Contrasting response mechanisms to root-zone salinity in three co-occurring Mediterranean woody evergreens: a physiological and biochemical study

The present study investigated the extent to which physiological and biochemical traits varied because of root-zone salinity in three Mediterranean evergreens differing greatly in their strategies of salt allocation at an organismal level: the 'salt-excluders', Olea europaea L. and Phillyrea latifolia L. (both Oleaceae), and Pistacia lentiscus L., which, instead, largely uses Na⁺ and Cl^- for osmotic adjustment. Both Oleaceae spp. underwent severe leaf dehydration and reduced net photosynthesis and whole-plant growth to a significantly greater degree than did P. lentiscus. Osmotic adjustment in Oleaceae mostly resulted from soluble carbohydrates, which, in turn, likely feedback regulated net photosynthesis. Salt stress reduced the actual efficiency of PSII photochemistry (Φ_PSII) and enhanced the concentration of de-epoxided violaxanthin-cycle pigments in O. europaea and P. latifolia. Phenylpropanoid metabolism was upregulated by salt stress to a markedly greater degree in O. europaea and P. latifolia than in P. lentiscus. In contrast, species-specific variations in leaf lipid peroxidation were not observed in response to salinity stress. The results suggest that the species-specific ability to manage the allocation of potentially toxic ions out of sensitive leaf organs, other than affecting physiological responses, largely determined the extent to which leaf biochemistry, mostly aimed to counter salt-induced oxidative damage, varied in response to salinity stress.