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

Highlights

• The foliar spray with K⁺ or Ca²⁺ improve the salt tolerance of thymus vulgaris.

• The KCl and CaCl enhanced the uptake of Ca²⁺ and K⁺ under salt stress.

• The foliar spary with KCl or CaCl₂ improved the non-ezymes and enzymes antioxidants in Thymus vulgaris.

• KCl sprayed regulated the ascorbate–glutathione cycle at short time.

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.

Introduction

Salinity is a major environmental stress that affects plant growth and development through the induction of ion toxicity nutritional unbalances, osmatic stress and reduced water uptake (AbdElgawad et al., 2016). One of the most deleterious serious consequences of salt toxicity is the generation of reactive oxygen species (ROS) which deleterious causes was the oxidative stress within the cells damaging their membrane integrity and ion imbalances, which results in osmotic stress, and generation of reactive oxygen species (ROS) (Ahmad et al., 2010). However, plants are equipped with a variety of antioxidant defense systems which protect sub-cellular structures such as chloroplastic and mitochondrial membranes from the stress-generated toxic ROS (Demidchik et al., 2014). These antioxidant defense systems comprise both enzymatic and non-enzymatic components. In fact, the ascorbate -glutathione cycle is an important ROS scavenging pathway which played a crucial role in maintaining the balance of these ROS within harmless levels helping thereby in averting oxidative damage to the cell (Ashraf, 2009). This cycle comprises the enzymes ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione-S-transferase (GST) which are the key enzymes linked to the non-enzymatic antioxidant components, ascorbic acid (AsA) and glutathione (GSH) (Miyake, 2010). This cycle also operates for scavenging of H₂O₂ where net electron flow is from NADPH to H₂O₂ (Mittler, 2002). Among the antioxidants enzymes, superoxide dismutase (SOD) and catalase (CAT) are important for maintaining the steady state level of superoxide radicals and H₂O₂ radicals thereby preventing the formation of the more toxic hydroxyl radical via the Fenton or metal-dependent Haber–Weiss reactions. The several physiologically important functions of the enzymatic and non-enzymatic antioxidant systems contribute to the regulation of photosynthesis as well as the mitigation of photoinhibition by scavenging ROS in chloroplasts.

ROS induced by salt stress, activate calcium- (Ca²⁺) and potassium- (K⁺) permeable cation channels at the plasma membrane, mediating Ca²⁺⁻based signaling events, K⁺ ion leakage, and programmed cell death (Zorrig et al., 2012). K deficiency obstructs plant metabolism and decreases chlorophyll content and photosynthetic efficiency by affecting the activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISO) and the associated photosynthetic attributes (Zorrig et al., 2012). For this reason, supplying plants with K and Ca fertilizers generally improves their growth and development. Furthermore, increased supply of K and Ca alleviates abiotic stresses and helps plants maintain their growth under salinity stress in particular (Tounekti et al., 2011; Ahanger and Agarwal, 2016; Zrig et al., 2015). More K uptake helps the plant in maintaining cell turgor, stomatal movements, and gas exchange rates. Furthermore, K was shown to contribute to mitigating oxidative stress by modulating ROS metabolism (Soledad et al., 2015) and increasing K⁺/Na⁺ ratio (Ahanger and Agarwal, 2016).

Likewise, Ca plays a major role in maintaining the structure and integrity of cell membranes. Under salinity stress conditions, it was reported that increasing Ca²⁺ supply helps maintain ionic homeostasis (Ahmad et al., 2016) and modulates the antioxidant metabolism (Tounekti et al., 2011; Zorrig et al., 2012; Zrig et al., 2015).

Commonly, thyme is harvested from the wild, but due to increasing demand for culinary and pharmaceutical uses, it is being increasingly cultivated under greenhouse conditions and, at times, irrigated with low quality water with elevated salinity. Under such conditions, plant growth is reduced, and metabolic activity is impaired by salt stress (Alizadeh et al., 2010; Tounekti et al., 2011). It is with this backdrop that we investigated whether K and Ca supplementation modulates the antioxidant metabolism of the plant helping alleviate the adverse effects of NaCl-salinity stress on growth and yield. We report on the role of foliar sprays with K or Ca in mitigating salt-induced oxidative damage to thyme plant tissues.