The Positive Effect of Selenium on Nitrate Accumulation in Spinach (Spinacia oleracea L.) and Lettuce (Lactuca sativa L.)

High nitrate (NO3−) in vegetables, especially in leafy vegetables poses threaten to human health. Selenium (Se) is an important element for maintaining human health, and exogenous Se application during vegetable and crop production is an effective way to prevent Se deficiency in human bodies. Exogenous Se shows a positive function on plant growth and nutrition uptake under abiotic and biotic stresses. However, the influence of exogenous Se on NO3− accumulation in hydroponic leafy vegetables is still not clear.

The present study was conducted in a completely randomized design with four replications at Research Greenhouse of Lorestan University. In this study, hydroponic lettuce (Lactuca sativa L. var. Grin leek) and spinach (Spinacia oleracea L. var. Sirius) plants were subjected to six different concentrations (0, 0.1, 0.5, 5, 10 and 50 μmol L–1) of Se as Na2SeO3. Zero concentration was considered as control. The modified Hoagland nutrient solution were prepared and 8 liters was added to each pot. The nutrient solutions were replaced with fresh solution every five days throughout this experiment. After the emergence of new roots in the nutrient solution, treatment was applied and selenium was added to the pots at a specific concentration. After Se treatment for 40 days, three plants were randomly harvested from each treatment. The effects of Se on plant growth, NO3− content, activities of nitrogen metabolism enzymes, photosynthetic capacity and glutathione peroxidase enzyme activity of lettuce (Lactuca sativa L.) and spinach (Spinacia oleracea L.) were investigated. The second youngest, fully expanded leaf was used to monitor photosynthetic capacity using chlorophyll meter and portable photosynthetic apparatus, respectively. Nitrate concentration in dried samples was determined based on nitrate to nitrite reductions in the vicinity of zinc powder and hydrogen ion. Data preparation was done in the Excel program and data analysis was done using SPSS 16 software. The means comparison of the treatments was done by LSD test and finally, the figures were drawn using MS Excel.

The results showed that the lowest and highest biomass in both plants were observed in 50 and 5 μmol L–1 Se treatments, respectively. Different levels of selenium had no significant effect on root fresh weight in spinach. However, in the lettuce, at 50 μmol L–1, the root fresh weight significantly decreased compared to the 5 μmol L–1. Moreover, exogenous Se positively decreased NO3− content and this effect was concentration-dependent. The lowest NO3− content was obtained under 5 μmol L–1 Se treatment.NO3− content in lettuce was lower than that of spinach in all treatments. The application of Se enhanced photosynthetic capacity by increasing the stomatal conductance, photosynthesis rate and chlorophyll content of plants. No significant difference (p ≤0.05) was observed in spinach in chlorophyll, stomatal conductance and photosynthetic rate between 10 and 5 μmol L–1, however, in lettuce this difference was significant. Chlorophyll content in lettuce was higher than spinach in all treatments. The best concentration for photosynthetic capacity in both plants was five μmol L–1, which could explain similar changes in root and shoot dry weight in the different treatments. The results showed that low selenium concentrations (≤5 μmol L–1) stimulated NO3− assimilation by enhancing nitrate reductase (NR), nitrite reductase (NiR) and glutamine synthetase (GS) activities. The lowest nitrate reductase activity in lettuce and spinach was 50 μmol L–1 and control, respectively. In all treatments, nitrate reductase activity in lettuce was higher than in spinach. The reaction of nitrate reductase to selenium application was similar to nitrate reductase activity. Glutamine synthetase activity in both plants increased with increasing selenium concentration. However, as the concentration increased (≥ 5 μmol L–1), the activity of this enzyme began to decrease. In addition, the activity of this enzyme was higher than control in all treatments. Selenium also increased glutathione peroxidase concentration. A significant increase in glutathione peroxidase activity at five μmol L–1 was observed compared to the other treatments in lettuce. However, there was no significant difference in the activity of this enzyme in spinach at concentrations of 5 and 0.5 μmol L–1.

These results provided direct evidence that exogenous Se showed positive function on decreasing NO3− accumulation via enhancing activities of nitrogen metabolism enzyme in both plants. This study suggested that five μmol L–1 Se could be used to reduce NO3− content and increased hydroponic lettuce and spinach yield.