Elevated CO2 improves assimilation rate and growth of tomato plants under progressively higher soil salinity by decreasing abscisic acid and ethylene levels

High salinity in the soil hampers crop performance. In this context, elevated atmospheric CO2 concentration can alter plant responses to excess salinity, as shown by studies exposing plants to high concentration of NaCl (osmotic shock) in controlled conditions under constant irradiance and temperature. These experiments, however, may not reflect the conditions that occur in natural environments. Here, we studied the impact of progressively imposed salt stress on tomato plant biomass, hormone biosynthesis and primary metabolism under elevated CO2 concentration with fluctuating irradiance and temperature. Na+ concentration and Na+: K+ ratios in plant tissues were increased by high salinity under both ambient and elevated CO2 concentration. Elevated CO2 led to enhanced growth in tomato plants under salt stress through stimulation of photosynthesis and reduced concentrations of abscisic acid and the ethylene precursor 1-aminocyclopropane-l-carboxylic acid in both leaves and roots. Furthermore, whereas high salinity reduced the concentration of Krebs Cycle intermediates and increased the concentration of photorespiratory metabolites glycine and serine, all were restored to control levels by elevated CO2. Our results demonstrate a beneficial effect of rising atmospheric CO2 concentration for crops and reveal new interactions in the physiological network controlling salinity stress responses.