Effect of selenium on cadmium uptake, growth and photosynthetic characteristics of garlic (Allium sativum L.) seedlings exposed to cadmium and sodium chloride under hydroponic conditions

Introduction Salinity is one of the most important factors reducing the yield of many crops. The usual response of plants to increasing concentrations of salt at the root zone is osmotic stress, ionic toxicity and nutrient deficiencies. Also, cadmium is one of the major heavy metal pollutants by high toxicity and accumulation, caused retard in the plant growth.The problem of Cd toxicity becomes even more severe when plants are exposed to salinity. The different materials are considered to mitigate the negative effects of stress on plants. In this regard, the selenium introduced as one of the trace elements beneficial to plants so that it can protect plants against adverse effects of stress. The purpose of this study was to determine uptake of cadmium, sodium, potassium and developmental and physiological changes of garlic seedlings exposed to cadmium under salt stress in the presence of selenium. Materials and methods A factorial experiment in based on a completely randomized design with three replications was done in Crop Physiology Laboratory, Faculty of Agriculture, Bu Ali-Sina University. Cadmium as cadmium chloride in 0, 10-4, 10-3 and 10-2 M concentrations and sodium chloride at two levels (0 and 25 mM) and sodium selenate (0 and 5 mg/ l , Na2Se2O4) were added to the growth medium. The traits growth of root and shoot dry weight was measured. In order to determine the content of cadmium, sodium and potassium, 0.2 g of tissue dry matter ground and by the wet digestion method and atomic absorption spectrophotometer (Varian, 220) was read content elements. The amount of chlorophyll in the shoot was calculated using Arnon method. Parameters of net photosynthesis, stomatal conductance and transpiration were measured in a fully open mature leaf by using an infrared analyzer. Data analysis was performed using SAS software and mean comparisons test data was used by least significant difference (LSD) and LSMEANS. Results and discussion The results showed that salinity and cadmium have caused a significant reduction in root and shoot dry weight, total chlorophyll content, net photosynthesis, stomatal conductance and transpiration. Interaction between salinity and Cd at 10-4 M decreased the absorption of Cd in seedling and followed by more improving chlorophyll content, net photosynthetic as a result from root and shoot dry weight increased compared to non-saline condition. But this effect was not observed at 10-3 and 10-2M Cd concentrations. On the other hand, selenium alone or in the presence of salinity or cadmium caused a significant increase in above mentioned growth traits. Also, salinity and cadmium raised sodium to potassium ratio in the shoot. The interaction of between salinity and cadmium with increasing concentration of Cd, sodium to potassium ratio enhanced in the aerial parts of the plant. Conclusions Overall, salinity and cadmium stresses in garlic seedling media, reduced root and shoot dry weight, chlorophyll, photosynthesis, transpiration and stomatal conductance. Application of selenium alone and also in present salinity and cadmium caused a significant increase in root and shoot dry weight, chlorophyll, net photosynthesis, transpiration and stomatal conductance. Although low concentrations of salinity and cadmium (10-4M) revealed a significant increase in root and shoot dry weight, which implies an increase in chlorophyll content and photosynthesis in plant garlic. Selenium also improved the total chlorophyll, photosynthesis in the interaction between salinity and cadmium. This subject confirms the positive effect of selenium on garlic seedling growth under cadmium and salt stresses, which in subsequent studies the physiological and molecular processes should be further investigated.