Light as a resource in plants

Light is a fundamental resource in the physiology and ecology of plants, serving both as an energy source and as an environmental signal. Through photosynthesis, plants convert light into chemical energy, while also using light cues to regulate vital processes such as germination, phototropism, flowering, and senescence (Teixeira, Reference Teixeira2020). Plants perceive changes in light intensity, quality, and duration – especially at dawn and dusk – to synchronize internal processes such as circadian rhythms and organelle movements, coordinating both metabolic and developmental pathways (Kong and Okajima, Reference Kong and Okajima2016; Teixeira, Reference Teixeira2020).

Photoreceptors mediate many of these responses. Phytochromes are primarily activated by red and far-red light (600–750 nm), while cryptochromes and phototropins respond to blue light (390–500 nm) (Chaves et al., Reference Chaves, Pokorny, Byrdin, Hoang, Ritz, Brettel, Essen, Van Der Horst, Batschauer and Ahmad2011; Christie et al., Reference Christie, Blackwood, Petersen and Sullivan2015). These proteins, containing chromophores, enable plants to perceive subtle changes in the light environment and trigger complex signalling cascades (Paradiso et al., Reference Paradiso, Meinen, Snel, De Visser, Van Ieperen, Hogewoning and Marcelis2011).

Photosynthetically Active Radiation (PAR), ranging from 400 to 700 nm, is traditionally considered the most effective spectrum for photosynthesis (Burgie et al., Reference Burgie, Bussell, Walker, Dubiel and Vierstra2014). Chlorophyll pigments located in chloroplasts absorb this light to drive the light-dependent reactions of photosynthesis (Finch et al., Reference Finch, Bailey, Mcarthur and Nasitwitwi2004). These reactions convert light energy into ATP and NADPH, which are then used in the Calvin-Benson cycle to fix carbon into sugars (Ruan, Reference Ruan2014). Under low light, insufficient energy impairs cellular processes, whereas excess light can cause photoinhibition and damage to the photosynthetic apparatus (Kono and Terashima, Reference Kono and Terashima2014; Erickson et al., Reference Erickson, Wakao and Niyogi2015).

Plants adjust their morphology and physiology in response to light availability. Under low light conditions, they may exhibit shade-adaptive traits such as increased chlorophyll concentration (per unit mass), leaf expansion, and reorientation toward light (Solymosi and Schoefs, Reference Solymosi and Schoefs2010; Higa et al., Reference Higa, Suetsugu, Kong and Wada2014). In contrast, exposure to high light often induces protective responses, including chloroplast repositioning and the synthesis of photoprotective pigments (Cowie et al., Reference Cowie, Byrne, Witkowski and Venter2016). Physiological traits such as photosynthetic rate, stomatal conductance, and water use efficiency are strongly influenced by light intensity and spectral composition (Zhang et al., Reference Zhang, Liu, Yang, Du and Yang2016; Xu et al., Reference Xu, Liang and Yang2019).

Light not only influences photosynthesis but also modulates sugar production, transport, and signalling. Sugars like glucose and sucrose function as key regulatory molecules that influence growth, development, and stress responses (Baena-González and Hanson, Reference Baena-González and Hanson2017). High sugar concentrations can repress photosynthetic gene expression, linking carbon status with metabolic feedback (Rolland et al., Reference Rolland, Leuven, Rolland and Sheen2005). These molecules also regulate the cell cycle, flowering, fruit development, and senescence, and act as osmoprotectants and signalling mediators under stress conditions (Lukaszuk et al., Reference Lukaszuk, Rys, Możdżeń, Stawoska, Skoczowski and Ciereszko2017; Fernandez et al., Reference Fernandez, Ishihara, George, Mengin, Flis, Sumner, Arrivault, Feil, Lunn, Zeeman, Smith and Stitt2017). Thus, artificial lighting at night, by altering sugar production and signalling, may influence crop development and stress adaptation in ways that warrant further investigation.

Starch metabolism is tightly regulated by the diel cycle. During the day, starch accumulates in leaves, while at night it is degraded to maintain metabolic activity (Stitt and Zeeman, Reference Stitt and Zeeman2012). Fluctuations in light availability – such as prolonged darkness – can lead to energy stress, impacting growth and development. Central regulatory pathways sense sugar and energy levels, linking environmental cues to growth responses (Robaglia et al., Reference Robaglia, Thomas and Meyer2012; Rolland et al., Reference Rolland, Leuven, Rolland and Sheen2005).

Finally, root system architecture is also influenced by light indirectly through carbon allocation and sugar signalling (Gupta et al., Reference Gupta, Singh and Laxmi2015). Biomass accumulation and the balance between sugar storage (as starch) and export (as sucrose) are key indicators of a plant’s light-use efficiency, particularly under variable light conditions such as those observed in soybean cultivation (Fang et al., Reference Fang, Ma, Wang, Nian, Ma, Huang and Mu2021).

