Partial rootzone drying increases water-use efficiency of lemon Fino 49 trees independently of root-to-shoot ABA signalling

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, ETc), deficit irrigation (DI) and partial rootzone drying (PRD) treatments, which received 75% ETc during the period of highest evaporative demand and 50% ETc 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 (g(s)) 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 (<2.5 kPa) atmospheric vapour pressure deficit (VPD). Differences in [X-ABA](leaf) were not detected between treatments either before or immediately after (<12 h) rewatering the dry side of PRD trees. Leaf water potential was higher in control trees, but decreased similarly in all irrigation treatments as daily LAVPD increased. In contrast, DI and PRD trees showed lower stomatal sensitivity to LAVPD than control trees. Although DI and PRD decreased stomatal conductance and photosynthesis, these treatments did not significantly decrease yield, but PRD increased crop water use efficiency (WUE) by 83% compared with control trees. Thus PRD-induced enhancement of crop WUE in a semiarid environment seems to involve physiological mechanisms other than increased [X-ABA](leaf).