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Salt stress limits crop production worldwide, and the problem is ever increasing because of irrational human acts causing secondary salinization, as well as because of global warming, with the consequent rise in sea level and increase in storm incidences (Pessarakli and Szabolcs, 1999).
Salt stress adversely affect crop productivity and quality due to ion cytotoxicity and osmotic stress (Zhu, 2002).The response of all plants to decreased osmotic potential is turgor loss, which results in stomatal closure, followed by reduction in gas exchange (Ashraf, 1994). Because of these stresses, which are primary effects of salt stress, secondary stresses such as oxidative damage may occur. Reactive oxygen species are highly reactive and can alter normal cellular metabolism through oxidative damage to membranes, proteins and nucleic acids (Dat et al. 2000, Imlay 2003).
Plants facing adverse situations including high salt concentrations lower their osmotic potential by accumulating osmolytes that do not perturb enzyme functions to maintain continuous water absorption at the low water potential. Additionally, salinity stress triggers defense or adaptive mechanisms in plants resulting in the accumulation of a wide range of antioxidants. They include enzymatic antioxidants (SOD, CAT, POX, APX) and non-enzymatic such as ascorbate. As the most abundant and strong reductant, ascorbate can diminish reactive oxygen species directly or with corporation to some antioxidant enzymes. Stress conditions may reduce pool of ascorbate therefore exogenous ascorbate may be able to increase plant tolerance. Some studies on the effect of exogenesis of ascorbate in plants under salt stress have shown an increase resistance to salt stress and reduce oxidative stress directly and with changing in enzymes activity (Shalata and Neumann, 2001, Dolatabadian, 2007; Athar, 2010). Nevertheless, it is not known if ascorbate treatment also induces salt stress tolerance in Chinese broccoli.
The ROS hydrogen peroxide (H2O2) has generally been viewed as a toxic cellular metabolite. However, it is now clear that it may also function as a signal molecule in both plant and animal cells (Finkel, 2000; Neill et al., 2002). The generation of H2O2 is increased in response to a wide variety of abiotic and biotic stresses, and some authors have suggested that H2O2 plays a dual role in plants: at low concentrations, it acts as a messenger molecule involved in acclimatory signaling, triggering tolerance against various abiotic stresses. Thus, it appears likely that H2O2 accumulation in specific tissues, and in the appropriate quantities, may benefit plants by mediating plant acclimation and cross-tolerance to both biotic and abiotic stresses (Bowler and Fluhr, 2000).
Previous studies shown exogenously applied H2O2 increased chilling stress tolerance (Prasad et al., 1994). H2O2 injection in Arabidopsis leaves led to protection from a subsequent excess light-induced photo bleaching effect (Karpinski et al., 1999). Recently, important evidence that H2O2 can induce acclimation to salt and heat stresses was obtained on rice seedlings pre-treated with various levels of H2O2 (Uchida et al., 2002 Azevedo Neto, 2005). However, it is not known if H2O2 pre-treatment also induces salt stress tolerance in kailan plants.
In recent years, the importance of vegetables including Brassica.oleracea in the daily diet is being increasingly recognized. Numerous epidemiological studies indicate that brassicas have potential for chemoprevention of degenerative diseases and certain types of cancer since they are rich sources of natural compounds (Higdon et al., 2007).
Knowledge on sensitivity, of various growth, physiological and biochemical processes to stress, including salinity stress, are vital for efficient crop management.
Despite of the large body of literature on salinity stress, our knowledge about the effects of salt stress on Chinese broccoli is still little. Therefore, there is a pressing need to know in more detail how an important vegetable plant as Chinese broccoli responds and adapts to such conditions.
With regard to the importance of Brassica oleracea in term of their benefits and highly consumed in human dietary, understanding the possible effects of salt stress on Chinese broccoli that seems never exposed to salt stress, can be significant. Therefore attempt to provide some practical, safe and environmentally sound techniques to retain productivity under salt stress is critical.
Aim of study
The objective of these studies was to evaluate the agronomical, physiological and biochemical responses of Chinese broccoli to salinity stress. Attempts were also taken to understand crop tolerance mechanisms to the increasing salinity at both enzymatic and non enzymatic biochemical levels.
