Recurrent deficit irrigation and fruit harvest affect tree water relations and fruitlet growth in ‘Valencia’ orange

Background - Partial rootzone drying is an irrigation strategy known for increasing water use efficiency without significantly affecting tree water status. 'Valencia' oranges have a very long development period and nearly mature fruit and new fruitlets may be present at the same time on the tree, competing for water and assimilates. Objectives - The present study investigates the effect of recurrent deficit irrigation and fruit harvest on tree water status and fruitlet growth of 'Valencia' orange. Methods - Forty-eight adult trees were exposed to three irrigation treatments for seven years (2007-2013): irrigation with 100% of ETc (CI), continuous deficit irrigation (DI, 50% of CI) and partial root-zone drying (PRD, 50% of CI on alternated sides of the root-zone). In spring 2014, stem water potential (Psi(stem)) and continuous measurements of sap flow and fruitlet growth were recorded before (May) and after (June) the harvest of mature fruit. Results - No differences in Psi(stem) were found among irrigation treatments, while Psi(stem) was lower in June than in May at midday. In both May and June, sap flow density (not sap flow per tree) was higher in DI than in Cl and PRD trees suggesting more efficient water uptake/transport in the former. In May, DI and PRD fruit showed lower daily relative growth rate (RGR) than CI fruit due to a possible shortage of carbon and nutrients. After removing mature fruits, differences among irrigation treatments were canceled. Sap flow was directly related to fruit RGR at low sap flow rates, but inversely related to RGR at high sap flow rates. Conclusions - Our data show that the presence of maturing fruit does not impact the water status of 'Valencia' trees, while it may transiently limit fruitlet growth (by source limitation) in deficit irrigated trees.