Behavior of Four Olive Cultivars During Salt Stress

Environmental conditions may strongly impact plant crop growth (Kachaou et al., 2010; Feller and Vaseva, 2014; Pandolfi et al., 2017). In particular, abiotic constraints, such as drought, soil salinity and extreme temperatures, which cause water depletion in cells, are responsible for a large proportion of losses in agricultural productivity (Bose et al., 2014). In order to overcome water shortages and to satisfy the increasing water demand for agricultural development, the use of water of low quality (brackish, reclaimed, drainage) that frequently has an high salinity level is becoming important in many countries (Chartzoulakis, 2005). In particular, plants under high salinity conditions are subject to significant physiological and biochemical changes, for example a marked decrease in photosynthesis rate and transport of salt ions from roots to shoots (Ben Ahmed et al., 2009; Anjum et al., 2011; Singh and Reddy, 2011; Goltsev et al., 2012; Abdallah et al., 2018). A major biochemical alteration, also induced by other types of stress, is the production of reactive oxygen species (ROS) (Gill and Tuteja, 2010; Boguszewska and Zagda?ska, 2012; Ozgur et al., 2013; Bose et al., 2014). An excess of ROS leads to lipid peroxidation, inhibition of enzymes, and modifications of nucleic acids (Proietti et al., 2013; Bose et al., 2014; Tedeschini et al., 2015). Under stress conditions, plants can nonetheless develop tolerance, that is, the ability to adequately survive, and often prosper, under an unfavorable environment, following a robust production of antioxidant enzymes (Ben Ahmed et al., 2009; Bhaduri and Fulekar, 2012; Keunen et al., 2013). Among these enzyme, superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GSH) are localized in chloroplasts and mitochondria (Pang and Wang, 2010; Del Buono et al., 2011; Proietti et al., 2013), whereas catalase (CAT) and guaiacol peroxidase (GPX) are generally present within microbodies and cytosol, respectively (Bhaduri and Fulekar, 2012; Hameed et al., 2013; Nath et al., 2016). Mechanistically, tolerance may also include osmotic adjustments at cellular level (Ayala-Astorga and Alcaraz-Meléndez, 2010). Some plants implement this process by increasing the amount of solutes and lowering the water potential of root cells, thereby counteracting the water outflow. These substances, reported as osmolytes, can accumulate in large amount, but do not generally interfere with enzymatic activities and cytoplasmic pH, due to their zwitterionic nature. Osmolytes commonly used by plants are sugars, alcohols, quaternary amines, betaine, glycine and proline (Warren, 2014). In this regard, the concentration of proline in leaves and roots was reported as a response by the olive tree to saline stress (Ayala-Astorga and Alcaraz-Meléndez, 2010; Hayat et al., 2012; Iqbal et al., 2014; Abdallah et al., 2018). In fact, proline facilitates water retention in the cytoplasm and, therefore, its concentration is indicative of response to saline stress (Ben Ahmed et al., 2009; Gupta and Huang, 2014). Cultivated olive (Olea europaea subsp. europaea var. europaea) is a long-living, evergreen, thermophilic species. In the Mediterranean basin where olive is mostly cultivated salinity is becoming a relevant problem due to high rates of evaporation and insufficient leaching (Mousavi et al., 2019). In addition in costal areas the need for water of good quality for urban use is increasing while there is a large amount of low quality water mostly saline (EC > 2.0 dS m-1) that can be use for irrigation (Chartzoulakis, 2005). Olive is considered as a moderately salt tolerant plant and the tolerance appear to be cultivar dependent (Rugini and Fedeli, 1990). The olive crop counts a very rich varietal herita