Advancing Light-Mediated Technology in Plant Growth and Development: The Role of Blue Light

Abstract

In controlled environment agriculture (CEA), supplementary lighting, particularly light-emitting diode (LED) technology, is essential for optimizing plant growth and development. Among the spectral components, blue light (400–500 nm) plays an important role in affecting plant morphogenesis, photosynthesis, and key physiological processes. However, species-specific guidelines for optimizing blue light parameters such as intensity, duration, and spectral ratios remain insufficiently developed. Furthermore, plant spectral requirements shift across developmental stages, highlighting distinct blue light management strategies for each phase. This review synthesizes existing knowledge on the impacts of blue light on morphological adaptation, photosynthetic efficiency, flowering, and secondary metabolism, with an emphasis on differential responses across diverse plant species. We emphasize the need for growth-stage-specific lighting protocols and scalable strategies applicable to commercial CEA systems. Interdisciplinary collaboration, integrating molecular biology, genomics, and horticultural engineering, is necessary to enhance understanding of blue light-driven regulatory networks, optimize photoreceptor responses, and facilitate systematic validation of adaptive lighting approaches, ultimately advancing sustainable horticulture and next-generation CEA innovations.

1. Introduction

Controlled environment agriculture (CEA) refers to advanced crop production systems operated within enclosed structures, such as greenhouses, vertical farms, and growth chambers, that allow precise regulation of environmental conditions beyond the capabilities of conventional open-field agriculture [1,2]. Among the key controllable factors in CEA, supplemental lighting plays an important role, as plant growth and development are highly sensitive to light quality, intensity, and duration under controlled conditions [3,4]. In recent years, light-emitting diodes (LEDs) have become the dominant lighting technology in CEA due to their high spectral precision, energy efficiency, and low heat loss [5,6].

Light quality, defined as the spectral composition or color spectrum, shows distinct and wavelength-specific responses in plants. Blue light (400–500 nm) is known to regulate a range of developmental processes including chlorophyll formation, stomatal opening, and photomorphogenesis [7,8,9,10]. Despite these well-documented effects, translating fundamental knowledge into actionable lighting strategies remains a major challenge in CEA. A key limitation is the lack of species-specific protocols that define optimal blue light intensity, duration, and spectral ratios across different developmental stages. In addition, the physiological effects of blue light are influenced by complex interactions with other wavebands, such as red (600–700 nm), far-red (700–800 nm), and green light (500–600 nm). For example, green light has been shown to counteract the effects of blue light on stomatal opening [11,12], which further complicates efforts to establish universal ‘light recipes’ for horticultural crops.

Plant responses to blue light are also highly dynamic, varying not only across species but also across developmental stages [13]. Different growth phases—such as seedling establishment, vegetative growth, flowering, and secondary metabolite production—require customized spectral strategies to optimize physiological responses and resource-use efficiency. These complexities necessitate the development of tailored, stage-specific blue light protocols that go beyond generalized or static lighting applications.

Although previous reviews have advanced our understanding of LED lighting in horticultural production, they often fail to provide species- and development-stage-specific guidance for blue light management in CEA. Some studies have broadly reviewed the effects of various light spectra on plant physiology and metabolism but typically lack sufficient detail to inform precision lighting protocols [14,15,16]. Others have focused narrowly on specific traits, such as blue light-mediated stem elongation [13], without addressing its broader roles in photosynthesis, flowering, and stress resilience. Consequently, existing literature does not yet provide a cohesive framework for adaptive, evidence-based lighting strategies in commercial CEA systems.

This review addresses these gaps by providing a comprehensive understanding of current knowledge on blue light responses in plant growth and development, with emphasis on variation across species and developmental stages. The specific objectives are to (1) characterize blue light-driven physiological and morphological adaptations, including stem elongation, leaf expansion, and root architecture; (2) examine its role in enhancing photosynthetic efficiency; (3) explore the role of blue light in modulating flowering and photoperiodic responses; (4) evaluate its impact on secondary metabolic pathways; and (5) evaluate its contribution to stress resilience (e.g., drought and salt stresses).

2. Blue Light

Blue light regulates various aspects of plant development, including morphogenesis, flowering, and the biosynthesis of secondary metabolites (Figure 1). Unlike red light, which is primarily absorbed by phytochromes [17], blue light triggers a different set of physiological pathways that are essential for optimizing light use and environmental adaptation. Plants perceive blue light using two major classes of specialized photoreceptors: cryptochromes (CRY1 and CRY2) and phototropins (Phot1 and Phot2) [18]. These photoreceptors decode light signals and trigger downstream signaling cascades that regulate gene expression, hormone activity, and developmental processes.

CRY-mediated pathways primarily influence morphogenesis, flowering time, and secondary metabolism [19]. CRY1 and CRY2 differentially respond to blue light intensity and stability—CRY1 is more light-stable and active under high-intensity conditions, while CRY2 is light-labile and is involved in low-light and circadian processes. Phototropins (Phot1 and Phot2), which contain light–oxygen–voltage (LOV) domains, govern rapid, light-induced physiological adjustments. These include phototropism, stomatal opening, chloroplast relocation, and leaf positioning, all of which improve photosynthetic efficiency and gas exchange.

