Effects of different levels selenium and nitrogen on some growth and biochemical characteristics of onion (Allium cepa L.) plant

Selenium is an essential micronutrient for animals and human. Although, effects of Se on plants is scarcely distinguished, there are increasing evidences that Se functions as an antioxidant in plants. Selenium has been show a positive effect on crop growth and stress tolerance at low concentrations, whereas at higher concentration it acts as an antioxidant and reduces plant growth. Plants play a key role in recycling and delivering Se from the soil to the food chain. Since selenium affects nitrogen metabolism; in this study, the time interaction of selenium by nitrogen was investigated on growth and biochemical characteristics of onion leaf for the first time. Because onion leaves are used as vegetables in Iran.
In the present experiment, the effects of different levels of selenium (Se) and nitrogen (N) on some growth and biochemical characteristics of onion (Allium cepa L.) cv.Ghermez Azarshahr were studied under hydroponic. selenium at two levels (0, 1 and 1.5 mg L-1 sodium selenate, and four levels nitrogen (56, 112, 168 and 224 mg L-1 N) were studied in a factorial experiment based on a completely randomized design with three replications. At the early stage of blubbing (three months after seed sowing) some growth characteristics (Plant fresh weight, Plant height, leaf fresh weight and Pseudostem fresh weight), yield and biochemical characteristics (chlorophyll, antioxidant activity, total phenol, total flavonoids and ascorbic acid) and nitrate of onion leaves were measured.
Based on the results, seedling growth characteristics of onion including Plant fresh weight (29.01 g), Leaf fresh weight (18.37 g), Plant height (54.14 cm) and yield (148.04 g/m2) at concentrations of 1 and 1.5 mgL-1 sodium selenite with nitrogen increased compared to 0 mgL-1 sodium selenite with nitrogen. The simultaneous application of selenium and nitrogen had a significant effect on chlorophyll b content and the highest chlorophyll b levels were observed in treatments 1 mg L-1 sodium selenate along with 224 mg L-1 nitrogen and 1.5 mg L-1 sodium selenate along with 224 mg L-1 nitrogen, the lowest chlorophyll b levels was observed at 0 mg L-1 sodium selenate along with 56 mg L-1 nitrogen. Addition, the highest total phenol (9.03 mg fresh weight), total flavonoid (0.0094 mg fresh weight), ascorbic acid (0.70 mg fresh weight) and antioxidant activity (69.08%) of onion leaves were observed at sodium selenate treatments with nitrogen. While, the highest amount of nitrate was recorded in the 224 mg L-1 nitrogen without any sodium selenate.
Use of sodium selenate along with nitrogen plays an important role in improving biochemical characteristics of onion leaf. The maximum growth rate of onion seedlings was observed in selenium treatments with nitrogen that indicating positive effect of both elements on increasing of chlorophyll synthesis and consequently increasing of photosynthesis and carbon fixation which leads to increase of onion growth. Generally, it can be concluded that the positive effect of selenium on the growth of onion seedlings has been associated with an increase in antioxidant activity and phenolic compounds. Nitrate levels increased at three levels of selenium with increasing nitrogen concentration, but decreased with increasing selenium levels. Finally, we recommend the use of 1 mg L-1 sodium selenate with 224 mg L-1 nitrogen in soilless culture system for the production of onion seedlings with maximum antioxidant compounds and yield.