Arsenic Application Changed Growth, Photo-Synthetic Pigments and Antioxidant Enzymes Activity in Sorghum (Sorghum bicolor L.) Under Salinity Stress

The elevation of arsenic (As) content in soils is of considerable concern with respect to its uptake by plant and subsequent entry into wildlife and human food chains. The treatment of sorghum seedlings with As as NaH₂As₄O. 7H₂O at various concentrations (A₁ = 0, A₂ = 20, A₃ = 40 and A₄ = 60 mg As kg⁻¹ soil) and salinity at four different levels (S₁ = 0, S₂ = 3, S₃ = 6 and S₃ = 9 dS m⁻¹) reduced fresh and dry weights of sorghum plants. The co-application of As and salinity increased the guaiacol peroxidase (GPX) activity in shoot and root tissues. The highest GPX activity in shoot and root tissues was obtained at S₂A₄ and S₃A₃ treatments, respectively. The activity of catalase (CAT) in shoot was not changed, but unlike the GPX activity, salinity and As decreased the CAT activity in root tissues. Concerning the photosynthesis pigments, salinity had no effect on the chlorophyll ‘a’, chlorophyll ‘b’ and carotenoid content in leaves, but the As treatment significantly decreased the content of both chlorophyll types. Salinity increased the anthocyanin content in leaves. There were negative correlation between soluble carbohydrates (r² = −0.78**) and stomata conductance (r² = −0.45**) and dry weight of the plant biomass in this study. By increasing the salinity and As concentration in root medium, soluble carbohydrate in leaves increased but salinity decreased the leaf stomata conductance.