Many studies have shown that plant responses to light involve an intricate network of signalling pathways across molecular, cellular, and physiological levels (Chory, Reference Chory2010; Teixeira, Reference Teixeira2020) (Fig. 1). Understanding these mechanisms is essential for exploring the potential of supplemental lighting in agriculture.

Figure 1. Diagram illustrating the main processes and responses related to the effects of light on plants. ‘Information source’ refers to the role of light as a signal that regulates physiological and developmental processes in plants.

Relationship between light and water availability

Numerous vital processes for proper plant functioning – such as metabolic reactions, ion transport, nutrient metabolism, cell development, and solute translocation – depend on an adequate water in supply. As a result, water deficit induces plants to modify their biochemical pathways, including transcription and translational processes, and, consequently, alters physiological mechanisms (Farooq et al., Reference Farooq, Wahid, Kobayashi, Fujita and Basra2009; Zargar et al., Reference Zargar, Gupta, Nazir, Mahajan, Malik, Sofi, Shikari and Salgotra2017). One of the most significant detrimental effects of a water deficit is the impairment of photosynthesis, a fundamental mechanism for plant growth and survival. Thus, water deficit represents a major threat to crop production (Shao et al., Reference Shao, Guangcheng, Doudou, Xi, Jingtao and Zhenhua2016).

Under experimental water deficit conditions, soybean plants exposed to high light intensity and water deficit grew in environments with lower relative humidity and higher temperatures (Zhang et al., Reference Zhang, Liu, Yang, Du and Yang2016). As a consequence, the relative water content in the leaves was lower, demonstrating that both high light intensity and soil water deficiency led to tissue dehydration (Nicolás et al., Reference Nicolás, Torrecillas, Dell Amico and Alarcón2005). The authors also observed a decrease in stomatal conductance and an increase in intercellular CO₂ concentration in plants under water deficit stress. Given that higher intercellular CO₂ typically enhances assimilation, the observed decline in photosynthesis strongly suggests that the water deficit impacted not only stomatal conductance but also mesophyll conductance and/or the biochemical aspects of photosynthesis. Reduced water availability has also been shown to lower the light saturation point (Zhang et al., Reference Zhang, Liu, Yang, Du and Yang2016; Engels et al., Reference Engels, Rodrigues, Ferreira, Inagaki and Nepomuceno2017; Schneider et al., Reference Schneider, Caverzan and Chavarria2019; Qiao et al., Reference Qiao, Hong, Jiao, Hou and Gao2024).

Intercellular CO₂ concentration serves as a reliable indicator of changes in net photosynthetic rate. These reductions may arise not only from decreased stomatal conductance, but also from anatomical and biochemical changes in mesophyll tissues – such as reduced intercellular air space, altered cell wall thickness, or changes in chloroplast distribution – all of which can affect CO₂ diffusion and photosynthetic capacity (Zhang et al., Reference Zhang, Liu, Yang, Du and Yang2016). Leaf relative water content influences stomatal behaviour, as reduced water availability triggers stomatal closure, resulting in lower conductance and decreased photosynthesis under high light conditions (Mahajan and Tuteja, Reference Mahajan and Tuteja2005).

In addition to this indirect effect, light also plays a direct role in regulating stomatal opening – particularly blue light, which activates specific photoreceptors in guard cells to promote stomatal opening under favourable water conditions (Shimazaki et al., Reference Shimazaki, Doi, Assmann and Kinoshita2007). Furthermore, under water deficit conditions, the decrease in photosynthetic activity and increase in photoinhibition are more pronounced in plants exposed to intense light than in those under shaded conditions (Sofo et al., Reference Sofo, Dichio, Montanaro and Xiloyannis2009).

In soybean, changes in photosynthetic rate under unbalanced water deficit conditions were closely associated with alterations in chlorophyll fluorescence parameters (Iqbal et al., Reference Iqbal, Hussain, Raza, Yang, Safdar, Brestic, Aziz, Hayyat, Asghar, Wang, Zhang, Yang and Liu2019). Chlorophyll fluorescence serves as a sensitive indicator of water stress on photosynthesis (Kalaji et al., Reference Kalaji, Rastogi, Živčák, Brestic, Daszkowska-Golec, Sitko, Alsharafa, Lotfi, Stypiński, Samborska and Cetner2018). Net photosynthetic rate, stomatal conductance, and transpiration are commonly reduced under water deficit conditions (Iqbal et al., Reference Iqbal, Hussain, Raza, Yang, Safdar, Brestic, Aziz, Hayyat, Asghar, Wang, Zhang, Yang and Liu2019). Similarly, in wheat, water deficit stress negatively impacts stomatal conductance, relative chlorophyll content, initial fluorescence, PSII quantum yield, and fresh and dry stem biomass (Pour-Aboughadareh et al., Reference Pour-Aboughadareh, Omidi, Naghavi, Etminan, Mehrabi, Poczai and Bayat2019). Although Stallmann et al., (Reference Stallmann, Schweiger, Pons and Müller2020) observed that PSII quantum yield remained stable, they reported substantial reductions in above ground biomass in water-stressed plants. Zhao et al., (Reference Zhao, Liu, Shen, Yang, Han, Tian and Wu2020) found that increasing levels of water deficit significantly decreased stomatal conductance, intercellular CO₂ concentration, photosynthesis, and transpiration in wheat. Plant height, aboveground biomass, and grain weight were also reduced, highlighting the direct impact of water stress on crop development and yield. Abid et al., (Reference Abid, Ali, Qi, Zahoor, Tian, Jiang, Snider and Dai2018) further showed that water deficit stress disrupts leaf water relations and membrane stability, increases reactive oxygen species (ROS) and lipid peroxidation, and triggers osmotic adjustment through accumulation of soluble sugars, proline, and free amino acids.

