Will carbon isotope discrimination be useful as a tool for analysing the functional response of barley plants to salinity under the future atmospheric CO₂ conditions?

The objective of this study was to determine the response of barley's carbon isotope composition and other physiological parameters to the interaction of salt stress and elevated CO2 levels, and the usefulness of carbon isotope discrimination (Δ(13)C) as indicative of the functional performance of barley (Hordeum vulgare L.). Barley plants were grown under ambient (350 μmol mol(-1)) and elevated (700 μmol mol(-1)) CO2 conditions and subjected to salt stress (0, 80, 160, and 240 mM NaCl) for 14 days. Elevated CO2 levels increased biomass production, water use efficiency and the photosynthetic rate, although this parameter was partly acclimated to elevated CO2 levels. Salt stress decreased this acclimation response because it enhanced the sink strength of the plant. Elevated CO2 significantly decreased the (13)C isotopic composition (δ(13)C) in all plant organs; however, the ratio of δ(13)C between the root and the leaf was increased, indicating a higher allocation of δ(13)C to the below-ground parts. Conversely, salt stress increased plant δ(13)C, showing differences between plant organs. From the strong correlations between Δ(13)C and biomass production, the photosynthetic rate or water use efficiency both at ambient and elevated CO2, we concluded that Δ(13)C is a useful parameter for evaluating leaf and whole plant responses to salinity and can provide an integrated index of processes to understand the mechanisms underlying salt tolerance of barley both under current and future environmental CO2 conditions.