Growth and water relations of field-grown Valencia orange trees under long-term partial rootzone drying

Climate, soil water potential (SWP), leaf relative water content (RWC), stem water potential (WPstem), stomatal conductance (gs), trunk, shoot and fruit growth of 'Valencia' orange trees were monitored during five consecutive seasons (2007–2012) to study water status and growth responses to irrigation placement or volume. 48 adult trees were exposed to conventional irrigation (CI, 100% of crop evapotranspiration on both sides of the rootzone), partial rootzone drying (PRD, 50% of CI water only on one alternated side of the rootzone) and continuous deficit irrigation (DI, 50% of CI water on both sides of the rootzone). Reducing irrigation volumes by 55% (DI) over CI increased leaf water deficit by 27% and reduced 'Valencia' fruit growth by 15% but not shoot or trunk growth. Similar water savings by PRD did not induce significant growth reductions. Differences in fruit growth rates determined 17% yield reduction in DI but not PRD trees. If we consider integrals of data across each season, PRD induced milder soil and leaf water deficit than DI but similar stomatal conductance. Tree daily water consumption (Etree) estimated from daily leaf transpiration was significantly lower in PRD and DI than in CI. Fruit growth efficiency (growth rate per unit Etree) was similar in all irrigation treatments, while shoot growth efficiency was higher in PRD than in CI. In PRD, an increased shoot growth efficiency rather than fruit growth efficiency is most likely due to water and assimilates being diverted from fruit to shoot growth under high VPD conditions. Although these results show good evidence of an irrigation placement effect inducing an advantage of the PRD strategy in 'Valencia' orange in terms of milder soil and leaf water deficit and more sustainable fruit growth compared to DI, PRD did not induce any significant advantage in terms of final yield over a simple reduction of irrigation volumes.