Despite these insights, little is known about how variations in light intensity and spectral composition modulate the impact of water deficit on plant photosynthetic performance (Zhang et al., Reference Zhang, Liu, Yang, Du and Yang2016; Zhao et al., Reference Zhao, Liu, Shen, Yang, Han, Tian and Wu2020), particularly regarding how specific light spectra interact with soil water availability to influence plant physiological responses. Moreover, the effects of water deficit stress on chlorophyll fluorescence and other photosynthetic parameters in soybean (Agric et al., Reference Agric, Eng, Wang, Wang, Pan, Zhang, Luo and Ji2018; Iqbal et al., Reference Iqbal, Hussain, Raza, Yang, Safdar, Brestic, Aziz, Hayyat, Asghar, Wang, Zhang, Yang and Liu2019) and wheat (Jafari and Khabiri, Reference Jafari and Khabiri2014; Stefański et al., Reference Stefański, Siedlarz, Matysik and Rybka2019) remain insufficiently explored. Given that water deficit is a major cause of yield loss and that plants in the field are exposed to rapid fluctuations in light intensity (Grieco et al., Reference Grieco, Roustan, Dermendjiev, Rantala, Jain, Leonardelli, Neumann, Berger, Engelmeier, Bachmann, Ebersberger, Aro, Weckwerth and Teige2020), understanding how light quality and intensity interact with water availability is essential for improving crop management strategies and productivity in extensive agricultural systems.

Artificial lighting in agronomic plants

Light quality and intensity significantly influence plant growth and biochemical composition. Light Emitting Diodes (LEDs) have gained prominence in controlled plant production systems due to their spectral specificity and superior energy efficiency compared to traditional lighting sources (Nelson and Bugbee, Reference Nelson and Bugbee2014; Singh et al., Reference Singh, Basu, Meinhardt-Wollweber and Roth2015; Monostori et al., Reference Monostori, Heilmann, Kocsy, Rakszegi, Ahres, Altenbach, Szalai, Pál, Toldi, Simon-Sarkadi, Harnos, Galiba and Darko2018; Al Murad et al., Reference Al Murad, Razi, Jeong, Samy and Muneer2021). These characteristics make LEDs a practical and economically viable tool for optimizing both yield and quality in the production of leafy greens, vegetables, fruits, ornamental crops, and flowers (Li and Kubota, Reference Li and Kubota2009; Bantis et al., Reference Bantis, Smirnakou, Ouzounis, Koukounaras, Ntagkas and Radoglou2018).

Many studies have revealed that different LED spectra can modulate plant growth and quality attributes (Sharma et al., Reference Sharma, Kumar, Shahzad, Ramakrishnan, Singh Sidhu, Bali, Handa, Kapoor, Yadav, Khanna, Bakshi, Rehman, Kohli, Khan, Parihar, Yuan, Thukral, Bhardwaj and Zheng2019; Stefański et al., Reference Stefański, Siedlarz, Matysik and Rybka2019; Fang et al., Reference Fang, Ma, Wang, Nian, Ma, Huang and Mu2021). Despite these promising results in controlled indoor production systems, the widespread adoption of LEDs in agricultural production remains limited, and further studies are needed to elucidate their effects across species and genotypes (Hasan et al., Reference Hasan, Bashir, Ghosh, Lee and Bae2017). LEDs are long-lasting light sources, ranging from ultraviolet (UV) to infrared (Stutte et al., Reference Stutte, Edney and Skerritt2009; Paradiso et al., Reference Paradiso, Meinen, Snel, De Visser, Van Ieperen, Hogewoning and Marcelis2011). Among these, blue and red wavelengths are the most influential for plant development, as they serve as primary energy sources for photosynthetic CO₂ assimilations (Kasajima et al., Reference Kasajima, Inoue, Mahmud and Kato2008; Paradiso et al., Reference Paradiso, Meinen, Snel, De Visser, Van Ieperen, Hogewoning and Marcelis2011; Pennisi et al., Reference Pennisi, Blasioli, Cellini, Maia, Crepaldi, Braschi, Spinelli, Nicola, Fernandez, Stanghellini, Marcelis, Orsini and Gianquinto2019).

