Effects of summer pruning on the water status and physiology of olive trees and on fruit characteristics and oil quality

An investigation was carried out in 2017 in central Italy, in an adult olive orchard with trees of the cultivar Leccino, spaced 5 × 5 m, trained to a vase and pruned biennially. The aim was to evaluate the effects of summer pruning (i.e. eliminating suckers, in the central part of the canopy, in the non-pruning year), on the water status and physiology of the leaves, on the yield/tree, and on fruit characteristics and oil quality. Unpruned trees were used as the control. During the investigation (July-October), rainfall was low (less than 100 mm) and the orchard was not irrigated. During the summer, the leaves of summer-pruned trees had higher relative water content (RWC) and greater photosynthetic activity than those of control trees. Summer-pruned trees had similar yield, but higher oil content on a dry-weight basis. Oil qualitative parameters (free acidity, peroxide value and spectrophotometric absorbances in the ultra-violet) were similar, whereas the total content of phenolic compounds was higher in control trees. The oils from summer-pruned trees were more herbaceous and fruity, and less bitter and spicy than oils from control trees. The results show that in situations where there is considerable formation of suckers, such as in the case of biennial, triennial or plurennial pruning, summer pruning can contribute to improving tree water status and the physiological condition, favoring photosynthetic activity. This practice also has effects on fruit oil content and oil quality, producing more aromatic and less bitter and spicy oil. Overall, the results indicate that summer pruning can have effects similar to irrigation.