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Increasing atmospheric CO₂ concentration is expected to impact agricultural systems through a direct effect on leaf gas exchange and also due to effects on the global availability of good-quality water as a result of climate warming. Thus, the planning of land use for agriculture requires new tools to identify the capability of current cultivars to adapt to growth restrictions under new ambient conditions. We hypothesized that salinity stress may produce a specific pattern of carbon isotopic composition (δ¹³C) in tomato (Solanum lycopersicum L.) at elevated CO₂ concentration ([CO₂]) that could be used in the breeding of salinity tolerance in a near-future climate scenario. Five commercial tomato cultivars were evaluated at elevated (800μmolmol⁻¹) or standard (400μmolmol⁻¹) [CO₂], being irrigated with a nutrient solution containing 0, 60 or 120mM NaCl. The biomass enhanced ratio, leaf net CO₂ assimilation and stomatal conductance, leaf NO₃ ⁻ and Cl⁻ concentrations and leaf free amino acid profile were analyzed in relation to the pattern of δ¹³C, under different saline stress conditions. The results indicate that at high [CO₂]: (i) salinity tolerance was enhanced, but the response was strongly cultivar dependent, (ii) leaf NO₃ ⁻ concentration was increased whilst Cl⁻ and proline concentrations decreased, and (iii) leaf δ¹³C was highly correlated with plant dry matter accumulation and with leaf proline concentration, leaf gas exchange and ion concentrations. This study shows that δ¹³C is a useful tool for the determination of the salinity tolerance of tomato at high [CO₂], as an integrative parameter of the stress period, and was validated by traditional physiological plant stress traits.
