In-field autonomous UV-B irradiation and phenolic compounds optical detection of lettuce crops by an agricultural rover prototype

Plants cultivated under protected environments are usually deprived of the solar UV-B radiation. Consequently, they contain reduced amounts of beneficial antioxidant compounds. To avoid this, an autonomous agricultural rover prototype was designed and developed to treat by artificial UV-B radiation baby leaf vegetables, under high tunnels, over few days before harvest. Lettuce crops were irradiated for 5 consecutive days with a daily energy dose of 6.75 kJ m-2, delivered by LEDs emitting at 306 nm. The effects of the irradiation in inducting the accumulation of leaf phenolics was monitored in situ on-line by a fluorescence sensor installed on the rover. UV-B-treated and control blocks were spatially mapped daily by the fluorescence sensor providing an index of the epidermal phenolic compounds (EPhen). Due to the fast growing rate of the lettuce plants, mapping of the EPhen Index showed a significant spatial variability, for both treatments. UV-B treatments led to a progressive rise of the EPhen Index, with stronger effects observed as the number of treatments increased. This approach can be applied to increase the quality of baby leaf vegetables, used for the production of Ready-To-Eat (RTE) salads, by enhancing their content of bioactive molecules. It would be advantageous for growers that could offer higher quality and more profitable products, as well as for consumers that could benefit from RTE salads rich in health-promoting antioxidants. The non-destructive monitoring of leaf epidermal phenolics, both as spatial average and distribution represents a rapid and useful tool for precision horticulture. It allows to schedule the best harvest time for obtaining high-quality products, especially important under the actual rapid and unpredictable changes in weather conditions.