Photosynthetic performance and biochemical adjustments in two co-occurring Mediterranean evergreens, Quercus ilex and Arbutus unedo, differing in salt-exclusion ability

The responses to mild root zone salinity stress were investigated in two co-occurring Mediterranean woody evergreens, Quercus ilex L. and Arbutus unedo L., which differ in morpho-anatomical traits and strategies to cope with water deficit. The aim was to explore their strategies to allocate potentially toxic ions at organism level, and the consequential physiological and biochemical adjustments. Water and ionic relations, gas exchange and photosystem II performance, the concentration of photosynthetic pigments, and the activity of antioxidant defenses, were measured. Quercus ilex displayed a greater capacity to exclude Na+ and Cl- from the leaf as compared with Arbutus unedo, in part as a consequence of greater reductions in transpiration rates. Salt-induced reductions in CO2 assimilation resulted in Q. ilex suffering from excess of light to a greater extent than A. unedo. Consistently, in Q. ilex effective mechanisms of nonphotochemical quenching, also sustained by the lutein epoxide-lutein cycle, operated in response to salinity stress. Q. ilex also displayed a superior capacity to detoxify reactive oxygen species with respect to A. unedo. Our data suggest that the ability to exclude salt from actively growing shoot organs tightly depends on metabolic cost sustaining leaf construction, i.e., species-specific leaf life-span, and the relative strategies to cope with salt-induced water stress.We discuss about how contrasting abilities to restrict the entry and transport of salt in sensitive organs relate with species-specific salt tolerance.