Effects of solar and supplemental UV-B radiation on UV-B-absorbing compounds and malondialdehyde (MDA) accumulations in the peel of lemons collected in summer and winter were analyzed. UV-B-absorbing compounds were higher in flavedo than in albedo tissue in both seasons; however, the highest values were observed in summer. These compounds were also higher in outer than in inner flavedo surface. Lemons were categorized as sun-, semisun- and shaded-lemon according to localization inside the tree canopy. Depending on-tree localization UV-B-absorbing compounds were higher in flavedo of sun-lemon than in semisun- and shaded-lemon. Supplementary UV-B radiation (22 kJ m−2 day−1 UV-BBE) induced UV-B-absorbing compound synthesis in on-tree and postharvest lemons. Two minutes of supplemental UV-B irradiation in summer lemons produced a strong increment (300%) of UV-B-absorbing compound content, whereas in winter lemons a slight increase (30%) was observed only after 3 min of irradiation. By contrast, UV-B-absorbing compound accumulation was not observed in albedo. MDA accumulation showed approximately a similar trend of UV-B-absorbing compounds. According to our results, solar UV-B was not required for UV-B-absorbing compound accumulation in lemon peel. Relationships between UV-B-absorbing compounds, MDA, reactive oxygen species and pathogen protection are also discussed.