Growth and physiological response of hydroponically-grown sunflower as affected by salinity and magnesium levels

The effects of NaCl and magnesium levels (Mg2+) on the physiological response of sunflower were investigated. Plants were grown for 54 days in hydroponic culture with NaCl (100 mM) or without NaCl and four concentrations of Mg2+: 0, 0.4, 1.0 and 5.0 mM. At the end of the vegetative growing cycle of sunflower, salt stress reduced leaf area development by 51% and dry matter accumulation by 37% as compared to non saline-treated plants; at this stage, considering the percent reduction of partitioned plant dry matter, roots (42%) and leaves (35%) showed to be more salt-sensitive than stem. Growth reduction was related to the drop in net CO2 assimilation rate and stomatal conductance, which started declining later during the vegetative growth period when leaf ion concentration started increasing. The investigated genotype was unable to exclude ions and significant amounts of Cl− (about 1700 μmol g−1 DW) and lesser Na+ (700 μmol g−1 DW) accumulated in the leaves. The decline in net CO2 assimilation was well correlated to the increase in leaf Cl− concentration (r 2 = 0.71) and not to leaf Na+ concentration (r 2 = 0.33). The results suggest that, though sunflower develops an endogenous protection system by which it redistributes this ions in the whole plant, with more ions accumulating in roots and older leaves, growth reduction may be attributed to specific toxic effects of Cl− on photosynthetic functionality. In both saline and non saline conditions, little or no significant differences in growth parameters of plants exposed to a range from 0.4 to 5 mM of Mg2+ were observed. Whereas, its deficiency caused a drastic reduction of dry matter accumulation up to 90%, due to progressive decline in CO2 assimilation rate and chlorophyll content, with imbalances in Ca2+, Mg2+ and K+