Spectral Composition of Light Affects Sensitivity to UV-B and Photoinhibition in Cucumber

Abstract

Ultraviolet B (UV-B) (280-315 nm) and ultraviolet A (UV-A) (315-400 nm) radiation comprise small portions of the solar radiation but regulate many aspects of plant development, physiology and metabolism. Until now, how plants respond to UV-B in the presence of different light qualities is poorly understood. This study aimed to assess the effects of a low UV-B dose (0.912 ± 0.074 kJ m^-2 day^-1, at a 6 h daily UV exposure) in combination with four light treatments (blue, green, red and broadband white at 210 μmol m^-2 s^-1 Photosynthetically active radiation [PAR]) on morphological and physiological responses of cucumber (Cucumis sativus cv. "Lausanna RZ F1"). We explored the effects of light quality backgrounds on plant morphology, leaf gas exchange, chlorophyll fluorescence, epidermal pigment accumulation, and on acclimation ability to saturating light intensity. Our results showed that supplementary UV-B significantly decreased biomass accumulation in the presence of broad band white, blue and green light, but not under red light. UV-B also reduced the photosynthetic efficiency of CO₂ fixation (α) when combined with blue light. These plants, despite showing high accumulation of anthocyanins, were unable to cope with saturating light conditions. No significant effects of UV-B in combination with green light were observed for gas exchange and chlorophyll fluorescence parameters, but supplementary UV-B significantly increased chlorophyll and flavonol contents in the leaf epidermis. Plants grown under red light and UV-B significantly increased maximum photosynthetic rate and dark respiration compared to pure red light. Additionally, red and UV-B treated plants exposed to saturating light intensity showed higher quantum yield of photosystem II (PSII), fraction of open PSII centres and electron transport rate and showed no effect on the apparent maximum quantum efficiency of PSII photochemistry (F_v/F_m) or non-photochemical quenching, in contrast to solely red-light conditions. These findings provide new insights into how plants respond to UV-B radiation in the presence of different light spectra.

Keywords: LEDs; UV-B; chlorophyll fluorescence; cucumber; gas exchange; light quality; morphology.

FIGURE 1

(A) Spectral irradiance (in W(cm² nm)^–1) in Perspex covered (control) and cellulose acetate covered (UV-B) boxes under four different PAR backgrounds (Table 1); broadband White (gray line), Blue, Green, Red (with lines of respective color) and UV-B (violet line). (B) Spectral irradiance in the UV range (violet line) with enlarged scale and the plant-weighted UV dose (black dotted line) in the UV-B treatments.

FIGURE 2

Biomass accumulation of cucumber plants grown under four PAR backgrounds after 14 days without (solid bars) or with (dashed bars) exposure to supplementary UV-B irradiation. (A) Height (cm); (B) Internode length (INL, cm); (C) Total dry mass (TDM, g); (D) Total leaf area (TLA, cm²); (E) Specific leaf mass (SLM, g cm^–2); (F) Leaf number. Data are mean values (n = 21 ± SE). Capital letters indicate significant differences between growth light qualities without UV-B and lower-case letters between non-UV-B-exposed plants and UV-B-exposed plants within the same light backgrounds, both at P < 0.05.

FIGURE 3

(A) Fitted light response curves of cucumber grown under four LED light backgrounds (White, Blue, Green, and Red) without (solid bars) and with (dashed bars) exposure to supplementary UV-B radiation for 14 days [for variation of the data at light saturation refer from panels (B–F)], where the arrow indicates the growth PAR. Curve fitted parameters: (B) Maximum net assimilation rate (A_max); (C) Dark respiration (R_dark); (D) Light compensation point (LCP); (E) Apparent quantum yield of photosynthesis (α); and (F) convexity (θ). Bars represent the mean values (n = 21 ± SE). Capital letters indicate significant differences between growth light qualities without UV-B and lower-case letters between non-UV-B-exposed plants and UV-B-exposed plants within the same PAR background, both at P < 0.05.

FIGURE 4

Chlorophyll fluorescence parameters measured in cucumber plants grown under four different PAR qualities (White, Blue, Green, and Red) and without (solid bars) or with (dashed bars) exposure to supplementary UV-B radiation for 14 days, prior to and after a 5 h saturating light treatment (1600 μmol m^–2 s^–1). (A) Maximum photochemical efficiency (F_v/F_m); (B) Apparent electron transport rate (ETR); (C) Fraction of oxidized PSII (q_L), (D) Non-photochemical quenching (NPQ). Bars represent mean values (Before saturating light: n = 21 ± SE; After saturating light n = 15 ± SE). Capital letters indicate significant differences between growth light qualities without UV-B and lower-case letters between non-UV-B-exposed plants and UV-B exposed plants within the same PAR background, both at P < 0.05.

FIGURE 5

(A) Epidermal chlorophyll, (B) flavonol and (C) anthocyanin content of cucumber plants, as measured with a DUALEX instrument, and grown under different light quality backgrounds (White, Blue, Green, and Red) and without (solid bars) or with (dashed bars) exposure to supplementary UV-B radiation for 14 days. Data are mean values (n = 15 ± SE). Capital letters indicate significant differences between growth light qualities without UV-B and lower-case letters between non-UV-B-exposed plants and UV-B exposed plants within the same PAR background, both at P < 0.05.