Fruit yield and quality responses of apple cvars Gala and Fuji to partial rootzone drying under Mediterranean conditions

Increasing irrigation water productivity is a major goal for fruit production in dry Mediterranean environments. The study was conducted in three consecutive years (2007-2009) under typical Mediterranean conditions and tested the effect of partial rootzone drying (PRD) on yield and fruit quality of two apple cultivars: Gala, maturing fruit in summer, and Fuji, maturing fruit in fall. Three irrigation treatments were imposed, conventional irrigation (CI), partial rootzone drying (PRD, 0.50 of CI water on one side of the rootzone, which was alternated periodically), and continuous deficit irrigation (DI, 0.50 of CI water on both sides of the rootzone). During the 2008 and 2009 irrigation seasons, DI reduced tree water status, and to some extent soil moisture, compared to CI and PRD. In all years and both cultivars, DI reduced crop load by 11 and 5 % over CI and PRD, respectively. In cvar Fuji, DI reduced production per tree by 9 % and yield efficiency by 16 % compared to CI. In all years for cvar Gala and in two of the three years for cvar Fuji, PRD and DI increased fruit soluble solid content by 5-6 %, whereas PRD improved peel color only in cvar Fuji and in two of the three years. In cvar Gala, DI fruit showed 27 % more sorbitol and 55 % more sucrose than PRD fruit. In both cultivars, PRD determined greater marketable yield and profit than DI. Irrigation water productivity (IWP) was increased by both PRD and DI, and in 'Fuji', PRD induced a 18 % greater IWP than DI. The different responses of the two cultivars to irrigation treatments can be attributed to differences in canopy size, crop load and mostly to the different timing of fruit growth. In particular, undergoing fast fruit growth during the irrigation period seems to induce permanent yield reductions in DI (but not PRD) trees of cvar Fuji, whereas water deficit during late fruit growth and lower crop load may have canceled the negative effect of DI in the smaller trees of cvar Gala.