Ion distribution and gas exchange of hydroponically grown sunflower plants as affected by salinity

This paper reports the results of a trial carried out on sunflower plants (Helianthus annuus L., Romsun HS90) grown in the greenhouse using inert substrate and two automatic and closed hydroponic systems: one of them hosting the control (C) with plants grown under optimal conditions on Hoagland nutrient solution, the other one, the salt treatment (S), with plants exposed to constant salt stress through adding 150 mM of NaCl to the nutrient solution. Salt supply caused a sharp reduction in leaf area development and dry matter production, especially in the first 4 weeks when leaves showed to be more sensitive than stem and roots. Such a reduction is attributable to the drop in net CO2 assimilation rate, transpiration and stomatal conductance and it was, on average, equal to 30, 26 and 40%, respectively, with respect to the control. The investigated genotype was not able to exclude Cl- and Na+ and considerable amounts accumulated in leaves, stem and roots. Concentration increased in leaves in the basipetal direction. Though sunflower has an efficient endogenous adaptation system by which it redistributes ions in the whole plant, with greater accumulation in older leaves, growth inhibition could be attributed to specific ion toxicity effects, and of chlorine in particular, on metabolic processes and thus on photosynthesis