Effect of selenium and melatonin on growth and nitrate accumulation in spinach (Spinacea oleracea L.)

Background and purpose: Spinach (Spinacia oleracea L.) is very important in the diet due to its high nutritional value, so it is consumed in large quantities in most countries of the world. But this vegetable has a high ability to accumulate nitrates, and if we don't pay attention to its nitrates, it can pose risk to human health. This has led to extensive research on the possibility of reducing nitrate accumulation in this vegetable. Some of these researches are focused on the application of nutritional elements and plant hormones. It has been reported that the application of selenium and melatonin hormone is effective in reducing nitrate accumulation in various plants. This research aims to investigate the effect of selenium nutrition and melatonin spraying, as well as the interaction between the two, on the growth and nitrate accumulation in spinach.

Materials and Methods: This study was conducted as a factorial experiment in a complete randomized design, including selenium as sodium selenite (0, 5, and 10 μM) and melatonin (0, 50, and 100 μM) in hydroponic cultivation and in a substrate of coco peat and perlite. The melatonin treatment was applied as a foliar spray at the three-leaf stage and after sunset, twice with a one-week interval. Selenium treatment was given to the plants in combination with the Hoagland nutrient solution. Growth indices, chlorophyll, leaf antioxidant capacity, total flavonoid concentration, total phenol concentration, vitamin C content, nitrate, and nitrate reductase enzyme activity were measured after harvesting the plants.

Results: The results of this research showed that the use of selenium and melatonin increased the growth traits such as total fresh weight biomass, shoot fresh weight, leaf fresh weight, and total dry weight biomass compared to the control. Application of selenium and melatonin increased some biochemical traits such as total phenol and flavonoid, antioxidant capacity and selenium of leaves. Data analysis showed that selenium and melatonin are effective in reducing nitrate accumulation in leaves and stems, and their effectiveness depends on their concentration. The findings of this research showed that selenium and melatonin have a better effect in reducing nitrate accumulation in leaves compared to stem. One reason for the effectiveness of melatonin in leaves compared to stems may be due to the larger surface area of the leaves being exposed to the melatonin spray. It seems that selenium has a negative effect on the plant at a concentration of 10 mM, although it has reduced nitrate accumulation; however, it was not as effective as at the level of 5 mM. The application of 50 mM melatonin resulted in a decrease in nitrate levels, but the concentration of 100 mM melatonin was more effective than 50 mM. It is possible that higher levels of melatonin are more effective in the expression of genes related to nitrate and nitrite reductase enzymes and thereby reduce nitrate levels. The highest amount of nitrate was found in the leaves and stems and the lowest amount of nitrate reductase enzyme activity was found in the leaves and stems of control plants, while the lowest amount of nitrate in leaves and stems was obtained in the treatment combination of 5 μM selenium and 100 μM melatonin, which decreased the amount of nitrate in the leaf to 65% and in the stem to 52%, And nitrate reductase enzyme activity showed a 32% increase in the leaf and a 65% increase in the stem compared to the control.Conclusion: Present research show that, the treatment combination of 5 μM selenium and 100 μM melatonin had the best effect in increasing total biomass (wet and dry) and reducing nitrate accumulation in spinach.