Dose dependent rhizospheric Ni toxicity evaluation: Membrane stability and antioxidant potential of Vigna species
Different concentrations of Ni are found in agricultural soil released from various environmental sources. The responses of plants grown in such soil-substratum vary accordingly depending upon the concentration of metal present and plant species. To assess the toxic effects of Ni and extent of plant defensive strategies, increasing Ni doses (50, 100, and 150 mg kg-1) were used in sandy-loam soil-substratum at pH 7.9 to evaluate the performance of Vigna species (V. cylindrica [L.] Skeels, V. mungo [L.] Hepper, and V radiata [L.] R. Wilczek). The experiment was conducted in a complete randomized design. Nickel stress was induced by adding various concentrations of Ni chloride in soil substratum. Malondialdehyde (MDA) and antioxidant levels were determined in roots and leaves. Escalating levels of Ni in soil resulted in an affirmative relationship between MDA with that of antioxidants. A dose-dependent increase in the activity of Superoxide Dismutases (SOD), Catalase (CAT), and peroxidases (POD) suggested the existence of a sequence response of these enzymes to scavenge oxidative stress in the roots. However, inadequate production of SOD and CAT appeared to be compensated by the enhanced activity of POD, which acted as potent quencher to reactive oxygen species (ROS) in leaves. At the most elevated Ni dose, SOD, CAT and POD activities were insufficient to counteract ROS generated that led to membrane damage manifested by elevated MDA levels. Nevertheless, SOD and CAT alleviated Ni toxicity in roots while SOD, CAT and POD acted in a concurrent manner to protect leaves from oxidative damage in V. cylindrica. The study clearly indicated a Ni dependent antioxidant enzymes defense system in V. cylindrica.