Effect of manganese on yield and quality of hydroponically grown lettuce

Hydroponic production of leafy vegetables sustains consumption of fresh produce all year round. However, cultivation of leafy vegetables in a growing medium prepared from water contaminated with trace elements, particularly manganese (Mn), may affect both the plant products and their consumers. Therefore, an attempt was made in the present study to evaluate the effect of treating two cultivars of lettuce (cv. Locarno and cv. Satine) with Mn (0.5 – control, 5, 25 and 50 mg dm-3) on (i) the concentration of Mn and other elements, (ii) biomass accumulation, (iii) the efficiency of the photosynthetic apparatus, (iv) the levels of ROS (reactive oxygen species), (v) the activity of antioxidative enzymes, and (vi) the content of phenolic compounds. Increased concentrations of Mn in the growing medium led to a significantly higher concentration of this element in lettuce leaves (up to a 9.4-fold increase), but concentrations of potassium, calcium and magnesium were almost unchanged. Although the intensity of photosynthesis was lower in both of the examined cultivars, biomass accumulation was slightly reduced only in cv. Locarno. The recorded toxic effect of Mn most probably resulted from the induced generation of ROS, levels of which usually increase albeit insignificantly in plants treated with the tested metal. Higher generation of ROS was to some extent counteracted by the increased activity of antioxidative enzymes (ascorbate peroxidase and catalase) and a higher content of phenolic compounds. The results of this study demonstrate that increased accumulation of Mn in lettuce not only impairs the plant growth, but may also pose a risk to human health, as these plant products may become dietary sources of Mn and elevated levels of ROS.