Photo-oxidative stress leading to oxylipin accumulation may trigger drought-induced leaf abscission in olive trees

Olive trees shed their leaves under severe drought as a defence mechanism. Foliar drought-induced abscission is a programmed process occurring in a differentiated cell layer at the base of the petiole. Considering the antioxidant properties of vitamin E and its interplay with lipid peroxidation-derived jasmonates in abiotic stress responses, we hypothesized about their possible role in abscission signaling by forming a jasmonates-increasing basipetal gradient along the leaf up to the abscission zone. We exposed young olives trees to water withdrawal for 21 days, after which five leaf sections, from the apex to the petiole, were sampled on both attached and detached leaves of irrigated and water-stressed trees. We found that prolonged drought stress resulted in a sharp reduction in the photosystem II efficiency, chlorophyll and vitamin E contents in leaves, leading to photo-oxidative stress, reflected by the increase in lipid peroxidation. In addition, the content of chloroplast-derived oxylipins and phytohormones, such as jasmonoyl-isoleucine and salicylic acid, increased. At the same time, a-tocopherol decreased in the petiole of water-stressed attached leaves, suggesting a conditioning for the abscission process to occur. Although no differences were observed in petioles from attached and detached leaves, the dropped ones showed higher oxidative stress in the leaf blade. It is concluded that redox signaling through oxylipins accumulation may trigger leaf abscission in drought-stressed olive trees. Mechanical stress is, however, additionally needed to execute leaf abscission once the abscission zone is properly conditioned.