Alleviation of allelochemical stress-induced growth inhibition and oxidative damage in lettuce under closed hydroponics through electro-degradation

Successive lettuce cultivation in closed hydroponics using the same nutrient solution causes the excess production and accumulation of allelochemicals. The accumulated allelochemicals induce oxidative damage and lipid peroxidation in plants leading to growth inhibition. In this study, we investigated the allelochemicals that induced oxidative damage and lipid peroxidation in lettuce grown in a once used non-renewed nutrient solution (1NR) and a twice used non-renewed nutrient solution (2NR) obtained from the successive cultivation and the alleviation of these damages through electro-degradation (ED). The 1NR solution was used for six weeks for a one-time lettuce cultivation while the 2NR solution was used for twelve weeks for a two-times lettuce cultivation. The results showed that the allelochemical stress caused growth inhibition in the lettuce in both the 1NR and 2NR solutions. It was observed that there was a higher generation of H2O2 and O2.- as well as a lower activity of the antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (POD), and ascorbate peroxidase (APX) in the roots of the plants grown in both the 1NR and 2NR solutions compared to plants grown in the new nutrient solution. The higher level of lipid peroxidation due to the higher MDA (malondialdehyde) content and higher soluble protein content were also observed in the roots of those plants. It was evident that lettuce root damage occurred due to accumulation of the allelochemicals in the 1NR and 2NR solutions. These damaged roots could not function normally nor uptake water and minerals from the culture solution. As a result, retarded lettuce growth was observed in the 1NR and 2NR solutions. The oxidative damage, soluble protein content, lipid peroxidation and ultimately growth retardation were more pronounced in the plants grown in the 2NR solution compared to the plants grown in the 1NR solution. The application of ED to the 1NR and 2NR solutions maintained the plant growth through less oxidative damage, soluble protein production and lipid peroxidation as was observed in the plants grown with the new nutrient solution. Therefore, the ED of a non-renewed culture solution would alleviate the allelochemical stress in lettuce under recycled hydroponics.