Can elevated CO2 improve salt tolerance in olive trees?

We compared growth, leaf gas exchange characteristics, water relations, chlorophyll fluorescence, and Na+ and Cl- concentration of two cultivars ('Koroneiki' and 'Picual') of olive (Oleo europoea L.) trees in response to high salinity (NaCl 100 mM) and elevated CO2 (eCO(2)) concentration (700 mu LL-1). The cultivar 'Koroneiki' is considered to be more salt sensitive than the relatively salt-tolerant 'Picual'. After 3 months of treatment, the 9-month-old cuttings of 'Koroneiki' had significantly greater shoot growth, and net CO2 assimilation (A(CO2)) at eCO(2) than at ambient CO2, but this difference disappeared under salt stress. Growth and A(CO2) of 'Picual' did not respond to eCO(2) regardless of salinity treatment. Stomatal conductance (g(s)) and leaf transpiration were decreased at eCO(2) such that leaf water use efficiency (WUE) increased in both cultivars regardless of saline treatment. Salt stress increased Leaf Na+ and Cl- concentration, reduced growth and leaf osmotic potential, but increased Leaf turgor compared with non-salinized control plants of both cultivars. Salinity decreased A(CO2), g(s), and WUE, but internal CO2 concentrations in the mesophytt were not affected. eCO(2) increased the sensitivity of PSII and chlorophyll concentration to salinity. eCO(2) did not affect leaf or root Na' or Cl- concentrations in salt-tolerant 'Picual', but eCO(2) decreased leaf and root Na+ concentration and root Ct- concentration in the more salt-sensitive 'Koroneiki'. Na' and Cl- accumulation was associated with the tower water use in 'Koroneiki' but not in 'Picual'. Although eCO(2) increased WUE in salinized Leaves and decreased salt ion uptake in the relatively salt-tolerant 'Koroneiki', growth of these young olive trees was not affected by eCO(2). (C) 2007 Elsevier GmbH. All rights reserved.