Differential responses of components of the antioxidant defense system to high salinity stress in the lesser duckweed (“Lemna minor” L.)

Abstract

Salt stress causes oxidative damage in plants, and it induces protective mechanisms through enzymatic and non-enzymatic components of the antioxidant system. Different components of this system exhibit specific degrees of tolerance toward certain salt concentrations. Their differential responses may contribute not only to a better understanding of the functional interconnections in the antioxidant defense system, but also to a more efficient selection of physiological and biochemical markers of stress reactions of plants, in the effort for an early and precise bioindication of oxidative damage caused by high salinity of the environment. In this context, the molar ratio between the reduced and the oxidized form of ascorbic acid is a more sensitive marker of oxidative stress than the total amount of this vitamin in the biomass of lesser duckweed. Glutathione content exhibits a more moderate variation with increasing salt stress than the concentration of carotenoid pigments in the fronds exposed to constant photon flux density. From among the antioxidant enzymes, ascorbate peroxidase was found to be the most sensitive, and superoxide dismutase was the most resistant to oxidative stress caused by increasing salinity. Catalase and glutathione reductase activities decreased under severe salt stress. Efficiency of the antioxidant system can be monitored by membrane damage through lipid peroxidation. Antioxidants of duckweed are useful tools for indication of increasing salinity of aquatic environments.