However, the classical action spectrum by McCree (Reference Mccree1971) demonstrated that green light can be at least 70% as efficient as red or blue for photosynthesis, with broad peaks in the PAR range. More recent studies indicate that adding green light to red and blue spectra can increase overall quantum yield by promoting light penetration into deeper leaf layers (Terashima et al., Reference Terashima, Fujita, Inoue, Chow and Oguchi2009). Despite this, green LEDs are rarely used in horticulture due to their lower energy conversion efficiency (µmol/J), which is approximately half that of red and blue LEDs (Weinold et al., Reference Weinold, Kolesnikov and Anadón2025). Instead, white LEDs, which include some green wavelengths, are becoming increasingly adopted for their balance between efficiency and spectral coverage (Park and Runkle, Reference Park and Runkle2018).

Red light (610–720 nm) enhances biomass accumulation (Fig. 2), promotes photosynthetic apparatus development (Wollaeger and Runkle, Reference Wollaeger and Runkle2015; Kaiser et al., Reference Kaiser, Ouzounis, Giday, Schipper, Heuvelink and Marcelis2019; Al Murad et al., Reference Al Murad, Razi, Jeong, Samy and Muneer2021), and induces flowering (Liao et al., Reference Liao, Suzuki, Yu, Zhuang, Takai, Ogasawara, Shimazu and Fukui2014; Zheng et al., Reference Zheng, He and Song2019). In lettuce, red light increases anthocyanin accumulation (Hasan et al., Reference Hasan, Bashir, Ghosh, Lee and Bae2017), suppresses lesion development, and induces defence-related gene expression (Ahn et al., Reference Ahn, Kim and Yun2015). It also reduces nitrate content while enhancing sugars and phenolic compounds (Samuoliene et al., Reference Samuoliene, Brazaityte, Sirtautas, Novičkovas and Duchovskis2012). In cereals, red light improves frost tolerance (Novák et al., Reference Novák, Boldizsár, Gierczik, Vágújfalvi, Ádám, Kozma-Bognár and Galiba2017) and enhances photosynthetic pigment concentrations, CO₂ assimilation, grain yield, and plant height (Dong et al., Reference Dong, Fu, Liu and Liu2014; Monostori et al., Reference Monostori, Heilmann, Kocsy, Rakszegi, Ahres, Altenbach, Szalai, Pál, Toldi, Simon-Sarkadi, Harnos, Galiba and Darko2018). Moreover, red light has been associated with improved disease resistance, especially when applied during the night (Gallé et al., Reference Gallé, Czékus, Tóth, Galgóczy and Poór2021).

Figure 2. Main induced processes in plants under each light spectrum and the potential use in irrigation pivot for nighttime supplementation. Created with BioRender.com https://BioRender.com.

Blue light is crucial for stomatal regulation, promoting opening and increasing photosynthetic rates (Zheng et al., Reference Zheng, He and Song2019; Kalaitzoglou et al., Reference Kalaitzoglou, Taylor, Calders, Hogervorst, Van Ieperen, Harbinson, De Visser, Nicole and Marcelis2021; Appolloni et al., Reference Appolloni, Orsini, Pennisi, Gabarrell Durany, Paucek and Gianquinto2021; Paradiso and Proietti, Reference Paradiso and Proietti2022). It also increases stomatal density (Jensen et al. Reference Jensen, Clausen and Kjaer2018), stimulates photomorphogenesis, chlorophyll synthesis, and the production of secondary metabolites (Islam et al., Reference Islam, Kuwar, Clarke, Blystad, Gislerød, Olsen and Torre2012; Nascimento et al., Reference Nascimento, Leal-Costa, Coutinho, Moreira, Lage, Barbi, Costa and Tavares2013). Additionally, blue light contributes to protection against diseases (Tokuno et al., Reference Tokuno, Ibaraki, Ito, Arakj, Yoshimijra and Osaki2012; Xu et al., Reference Xu, Fu, Li and Wang2017). In cabbage and tobacco it enhances antioxidant activity and ascorbic acid content, contributing to resistance against pathogens such as cucumber mosaic virus (Chen et al., Reference Chen, Ren, Deng, Li, Cha, Lin and Xi2015), In lettuce, it promotes both shoot and root biomass accumulation (Sellaro et al., Reference Sellaro, Crepy, Trupkin, Karayekov, Buchovsky, Rossi and Casal2010), and stimulates anthocyanin synthesis (Li and Kubota, Reference Li and Kubota2009). In tomatoes, blue light fosters proline accumulation and mitigates grey mould symptoms (Hee-Sun Kook, Reference Hee-Sun Kook2013).

