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 (Olea europaea L.) trees in response to high salinity (NaCl 100 mM) and elevated CO2 (eCO2) concentration (700 μL L−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 (ACO2) at eCO2 than at ambient CO2, but this difference disappeared under salt stress. Growth and ACO2 of 'Picual' did not respond to eCO2 regardless of salinity treatment. Stomatal conductance (gs) and leaf transpiration were decreased at eCO2 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 ACO2, gs, and WUE, but internal CO2 concentrations in the mesophyll were not affected. eCO2 increased the sensitivity of PSII and chlorophyll concentration to salinity. eCO2 did not affect leaf or root Na+ or Cl− concentrations in salt-tolerant 'Picual', but eCO2 decreased leaf and root Na+ concentration and root Cl− concentration in the more salt-sensitive 'Koroneiki'. Na+ and Cl− accumulation was associated with the lower water use in 'Koroneiki' but not in 'Picual'. Although eCO2 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 eCO2.