Salt stress modifies apoplastic barriers in olive (Olea europaea L.): a comparison between a salt-tolerant and a salt-sensitive cultivar

•We found that Frantoio and Leccino differentially avoid salt intrusion into the roots and translocation to the shoot.•We investigated the anatomical changes in olive roots under salt stress.•X-ray microanalysis of frozen-hydrated samples give information on roots ion selectivity.•We found genotypes-depended apoplastic adjustments in roots.

Olea europaea L. (Olive tree) is a glycophytic species showing differences in salinity response depending on the genotype and the cultivar. Despite several studies have demonstrated which olive cultivars are tolerant and which are sensitive to salt stress, the role of anatomical adjustment in olive roots under salinity has not been completely clarified by scientific evidence. To address this problem one-year cuttings of olive cultivar Leccino (salt-sensitive) and Frantoio (salt-tolerant), were grown in an aeroponic system under controlled condition. Plants were sprayed (5 s of spray every 15 min-intervals) with the nutrient solution with and without 120 mM NaCl for 40 days. Free hand section of roots were taken and stained with 0.01% Fluorol yellow 088 to allow suberin lamellae visualization under a fluorescence microscope. Moreover, Cryo-SEM and energy dispersive X-ray microanalysis were used for ion localization in root cells. Both treated cultivars with Na+ showed a significant reduction in shoot length, besides that, Na+ concentration in leaves increased only in Leccino (salt-sensitive) as confirmed by translocation factor. Microscopic analyses showed that apoplastic barriers in the endodermis developed closer to the root apex in Leccino compared to Frantoio. Moreover, a significant Na+ gradient concentration from exodermis to stele tissues between genotypes and cell types has been detected and it resulted stronger in Leccino than in Frantoio. In conclusion the different apoplastic adjustments in roots play a role in reducing ion fluxes to the shoots depending on genotype and Na+ concentration.