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Abstract
The present study investigates the regulatory role of exogenous calcium (Ca) in developing salt stress tolerance in rice seedlings. Hydroponically grown 13-day-old rice (Oryza sativa L. cv. BRRI dhan47) seedlings were exposed to 200 mM NaCl alone and combined with 2 mM CaCl2 and 2 mM ethylene glycol tetraacetic acid (EGTA, a Ca scavenger) for 3 days. The salt stress caused growth inhibition, chlorosis and water shortage in the rice seedlings. The salt-induced stress disrupted ion homeostasis through Na+ influx and K+ efflux, and decreased other mineral nutrient uptake. Salt stress caused oxidative stress in seedlings through lipid peroxidation, loss of plasma membrane integrity, higher reactive oxygen species (ROS) production and methylglyoxal (MG) formation. The salt-stressed seedlings supplemented with exogenous Ca recovered from water loss, chlorosis and growth inhibition. Calcium supplementation in the salt-stressed rice seedlings improved ion homeostasis by inhibition of Na+ influx and K+ leakage. Exogenous Ca also improved ROS and MG detoxification by improving the antioxidant defense and glyoxalase systems, respectively. On the other hand, applying EGTA along with salt and Ca again negatively affected the seedlings as EGTA negated Ca activity. It confirms that, the positive responses in salt-stressed rice seedlings to exogenous Ca were for Ca mediated improvement of ion homeostasis, antioxidant defense and glyoxalase system.
Highlights
Gradual changes in climate challenges crop production through various abiotic stresses, which become major constraints to crop production due to the unpredictable and complex nature of the environment (Mittler and Blumwald, 2010)
Plants exposed to higher levels of salinity are affected by both hyperionic and hyperosmotic stress through accumulating Na+ and Cl− which causes membrane damage, nutrient imbalance, enzymatic inhibition, metabolic dysfunction, photosynthesis inhibition, and hampers other major physiological and biochemical processes that leads to growth inhibition or death of the plant
Under non-stress conditions, exogenous Ca and EGTA had no visual effect on the rice seedlings
Summary
Gradual changes in climate challenges crop production through various abiotic stresses, which become major constraints to crop production due to the unpredictable and complex nature of the environment (Mittler and Blumwald, 2010). Around 20% of irrigated land has been affected by salinity (Pitman and Lauchli, 2002), and it is estimated that salt stress will cause up to 50% loss of cultivable land in the middle of the 21st century (Mahajan and Tuteja, 2005). Plants exposed to higher levels of salinity are affected by both hyperionic and hyperosmotic stress through accumulating Na+ and Cl− which causes membrane damage, nutrient imbalance, enzymatic inhibition, metabolic dysfunction, photosynthesis inhibition, and hampers other major physiological and biochemical processes that leads to growth inhibition or death of the plant. With higher levels of NaCl, Na displaces Ca from membranes, which increases intracellular Na. As a result, under salt-stress conditions Na content exceeds that of K, resulting in a higher Na/K ratio as well as nutrient imbalance (Cramer et al, 1985; Shabala et al, 2006; Wu and Wang, 2012). Salinity accelerates the generation of cytotoxic methylglyoxal (MG) through the glycolysis pathway and causes oxidative damage through degradation of protein synthesis (Yadav et al, 2005; Hasanuzzaman et al, 2014)
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