Introduction
Plants, as phototrophic organisms, heavily rely on light as a source of energy for photosynthesis. Light that is used in photosynthesis is absorbed by photosynthetic pigments — chlorophylls (Chls) and carotenoids (Cars). Chls absorb red and blue light most efficiently; Cars absorb blue light. Green light, although less efficient, can also be a source of energy in photosynthesis. Its absorption efficiency by the leaf depends on leaf structure which determines light scattering inside the leaf. Light is also an important source of information about the environment. Plants are able to evaluate the spectral composition, intensity or dose, duration and direction of light, photoperiod and its changes, which allows them to regulate their development in accordance with current light conditions. The signal, or regulatory, role of light is carried out by a system of photoreceptors that specifically perceive different ranges of the spectrum and evaluate their ratios: plants sense red and far-red light by phytochromes; blue and ultraviolet A (UV-A) light — by cryptochromes, phototropins, Zeitlupes and partly by phytochromes; ultraviolet B (UV-B) — by the photoreceptor UV-B; green — partly by cryptochromes and phototropins; and partly by phytochromes (Galvão and Fankhauser 2015). The photosynthetic apparatus (PSA) is also a kind of photoreceptor, the signals from which affect the expression of nuclear and plastid genes — primarily associated with photosynthesis, but not exclusively (Szechynska-Hebda and Karpinski 2013). Another role of light is biosynthesis, as photons directly take part in the synthesis of chemicals crucial for organism functioning. For example, one of the reactions in the Chl biosynthesis pathway — conversion of protochlorophyllide into chlorophyllide — requires a photon, and in angiosperms, it is the only pathway of Chl synthesis (Yuan et al. 2017; Solymosi and Mysliwa-Kurdziel 2021). Light also may be harmful for all living organisms — excessive light, particularly high-energy blue and ultraviolet, causes photodamage due to nonspecific absorption of these photons by various cell components (Yadav et al. 2020).
Studies of the physiology and productivity of plants grown with light of different spectral bands have a long history. In earlier works, light of different spectral quality was yielded by full-spectrum light sources (natural sunlight, incandescent, white fluorescent, high pressure sodium-vapour lamps or others) in combination with broad-band filters, or by fluorescent lamps of a certain colour (Voskresenskaya et al. 1968; Lichtenthaler et al. 1980; Leong et al. 1985; Deng et al. 1989; Bukhov et al. 1995).
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During the last three decades, there has been a new interest in such works because of the advent of new light sources — light-emitting diodes (LEDs), which emit narrow-band light (with half bandwidth ~ 10-30 nm) of different wavebands and have a light intensity high enough for long-term plant growth (Goins et al. 1997; Wang et al. 2009; Berkovich et al. 2017; Tarakanov et al. 2022). With such light sources, researchers can treat plants with narrow-band light of different wavelengths, which is important for photobiological research. Also, LEDs are increasingly used in horticultural lighting (greenhouses, vertical farms and space greenhouses) due to their beneficial technical characteristics: electrical efficiency, small mass and volume, solid state construction, low heat production, longevity and safety (Goins et al. 1997; He et al. 2015; Berkovich et al. 2017). Knowledge gained through fundamental research can help build LED-based horticultural light sources with an optimal spectrum. By manipulating the spectral composition of the lighting source, it is possible to create plants with certain growth characteristics, induce accumulation of important chemicals or increase plant yield (Landi et al. 2020).
The first works involving LEDs studied the effects of individual spectral bands and their combinations on plant growth and development, photosynthetic carbon assimilation, metabolite accumulation and yield. The spectral bands studied were mostly red and blue, due to their primary importance for photosynthesis; later studies included green, far-red, orange, yellow and violet spectral bands. The aim of these works was to find a spectrum that would allow optimal plant performance (Goins et al. 1997; Matsuda et al. 2007; Wang et al. 2009; Hogewoning et al. 2010; Liu et al. 2011a, b; Xiaoying 2012). Works aimed at optimising LED horticultural lighting are still being carried out. At the same time, more works focused on understanding the physiological, biochemical and genetic mechanisms behind the effects of narrow-band light on plants (Avercheva et al. 2009; 2010; 2016; Savvides et al. 2012; Muneer et al. 2014; Su et al. 2014; Miao et al. 2016; He et al. 2017; Kochetova et al. 2018, 2022; Lanoue et al. 2018; Hamdani et al. 2019; Gao et al. 2020; Karlický et al. 2021; Tantharapornrerk et al. 2021; Trojak and Skowron 2021; Tarakanov et al. 2022). Many of them focus on the PSA as the source of organic compounds for plant growth and development. The PSA is itself a complex, multi-component structure, and its components can be affected by light spectrum in different ways (summarised in Fig. 1).
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Despite there being a large body of literature concerning LEDs as light sources for growing plants, the results of these works are often hard to compare. This is, in part, because plants of different species and age are used in these studies. Light spectrum, irradiance, photoperiod and length of plant exposure to narrow-band light may also vary. Control plants can be grown with light sources with different emission spectra. It has also been shown that effects of narrow-band light on plants are often specific to species, ecotype and cultivar (Landi et al. 2020; Yavari et al. 2021), which further complicates data systematisation. However, the accumulated knowledge allows putting together a general, though incomplete, scheme of the interaction between light of different spectral wavebands and plant physiological state. It is especially important to understand the interaction between the role of light as a source of energy and as an environmental signal.
In this review, we aim to analyse the current data on PSA development and function under narrow-band light of different spectral wavebands — red (600-700 nm), green (500-570 nm), blue (420-500 nm) and near ultraviolet (UV-A, 315-400 nm). This requires considering two topics: (1) what is the structure and function of the photosynthetic apparatus and its components in plants grown with light of different spectral bands; (2) what regulatory mechanisms light can use to control photosynthetic apparatus development. Here, we summarise the recent literature on these topics, considering separately the effects of light spectrum on different PSA components (as shown in Fig. 1) and attempt to integrate them where possible. As the photosynthetic apparatus relies on stomata for sufficient CO2 uptake, the regulation of their development and function by light spectrum is also considered.
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