Upon activation, these photoreceptors initiate downstream signaling cascades involving key transcription factors, e.g., ELONGATED HYPOCOTYL 5 (HY5) and PHYTOCHROME INTERACTING FACTORS (PIFs), hormonal regulators, e.g., auxin, gibberellin (GA), abscisic acid (ABA), and secondary messengers such as reactive oxygen species (ROS). Through these interactions, blue light enables plants to dynamically adjust their architecture, metabolism, and defense mechanisms in response to environmental changes. Figure 1 provides a conceptual overview of how blue light is perceived, transduced, and integrated into key physiological and developmental outcomes in plants.

3. Effects of Blue Light on Plant Morphology and Development

Blue light has long been recognized for its role in inhibiting stem elongation [20,21,22]. This phenomenon was initially observed in studies using broadband light or impurity blue light sources, such as blue-colored fluorescent lamps, which inadvertently emit residual red and far-red wavelengths [20]. These spectral impurities complicated early interpretations, as the activation of phytochromes by non-blue wavelengths potentially affected the effects of blue light.

With the advent of LED technology, studies on blue light-mediated plant elongation have reported inconsistent results, which can be attributed to the use of different plant species, genotypes (varieties), growth stages (e.g., seedlings and mature plants), LED lighting characteristics [23,24,25], and cultivation conditions [13]. Even within the same trial, different durations of lighting can lead to varying plant elongation responses [26,27,28].

3.1. Growth and Development of Young Plants

To determine whether blue light promotes growth and development more effectively than red light, we reviewed studies that utilized blue and red LED lights alone across various species and experimental conditions (Table 1). Overall, blue light shows highly species-specific and condition-dependent effects on young plant morphology. Leafy vegetables and ornamentals generally show a tendency toward hypocotyl or stem elongation under blue light, particularly under continuous photoperiods (24 h) and moderate photosynthetic photon flux density (PPFD) levels (50–150 µmol m⁻² s⁻¹). For example, species such as arugula (Brassica eruca) [29,30,31,32], calibrachoa (Calibrachoa × hybrida) [33,34], geranium (Pelargonium hortorum) [34], marigold (Tagetes erecta) [33,34], and petunia (Petunia × hydrida) [33,34,35] consistently exhibited enhanced hypocotyl or stem elongation. This elongation is likely due to the partial shade avoidance response (SAR) mediated by Phot1 and potentially reduced suppression from CRY1, allowing for gibberellin (GA)-driven stem extension.

In contrast, growth inhibition is frequently observed in cereals, fruit crops, and certain herbs, particularly under shorter photoperiods (≤16 h) or higher PPFD (≥200 µmol m⁻² s⁻¹). Examples include rice (Oryza sativa) [36,37], barley (Hordeum vulgare) [38], maize (Zea mays) [39], bitter gourd (Momordica charantia) [40], kiwi (Actinidia chinensis) [41], mulberry (Morus alba) [42], and coriander (Coriandrum sativum) [43], where blue light suppressed hypocotyl length, stem elongation, or plant height. The primary molecular mechanism underlying this inhibition involves CRY1-mediated stabilization of ELONGATED HYPOCOTYL 5 (HY5), a positive regulator of photomorphogenesis, coupled with repression of PHYTOCHROME INTERACTING FACTORs (PIFs) transcription factors and suppression of auxin-responsive pathways.

Inconsistent responses have been reported in several species, including arabidopsis (Arabidopsis thaliana) [13,44], cabbage (Brassica oleracea var. Capitata) [29,30,31,45,46], cherry tomato (Solanum lycopersicum var. cerasiforme) [47,48,49], cucumber (Cucumis sativus) [24,50,51,52], kale (Brassica napus) [29,30,45,53], lettuce (Lactuca sativa) [52,54,55,56,57], mustard (Brassica juncea) [29,31,32,58], pepper (Capsicum annuum) [52,59,60], soybean (Glycine max) [27,52], and tomato (Solanum lycopersicum) [44,50,51,52,61,62,63,64]. These discrepancies are likely due to variations in photoperiod length, PPFD levels, peak wavelengths, and species-specific light sensitivities. Interestingly, even within species, genotypic variation was evident: for instance, different cabbage [45] or lettuce [56] genotypes showed opposing responses to blue light under the same lighting conditions. These findings demonstrate that the response of young plants to blue light is not universally inhibitory or promotive, but instead highly dependent on species, genotype, light intensity, photoperiod, and peak wavelength. A mechanistic understanding involving the balance of CRY1/CRY2 and Phot1 activity, hormonal cross-talk (GA, auxin), and photoperiodic signaling is crucial for optimizing BL-based light recipes in young plants.

Table 1. Effects of sole-source blue light on the growth and development of young plants as compared with those grown under red light.

