Water relations of apple trees (Malus x domestica Borkh., cv. Gala) under partial rootzone drying irrigation

Abstract

Single-leaf water relations, fruit growth and quality were monitored from 2001-2003 for field-grown apple (Malus x domestica Borkh. cv. 'Gala') trees. Trees were subjected to three irrigation treatments: 1) well-watered Control, 2) (DI50) 50 % of irrigation volume applied the same way, and 3) (PRD50) 50 % irrigation volume applied to half the surface area of the Control. In all years, average fruit size, yield and quality were not affected by irrigation treatments. Gas exchange was very low, irrespective of treatments, however leaf water potential (\p) did not attain levels previously suggested to close stomata. Leaf abscisic acid (ABA) concentration did not differ among treatments. Stomata of apple leaves responded sensitively to the vapor pressure deficit (VPD) of the atmosphere regardless of soil moisture status. Water use efficiency (WUE) was not increased by deficit irrigation, but regulation of gas exchange by VPD allowed apple orchards to be irrigated at approximately 30 % of reference evapo-transpiration (ETo). In 2003, a split-rooted containerized system was developed by approach-grafting, one-year-old 'Gala' apple trees. Four irrigation treatments were imposed in 2004 over a 30-day experimental period: Control (>100 % of ETc) applied to both containers; PRD100 (>100 % ETc) applied to one container only; and two treatments receiving 50 % ETc applied to either one (PRD50) or both containers (DI50). Both PRD treatments had water alternated between containers when a threshold soil water content was attained. Monitoring of sap flow under fully irrigated conditions showed the dependency of root sap flow on root cross-sectional area. Previously dried roots responded rapidly to irrigation events. Both 50 % ETc treatments experienced similar declines in predawn ψ (ψpd) and single-leaf gas exchange, relative to Controls. Leaf ABA concentration was similar for PRD50 and DI50, and was significantly higher than both >100 % ETC treatments. The PRD100 and Control had similar ψpd, however gas exchange was reduced in PRD100 by 30 % compared to Controls. Leaf ABA concentration was higher in PRD100 than Controls, but could not unequivocally account for the down-regulation of PRD100 stomata. As similarly reported in field experiments, WUE was not increased with deficit treatments.