Hardening of Olea europaea var. sylvestris seedling by application of water and humidity ambiental stress treatments

The purpose of this study was to examine the effect of irrigation and air humidity on the water relations, growth and survival of Olea europaea var. sylvestris during the nursery phase, in order to evaluate the degree of hardening resulting from these conditions. Seedlings of O. europaea were pot-grown (PVC-14 cm ¿) in two greenhouses. In one of the greenhouses the air humidity was controlled by a dehumidifying system. In each greenhouse half of the plants were irrigated four days per month to 100% water-holding capacity and the other half two days per month for four months. After the nursery phase, the plants were transplanted into PVC-22 cm ¿ pots placed outside the greenhouses and then watered by a drip irrigation system one day per week for one month, after which they received no irrigation until the end of the experiment. The combined effect of both hardening treatments (deficit irrigation and low air humidity) increased the rate of survival by around 63% compared with the control (0%). Low air humidity and deficit irrigation reduced all growth parameters. However, while the water deficit increased the number of stomata per plant, this parameter was not affected by low air humidity. Water stress induced tissue dehydration. The water-stressed plants developed foliar and root osmotic adjustment, but not to a sufficient degree to maintain turgor pressure. The effect of low air humidity on leaf water potential and its components was less marked. However, despite the fact that no osmotic adjustment was developed by these plants, leaf turgor potentials were higher, suggesting that low-air humidity preconditioning might trigger an elastic adjustment in this species. As a conclusion, in the conditions studied, both culture methods showed their validity as acclimatization processes and improved the post planting survival rate of seedlings.