Abstract

The protective role of silicon in relation to growth vigor of flag leaf, membrane characteristics (%), antioxidant enzymes, lipid peroxidation, membrane stability index (%) and membrane leakage, as well as non-enzymatic antioxidants and non-photosynthetic pigment contents were investigated in alkalinity stressed sorghum plants. In the majority of cases, alkalinity stress induced significant reduction (p ≤ 0.05) in leaf area and degree of succulence (non-significant increase in case of sensitive cultivar) of both sorghum cultivars during grain-filling. On the other hand, alkalinity stress induced a clear increase (p ≤ 0.05) in the degree of leaf sclerophylly of tolerant cultivar and a non-significant increase in case of sensitive cultivar. Alkalinity stress reduced bio-membranes stability through increasing its lipid peroxidation resulting in an increase in membrane leakage (ML) with a simultaneous decrease in membrane stability index (MSI) in leaves of both sorghum cultivars. Moreover, it was obvious that alkalinity significantly increased the activity of catalyse, ascorbic acid oxidase (AAO) and peroxidase (POD) activities and induced a non-significant reduction in polyphenol oxidase (PPO) activity in leaves of both sorghum cultivars comparing with the control plants. Among cultivars, tolerant one showed higher enzymes activity than sensitive one. Application of silicon markedly increased AAO and POD activities as well as a non-significant decrease in PPO activity in leaves of alkalinity stressed sorghum plants. Alkalinity stress caused a non-significant increase (p ≤ 0.05) in the amount of total phenols and flavonoids as well as non-photosynthetic pigment content (anthocyanin, β-carotene and lycopene contents) in flag leaf of both cultivars. In addition, application of silicon induced an additional increase (p ≤ 0.05) in total phenols and flavonoids as well as anthocyanin and β-carotene contents except for lycopene content which induced a non-significant decrease. These results suggest that the exogenous application of silicon assisted the plants to become more tolerant to alkalinity stress induced oxidative damage by up-regulating the membrane characteristics and enhancing their antioxidant defense system.