Effects of UV-B and UV-C Spectrum Supplementation on the Antioxidant Properties and Photosynthetic Activity of Lettuce Cultivars

Indoor farming systems enable plant production in precisely controlled environments, managing factors such as lighting, photoperiod, and nutrient supply, to enhance crop yield and quality. However, the implementation of stable growth conditions and the absence of stress stimulants can weaken plants' defense responses and limit the accumulation of bioactive, health beneficial phytochemicals. A potential solution is the controlled application of stressors, such as supplemental ultraviolet (UV) light, which stimulates secondary metabolites (SM) synthesis and increase overall antioxidant potential. To this end, we analyzed the efficiency of short-term pre-harvest supplementation of red-green-blue (RGB, LED) spectrum with ultraviolet B (UV-B, 311 nm) or C (UV-C, 254 nm) light to boost phytochemicals synthesis. Additionally, given the biological harm of UV radiation due to high-energy photons, we monitored plants’ photosynthetic activity during treatment. Our analyses showed that UV-B radiation, applied with a cumulative dose of 15.622 kJ m–2, did not negatively impact photosynthetic activity while significantly increasing the overall antioxidant potential of lettuce through enhanced levels of polyphenolic SM (total phenolics, flavonoids, anthocyanins) and ascorbic acid. Conversely, UV-C radiation, applied with a lower cumulative dose of 6.008 kJ m–2, induced anthocyanin accumulation in the green leaf cultivar but significantly harmed the photosynthetic apparatus and limited plant growth. These results are crucial for understanding the potential of UV light supplementation in producing functional plants.