Species	Varieties	Peak Wavelength (nm)	PPFD (µ mol m−2 s−1)	Photoperiod (h d−1)	Morphological Responses	Reference
Arabidopsis (Arabidopsis thaliana)	-	-	100	-	Promote stem length	[13]
	‘ler’	-	120	16	Inhibit hypocotyl length and plant height	[44]
	‘col-0’	-	120	16	Inhibit hypocotyl length and plant height	[44]
Arugula (Brassica eruca)	‘Rocket’	450	50/100	24	Promote hypocotyl length	[29]
	‘Rocket’	455	100	24	Promote hypocotyl length	[30]
	‘Rocket’	440	100	24	Promote hypocotyl length	[31]
	‘Rocket’	440	20–650	24	Promote hypocotyl length	[32]
Artichokes (Cynara cardunculus var. scolymus)	‘Green Globe’	448	41	16	Inhibit plant height	[65]
	‘Cardoon’	448	41	16	Inhibit plant height	[65]
	‘Violetto’	448	41	16	Inhibit plant height	[65]
Bamboo (Phyllostachys edulis)	‘Moso Bamboo’	450	30	-	Inhibit stem length	[66]
Barley (Hordeum vulgare)	‘Luch’	415	70	16	Inhibit plant height	[38]
Bitter gourd (Momordica charantia)	‘QX001’	465	50	12	Inhibit plant height	[40]
Cabbage (Brassica oleracea var. capitata)	-	450	100	24	Promote hypocotyl length	[29]
	-	455	100	16/24	Promote hypocotyl length	[30]
	-	440	100	24	Promote hypocotyl length	[31]
	-	455	100	24	Promote hypocotyl length	[46]
	‘Kinshun’	470	50	16	Promote stem length	[45]
	‘Red Rookie’	470	50	16	Inhibit stem length	[45]
Calibrachoa (Calibrachoa × hybrida)	‘Kabloom Deep Blue’	455	100	24	Promote hypocotyl length	[33]
	‘Kabloom Deep Blue’	440	100	16/24	Promote stem length	[34]
Cherry tomato (Solanum lycopersicum var cerasiforme)	‘Cuty’	456	205	12	Promote plant height	[49]
	-	450	320	12	Inhibit plant height	[47]
	-	-	320	12	Inhibit plant height	[48]
Coriander (Coriandrum sativum)	‘Sumai’	450	200	16	Inhibit plant height	[43]
Cucumber (Cucumis sativus)	‘Cumlaude’	455	100	18	Promote hypocotyl length	[24]
	‘Cumlaude’	455	100	18	Promote plant height, hypocotyl length, and epicotyl length	[50]
	‘Xiamei No. 2’	454	100	16	Promote stem length	[51]
	‘Sweet Slice’	-	200/500	16	Inhibit stem length	[52]
Eggplant (Solanum melongena)	‘Kokuyo’	470	20–150	16	Promote stem length	[54]
	‘Jingqiejingang’	458	300	12	Promote plant height	[67]
Geranium (Pelargonium hortorum)	‘Pinto Premium Salmon’	440	100	16/24	Promote stem length	[34]
Impatiens (Impatiens walleriana)	‘SuperElfin XP Red’	446	160	18	Inhibit plant height	[61]
	‘SuperElfin XP Red’	446	160	18	Inhibit plant height	[62]
Kiwi (Actinidia chinensis)	‘Hayward’	470	200	16	Inhibit stem length	[41]
Kale (Brassica napus)	‘Red Russian’	450	50/100	24	Promote hypocotyl length	[29]
	‘Red Russian’	455	100	24	Promote hypocotyl length	[30]
	‘Red Russian’	455	100	24	Promote hypocotyl length	[45]
	‘Scarlet’	430	100	16	Inhibit hypocotyl length	[68]
Lettuce (Lactuca sativa)	‘Okayamasaradana’	470	20–150	16	Inhibit stem length	[54]
	‘Okayamasaradana’	450	85/170	16	Inhibit stem length	[55]
	‘Waldmann’s Green’	-	200/500	16	Inhibit stem length	[52]
	‘Rouxai’	449	180	20	Inhibit stem length	[56]
	‘Rouxai’	449	180	20	Promote leaf length	[56]
	‘Cheong Chi Ma’	460	200	18	Promote shoot length	[57]
Maize (Zea mays)	‘Zheng58’	450	13	12	Inhibit mesocotyl length and coleoptile length	[39]
Marigold (Tagetes erecta)	Antigua Orange’	440	100	24	Promote stem length	[34]
	‘Antigua Orange’	440	100	16	Promote hypocotyl length	[34]
	‘Antigua Orange’	455	100	24	Promote stem length	[33]
Mulberry (Morus alba)	‘Longsang No. 1’	465	100	14	Inhibit stem length	[42]
Mustard (Brassica juncea)	‘Ruby Streaks’	440	100	24	Promote hypocotyl length	[31]
	‘Ruby Streaks’	440	250–650	24	Promote hypocotyl length	[32]
	‘Ruby Streaks’	450	50	24	Inhibit hypocotyl length	[29]
	‘Ruby Streaks’	450	110	12	Inhibit plant height	[58]
Pea (Pisum sativum)	-	-	-	8	Promote plant height	[69]
Pepper (Capsicum annuum)	‘Hangjiao No. 12’	460	180	12	Inhibit plant height	[59]
	‘HA-2502’	457	300	12	Inhibit plant height and hypocotyl length	[60]
	‘California Wonder’	-	200/500	16	Promote stem length	[52]
Petunia (Petunia × hydrida)	‘Duvet Red’	455	100	24	Promote hypocotyl length	[33]
	Duvet Red’	440	100	16/24	Promote stem length	[34]
	‘Dwarf varieties mix’	-	-	12	Promote hypocotyl length	[35]
Radish (Raphanus sativus)	‘Cherry Belle’	-	200	16	Inhibit stem length	[52]
Rice (Oryza sativa)	‘XZX24’	450	100	12	Inhibit plant height	[37]
	‘HZY261’	450	100	12	Inhibit plant height	[37]
	‘IR1552’	460	100	12	Inhibit plant height	[36]
	‘TS10’	460	100	12	Inhibit plant height	[36]
Salvia (Salvia splendens)	‘Red Vista’	446	160	18	Inhibit plant height	[61]
	‘Red Vista’	446	160	18	Inhibit plant height	[62]
Sesame (Sesamum indicum)	‘Gomazou’	470	80	24	Promote stem length	[70]
Soybean (Glycine max)	‘Pungwon’	447	50	24	Promote plant height	[27]
	‘Hoyt’	-	200/500	16	Inhibit stem length	[52]
Tomato (Solanum lycopersicum)	‘cry1’	447	150	18	Promote stem length	[64]
	‘Komeett’	455	100	18	Inhibit hypocotyl length	[50]
	‘Early Girl’	446	160	18	Inhibit plant height	[62]
	‘Early Girl’	-	200/500	16	Inhibit stem length	[52]
	‘Early Girl’	446	160	18	Inhibit plant height	[61]
	‘Piennolo’	446	190	12	Inhibit plant height and internode length	[63]
	‘Moneymaker’	454	100	16	Inhibit stem length	[51]
	‘Moneymaker’	-	120	16	Inhibit hypocotyl length and plant height	[44]
Zinnia (Zinnia elegans)	‘Art Deco’	-	-	12	Inhibit hypocotyl length and stem height	[35]

