Differential Antioxidant Response to Supplemental UV-B Irradiation and Sunlight in Three Basil Varieties

Three basil plant varieties (<i>Ocimum basilicum</i> var. Genovese, <i>Ocimum</i> × <i>citriodorum</i>, and <i>Ocimum basilicum</i> var. <i>purpurascens</i>) were grown under moderate light (about 300 µmol photons m⁻² s⁻¹) in a glasshouse or growth chamber and then either transferred to an open field (average daily dose: 29.2 kJ m⁻² d⁻¹) or additionally exposed to UV-B irradiation in a growth chamber (29.16 kJ m⁻² d⁻¹), to reveal the variety-specific and light-specific acclimation responses. Total antioxidant capacity (TAC), phenolic profile, ascorbate content, and class III peroxidase (POD) activity were used to determine the antioxidant status of leaves under all four light regimes. Exposure to high solar irradiation at the open field resulted in an increase in TAC, total hydroxycinnamic acids (HCAs, especially caffeic acid), flavonoids, and epidermal UV-absorbing substances in all three varieties, as well as a two-fold increase in the leaf dry/fresh weight ratio. The supplemental UV-B irradiation induced preferential accumulation of HCAs (rosmarinic acid) over flavonoids, increased TAC and POD activity, but decreased the ascorbate content in the leaves, and inhibited the accumulation of epidermal flavonoids in all basil varieties. Furthermore, characteristic leaf curling and UV-B-induced inhibition of plant growth were observed in all basil varieties, while a pro-oxidant effect of UV-B was indicated with H₂O₂ accumulation in the leaves and spotty leaf browning. The extent of these morphological changes, and oxidative damage depended on the basil cultivar, implies a genotype-specific tolerance mechanism to high doses of UV-B irradiation.