Different Irrigation Regimes Induce Changes in Vessel Size in Olive Trees (Olea europaea L.) from Southern Italy

Olive tree is a Mediterranean, evergreen species adapted to a semi-arid environment. If moderately watered, it increases fruit and oil yields, while maintaining oil quality. However, in the Mediterranean area a rational management of irrigation is difficult, because of frequent water shortages, recently increasing in frequency and severity. To address this problem, several studies over the past decade have been conducted to evaluate the possibility of controlled deficit irrigation. In order to study long term effects of irrigation, dendrochronological and wood anatomical analyses on nine 18-year-old olive trees (cv. 'Nocellara del Belice') were carried out. Three trees for each treatment (rainfed, irrigated at 66% of ETc, and irrigated of 100% of ETc) were selected in a plantation nearby Benevento (southern Italy), and one cross-section from each tree was analyzed. Using standard dendrochronological methods annual rings were dated and anatomical analyses performed. Micro-sections were cut from each sample with a sliding microtome and then analyzed using an optical microscope. Significant differences were found between irrigated and rainfed olive trees. In particular, vessel size variability within a ring in the irrigated olive trees was very low, whereas in rainfed trees a clear change in vessels size between earlywood and latewood was observed. This suggests that cambial activity stopped after the formation of earlywood. Latewood cells were formed under different environmental conditions in comparison with earlywood. Vessels in irrigated olive trees were smaller and more abundant. To our knowledge, this is the first dendrochronological study conducted in an irrigated plantation of olive trees, showing useful information for water management.