Effects of sodium sulfate and sodium bicarbonate on the growth, gas exchange and mineral composition of lettuce

The responses of two butterhead lettuce (Lactuca sativa L.) cultivars 'P' and 'L-2' were investigated under Na2SO4 and NaHCO3 salinity stress. The influences of salinity on gas exchange and distribution of mineral composition in plants were determined. The salinity treatments were applied through a nutrient solution containing 0, 20, 40 and 60 mM Na2SO4 or 0, 2.5, 5 and 7.5 mM NaHCO3. With increasing concentration of Na2SO4 or NaHCO3, leaf area, shoot dry weight, leaf length and leaf width decreased, photosynthetic rate and stomatal conductance also diminished. NaHCO3 was more toxic to lettuce shoot growth than Na2SO4. The decrease in water uptake per plant implied osmotic stress existed under Na2SO4 salinity, while in the NaHCO3 experiment, water uptake was not affected but pH of the nutrient solution increased markedly. Leaf necrosis appeared in both cultivars at Na2SO4 concentrations above 40 mM, and leaves were chlorotic at NaHCO3 concentrations above 5 MM. In both experiments, leaf injury symptoms were more severe in 'L-2' plants than in 'P' plants. The concentrations of K and Ca decreased, whereas Na concentration increased, as Na2SO4 concentration increased. The Na concentration in the root was greater than in the shoot of 'P' plants, whereas the trend was just the reverse in 'L-2' plants. The K concentration in the shoot decreased and Na concentration increased with increasing NaHCO3 concentration. Shoot Ca only decreased in 'L-2' plants. The growth reduction under Na2SO4 stress may be due to the combined effects of osmotic stress and excess accumulation of Na, while in the NaHCO3 experiment, the growth reduction may be related to the HCO3- toxicity and high pH rather than water stress or excessive Na. The cultivar 'L-2' was more sensitive to Na2SO4 and NaHCO3 salinity than the cultivar 'P'. (C) 2003 Elsevier B.V. All rights reserved.