Physiological and Biochemical Remarks on Environmental Stress in Olive (Olea europaea L.)

Olive (Olea europaea L.) is an evergreen tree traditionally cultivated in the Mediterranean basin where plants undergo, to some degree, stress due to unfavourable environmental conditions. Water deficit, freezing, salinity and air pollution are a few of the stress factors restricting growth, so that olive productivity at the end of the growing season expresses only a fraction of the plant’s genetic potential. Understanding the physiological and biochemical processes that enable olive adaptation and acclimation, as well as the mechanisms of stress injury, is therefore of relevant importance. However, the studies on the physiology and biochemistry of stress resistance and on the role of gene expression and protein synthesis in olive are almost in an initial phase when compared with the progress achieved for other cultivated plants. In this paper, the physiological and biochemical studies performed on olive plants exposed to the main environmental stresses will be analysed and discussed. INTRODUCTION Olive (Olea europaea L.) is an evergreen tree traditionally cultivated in the Mediterranean basin where several environmental constraints are limiting factors for its productivity. Water deficit and salinity stresses are probably the main restrictions for olive cultivation in the Mediterranean climate, though olive behaves as an intermediate droughtand salt-tolerant species when compared with other temperate fruit trees. Low (freezing) temperature becomes a stressful factor when olive cultivation areas approach climatic limits, as usually happens in the northern and central Italian regions and also in some southern zones of Europe. In these areas, plant survival can be seriously compromised when temperature approaches -10oC. In the last decades, human activities have increased the number of environmental constraints that cultivated plants have to cope with and atmospheric pollution is probably the main one. Significant changes in the composition of atmosphere have been observed for several gases, such as for tropospheric ozone (O3) and carbon dioxide (CO2). These changes have opened exciting areas for environmental studies and several articles have been published on this subject. However, the effects of O3, and CO2 on olive physiology and biochemistry, especially when considering long-term realistic experiments, are still almost unexplored. Despite the relevant importance of olive resistance to environmental stress, the studies in this field are still limited when compared to the progress achieved in other cultivated species. In this paper, the physiological and biochemical studies performed on olive plants exposed to environmental stress will be analysed and discussed. WATER DEFICIT In Mediterranean climates water deficit is probably one of the main restrictions for olive cultivation, though olive behaves as an intermediate drought-tolerant species when compared with other temperate fruit trees. Olive leaves can, in fact, tolerate very low water potential (up to -6÷8 MPa) and lose almost 40% of tissue water, while maintaining a full rehydration capacity (Rhizopoulou et al. 1991). These adaptations enable olive Proc. 4 IS on Olive Growing Eds. C. Vitagliano & G.P. Martelli Acta Hort. 586, ISHS 2002 436 plants to establish a high water potential between leaves and roots and consequently to extract soil water up to –2.5 Mpa. Drought stress determines growth inhibition and slows down photosynthetic activity (Bongi and Palliotti 1994). However, olive plants still maintain a slight net assimilation (10% of well-water plants) rate at very low (–6.0 MPa) pre-dawn leaf water potential (Xiloyannis et al. 1999) and prevent excessive water loss by stomata closure (Fernandez et al. 1997; Giorio et al. 1999). Studies on the diurnal course of leaf water potential and gas exchange parameters in olive plants subjected to different levels of water deficit showed that the net photosynthetic rates and stomatal conductance reached maximum values early in the morning both in well-watered and in stressed plants, but declined more and faster in plants exposed to stress conditions. Moreover, the inactivation of photosynthetic activity during severe drought stress affected the non-stomatal component of photosynthesis and perhaps induced a light-dependent inactivation of the primary photochemistry associated with photosystem II (PSII) (Angelopoulos et al. 1996). The maintenance of a slight photosynthetic activity during drought stress enables olive plants to continue the production of assimilates and their accumulation in the root system, determining a higher root-leaf ratio in comparison with well-watered plants (Xiloyannis et al. 1999). These adaptations result in a better defence of the plants to drought conditions (Celano et al. 1999). Leaf activities and root contact with the soil particles depend on cell turgor, which can be regulated through active and passive osmotic adjustments. In water-stressed olive plants, mannitol and glucose have an important role in the active osmotic adjustments of leaves together with organic acids, such as malic and citric, while mineral elements are not involved (Xiloyannis et al. 1999). Rewatering of drought stressed olive plants induced a period of leaf activity inertia. This effect was probably dependent on hormone balance and conductivity of the xylem system. Studies on the short-term water use dynamics in olive trees after rewatering by heat-pulse measurements of the sap flux (Moreno et al. 1996) showed differences between regularly irrigated and non-irrigated trees. Following irrigation, the regularly irrigated plants maintained, for 3 days after irrigation, a transpiration rate of 1.65 mm mm d. Subsequent to this phase, the rate of water use declined and transpiration fell. The sap flow in the near-surface root dropped concomitantly. In nonirrigated plants, irrigation lifted the transpiration rate to only 1.12 mm mm d and leaf water potential did not recover because of plant inability to refill cavitated vessels. These data showed that, even after rewatering, olive behaves as a parsimonious and cautious consumer of soil water. Further studies on the physiology and biochemistry of olive plants during water deficit and rewatering are important to better understand the cellular and molecular mechanisms involved in olive plant resistance.