Note: PPFD represents photosynthetic photon flux density; “-” indicates data is not provided in the reference.

Table 1. Effects of sole-source blue light on the growth and development of young plants as compared with those grown under red light.

Species	Varieties	Peak Wavelength (nm)	PPFD (µ mol m−2 s−1)	Photoperiod (h d−1)	Morphological Responses	Reference
Arabidopsis (Arabidopsis thaliana)	-	-	100	-	Promote stem length	[13]
	‘ler’	-	120	16	Inhibit hypocotyl length and plant height	[44]
	‘col-0’	-	120	16	Inhibit hypocotyl length and plant height	[44]
Arugula (Brassica eruca)	‘Rocket’	450	50/100	24	Promote hypocotyl length	[29]
	‘Rocket’	455	100	24	Promote hypocotyl length	[30]
	‘Rocket’	440	100	24	Promote hypocotyl length	[31]
	‘Rocket’	440	20–650	24	Promote hypocotyl length	[32]
Artichokes (Cynara cardunculus var. scolymus)	‘Green Globe’	448	41	16	Inhibit plant height	[65]
	‘Cardoon’	448	41	16	Inhibit plant height	[65]
	‘Violetto’	448	41	16	Inhibit plant height	[65]
Bamboo (Phyllostachys edulis)	‘Moso Bamboo’	450	30	-	Inhibit stem length	[66]
Barley (Hordeum vulgare)	‘Luch’	415	70	16	Inhibit plant height	[38]
Bitter gourd (Momordica charantia)	‘QX001’	465	50	12	Inhibit plant height	[40]
Cabbage (Brassica oleracea var. capitata)	-	450	100	24	Promote hypocotyl length	[29]
	-	455	100	16/24	Promote hypocotyl length	[30]
	-	440	100	24	Promote hypocotyl length	[31]
	-	455	100	24	Promote hypocotyl length	[46]
	‘Kinshun’	470	50	16	Promote stem length	[45]
	‘Red Rookie’	470	50	16	Inhibit stem length	[45]
Calibrachoa (Calibrachoa × hybrida)	‘Kabloom Deep Blue’	455	100	24	Promote hypocotyl length	[33]
	‘Kabloom Deep Blue’	440	100	16/24	Promote stem length	[34]
Cherry tomato (Solanum lycopersicum var cerasiforme)	‘Cuty’	456	205	12	Promote plant height	[49]
	-	450	320	12	Inhibit plant height	[47]
	-	-	320	12	Inhibit plant height	[48]
Coriander (Coriandrum sativum)	‘Sumai’	450	200	16	Inhibit plant height	[43]
Cucumber (Cucumis sativus)	‘Cumlaude’	455	100	18	Promote hypocotyl length	[24]
	‘Cumlaude’	455	100	18	Promote plant height, hypocotyl length, and epicotyl length	[50]
	‘Xiamei No. 2’	454	100	16	Promote stem length	[51]
	‘Sweet Slice’	-	200/500	16	Inhibit stem length	[52]
Eggplant (Solanum melongena)	‘Kokuyo’	470	20–150	16	Promote stem length	[54]
	‘Jingqiejingang’	458	300	12	Promote plant height	[67]
Geranium (Pelargonium hortorum)	‘Pinto Premium Salmon’	440	100	16/24	Promote stem length	[34]
Impatiens (Impatiens walleriana)	‘SuperElfin XP Red’	446	160	18	Inhibit plant height	[61]
	‘SuperElfin XP Red’	446	160	18	Inhibit plant height	[62]
Kiwi (Actinidia chinensis)	‘Hayward’	470	200	16	Inhibit stem length	[41]
Kale (Brassica napus)	‘Red Russian’	450	50/100	24	Promote hypocotyl length	[29]
	‘Red Russian’	455	100	24	Promote hypocotyl length	[30]
