Potassium and calcium improve salt tolerance of Thymus vulgaris by activating the antioxidant systems

Abstract

Thyme (Thymus vulgaris L.) is an important wild aromatic plant of the Mediterranean region which also has wide-ranging therapeutic properties. Its growth and chemical properties can be modified by agricultural practices especially mineral nutrition and irrigation. An experiment was undertaken to determine the effectiveness of exogenous foliar application of potassium and calcium against the adverse effects of NaCl-induced stress on Thyme. Salt stress decreased growth and photosynthetic assimilation rate. Leaf spray with K+ or Ca2+ reduced salt-induced damage by maintaining K+/Na+ and Ca2+/Na+ ratios, resulting in photosynthesis improvement under salt conditions. Salt-affected plants had higher concentrations of hydrogen peroxide (H2O2) and malondialdehyde (MDA) in their shoot tissues indicating oxidative damage to their cell membranes; Increased K+ and Ca2+ supply reduced the accumulation of both. Salinity appears to have upregulated the activity of antioxidant defense enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase, glutathione reductase, peroxide guaiacol, Monodehydroascorbate reductase, as well as Dehydroascorbate reductase. The application of K+ and Ca2+ solutions onto the foliage of NaCl-affected plants further enhanced the activity of the antioxidant enzymes of the ascorbate glutathione cycle potentially allowing a better protection of the cell membranes from reactive oxygen species.