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Effects of drought and salinity on water status, growth and physiological activity of olive can be temporary or permanent and may impact olive production sustainability, especially in southern Mediterranean areas. Tunisia has a Mediterranean climate with high temperatures and low summer rainfall. Thus water stress problems are likely to be more severe for cultivated olive trees. In addition, the reduction in the availability of good quality irrigation water will increase the use of saline water. Olive trees are able to tolerate low soil water availability and quality and develop physiological adaptations to cope with water and salt stress. However, these adaptation strategies are limited and permanent damages can be observed. The permanent effects caused to olive leaves due to drought, and the capacity of irrigation with saline water to avoid these impacts is not well known. The aim of this work was to compare olive leaves performance after a severe drought with, and without, irrigation and to assess recovering capacities after a rainy period. Moreover, irrigation water quality effects were also evaluated. The results showed that, photosynthetic rate was very low for rainfed plants, during drought, as compared to irrigated ones. After re-watering, rainfed trees photosynthetic rate was only 55% of that of trees irrigated with fresh water. Irrigation with saline water (EC = 7.5 dS m−1) reduced drought impact by increasing photosynthesis by 55% but remained lower than that of fresh water by 23%. Thus olive leaves were unable to recover their whole photosynthetic capacity after being exposed to severe water or salt stress. Furthermore, young leaves had the same photosynthetic capacity at the beginning. This indicated that olive leaves lost permanently half of their photosynthetic activity during to drought without irrigation. The use of saline water reduced this gap to 23% as compared to fresh water.

The treated wastewater (TWW) reuse in crop irrigation is becoming a frequent practice in areas with limited water resources. This alternative water may be considered as nutrients source that can fill the water deficit. However, TWW contains high salts concentration and may negatively affect plant performances. The ability of short term irrigation with TWW in order to improve young olive (Olea europaea L. cv. Chetoui) performance was studied. One-year-old olive plants were subjected, for four months, to different concentrations (33, 50 and 100%) of TWW. As compared to the control olive plants, results showed that irrigation with TWW induced an enrichment of plant tissues with macro- and micronutrients. This enhancement significantly (p≤0.05) improved the photosynthesis activity, soluble sugars and thus plant growth. The irrigation with undiluted TWW was more efficient than diluted TWW in ameliorating the physiological performance of young olive plants. Therefore, TWW seems to be an interesting bio-fertilizer of young olive plant when applied for a short period.

The reuse of treated wastewater for olive irrigation is becoming a common practice in areas with limited water resources. However, this water may contain high level of salts (Na+ and Cl−) that could affect plant performances when used for a long period. In order to enhance the tolerance of plants to salt stress induced by treated wastewater, the application of arbuscular mycorrhizal fungi (AMF) may be a suitable solution. In this study, the ability of different AMF inoculums to improve young olive plants (Olea europaea L. cv. Chetoui) performances under long term irrigation with treated wastewater was studied. One-year-old olive trees inoculated with Glomus deserticola and/or Gigaspora margarita were irrigated with treated wastewater for one year. As compared to plants irrigated with tap water, treated wastewater irrigation caused a significant decrease in relative water content (RWC), total fresh and dry weights, gas exchange parameters, and chlorophyll and starch contents. Nevertheless, a significant increase in Na+ and Cl−, proline, soluble sugars, total polyphenols as well as flavonoids contents was observed under treated wastewater irrigation. Interestingly, colonization with different AMF inoculums, particularly the 1:1 mixture of G. deserticola and G. margarita, alleviated the negative effect of saline treated wastewater on young olive plants and significantly improved the above parameters. In fact, mycorrhizal symbiosis decreased the Na+ and Cl− contents and improved the RWC, the total fresh and dry weights and the photosynthetic activity. Furthermore, mycorrhizal plants showed higher concentrations of proline and soluble sugars as well as higher antioxidant defense systems as compared to the non-inoculated plants.

Plants are subjected to many stresses (salinity, drought, heavy metals, temperature, etc.) that may limit their growth. As a result, plants exhibit a variety of adaptive strategies to mitigate the adverse effects of abiotic stresses. Among those strategies, the accumulation of compatible solute such as proline is the most common defensive mechanism adopted. Indeed, proline plays a highly beneficial role as an osmolyte. In addition, proline helps to stabilize subcellular structures and acts as a metal chelator. Proline may also activate the cellular antioxidant system and scavenge the reactive oxygen species (ROS). Accordingly, application of exogenous proline was investigated on plants to induce tolerance. Therefore, under salt conditions, exogenous proline might alleviate growth inhibition and ameliorate photosynthetic activity, mineral nutrition, and water status. Furthermore, it may regulate osmotic potential and decrease the effect of toxic ions by activating the enzymatic antioxidant system. Exogenous proline applied under drought stress has successfully improved physiological performance. In these conditions, these applications increased proline accumulation in cells and therefore reduce cell osmotic potential and maintain turgor. This osmotic adjustment may help plants to improve their biomass. Finally, the beneficial role of exogenous proline was also proved in plants subjected to metal stress. This organic compound may reduce the phytotoxic effects of metals through the formation of proline-metal complexes and the stimulation of enzymatic and nonenzymatic antioxidant systems. Besides, exogenous proline may protect plants subjected to metal contamination by stabilizing membranes and thereby preventing electrolyte leakage, improving water status, photosynthetic activity, and growth rate.