	‘Red Russian’	455	100	24	Promote hypocotyl length	[45]
	‘Scarlet’	430	100	16	Inhibit hypocotyl length	[68]
Lettuce (Lactuca sativa)	‘Okayamasaradana’	470	20–150	16	Inhibit stem length	[54]
	‘Okayamasaradana’	450	85/170	16	Inhibit stem length	[55]
	‘Waldmann’s Green’	-	200/500	16	Inhibit stem length	[52]
	‘Rouxai’	449	180	20	Inhibit stem length	[56]
	‘Rouxai’	449	180	20	Promote leaf length	[56]
	‘Cheong Chi Ma’	460	200	18	Promote shoot length	[57]
Maize (Zea mays)	‘Zheng58’	450	13	12	Inhibit mesocotyl length and coleoptile length	[39]
Marigold (Tagetes erecta)	Antigua Orange’	440	100	24	Promote stem length	[34]
	‘Antigua Orange’	440	100	16	Promote hypocotyl length	[34]
	‘Antigua Orange’	455	100	24	Promote stem length	[33]
Mulberry (Morus alba)	‘Longsang No. 1’	465	100	14	Inhibit stem length	[42]
Mustard (Brassica juncea)	‘Ruby Streaks’	440	100	24	Promote hypocotyl length	[31]
	‘Ruby Streaks’	440	250–650	24	Promote hypocotyl length	[32]
	‘Ruby Streaks’	450	50	24	Inhibit hypocotyl length	[29]
	‘Ruby Streaks’	450	110	12	Inhibit plant height	[58]
Pea (Pisum sativum)	-	-	-	8	Promote plant height	[69]
Pepper (Capsicum annuum)	‘Hangjiao No. 12’	460	180	12	Inhibit plant height	[59]
	‘HA-2502’	457	300	12	Inhibit plant height and hypocotyl length	[60]
	‘California Wonder’	-	200/500	16	Promote stem length	[52]
Petunia (Petunia × hydrida)	‘Duvet Red’	455	100	24	Promote hypocotyl length	[33]
	Duvet Red’	440	100	16/24	Promote stem length	[34]
	‘Dwarf varieties mix’	-	-	12	Promote hypocotyl length	[35]
Radish (Raphanus sativus)	‘Cherry Belle’	-	200	16	Inhibit stem length	[52]
Rice (Oryza sativa)	‘XZX24’	450	100	12	Inhibit plant height	[37]
	‘HZY261’	450	100	12	Inhibit plant height	[37]
	‘IR1552’	460	100	12	Inhibit plant height	[36]
	‘TS10’	460	100	12	Inhibit plant height	[36]
Salvia (Salvia splendens)	‘Red Vista’	446	160	18	Inhibit plant height	[61]
	‘Red Vista’	446	160	18	Inhibit plant height	[62]
Sesame (Sesamum indicum)	‘Gomazou’	470	80	24	Promote stem length	[70]
Soybean (Glycine max)	‘Pungwon’	447	50	24	Promote plant height	[27]
	‘Hoyt’	-	200/500	16	Inhibit stem length	[52]
Tomato (Solanum lycopersicum)	‘cry1’	447	150	18	Promote stem length	[64]
	‘Komeett’	455	100	18	Inhibit hypocotyl length	[50]
	‘Early Girl’	446	160	18	Inhibit plant height	[62]
	‘Early Girl’	-	200/500	16	Inhibit stem length	[52]
	‘Early Girl’	446	160	18	Inhibit plant height	[61]
	‘Piennolo’	446	190	12	Inhibit plant height and internode length	[63]
	‘Moneymaker’	454	100	16	Inhibit stem length	[51]
	‘Moneymaker’	-	120	16	Inhibit hypocotyl length and plant height	[44]
Zinnia (Zinnia elegans)	‘Art Deco’	-	-	12	Inhibit hypocotyl length and stem height	[35]

Note: PPFD represents photosynthetic photon flux density; “-” indicates data is not provided in the reference.

