Defence strategies of olive against water stress

The response of plants to water deficit varies with species. The olive tree is able to resist water stress by lowering the water content and water potentials of its tissues. This mechanism permits the plant to establish a high potential gradient between leaves and roots, and therefore to utilize soil water up to -2.5 MPa. In water stress conditions the olive tree stops shoot growth but not its photosynthetic activity: net assimilation at -6.0 MPa predawn leaf water potential is 10% that of well-watered plants. This allows the continued production of assimilates as well as their accumulation in various plant parts, in particular in the root system, creating a higher root/leaf ratio compared to well-watered plants. Active and passive osmotic adjustment play an important role in maintaining cell turgor and leaf activities which depend on it. Sugars, especially mannitol and glucose, play a major part in the osmotic adjustment of leaves. Organic acids, such as citric and malic, also play a role in active osmotic adjustment, whereas minerals do not seem to be involved. In addition, the osmotic adjustment observed in the root system allows maintenance of cell turgor, avoiding or delaying the separation of roots from soil particles. After a period of water stress and the following rewatering of the soil, olive, like other tree species, shows a period of inertia of leaf activity. This is not due to a lack of cell turgor but to other causes related to the hormone balance and to the conductivity of the xylem system. How fast leaf functionality is restored depends on the previously reached level of stress.