Deficit irrigation does not affect alternate bearing of ‘Valencia’ orange trees

This study evaluated the effect of long-term partial rootzone drying and continuous deficit irrigation on the degree of alternate bearing of adult ‘Valencia’ orange trees grown in northern Sicily. Three irrigation strategies were imposed in summer from 2007 to 2011: irrigation with volumes corresponding to 100% of crop evapotranspiration applied to entire rootzone (CI), partial rootzone drying (PRD) with 50% of CI water applied to one alternated side of the rootzone, and continuous deficit irrigation (CDI) with 50% of CI water applied to both sides of the rootzone. The experiment was conducted on 48 adult orange trees arranged according to a randomized block design. Weather and soil parameters were monitored continuously in the field. Plant water status and shoot growth were monitored regularly throughout the seasons. Fruit weight and number were recorded every year at harvest. Alternate bearing index (ABI) was calculated as a measure of the intensity of alternation in each tree in terms of yield, number of fruits, and kilos of fruit per unit of irrigation water applied (IWP). All trees in trial exhibited the typical alternation of biennial bearing. As a general effect, CDI significantly reduced leaf relative water content (RWC), stomatal conductance and fruit size and yield, but not fruit number. PRD significantly reduced stomatal conductance, but induced intermediate RWC and yield, between CI and CDI. A negative but non-linear relationship was found between shoot growth rate and fruit yield. In spite of any reduction of leaf hydration and yield, deficit irrigation did not affect ABI, and trees under all treatments exhibited a similar degree of alternation in terms of yield (ABI = 0.39), number of fruits (ABI = 0.44) or IWP (ABI = 0.40). These results show that biennial bearing of ‘Valencia’ orange trees is not related to water supply at the tested levels, suggesting that it may not depend on any possible change in carbohydrate reserves or abscisic acid production generated by water deficit.