3.2. Stem Elongation and Leaf Expansion of Mature Plants

Compared with young plants, mature plants show more consistent growth promotion under blue light, particularly in ornamentals and the model plant Arabidopsis (Arabidopsis thaliana) (Table 2). Blue light-induced elongation responses are typically observed under 440–455 nm blue light at long photoperiod (>16 h) and moderate PPFD levels (50–150 µmol m⁻² s⁻¹), enhancing stem elongation, canopy height, and shoot architecture. Gene-specific studies using cry1, cry2, and phot1 mutants demonstrate that low-activity phytochromes (PHY), low-activity CRY1, high-activity CRY2, and Phot1 and Phot 2 are associated with blue light-induced morphological responses [71,72,73]. Similarly, ornamental species, such as calibrachoa (Calibrachoa × hybrida) [34,74,75], geranium (Pelargonium hortorum) [34,74,75], marigold (Tagetes erecta) [34,74,75,76] and petunia (Petunia × hydrida) [34,74,75,77,78,79], show strong elongation responses to blue light, especially under conditions that optimize CRY1/2 and Phot activation.

At higher light intensities (>200 µmol m⁻² s⁻¹) or non-optimal wavelengths (>460 nm or <420 nm), elongation is typically suppressed in most crops, e.g., cannabis [80], chrysanthemum [81], and mint [82]. However, some species, such as petunia [77,79] and tulip [83] retain elongation responses under these conditions, suggesting genotype-specific plasticity and potential phytochrome-mediated cross-regulation.

Mature plants are less sensitive to photoperiod variation compared with young plants. For example, petunia (Petunia × hydrida) can elongate under a 12 h photoperiod, while seedlings of many species require >16 h for elongation [77]. Blue light also affects leaf morphology, although its effects vary depending on the stage of development. In lettuce, blue light enhances leaf thickness and cell division at later growth stages but inhibits new leaf initiation and reduces fresh weight during early growth [56].

Table 2. Effects of sole-source blue light on the growth and development of mature plants as compared with those grown under red light.

Species	Varieties	Peak Wavelength (nm)	PPFD (µ mol m−2 s−1)	Photoperiod (h d−1)	Morphological Responses	Reference
Arabidopsis (Arabidopsis thaliana)	‘col-0’	455	100	24	Promote stem length	[71]
	‘col-0’	455	100	24	Promote stem length Inhibit hypocotyl length	[73]
	‘col-0’	455	100	24	Promote stem length	[72]
	‘phot1’	455	100	24	Promote stem length	[71]
	‘cry1’	455	100	24	Promote stem length Promote hypocotyl length	[73]
	‘cry2’	455	100	24	Promote stem length Inhibit hypocotyl length	[73]
	‘cry1cry2’	455	100	24	Promote stem length Promote hypocotyl length	[73]
	‘CRY1OX’	455	100	24	Promote stem length Inhibit hypocotyl length	[73]
	‘CRY2OX’	455	100	24	Inhibit hypocotyl length	[73]
	‘phyAphyBphyCphyDphyE’	455	100	24	Inhibit hypocotyl length	[72]
Calibrachoa (Calibrachoa × hybrida)	‘Kabloom Deep Blue’	450	50/100	24	Promote stem length and canopy height	[74]
	‘Kabloom Deep Blue’	455	100	24	Promote stem length	[75]
	‘Kabloom Deep Blue’	440	100	16/24	Promote stem length	[34]
Cannabis (Cannabis sativa)	‘Babbas Erkle Cookies’	430	250–270	18	Inhibit plant height	[80]
Chrysanthemum (Dendranthema grandiflorum)	‘Token’	469	25	-	Inhibit shoot height	[81]
Geranium (Pelargonium hortorum)	‘Pinto Premium Salmon’	450	50/100	24	Promote stem length and canopy height	[74]
	Pinto Premium Salmon’	455	100	24	Promote stem length and canopy height	[75]
	Pinto Premium Salmon’	440	100	16/24	Promote stem length	[34]
Lettuce (Lactuca sativa)	‘Green Oak Leaf’	460	133	14	Inhibit stem length	[82]
Mint (Mentha)	‘Spear mint’	460–475	500	16	Inhibit plant height	[82]
	‘Pepper mint’	460–475	500	16	Inhibit plant height	[82]
	‘Horse mint’	460–475	500	16	Inhibit plant height	[82]
Marigold (Tagetes erecta)	‘Antigua Orange’	450	50/100	24	Promote canopy height and stem length	[74]
	‘Antigua Orange’	455	100	24	Promote canopy height	[75]
	‘Antigua Orange’	440	100	24	Promote stem length	[34]
	‘Orange Boy’	440	90	16	Promote plant height	[76]
Petunia (Petunia × hydrida)	‘Duvet Red’	450	50/100	24	Promote stem length and canopy height	[74]
	Duvet Red’	455	100	24	Promote stem length	[75]
	Duvet Red’	440	100	16/24	Promote stem length	[34]
	‘Baccarat blue’	470	70/150	12	Promote hypocotyl length	[77]
	‘Baccarat blue’	450	100/150	14	Promote plant height	[78]
	‘Baccarat blue’	470	100	14	Promote plant height	[79]
	‘Merlin blue Moon’	470	100	14	Promote plant height	[79]
Salvia (Salvia splendens)	‘Red Vista’	440	90	16	Promote plant height	[76]
Sunflower (Helianthus annuus)	‘Pacino Gold’	450	60	22	Promote stem length	[84]
	‘Pacino Cola’	460	60	18	Promote stem length and internode length	[85]
Tulip (Tulipa × gesneriana)	‘Lasergame’	447	200	12	Promote internode length	[83]

