نقش حفاظتی کلسیم در مقابل نمک مسموم در کنگر فرنگی اورشلیم توسط تنظیم آنزیم آنتی اکسیدان

The ameliorative effect of external Ca2+ on Jerusalem artichoke (Helianthus tuberosus L.) under salt stress was studied through biochemical and physiological analyses of Jerusalem artichoke seedlings treated with or without 10 mol L−1 CaCl2, 150 mmol L−1 NaCl, and/or 5 mmol L−1 ethylene-bis(oxyethylenenitrilo)-tetraacetic acid (EGTA) for five days. Exposure to NaCl (150 mmol L−1) decreased growth, leaf chlorophyll content, and photosynthetic rate of Jerusalem artichoke seedlings. NaCl treatment showed 59% and 37% higher lipid peroxidation and electrolyte leakage, respectively, than the control. The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were decreased by NaCl, indicating an impeded antioxidant defense mechanism of Jerusalem artichoke grown under salt stress. Addition of 10 mmol L−1 CaCl2 to the salt solutions significantly decreased the damaging effect of NaCl on growth and chlorophyll content and simultaneously restored the rate of photosynthesis almost to the level of the control. Ca2+ addition decreased the leaf malondialdehyde (MDA) content and electrolyte leakage from NaCl-treated seedlings by 47% and 24%, respectively, and significantly improved the activities of SOD, POD, and CAT in NaCl-treated plants. Addition of EGTA, a specific chelator of Ca2+, decreased the growth, chlorophyll content, and photosynthesis, and increased level of MDA and electrolyte leakage from NaCl-treated plants and from the control plants. EGTA addition to the growth medium also repressed the activities of SOD, POD, and CAT in NaCl-treated and control seedlings. External Ca2+ might protect Jerusalem artichoke against NaCl stress by up-regulating the activities of antioxidant enzymes and thereby decreasing the oxidative stress.

Soil salinity is one among the major abiotic stresses that influence plant productivity (Bohnert et al., 1995). Saline conditions reduce the water absorption ability of plants, cause rapid reductions in growth, and induce many metabolic changes similar to those caused by water stress. High salt concentrations in soil adversely affect several metabolic processes in plants, most often leading to death. Cellular ionic imbalance in plant cells is the first consequence of salt stress (Niu et al., 1995; Zhu et al., 1997). Increased concentrations of Na+ and Cl− in cells exert numerous deleterious impacts on vital cellular processes and functions (Serrano et al., 1999). It has been shown that under high salinity not only the homeostasis of Na+ and Cl− but also the distribution of Ca2+ and K+ is disturbed (Rodriguez-Navarro, 2000). Higher concentrations of salt in the external environment of roots impose a hyperosmotic shock in plants because of a decrease in the chemical activity of water and loss of cell turgor. Because of the reduction in chloroplast stromal volume and/or increased generation of reactive oxygen species (ROS) (Price and Hendry, 1991) under salt stress, the rate of photosynthesis is also decreased.Salt adaptation in plants is mainly dependent on the regulatory mechanisms of ionic and osmotic homeostasis (Niu et al., 1995; Hare et al., 1998; Yeo, 1998; Zhu, 2003). In addition to ionic and osmotic imbalance, salt stress also causes oxidative stress through an increase in cellular level of ROS, such as superoxide radicals, hydrogen peroxide, and hydroxyl radicals (Alscher et al., 1997; Mittler, 2002; Neill et al., 2002). These ROS are highly reactive and can alter normal cellular metabolism through oxidative damage to lipids, proteins, and nucleic acids (Alscher et al., 1997; Imlay, 2003). During salt stress, ROS generation in plant cells increases by several folds (Smirnoff, 1993). Thus, it is imperative to assume that regulation of antioxidant enzymes could be an important strategy of plants in fighting against salt stress. Some earlier studies have also shown that resistance to oxidative stress, at least in part, is involved in salt stress tolerance of plants (Gosset et al., 1994; Gueta-Dahan et al., 1997; Hern´andez et al., 2000; Mittova et al., 2002; Badawi et al., 2004; de Azevedi Neto et al., 2006). It is well-known that calcium plays an important role in the adaptation of plants to diverse kinds of environmental stresses including salt stress (Bowler and Fluhr, 2000). Ameliorating effect of Ca2+ on Na+-induced growth inhibition of root has been shown (Cramer et al., 1988); Ca2+ prevents the salinity-induced shortening of growth zones of root (Bernstein et al., 1993). Volume and length of cell are generally reduced in salinity conditions; however, Ca2+ prevents salinity-induced decrease in cell volume and length (Azaizeh et al., 1992). Similarly, it has previously been shown that Ca2+ restores the photosynthesis under NaCl stress through increasing stomatal conductance (Perera et al., 1995). It was also shown that Na+/Ca2+ ratio largely affects the transport of water in the root and to the leaf growing zones (Cramer, 1992). There exists sufficient evidence showing that salinity alters the ionic content and transport in plants. Calcium has several regulatory functions over membrane property and ionic transport in halophytes and glycophytes. For example, increasing the external Ca2+ concentration reduces the transport of Na+ through ion channels, thereby reducing Na+ influx in root cells (Tyerman et al., 1997). Although a general protective role of Ca2+ against NaCl toxicity in plants is well-known (Hyder and Greenway, 1965), the role of Ca2+ in salt stress-induced oxidative stress tolerance of plants has not been confirmed. Jerusalem artichoke (Helianthus tuberosus L.) is a C3 warm-season plant that can be cultivated at relatively low cost with zero irrigation (Monti et al., 2005). Jerusalem artichoke has been recognized recently as a good source of fructose and inuline (Baldini et al., 2004; Saengthongpinit and Sajjaanantakul, 2005), and consequently has a potential application in several industries. Some evidences suggest that Jerusalem artichoke possesses a certain degree of salt tolerance, and can be grown in unused saline soils of costal areas (Liu et al., 2004; Long et al., 2005). To the best knowledge of the authors, no attempt has previously been made to study the antioxidant system of Jerusalem artichoke under salt stress. The aim of the present study was to investigate the effect of hyper salinity (NaCl) on the activity of some key antioxidant enzymes of plants and the effect of Ca2+ enrichment on NaCl-induced salt stress adaptation and antioxidant system of Jerusalem artichoke.

The enrichment of the growth medium with 10 mmol L−1 of Ca2+ prevented chlorophyll loss,maintained optimum photosynthesis and growth, and preserved membrane permeability in Jerusalem artichoke grown under NaCl stress. The above protective effects of Ca enrichment could be related to the improved activities of key antioxidant enzymes, and thereby their free radical scavenging in the stressed plants. The present study did not discriminate between osmotic and ionic effects of NaCl stress but the obtained results showed that calcium had a regulatory control over activities of antioxidant enzymes, and, thereby, salt stress tolerance of plants. Although a distinct protective effect of Ca2+ on antioxidant enzymes and, thereby, overall better growth performance of the NaCl-treated plants were found, mechanism(s) involved in the process are largely unknown and need to be elucidated.