However, the effects of blue light can be cultivar-dependent. For example, Ouzounis et al., (Reference Ouzounis, Heuvelink, Ji, Schouten, Visser and Marcelis2016) observed differential responses among nine tomato genotypes exposed to red and red+blue light, highlighting variations in biomass, leaf area, and stomatal conductance. Appolloni et al., (Reference Appolloni, Orsini, Pennisi, Gabarrell Durany, Paucek and Gianquinto2021), in a meta-analysis, reinforced that LED supplementation enhances key traits such as soluble solids, ascorbic acid, chlorophyll content, photosynthetic efficiency, and productivity in tomatoes.

Green light penetrates deeper into the mesophyll, enhancing photosynthesis in lower canopy layers and influencing morphogenesis and physiological processes (Johkan et al., Reference Johkan, Shoji, Goto, Hahida and Yoshihara2012; Smith et al., Reference Smith, Mcausland and Murchie2017). Terashima et al., (Reference Terashima, Fujita, Inoue, Chow and Oguchi2009) further demonstrated that, under moderate to strong white light, additional green light can drive photosynthesis more effectively than red light in lower chloroplasts, due to its deeper penetration into the leaf and more uniform excitation of chloroplasts throughout the mesophyll.

In lettuce, green light increased net photosynthetic rates, photochemical efficiency, and antioxidant enzyme activity (Bian et al., Reference Bian, Yang, Li, Cheng, Barnett and Lu2018) (Table 1). Recent evidence from a comprehensive meta-analysis has challenged the long-standing assumption that green light is less effective than red or blue light in promoting plant growth. Chen et al. (Reference Chen, Bian, Marcelis, Heuvelink, Yang and Kaiser2024) analysed data across a wide range of studies and demonstrated that green light can be equally effective in promoting plant biomass accumulation, particularly when combined with red and blue wavelengths. This supports the view that green light penetrates deeper into the canopy and may contribute significantly to whole-plant photosynthesis and morphological responses, especially under dense planting conditions or in shaded environments.

Studies have consistently shown that plant responses to light spectra and intensities are species- and trait-dependent. In a comparative study with tomato, cucumber, radish, pepper, lettuce, soybean, and wheat, Snowden et al., (Reference Snowden, Cope and Bugbee2016) concluded that light quantity has a more pronounced effect on plant form than spectral quality. Light fluctuations can modulate thylakoid redox status, trigger reactive oxygen species (ROS) production, and lead to photodamage under suboptimal conditions (Alter et al., Reference Alter, Dreissen, Luo and Matsubara2012; Takagi et al., Reference Takagi, Ihara, Takumi and Miyake2019). Additionally, environmental variables such as water availability and planting density influence these light-mediated responses (Ouzounis et al., Reference Ouzounis, Heuvelink, Ji, Schouten, Visser and Marcelis2016; Takagi et al., Reference Takagi, Ihara, Takumi and Miyake2019; Arenas-Corraliza et al., Reference Arenas-Corraliza, Rolo, López-Díaz and Moreno2019; Hitz et al., Reference Hitz, Hartung, Graeff-Hönninger and Munz2019).

The effects of light quality on plant growth and development have been extensively investigated in agronomic plants (Ma et al., Reference Ma, Nian, Luo, Ma, Cheng and Mu2018; Monostori et al., Reference Monostori, Heilmann, Kocsy, Rakszegi, Ahres, Altenbach, Szalai, Pál, Toldi, Simon-Sarkadi, Harnos, Galiba and Darko2018; Stefański et al., Reference Stefański, Siedlarz, Matysik and Rybka2019; Etae et al., Reference Etae, Wamae, Khummueng, Utaipan and Ruangrak2020; Fang et al., Reference Fang, Ma, Wang, Nian, Ma, Huang and Mu2021). Soybean plants exhibit phenotypic plasticity in response to light conditions. Under low light, they display typical shade-avoidance responses, such as elongated stems, while high light intensity increases photosynthetic activity and the accumulation of sugars and starch (Fang et al., Reference Fang, Ma, Wang, Nian, Ma, Huang and Mu2021).

The combination of red and blue light promotes the growth of soybean seedlings and has a positive effect on their overall development, such as agronomic traits, physiological characteristics and photosynthetic capacity (root growth and dry weight, stem diameter, hypocotyl and epicotyl length, plant height, shoot dry weight, and photosynthetic pigment) (Ma et al., Reference Ma, Nian, Luo, Ma, Cheng and Mu2018). Blue LEDs have been shown to enhance isoflavone and phenolic content in soybean sprouts (Azad et al., Reference Azad, Kim, Park and Cho2018), compounds associated with antioxidant activity and stress protection (Paradiso et al., Reference Paradiso, Meinen, Snel, De Visser, Van Ieperen, Hogewoning and Marcelis2011; Paradiso and Proietti, Reference Paradiso and Proietti2022).