Note: PPFD represents photosynthetic photon flux density; “-” indicates data is not provided in the reference.

Table 2. Effects of sole-source blue light on the growth and development of mature plants as compared with those grown under red light.

Species	Varieties	Peak Wavelength (nm)	PPFD (µ mol m−2 s−1)	Photoperiod (h d−1)	Morphological Responses	Reference
Arabidopsis (Arabidopsis thaliana)	‘col-0’	455	100	24	Promote stem length	[71]
	‘col-0’	455	100	24	Promote stem length Inhibit hypocotyl length	[73]
	‘col-0’	455	100	24	Promote stem length	[72]
	‘phot1’	455	100	24	Promote stem length	[71]
	‘cry1’	455	100	24	Promote stem length Promote hypocotyl length	[73]
	‘cry2’	455	100	24	Promote stem length Inhibit hypocotyl length	[73]
	‘cry1cry2’	455	100	24	Promote stem length Promote hypocotyl length	[73]
	‘CRY1OX’	455	100	24	Promote stem length Inhibit hypocotyl length	[73]
	‘CRY2OX’	455	100	24	Inhibit hypocotyl length	[73]
	‘phyAphyBphyCphyDphyE’	455	100	24	Inhibit hypocotyl length	[72]
Calibrachoa (Calibrachoa × hybrida)	‘Kabloom Deep Blue’	450	50/100	24	Promote stem length and canopy height	[74]
	‘Kabloom Deep Blue’	455	100	24	Promote stem length	[75]
	‘Kabloom Deep Blue’	440	100	16/24	Promote stem length	[34]
Cannabis (Cannabis sativa)	‘Babbas Erkle Cookies’	430	250–270	18	Inhibit plant height	[80]
Chrysanthemum (Dendranthema grandiflorum)	‘Token’	469	25	-	Inhibit shoot height	[81]
Geranium (Pelargonium hortorum)	‘Pinto Premium Salmon’	450	50/100	24	Promote stem length and canopy height	[74]
	Pinto Premium Salmon’	455	100	24	Promote stem length and canopy height	[75]
	Pinto Premium Salmon’	440	100	16/24	Promote stem length	[34]
Lettuce (Lactuca sativa)	‘Green Oak Leaf’	460	133	14	Inhibit stem length	[82]
Mint (Mentha)	‘Spear mint’	460–475	500	16	Inhibit plant height	[82]
	‘Pepper mint’	460–475	500	16	Inhibit plant height	[82]
	‘Horse mint’	460–475	500	16	Inhibit plant height	[82]
Marigold (Tagetes erecta)	‘Antigua Orange’	450	50/100	24	Promote canopy height and stem length	[74]
	‘Antigua Orange’	455	100	24	Promote canopy height	[75]
	‘Antigua Orange’	440	100	24	Promote stem length	[34]
	‘Orange Boy’	440	90	16	Promote plant height	[76]
Petunia (Petunia × hydrida)	‘Duvet Red’	450	50/100	24	Promote stem length and canopy height	[74]
	Duvet Red’	455	100	24	Promote stem length	[75]
	Duvet Red’	440	100	16/24	Promote stem length	[34]
	‘Baccarat blue’	470	70/150	12	Promote hypocotyl length	[77]
	‘Baccarat blue’	450	100/150	14	Promote plant height	[78]
	‘Baccarat blue’	470	100	14	Promote plant height	[79]
	‘Merlin blue Moon’	470	100	14	Promote plant height	[79]
Salvia (Salvia splendens)	‘Red Vista’	440	90	16	Promote plant height	[76]
Sunflower (Helianthus annuus)	‘Pacino Gold’	450	60	22	Promote stem length	[84]
	‘Pacino Cola’	460	60	18	Promote stem length and internode length	[85]
Tulip (Tulipa × gesneriana)	‘Lasergame’	447	200	12	Promote internode length	[83]

Note: PPFD represents photosynthetic photon flux density; “-” indicates data is not provided in the reference.

