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The ability of exogenous compatible solutes, such as proline, to counteract salt inhibitory effects in olive plants ( Olea europaea L. cv. Chemlali) was investigated. Two-year-old olive trees were subjected to different saline water irrigation levels supplied or not with exogenous proline. Leaf water relations (relative water content, water potential), photosynthetic activity, and leaf chlorophyll content decreased under either saline water level. The proline supplement mitigated the reduction of growth and photosynthetic activity under salt stress, and the mitigating effect of proline was different among treatments. The increment rate of leaf relative water content (RWC) in the presence of 25 and 50 mM proline was 4.45 and 6.67%, respectively, in comparison to values recorded in SS1-treated plants (plants irrigated with water containing 100 mM NaCl). In SS2 (200 mM NaCl) plus proline-treated plants, this increase was 1.14 times for 25 mM proline and 1.19 times for 50 mM proline higher than those recorded in severe salt stress treatment (SS2). In response to salt stress, Chemlali olive plants seem to activate a complex antioxidative defense system that was displayed via the increase of activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) and the decrease of polyphenol oxidase (PPO) under either salt stress treatment. The exogenous application of proline improved the antioxidative enzyme activities of salt-stressed olive plants. Indeed, in young or old leaf tissues, the highest levels of these antioxidant enzymes activities were recorded in (SS2 + P2)-treated plants (plants irrigated with water containing 200 mM NaCl plus 50 mM proline). In young leaves, this increase was 2.11, 2.96, and 2.76 times, respectively, for SOD, APX, and CAT enzyme activities in comparison to their respective activities in control plants (nonstressed plants irrigated with fresh water). In old leaves, this increase was 2, 2.41, and 2.48 times, respectively, for the various enzymes. If compared to high water salinity-treated plants (SS2), this increase was 1.1, 1.3, and 1.4 times in young leaves, respectively, for SOD, APX, and CAT activities. From these results, the proline supplements seem to improve olive salt tolerance by amelioration of some antioxidative enzyme activities, photosynthetic activity, and, so, plant growth and the preservation of a suitable plant water status under salinity conditions. More to the point, the decrease of soluble sugars contents in proline treated-plants revealed the important osmoprotectant effect played by the added proline in such a way that limited the need of salt-stressed plants for soluble sugars synthesis.

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The ability of exogenous compatible solutes, such as proline, to counteract salt inhibitory effects was investigated in 2-year-old olive trees (Olea europaea L. cv. Chemlali) subjected to different saline water irrigation levels supplied or not with exogenous proline. Leaf water relations [relative water content (RWC), water potential], photosynthetic activity, leaf chlorophyll content, and starch contents were measured in young and old leaves. Salt ions (Na(+), K(+), and Ca(2+)), proline and soluble sugars contents were determined in leaf and root tissues. Supplementary proline significantly mitigated the adverse effects of salinity via the improvement of photosynthetic activity (Pn), RWC, chlorophyll and carotenoid, and starch contents. Pn of young leaves in the presence of 25 mM proline was at 1.18 and 1.38 times higher than the values recorded under moderate (SS1) and high salinity (SS2) treatments, respectively. Further, the proline supply seems to have a more important relaxing effect on the photosynthetic chain in young than in old leaves of salt-stressed olive plants. The differential pattern of proline content between young and old leaves suggests that there would be a difference between these tissues in distinguishing between the proline taken from the growing media and that produced as a result of salinity stress. Besides, the large reduction in Na(+) accumulation in leaves and roots in the presence of proline could be due to its interference in osmotic adjustment process and/or its dilution by proline supply. Moreover, the lower accumulation of Na(+) in proline-treated plants, compared to their corresponding salinity treatment, displayed the improved effect of proline on the ability of roots to exclude the salt ions from the xylem sap flowing to the shoot, and thus better growth rates.