The olive tree (Olea europaea L.) is commonly grown in the Mediterranean basin and is able to resist severe and prolonged drought. Levels of proline (PRO) and malondialdehyde (MDA), and the lipoxygenase (LOX) activity were determined in 2-year-old olive plants (cv. 'Coratina') grown in environmental conditions characterized by high temperatures and high photosynthetic photon flux density levels and gradually subjected to a controlled water deficit for 20 days. Before and during the experimental period, leaf and root samples were collected and analysed for PRO and MDA. The levels of PRO increased in parallel with the severity of drought stress in both leaves and roots. Significant increases of LOX activity and MDA content were also observed during the progressive increment of drought stress in both leaf and root tissues. Measurements of transpiration and photosynthetic rate, stomatal conductance and substomatal CO(2) concentration were carried out during the experiment. The accumulation of PRO indicates a possible role of PRO in drought tolerance. The increases of MDA content and LOX activity show that the water deficit is associated with lipid peroxidation mechanisms.

The effect of high levels of natural light on leaf photosynthesis in olive trees {Olea europaea L. van Coratina), grown in pots outdoors in the summer and subjected to water, stress, was studied. Net photosynthetic rates reached maximum values early in the morning in both control and stressed plants and subsequently declined gradually. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics of the upper intact leaf surface. The maximum fluorescence yield {Fp) and the ratio Fv/Fp decreased at midday especially in water-stressed plants, but the initial fluorescence {Fo) rose to a maximum value at midday and declined again in the afternoon. In control plants the values of maximum fluorescence Fp and the ratio Fv/Fp increased again in the afternoon and had recovered almost completely by 8 p.m. as the leaf water potential recovered. In stressed plants this diurnal recovery was not complete, so that the photosynthetic rates and the ratio Fv/Fp declined gradually during the development of water stress. These results indicate that in olive trees subjected to severe water stress the non-stomatal component of photosynthesis was affected and perhaps a light-dependent inactivation of the primary photochemistry associated with photosystem II (PSII) occurred. Four to five days after rewatering severely stressed plants, the predawn leaf water potential, net photosynthetic rates and chlorophyll fluorescence indices recovered only partially.

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The effects of drought on the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD), indoleacetate oxidase (IAAox) and polyphenol oxidase (PPO) were studied in 2-year old Olea europaea L. (cv. ‘Coratina’) plants grown under high temperatures and irradiance levels and gradually subjected to a controlled water deficit. After 20 d without irrigation, mean predawn leaf water potential fell from –0.37 to –5.37 MPa, and decreases in net photosynthesis and transpiration occurred. The activities of SOD, APX, CAT and POD increased in relation to the severity of drought stress in both leaves and roots. In particular, a marked increase in APX activity was found in leaves of plants at severe drought stress. CAT activity increased during severe water deficit conditions in leaves and fine roots. The patterns of POD and IAA oxidase activity ran in parallel and showed increases in relation to the degree of drought. In contrast, PPO activity decreased during the progression of stress in all the tissues studied. The results show that the ability of olive trees to up-regulate the enzymatic antioxidant system might be an important attribute linked to drought tolerance. This could limit cellular damage caused by active oxygen species during water deficit.

Abstract.Olive trees (Olea europaea L.) are commonly grown in the Mediterranean basin where prolonged droughts may occur during the vegetative period. This species has developed a series of physiological mechanisms, that can be observed in several plants of the Mediterranean macchia, to tolerate drought stress and grow under adverse climatic conditions. These mechanisms have been investigated through an experimental campaign carried out over both irrigated and drought-stressed plants in order to comprehend the plant response under stressed conditions and its ability to recover. Experimental results show that olive plants subjected to water deficit lower the water content and water potentials of their tissues, establishing a particularly high potential gradient between leaves and roots, and stop canopy growth but not photosynthetic activity and transpiration. This allows the continuous production of assimilates as well as their accumulation in the various plant parts, so creating a higher root/leaf ratio if compared to well-watered plants. Active and passive osmotic adjustment due to the accumulation of carbohydrates (in particular mannitol and glucose), proline and other osmolytes have key roles in maintaining cell turgor and leaf activities. At severe drought-stress levels, the non-stomatal component of photosynthesis is inhibited and a light-dependent inactivation of the photosystem II occurs. Finally, the activities of some antioxidant enzymes involved in the scavenging of activated oxygen species and in other biochemical pathways increase during a period of drought. The present paper provides an overview of the driving mechanisms adopted by olive trees to face drought stress with the aim of better understanding plant-soil interactions.