Selenium (Se) is microelement beneficial to plants and essential to animals and humans. Supply of Se shows positive effects on plant growth, but high concentrations affect its growth. This study is based on the hypothesis that nutritional disorder and oxidative stress induced by toxic levels of selenium maintain a close correlation with inhibition of photosynthesis net and reduced growth in lettuce plants. In this study, we investigated impact of Se toxicity on gas exchange, oxidative stress indicators, nutritional status, and growth of lettuce plants. Two sources and ten selenium concentrations were evaluated in lettuce according to a completely randomized experimental design in a factorial scheme with two selenium sources (selenite and selenate) and ten selenium concentration (0, 2, 4, 6, 8, 16, 32, 64, 96, 128 μM). Results show that reduction in leaf area and shoot dry matter was high when selenite was supplied to plants. It was achieved due to oxidative stress and nutritional disorder that affected photosynthesis, which resulted in low photosynthesis net. These results were reinforced by strong correlation of photosynthesis with essential nutrient contents and indicators of oxidative stress in plants treated with selenite. However, photosynthesis net was increased with 8 μM concentration of selenate. Lettuce growth was reduced due to oxidative stress and nutritional disorder. The results of this study contribute to clarifying negative modulation of photosynthesis net by higher selenate or selenite concentrations in lettuce plants through growth analysis, nutritional composition, oxidative stress indicators, and gas exchange. The strong or very strong negative correlation between photosynthesis net and oxidative stress indicators (superoxide, peroxide and malondialdehyde), photosynthesis net and chlorophyll a, photosynthesis net, and selenium content support the hypothesis of this study in which selenium-induced damage to the photosynthetic apparatus reduces the growth of lettuce. These results show new evidence on the mechanism of action of selenium toxicity on the photosynthetic machinery of lettuce plants. In addition, the results found show that lettuce plants respond differently to the source and concentration of selenium, with symptoms of toxicity manifesting even in the short exposure time of lettuce plants.
