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

Abstract

Three basil plant varieties (Ocimum basilicum var. Genovese, Ocimum × citriodorum, and Ocimum basilicum var. purpurascens) 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.

Keywords: Ocimum basilicum var. Genovese; Ocimum basilicum var. purpurascens; Ocimum × citriodorum; ascorbate; epidermal flavonoids; hydrogen peroxide; polyphenols; supplemented and ecologically relevant UV-B irradiation; total leaf antioxidant capacity.

Figure 1

Representative images of three basil varieties (five plants for each variety) grown at constant PAR and artificial UV-B radiation (1.35 W m⁻²) 6 h per day after 8 days of treatment. Ocimum basilicum var. Genovese (GB) (top), Ocimum × citriodorum (CB) (middle), and Ocimum basilicum var. purpurascens (PB) (bottom).

Figure 2

The dynamics of epidermal flavonoid (EpFlav) accumulation, in the leaves of three basil varieties (Ocimum basilicum var. Genovese—GB, n = 20; Ocimum × citriodorum—CB, n = 20; and Ocimum basilicum var. purpurascens—PB, n = 20) grown in (A) glasshouse (GH—black line) and open field (OF—red line) during 15 days; (B) growth chamber (GC—black line) and growth chamber with additional UV-B supplementation (UV-B—red line) during 8 days. Values are given as means ± SE. Significant differences in EpFlav between plants grown in GH and OF, as well as between plants grown in GC and UV-B are indicated for all varieties according to the Mann–Whitney U-test (* p ≤ 0.05, ** p < 0.01, *** p < 0.001).

Figure 3

The content of hydroxybenzoic acids (HBAs) in the leaves of three basil varieties (Ocimum basilicum var. Genovese, GB, n = 7–9; Ocimum × citriodorum, CB, n = 7–9; and Ocimum basilicum var. purpurascens, PB, n = 7–9) grown under different conditions: glasshouse—GH (light purple), open field—OF (dark purple), growth chamber—GC (light green), and growth chamber + UV-B (dark green). Protocatechuic acid, PrcA; hydroxybenzoic acid, HBA; p-hydroxybenzoic acid, p-HBA; syringic acid, SyA; gallic acid, GA. Values are given as means ± SE. Different letters denote significant differences between the different treatments and basil varieties (n.d.—not detected, p ≤ 0.05), according to Tukey’s post hoc test.

Figure 4

The content of hydroxycinnamic acids (HCA) in leaves of three basil varieties (Ocimum basilicum var. Genovese, GB, n = 7–9; Ocimum × citriodorum, CB, n = 7–9; and Ocimum basilicum var. purpurascens, PB, n = 7–9) grown under different conditions: glasshouse—GH (light purple), open field—OF (dark purple), growth chamber—GC (light green), and growth chamber + UV-B (dark green). Rosmarinic acid, RA; caffeic acid, CA; other hydroxycinnamic acids, HCAs; p-coumaric acid, pCA; chlorogenic acid, CGA; ferulic acid, FA. Values are given as means ± SE. Different letters denote significant differences between the different treatments and basil varieties (p ≤ 0.05), according to Tukey’s post hoc test.

Figure 5

The content of epicatechin (ECat), quercetin (Q), and cyanidin (Cy) in the leaves of three basil varieties (Ocimum basilicum var. Genovese, GB, n = 7–9; Ocimum × citriodorum, CB, n = 7–9; and Ocimum basilicum var. purpurascens, PB, n = 7–9) grown under different conditions: glasshouse—GH (light purple), open field—OF (dark purple), growth chamber—GC (light green), and growth chamber + UV-B (dark green). Values are given as means ± SE. Different letters denote significant differences between the different treatments and basil varieties (p ≤ 0.05), according to Tukey’s post hoc test. n.d.—not detected.

Figure 6

Total antioxidant capacity (TAC), reduced ascorbate (Asc) content, and class III peroxidases’ (PODs) activity of three basil varieties (Ocimum basilicum var. Genovese, GB, n = 7–9; Ocimum × citriodorum, CB, n = 7–9; and Ocimum basilicum var. purpurascens, PB, n = 7–9) grown under different conditions: glasshouse—GH (light purple), open field—OF (dark purple), growth chamber—GC (light green), and growth chamber + UV-B (dark green). Values are given as means ± SE. Different letters denote significant differences between the different treatments and basil varieties (p ≤ 0.05), according to Tukey’s post hoc test.

Figure 7

Representative images of H₂O₂ accumulation using the DAB uptake method in the leaves of three basil varieties grown at constant PAR (a–c) and constant PAR with artificial UV-B irradiation (d–f) after 8 days of treatment. Ocimum basilicum var. Genovese (GB) (a,d), Ocimum × citriodorum (CB) (b,e), and Ocimum basilicum var. purpurascens (PB) (c,f).