Drought-Adaptive Mechanisms of Young Sweet Cherry Trees in Response to Withholding and Resuming Irrigation Cycles

The present work evaluates the main adaptive mechanisms developed by young sweet cherry trees (Prunus avium L.) to cope with drought. For this purpose, the young trees were subjected to two drought cycles with different water stress intensities followed by a recovery period. Three irrigation treatments were applied: control treatment (CTL) irrigated to ensure non-limiting soil water conditions; moderate water stress (MS) subjected to two drying cycles whose duration was dependent on the time elapsed until the trees reached values of midday stem water potential (psi(stem)) of -1.3 and -1.7 MPa for the first and second cycle, respectively; and severe water stress (SS) similar to MS, but with reference values of -1.6 and -2.5 MPa. In-between drought cycles, MS and SS trees were irrigated daily as the CTL trees until reaching psi(stem) values similar to those of CTL trees. The MS and SS trees showed an important stomatal regulation and lower vegetative growth. The decreasing leaf turgor potential (psi(turgor)) during the drought periods accounted for 40-100% of the reduction in leaf water potential at midday (psi md). The minimum osmotic potential for mature leaves was about 0.35 MPa lower than in well-irrigated trees. The occasional osmotic adjustment observed in MS and SS trees was not sufficient to maintain psi(turgor) values similar to the CTL trees or to increase the specific leaf weight (SLW). The leaf insertion angle increased as the water stress level increased. Severe water stress (psi(stem) < -2.0 MPa) resulted in clear early defoliation as a further step in water conservation.