Sustainable Reuse of Expresso Coffee By-products as a Natural Fertilizer to Improve Growth and Photosynthesis in Cucumber ( Cucumis sativus L.) Plants

Spent coffee grounds (SCGs), deriving from the high global coffee consumption, can have an appreciable environmental impact. One of the eco-friendly reuses of SCGs is their application as a natural fertilizer. This study adopted a soilless cultivation approach to growing Cucumis sativus seedlings using, as rooting media, different coffee solutions obtained by infusing fresh coffee powder or SCGs from mocha and pods for 7 or 30 days. Seedlings’ germination, eco-physiological and biochemical traits were evaluated and the potential toxicological risks or beneficial effects of SCGs on plants were assessed. Results indicate that, although germination was unaffected, fresh coffee medium inhibited seedling development and that 7 days mocha solution stopped seedling growth and induced leaf structural alterations. Conversely, 30 days mocha and pod suspensions promoted seedling biomass, and increased leaf area, leaf polyphenol concentration, and photochemical efficiency. In conclusion, findings indicate that fresh powder can induce toxic effects while SCGs can improve cucumber from a physiological point of view by acting as a natural fertilizer. This study opens future investigations that will help manage coffee waste released into the environment by sustainable practices. Graphical Abstract