Salt and drought stress in olive tree (Olea europaeaL.): an integrated approach.

Olive tree is a widespread cultivated tree among Mediterranean area. Despite this, Mediterranean climate is not the best for its cultivation, basically for drought and salinity environmental problem. The study of physiological, biochemical and molecular response to salinity in olive fruit it has been studied in salt-sensitive Leccino cultivar, able to translocate Na+ to aerial part. In our experimental conditions, results shown a stress response concentrated in early veraison stage (<50% of purple skin). In fact, irrigating with saline water from pit-hardening to harvest, Na+ concentration reach the highest values in fruit flesh of Maturation group 1 (MG1, <50% of purple skin). Particularly, up to 600 mg Kg -1 when subjected to 60 mM NaCl irrigation water and about 2214 mg kg-1 when is irrigated with 80 mM NaCl water. With the advance of ripening, Na+ concentration in flesh is reduced, suggesting a re-translocation in vegetative portion to permit a suitable accumulation of oil droplets in cells. Also antioxidant response is MG-related, showing an increase of DPPH%, PAL activity, Total Phenols and Anthocyanins content (respectively, +15%, +16%, +58% and around doubled). Anatomical traits under salt stress reveal a thickening in all fruit tissue layers (Cuticle, epidermis, Hypodermis and Outer Mesocarp), that could be useful for biotic agent protection. Fatty acid profile of Triacyl-glicerol (TAGs) under salt stress record major changes in MG1, showing an increment of oleic acid (+5%), a decrement of linoleic acid (-2.36%) and a consequent increment of oleic/linoleic ratio and a related transcriptional down-regulation of FAD6 genes. Low Electron Transport Rate (ETR) between photosystems recorded in the leaves closest to the infructescence could allow to a more oxidized state of those complexes, causing a reduction in the activity levels of the FAD6 enzyme. Also the up-regulation of FAD2-2 seems to have a role in salt stress response, mainly for maintaining plasmatic membrane stability. Anyway, total fatty acid richness fraction is not affected by salt stress. Simultaneous salt and drought stress response it has been studied in cultivars Leccino and Frantoio, differentially-sensitive to both stress.Water status of simultaneous stressed plants were monitored with non-destructive innovative methodologies (Zim-probes), revealing the typical inversion phenomena of Pp curve shape due to an unfavorable air/water ratio typical of plant’s osmotic injuries. Physiological and biochemical data shown that simultaneous drought and salt stress didn’t impair growth, total plant fresh and dry weight and Na+ concentration in Leccino leaves (6359 mg kg-1), although water loss in lysimeters recorded by wheighing (assumed as effective evapo-transpiration) at the end of experiment is influenced (-62% in Leccino vs -53.5% in Frantoio, respect to control). Anyway, Na+ content on a whole-plant basis is reduced by water scarcity (-47.6% in Leccino vs -31.6% in Frantoio), explaining the lack of effect on growth and biomass allocation. Major effects of simultaneous drought and salt stress is focused in leaves: non-structural carbon partitioning toward mannitol (39 mg g-1 in Leccino vs 22 mg g-1 in Frantoio on a dry weight basis) and cations stechiometrical ratios (K/Na ratio 22 times higher in Frantoio and Ca/Na ratio 11 times higher in Frantoio). PCA analysis help to conclude that in simultaneous drought and salt stress conditions only at leaves level is evident a cultivar related response, mainly for higher K/Na and Ca/Na ratios level in leaves.