Root metabolite secretion plays a critical role in increasing nutrient acquisition, allelopathy, and shaping the root-associated microbiome. While much research has explored the ecological functions of root exudates, their relevance to horticultural practices, particularly soilless cultivation, remains underexplored. Steering root exudation could help growers enhance the effectiveness of plant growth-promoting bacteria. This review summarizes current knowledge on root exudation in soilless systems, examining its process and discussing environmental influences in the context of soilless cultivation. Plants in soilless systems exhibit higher total carbon exudation rates compared to those in natural soils, with exudation profiles varying across systems and species. Root exudation decreases with plant age, with most environmental adaptations occurring during early growth stages. Several environmental factors unique to soilless systems affect root exudation. For instance, nutrient availability has a major impact on root exudation. Light intensity reduces exudation rates, and light quality influences exudation profiles in a species- and environment-dependent manner. Elevated CO₂ and temperature increase exudation. Factors related to the hydroponic nutrient solution and growing media composition remain insufficiently understood, necessitating further research.

• At an initial stage of development, soilless lettuce exudation rates are high. • At this stage, exudation rates are modulated by light and substrate composition. • Older plants show lower rates. • Rates of older plants are not influenced by these environmental conditions.

Plant root exudation is an inherent metabolic process that enhances various functions of the root system like the mobilization of nutrients and interactions with surrounding microbial communities. The role and extent of root exudation in soilless crop production is poorly investigated. Here, we analyzed soilless lettuce and show that the root exudation rate declines with plant age. Furthermore, the impact of light quality and substrate was assessed by growing soilless lettuce in 100% red light (660 nm), 100% blue light (450 nm), and white light (full-light spectrum) and in 100% perlite, 100% potting soil, or mixtures of both materials. Root exudates were collected at 10, 17 and 24 days after transplanting. The total carbon root exudation rate was influenced by light conditions and substrate composition at the earliest timepoint of the culture but not at later growth stages. The total carbohydrate exudation rate was significantly higher under pure blue and red light compared to white light. The impact of light depended on the presence of perlite in the substrate. The total phenolic compound exudation rate was most strongly influenced by the substrate composition and reached the highest level in either pure potting soil or pure perlite. These findings underscore the importance of root exudation during the initial stages of development. Light and growing media influence the exudation rate at this early stage, suggesting that exudation is an adaptive process of the soilless lettuce culture. HIGHLIGHT: At an initial stage of development, soilless lettuce exudation rates are high and modulated by light and substrate composition, whereas older plants show lower rates that are not influenced by these environmental conditions.