An ecophysiological analysis of salinity tolerance in olive

Olive (Olea europaea L.) is an important fruit-tree crop species widely distributed in marginal lands of the Mediterranean basin, and, hence, usually exposed to excess soil salinity concentration. Mechanisms of response to salinity stress have been explored in a relatively recent past, and basically involve (1) the salt-exclusion from the shoot, which is also supported by a reduced water-mass flow and inherently low relative growth rates; (2) the large use of the osmoprotectant mannitol to partially counter salt-induced declines in leaf osmotic potential. These mechanisms of response result in drastic reductions in net assimilation rate. and, hence, in biomass production, and are of adaptive value in olive under salinity stress, not a mere consequence of it. We suggest that survival, not growth performance under unfavorable conditions, is a key determinant conferring salt-tolerance in olive, which is faced to fluctuating soil salinity concentration during the growing season. The "low-Na+ strategy" adopted by olive to cope with excess soil salinity unlikely positively relates with biomass production during the stress period, but allows a rapid recovery of plant performance when good-quality water is available to the roots. The "low-Na+ strategy" is generally enhanced by increasing the root-zone Ca2+ concentration, but this key issue of salinity tolerance has been poorly investigated in olive, despite this species mostly inhabit calcareous soils. Actually, few data available show that high-Ca2+ supply enhances the ability of olive plants to recover net photosynthesis and whole-plant growth rate during a period of relief from salinity, due to the effect of Ca2+ in preserving actively growing and old leaves from a massive accumulation of potentially toxic ions during the stress period. Finally, we note that salt-treated olive under natural conditions are additionally exposed to "excess-light" stress, as the usage of sunlight irradiance in photosynthetic processes is drastically reduced by stomatal and mesophyll limitation to CO2 diffusion in the leaf. We discuss on major biochemical adjustments, namely changes in mannitol, violaxanthin-cycle pigment, and flavonoid concentrations, mostly aimed at countering the oxidative damage because of the concerted action of salinity stress and high sunlight. These adjustments have been recently suggested to protect salt-treated leaves in full sunshine from heat stress-induced oxidative damage to a greater extent than leaves growing under partial shading. These "unexpected" findings, which are of great significance in the ecology of O. europoea, open new and intriguing perspectives in the response mechanisms to salinity stress in this species.