Yield, Nutritional, and Thermal Responses of Lettuce (Lactuca sativa) and Eggplant (Solanum melongena) Under Greenhouse Covers with Different UV-B Transmittance

Ultraviolet-B (UV-B) radiation plays a pivotal role in plant growth, metabolism, and the accumulation of bioactive compounds, but its effects under greenhouse conditions are highly species- and dose-dependent. This study investigated the responses of eggplant (Solanum melongena L., cv. Lunga Napoletana) and lettuce (Lactuca sativa L., cv. Rosplus) cultivated under greenhouse films transmitting 3–39% of ambient UV-B. Leaf temperature was monitored throughout the growth cycle using infrared thermography, while physiological parameters (chlorophyll, flavonoids, anthocyanins, and nitrogen index) and post-harvest nutritional traits (antioxidant activity, vitamin C, carotenoids, and total chlorophyll) were assessed. Comparative analysis revealed species-specific responses. Eggplant exhibited peak nutraceutical quality at higher UV-B levels (35–39%) with minimal changes in yield, whereas lettuce achieved maximal yield and secondary metabolite accumulation under intermediate UV-B (30–35%). At the highest UV-B transmittance (39%), both species exhibited stable or slightly reduced thermal and physiological parameters, indicating dose-dependent regulatory mechanisms that maintain photoprotection and metabolic activity under elevated UV-B exposure. Results suggest an apparent optimal range of UV-B transmittance in greenhouse systems under the tested experimental conditions, contributing to improved crop productivity and nutritional quality.