressure deficit (LAVPD), lower leaf water potential and soil water availability, [X-ABA] leaf was only related to stomatal closure in well irrigated trees under moderate (
