S08 - Session P3 - Root exudation of hydroponic lettuce is stable under different light qualities

Authors: Brechtje de Haas *, Eva Oburger, Danny Geelen, Marie-Christine Van Labeke

Plant growth-promoting rhizobacteria are difficult to maintain in hydroponics. A way of steering the microbiome would be through root exudation. As light quality can be used to steer secondary metabolites in plants, it might also be a way to influence root exudation. This study investigated the effect of light quality on total carbohydrates, total proteins, and total phenolics in leaves and roots of lettuce as well as to monitor root exudation rates. Lactuca sativa Expertise RZ was grown in a Deep Flow Technique hydroponic system. Lettuce plants were exposed to four light qualities: full-spectrum light, 50% red and 50% blue light, 100% blue light, and 100 % red light at 200 µmol m -2 s -1 . After four weeks, leaf and root tissue and root exudates were sampled. Exudates were sampled in a Micropure (0.01 mg L -1 ) solution to inhibit microbial decomposition of exuded metabolites during exudation sampling period. Total proteins and phenolic content were spectrophotometrically determined in plant tissues. Total C, total carbohydrate, total proteins and total phenolic concentrations in exudate samples were determined using spectrophotometrical assays. Light treatment had no significant (p > 0.05) effect on total protein content in the roots and leaves. Total C, carbohydrate and protein exudation rates were not significantly (p > 0.05) affected by light treatment. Total phenolic content in the leaves was significantly (p < 0.05) higher in plants treated with 100% blue light than 100% red light. However, light quality had no significant effect (p > 0.05) on total phenolic content in roots and total phenol exudation rate. These results indicate that the root exudation rate of lettuce plants is not profoundly altered under different light qualities, however, a detailed metabolomic analysis might change this view. Further research will study the impact of these light conditions on the rhizosphere microbiome.