Bioassay of Root Exudates, their Impact in Susceptible Species and their Degradation in Hydroponics: A Review

Plants grown in hydroponics exhibit superior characters and yield compared to those grown in soil. However, root exudates accumulated particularly in renewed nutrient solution (RNS) hinders development of crops in hydroponics. These exudates are secondary metabolites released by the roots as a result of physiological processes, exhibiting autotoxic effects in plants. They are allelochemicals encompassing various chemical groups such as phenolic compounds, terpenoids, alkaloids, benzoxazinoids or other organic acids. The quantity and type of allelochemical released by plants varies depending on multiple factors such as temperature, light, nutrient deficiency, stress, physiological status of plant, pH and environment. The combined effect of more than one allelochemical is often additive or synergistic thus elevating the impact caused by individual allelochemical. These phytotoxic exudates are known to have inhibited the growth and development of plants by diverse pathways unique to their characteristic. There are several methods developed for degradation of exudates including AC (Activated Charcoal), O3/H2O2 treatment or other. This review discusses the bioassay of secondary metabolites causing autotoxicity, mechanism, impact in horticultural and ornamental crops, factors affecting their release and methods for elimination in hydroponics.