Under water-limited and low-light conditions, soybean plants exhibit reduced levels of chlorophyll a, carotenoids per unit mass, and key photosynthetic parameters per unit area (Zhang et al., Reference Zhang, Liu, Yang, Du and Yang2016). In wheat, light spectra and intensity modulate key developmental processes and productivity. LED lighting was associated with enhanced photosynthetic activity, tillering, biomass accumulation, and grain yield, as well as improved flour quality parameters, especially under optimized blue/red light ratios (Monostori et al., Reference Monostori, Heilmann, Kocsy, Rakszegi, Ahres, Altenbach, Szalai, Pál, Toldi, Simon-Sarkadi, Harnos, Galiba and Darko2018). Genotypic variability in light response has also been demonstrated in wheat and barley grown under shade conditions in agroforestry systems (Arenas-Corraliza et al., Reference Arenas-Corraliza, Rolo, López-Díaz and Moreno2019), indicating the potential for breeding programs to improve adaptability to low-light environments.

In addition to physiological and biochemical responses, light quality has also been explored as a tool to guide plant architecture during early growth stages in controlled environments. For instance, specific blue/red light ratios have been used to promote compact growth habits in cereal seedlings, which are desirable in greenhouse-based breeding programs. LED illuminators with relatively high blue radiation effectively reduced stem elongation and modulated time to heading in wheat, barley, and oat, although optimal light conditions varied slightly among species (Stefański et al., Reference Stefański, Siedlarz, Matysik and Rybka2019). These findings support the potential for designing universal lighting systems tailored to early-stage cereal breeding in controlled environments.

Takagi et al. (Reference Takagi, Ihara, Takumi and Miyake2019) found that although photosystems I and II efficiency remained stable under varying light intensities, chlorophyll content and chlorophyll a/b ratios differed significantly among wheat cultivars under high light conditions. Soybean genotypes also exhibit different morphological adjustments to row spacing and plant density, modulating leaf area index and canopy architecture (Balbinot et al., Reference Balbinot, Debiasi, Franchini, Prieto, De Moraes, Werner and Ferreira2018). These observations exemplify phenotypic plasticity – defined as the capacity of a genotype to produce different phenotypes in response to environmental variation (Gianoli, Reference Gianoli2004).

Although LEDs offer a powerful means to manipulate plant metabolism, there remains a gap in understanding how best to use them to optimize crop quality and productivity. The variability in species- and cultivar-level responses underlines the need for more targeted research (Ouzounis et al., Reference Ouzounis, Rosenqvist and Ottosen2015). A better understanding of photosynthetic regulation and metabolic responses under specific light environments could enable more sustainable and efficient agricultural systems.

The effects of artificial night lighting on plants

Understanding the ecological and physiological impacts of artificial lighting on plants requires detailed knowledge of light intensity, spatial distribution, spectral composition, duration, and timing (Bennie et al., Reference Bennie, Davies, Cruse and Gaston2016). This is due to the inherent spatial and temporal heterogeneity in energy distribution from artificial lighting systems (Blanchard and Runkle, Reference Blanchard and Runkle2010; Gaston et al., Reference Gaston, Davies, Bennie and Hopkins2012), which precludes the characterization of plant responses using a single, isolated parameter (Bennie et al., Reference Bennie, Davies, Cruse and Gaston2016).

Artificial light at night can sensitize the photosynthetic apparatus and activate secondary metabolic pathways, thereby influencing carbon assimilation and plant development (Poulin et al., Reference Poulin, Bruyant, Laprise, Cockshutt, Marie-Rose Vandenhecke and Huot2014). However, light perception in plants is highly complex, governed by multiple physiological and signalling pathways. While certain metabolic processes are regulated by specific photoreceptor system, other may be simultaneously promoted or inhibited by different photoreceptors (Song et al., Reference Song, Ito and Imaizumi2010), highlighting the intricate crosstalk in light signalling networks.

Blanchard and Runkle (Reference Blanchard and Runkle2010) assessed the impact of continuous and cyclic artificial lighting on ornamental plants. Continuous exposure lasted four hours per night, while cyclic exposure consisted of six-minute light pulses every 30 minutes over the same period. Plants were placed at five lateral distances from the light source (1, 4, 7, 10, and 13 m). In all treatments, flowering was initiated in at least 80% of individuals across species, indicating that even at 13 m, irradiance levels exceeded the threshold necessary to elicit physiological responses. This suggests that even intermittent artificial lighting can significantly influence plant phenology (Summerfield and Roberts, Reference Summerfield, Roberts and Atherton1987).