Exp I: Effects of NaCl salinity on growth and physiological traits of four Chinese broccoli varieties
Objective: Determination of the most productive cultivar of imported Chinese broccoli under salinity
Effects of salt stress on some agronomical physiological and biochemical traits of four cultivars were measured, results shown final growth parameters, relative water content significantly reduced by salinity while chlorophyll fluorescence approximately was constant moreover synthesis of proline, generation of hydrogen peroxide and accumulation of malondialdehyde were changed significantly by stress. Despite of changes in these traits by salt stress we have found just agronomical variables of these cultivars were analytically different may be due to inherent potential of each cultivar. Therefore cultivar number 11 was selected for the highest ability of production.
Exp II: Effect of salinity on growth and nutrient composition of Chinese broccoli in condition of salt stress
Objective: Evaluation of growth parameters and nutrient composition under salt stress
Growth, ions accumulation and nutrients composition were measured and analyzed. Growth severely dropped with increase in salt concentration. Reduction in leaf expansion, low photosynthesis rate and disruption in biological processes leads in growth diminution. Salt stress has altered ions concentration and nutrients composition in this plant. With increase in salt concentration potassium and calcium has shown a great drop in both leaf and root. Also Phosphorus decreased in leaf while nitrogen concentration has not shown and significant changes. Moreover sodium and chloride ions enhanced dramatically.
Exp III: Physiological and biochemical characterizations of Chinese broccoli under salt stress
Objective: Evaluation of salt stress effects on final growth, physiological and biochemical responses of Chinese broccoli
Salinity cause a significant changes in most of all agronomical, physiological and biochemical traits. Reduction in water relation and gas exchange leads a significant drop in photosynthesis, Protein synthesis, chlorophylls content and finally growth and productivity. Besides synthesis and accumulation of proline, anthocyanins were increased. Results shown generation and accumulation of hydrogen peroxide and MDA were increased with increase in salt concentration. Consequently antioxidant enzymes activities have amplified in these conditions.
Exp IV: Effects of foliar application of ascorbic acid on tolerance mechanism under salt stress
Objective: Estimation of exogenesis of ascorbic acid on antioxidative enzymes activites, energy harvesting system, water realtion and accumulation of proline, hydrogen peroxide and MDA in leaf cells under stress.
Interaction of foliar application of ascorbic acid and salt stress on Chinese broccoli were studied. Results shown exogenous of ascorbic acid can reduce generation or accumulation of hydrogen peroxide which leads a significant reduction in antioxidant enzymes activity, even if this reduction was not significant for CAT. Mover synthesis and accumulation of proline were reduced while stability of chlorophylls and carotenoids were grown.
Exp V: Exogenous of hydrogen peroxide changes antioxidant enzymes activities in Chinese broccoli in salt stress condition
Objective: Evaluation of antioxidative enzyme activites, energy harvesting system and accumulation of MDA with application of low concentration of hydrogen peroxide under salt stress.
Exogenous of hydrogen peroxide on Chinese broccoli under stress was amplified antioxidant enzymes activity. These changes were significant in all of three enzymes. Furthermore it made significant changes in chlorophyll stability under salt stress, while no changes were seen on accumulation of MDA.
Salt stress can diminish crop growth and productivity due to impact on a vast varies of yieldâ€™s effective elements. Impact on water relations, disruption in energy harvesting and flowing systems are resulting in ROS generation and consequently, defense system activation. This adaptation is energy demanding therefore, prosperity is mostly depending to the energy management in the stress conditions.
Application of ascorbic acid as an auxiliary oxidant extinguisher probably can retain plant's energy balance. Our results have shown that foliar application of AsA can increase the survival capacity of Chinese broccoli plants under conditions of salt stress. The increase in resistance to salt stress is associated with the antioxidant activity of AsA, a partial inhibition of salt-induced increases in lipid peroxidation by ROS, and a decrease in antioxidant activity.
According to these results it can be suggested that usage of AsA can reduce the harmful effects of ROS and improves plant resistance.
Pre-treatment of hydrogen peroxide seems to tempt plants for acclimation and provide defense machines before stress occur. Our results show that H2O2 pre-treatment under normal growth conditions enhanced salt tolerance in the Chinese broccoli, and that differences in the antioxidative enzyme activities may, at least in part, explain the increased tolerance of acclimated plants to salt stress. Additional evidence is provided here that H2O2 metabolism is involved as signal in the processes of brassica alboglabra salt acclimation.