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 (Fv/Fm) 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.
Highlights
Plants perceive signals from their surrounding environment and regulate their growth and development (Smith, 1982; Huché-Thélier et al, 2016)
Plants grown under red light had the highest θ while those grown under blue light had the lowest (Figure 3F)
Together with an unchanged maximum net assimilation rate (Amax), decreased electron transport rate (ETR) and fraction of open photosystem II (PSII) centres (qL), as well as an increased non-photochemical quenching (NPQ) in plants grown under blue photosynthetically active radiation (PAR), we show that monochromatic blue light does not improve plant acclimation or increase photoprotection to Ultraviolet B (UV-B)
Summary
Plants perceive signals from their surrounding environment and regulate their growth and development (Smith, 1982; Huché-Thélier et al, 2016). Plants are highly sensitive to the spectral distribution of light and perceive changes in the light spectra and intensity through several protein photoreceptors (Fankhauser and Chory, 1997). These photoreceptors are sensitive to specific regions of the spectrum and overlap of action spectra of different plant photoreceptors occur, allowing the plant to detect a wider and more complex range of changes in their light environment (Heijde and Ulm, 2012). Cryptochromes and phototropins are sensitive to blue light (400–500 nm) and ultraviolet (UV) radiation A (UV-A) (315–400 nm), whereas phytochromes perceive red (600–700 nm) and far-red (700– 800 nm) light. The levels of the photosynthetically active radiation (PAR) (400–700 nm) and the PAR/UV-B ratio are factors that strongly influence plant UV-B responses (Krizek, 2004; Lidon et al, 2012; Jenkins, 2017)
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