In a long-term field experiment, Bennie et al., (Reference Bennie, Davies, Cruse, Bell and Gaston2018) examined the impact of nighttime artificial lighting on species composition and flowering phenology in a semi-natural grasslands. Treatments included high- and low- intensity white light, intermittent light (four-hour interruptions), and a control (no light). The study revealed significant variations in biomass among grass species and differences in flowering dates across light treatments, underscoring the influence of nighttime lighting on plant development in field conditions.

The quantity of light (irradiance or intensity) is a fundamental determinant of plant responses, both in controlled environments and under field conditions. Light quantity directly affects photosynthetic carbon assimilation, photoperiodic signalling, and the activation of photoreceptors, and even low irradiance can lead to physiological and developmental changes when applied over sufficient time (Whitman et al., Reference Whitman, Heins, Cameron and Carlson1998; Blanchard and Runkle, Reference Blanchard and Runkle2010). This highlights the need to consider not only spectral quality and timing, but also how much light is delivered and how it is distributed across the canopy, especially when applying artificial lighting in heterogeneous field conditions.

Previous studies have demonstrated that relatively low levels of light (Whitman et al., Reference Whitman, Heins, Cameron and Carlson1998) or brief light exposures (Runkle et al., Reference Runkle, Heins, Cameron and Carlson1998) can effectively modify plant photoperiodic responses. For example, short flashes of red light at night inhibit flowering in short-day plants, an effect that can be reversed by subsequent exposure to far-red light (Borthwick et al., Reference Borthwick, Hendricks, Parker, Toole and Toole1952), emphasizing the precise role of light quality in photoperiod regulation.

A pioneering study in Brazil investigated the integration of artificial nighttime lighting with centre-pivot irrigation systems. The results indicated substantial increases in the number of internodes, plant height, pod number, and, notably, a 57.3% yield increase in soybeans under supplemental lighting compared to irrigation-only treatments (Lemes et al., Reference Lemes, Azevedo, Domiciano and Andrade2021). In this study, LED light panels were mounted on pivot structures 3 m above the canopy, and plants received approximately 40 hours of supplemental lighting throughout the growth cycle, with the lighting system activated nightly, regardless of cloud cover.

Taken together, these findings clearly demonstrate that artificial nighttime lighting can induce significant physiological and developmental responses in plants. These include enhanced flowering, altered biomass allocation, and increased productivity. Such responses reveal the potential of targeted nighttime lighting to optimize crop performance, particularly in field conditions. Nonetheless, further research is needed to fine-tune parameters such as light spectrum, intensity, duration, and exposure dynamics to fully realize the benefits across diverse cropping systems.

Additional insights into plant responses and implications for field-level lighting

Plant responses to light are highly plastic and depend on factors such as light intensity, spectral quality, duration, and the spatial distribution of light within the canopy. One aspect not yet addressed in this manuscript is the well-established difference between sun and shade leaves. Shade-acclimated leaves, which form under low light intensities within dense canopies, exhibit lower light compensation points, thinner anatomy, and reduced photosynthetic capacity per unit area compared to sun leaves (Massa et al., Reference Massa, Kim, Wheeler and Mitchell2008; Poorter et al., Reference Poorter, Niinemets, Ntagkas, Siebenkäs, Mäenpää, Matsubara and Pons2019). These differences impact light use efficiency in field settings, especially under dense planting systems where self-shading limits light penetration.

To mitigate these effects, within-canopy lighting has been explored in greenhouse systems and, to a limited extent, in open fields. Trouwborst et al. (Reference Trouwborst, Oosterkamp, Hogewoning, Harbison and Van Ieperen2010) demonstrated that intra-canopy lighting in tomato increases light interception and photosynthetic activity throughout the canopy. In soybean, Johnston et al. (Reference Johnston, Pendleton, Peters and Hicks1969) tested field-level within-canopy lighting, indicating early recognition of the potential for supplemental light below the canopy to improve yield. This opens up new avenues for integrating lighting systems, such as those mounted on irrigation pivots, to enhance vertical light distribution beyond just canopy-level exposure.

Another critical aspect involves the far-red (FR) region of the spectrum, which has been largely overlooked in field applications. Far-red light affects the phytochrome system and plays a key role in regulating shade avoidance responses, flowering, and stem elongation. End-of-day far-red (EOD-FR) treatments are known to promote biomass accumulation and photosynthetic capacity in several species (Kalaitzoglou et al., Reference Kalaitzoglou, Van Ieperen, Harbison, Van Der Meer, Martinakos, Weerheim, Nicole and Marcelis2019; Zhen and van Iersel, Reference Zhen and Van Iersel2017). Moreover, Zhen and Bugbee (Reference Zhen and Bubgee2020) emphasized that far-red light contributes to photosynthesis when coupled with red light, challenging the traditional PAR (400–700 nm) definition and reinforcing the need to include FR in artificial lighting strategies. Incorporating far-red LEDs in pivot-mounted systems may thus enhance morphological and physiological responses, particularly under dense canopies or short photoperiods.

