Environmental effect on ABA concentration and water potential in olive leaves (Olea europaea L. cv "Koroneiki") under non-irrigated field conditions

How does the olive tree respond to environmental stress in the Mediterranean climate under non-irrigated field conditions with respect to leaf abscisic acid (ABA) concentrations and water status? To answer this question we determined simultaneously ABA concentration and water potential (psi(l)) in olive leaves (Olea europaea L. cv “Koroneiki”) during three successive years and related them to environmental parameters. The experiment was carried out in the orchard of the Agricultural University of Athens (Greece). The environmental parameters we followed were: (i) soil temperature (soil T) at a depth of 20 cm at 08:00 h on the sampling day; (ii) air temperatures, i.e. maximum (Max air T) and minimum (Min air T) on the previous day; (iii) relative atmospheric humidity at 08:00 h on the sampling day (RH (08:00)) and at 14:00 h on the previous day (RH (14:00)); (iv) weekly rainfall; (v) wind velocity on the previous day and (vi) solar radiation on the previous day. Similar patterns of change of ABA levels and psi(l), as well as environmental parameters were observed during both the non-bearing (NBC) and the bearing (BC) growth cycles. Low leaf ABA content was detected during autumn and spring periods. The olive tree tended to acclimatize to prolonged hot-dry periods by reducing the level of ABA at the end of summer, in spite of the low psi(l). During winter, leaf ABA content remained low, while psi(l) values were at their highest level. Old leaves were found to sustain lower psi(l) values than young ones, especially during the hot period. Regression analysis revealed statistically significant correlation between leaf ABA and psi(l), as well as between environmental parameters and ABA or psi(l) at P < 0.05 or at P < 0.01 (except for wind velocity). Concerning ABA content, our results revealed young olive leaves (NBC) to be more sensitive to most environmental parameters than old ones (BC). Water status of leaves was affected more strongly by most environmental parameters during the bearing than the non-bearing growth cycle. Temperatures affected psi(l) more directly than leaf ABA content.