Tolerance responses of Brassica juncea to salinity, alkalinity and alkaline salinity

Soil salinity and alkalinity are common constraints to crop productivity in low rainfall regions of the world. These two stresses have been extensively studied but not the combined stress of alkaline salinity. To examine the effects of mild salinity (50mM NaCl) combined with alkalinity (5mM NaHCO3) on growth of Brassica juncea (L.) Czern., 30 genotypes were grown in hydroponics. Growth of all genotypes was substantially reduced by alkaline salinity after 4 weeks of stress. Based on large genotypic differences, NDR 8501 and Vaibhav were selected as tolerant and Xinyou 5 as highly sensitive for further detailed physiological study. Shoot and root biomass and leaf area of the selected genotypes showed greater reduction under alkaline salinity than salinity or alkalinity alone. Alkalinity alone imposed larger negative effect on growth than salinity. K+ and P concentrations in both shoot and root were significantly reduced by alkaline salinity but small difference existed among the selected genotypes. Leaf Fe concentration in Xinyou 5 decreased under alkaline salinity below a critical level of 50 mg kg(-1), which explained why more chlorosis and a larger growth reduction occurred than in NDR 8501 and Vaibhav. Relatively large shoot and root Na+ concentration also had additional adverse effect on growth under alkaline salinity. Low tissue K+, P and Fe concentrations by alkalinity were the major factors that reduced growth in the selected genotypes. Growth reduction by salinity was mainly caused by Na+ toxicity. Shoot Na+ concentration of NDR 8501 and Vaibhav was almost half those in Xinyou 5, suggesting NDR 8501 and Vaibhav excluded more Na+. However, Na+ exclusion was reduced by more than 50% under alkaline salinity than salinity in the selected genotypes. In conclusion, our results demonstrated that alkaline salinity reduced uptake of essential nutrients and Na+ exclusion that resulted in more negative consequences on growth than salinity alone.