Importantly, Kono et al. (Reference Kono, Yamori, Suzuki and Terashima2017) emphasized that far-red light should be considered in mechanistic studies of photosynthetic performance, as it plays a significant role under natural conditions. Their findings suggest that true photosystem activities and basic photosynthetic responses – such as light response curves and photoinhibition – should be evaluated under light conditions that include both PAR and far-red components. This reinforces the need to simulate natural light environments, particularly in eco-physiological studies that aim to understand plant function and productivity under realistic field scenarios.

Continuous or nighttime lighting can profoundly affect circadian-regulated processes in plants. The circadian clock orchestrates numerous physiological activities, including photosynthesis, hormone biosynthesis, sugar metabolism, and stress responses, ensuring they occur at optimal times of day (Velez-Ramirez et al., Reference Velez-Ramirez, Ieperen, Vreugdenhil and Millenaar2011). Disruption of this rhythmicity under prolonged light exposure can result in photodamage, reduced growth, altered carbon allocation, and stress susceptibility. These effects are often species- or genotype-dependent, and some plants exhibit better adaptation to continuous light, while others experience severe impairments. Thus, when considering nighttime lighting under field conditions, the circadian compatibility of crop species and cultivars becomes a critical factor for optimizing growth and avoiding metabolic imbalances.

Finally, the economic feasibility of artificial lighting in open fields is a crucial consideration (Ma et al., Reference Ma, Xu and Cheng2021). Although LED technology has advanced significantly (Morrow, Reference Morrow2008), energy costs remain a limiting factor. Nelson and Bugbee (Reference Nelson and Bugbee2014) analysed the cost-effectiveness of LED use in greenhouses, highlighting that light efficacy and crop productivity must be weighed against installation and operational expenses. In field scenarios, where energy dispersion is greater and control is limited, such considerations are even more critical. Future research should address not only the biological benefits but also the cost-benefit balance of using supplemental lighting, especially when integrated with systems like irrigation pivots, which already have energy infrastructure in place.

Together, these perspectives reinforce the complexity of plant-light interactions and suggest that strategic deployment of artificial lighting, when guided by physiological knowledge and economic evaluation, may improve productivity and sustainability in field-grown crops.

Concluding remarks and future perspectives

The reviewed literature underscores the central role of light quality and intensity in shaping plant growth and metabolism, especially under stress conditions such as water deficit. These effects vary significantly among cultivars, reflecting the influence of genetic variability and phenotypic plasticity in response to environmental stimuli. Although numerous morphological, physiological, and biochemical changes have been reported, studies directly linking these responses to final crop productivity remain scarce.

Emerging evidence from field-based studies on artificial nighttime lighting points to promising outcomes, suggesting that plants are highly sensitive to this stimulus. However, critical questions remain. Key among them is whether nighttime lighting acts primarily as a developmental cue or as effective supplemental illumination. It is essential to investigate how different lighting regimes – continuous vs. intermittent, varying intensities, and distances from the source – impact plant development and productivity.

Particularly in the context of lighting systems integrated with irrigation pivots, there is a need to identify the optimal spectral composition (e.g., red, blue, or combined light) and the most effective lighting strategies (e.g., monochromatic, simultaneous, or alternating spectra). These should be evaluated for their capacity to enhance crop resilience, particularly through the stimulation of secondary metabolism, as observed in horticultural crops.

It is also crucial to characterize the responses of different photoperiodic plant types – short-day, long-day, and day-neutral – to artificial nighttime lighting, including their physiological and biochemical responses. Moreover, scenarios in which irrigation is unnecessary (due to adequate soil moisture) offer an opportunity to isolate and evaluate the effects of lighting alone.

Finally, artificial nighttime lighting represents a potentially valuable tool for improving resource use efficiency, especially in high-demand systems requiring shortened cycles and increased productivity. Its use as a light supplement during overcast days, delivered via irrigation pivots, could further enhance yield potential. To capitalize on these possibilities, comprehensive experimental research is needed to refine application strategies, assess environmental trade-offs, and define the boundaries of effectiveness and sustainability of artificial lighting in modern agriculture.

Although this review focused on the physiological and agronomic responses of crops to light intensity and spectral quality, future research should incorporate quantitative analyses of economic viability. Cost-benefit assessments, such as those exemplified by Nelson and Bugbee (Reference Nelson and Bugbee2014), are essential to evaluate the feasibility of implementing advanced lighting technologies in agricultural systems. Additionally, studies exploring technical and operational constraints in field-level applications are needed to bridge the gap between experimental advances and real-world adoption.

Table 1. Main results of studies with different types of light-emitting diodes (LEDs) and their effects on different plant species