3.3. Root Development and Architecture

While blue light is widely recognized for its role in regulating aboveground morphology, emerging evidence underscores its systemic influence on root system architecture. Roots, although shielded from direct light in natural settings, exhibit distinct architectural responses to blue light exposure. In Arabidopsis (Arabidopsis thaliana), blue light suppresses primary root elongation while stimulating lateral root formation, a response mediated by auxin redistribution and signaling [86]. This hormonal reprogramming enhances root branching, potentially improving nutrient and water uptake in compact soils. Blue light also modulates root gravitropism, i.e., the orientation of root growth in response to gravity. Under microgravity conditions, Arabidopsis roots exhibit positive phototropism toward blue light, a response typically masked by the gravitational pull of Earth [87]. These findings suggest that gravity can obscure blue light-induced phototropic responses in roots.

The effects of blue light on root development vary across species, indicating species-specific responses. In cherry rootstock (Prunus avium), blue light enhances adventitious root formation under in vitro conditions, improving transplant success rates [88]. In contrast, blue light inhibits primary root growth in cucumber (Cucumis sativus), highlighting the different regulatory mechanisms among plant species [89]. These findings suggest that while blue light can enhance root branching in certain species, it may limit root growth in others, depending on specific light perception and auxin transport mechanisms [90]. In hydroponic and aeroponic systems, where root architecture is not constrained by soil, optimizing light exposure offers a unique opportunity to engineer root traits for maximum efficiency. Controlled blue light supplementation can be strategically used to promote lateral root proliferation and enhance root hair density, improving nutrient uptake efficiency and overall plant health. For example, compact yet highly branched root systems may improve oxygen and nutrient absorption in aeroponics, while minimizing root zone volume—an important trait in vertical farming environments. Future work should focus on defining species-specific blue light thresholds and timing to fine-tune root system traits for different CEA platforms.

3.4. Other Factors Influencing Blue Light Responses

3.4.1. Spectral Interactions and Light Recipes

The effects of blue light on plant development are not only species- and development-dependent but also significantly influenced by its interactions with other wavelengths, particularly red and far-red light, which modulate phytochrome activity. Phytochrome photostationary state (PPS)—a ratio representing the active/inactive forms of phytochromes—is a key factor in these interactions. Pure blue LED light exhibits a much lower PPS (~0.5) compared to red LED light (~0.9) [91]. While the threshold for triggering phytochrome-mediated responses remains debated, PPS values below 0.6 are generally considered insufficient for full activation [92]. Thus, elongation observed under pure blue light conditions is likely due to low phytochrome activity. Kong and Zheng [13] reported that increasing the proportion of blue light (0–100%) in red LED systems promoted stem elongation in petunias, calibrachoa, geraniums, and marigolds. They found pure blue light (PPS = 0.5) promoted elongation, while adding a small amount of red light (10%) increased the PPS to 0.7 and suppressed elongation [51]. Interestingly, further supplementing the red-blue mix with far-red light reduced the PPS to ~0.6 and reversed elongation, indicating that PPS-mediated phytochrome signaling determines morphological outcomes. Moreover, the addition of other low-level wavelengths such as UVA, UVB, or green light to blue light had minimal effects on elongation, with calculated PPS values remaining below 0.6, emphasizing the dominance of red light in modulating blue light responses.

These spectral interactions have been found across different species. For example, supplementing blue light with far-red light has been shown to increase stem elongation in certain species by modulating phytochrome activity [93]. In Cannabis sativa, blue light alone increased shoot dry mass but inhibited elongation, a limitation counteracted by supplemental far-red, which enhanced shoot length and yield without compromising biomass accumulation [94]. This interaction highlights the role of phytochrome-mediated signaling in balancing blue light’s growth-inhibiting effects, enabling tailored light recipes to optimize elongation and biomass simultaneously. However, in buckwheat and cucumber seedlings, optimal biomass and height were observed under blue LED treatment alone [23,24]. These findings underscore the importance of considering spectral interactions when designing light recipes for horticultural practices. Likewise, red–blue LED combinations often outperform monochromatic red or blue light in crops such as lettuce, promoting a larger leaf area, greater biomass accumulation, and improved chloroplast development, particularly when blue is optimized within the spectral composition alone [95,96]. However, increasing the blue light percentages can decrease tomato growth, primarily through its impact on morphology and subsequent light interception [97]. For example, the dry weight of plants grown under 27% blue light was 1.6 times greater than that of plants exposed to 61% blue light. Notably, green light, traditionally considered less efficient for photosynthesis, enhances carbon fixation in shaded tissues by penetrating deeper into leaf layers [98]. When added to red-blue lights, it increases both fresh and dry mass in lettuce compared to red light alone [99,100].

3.4.2. Environmental and Cultivation Conditions

Environmental conditions such as temperature and relative humidity may affect how plants respond to blue light. Temperature is a key modulator of phytochrome activity and photoreceptor-mediated growth. Studies in Arabidopsis (Arabidopsis thaliana) have shown that increasing temperatures from 17 °C to 27 °C suppress phytochrome activity, thereby enhancing red light-induced hypocotyl elongation [101,102]. However, blue light can counteract high-temperature-mediated elongation by activating cryptochrome pathways, indicating a temperature-specific role of blue light in growth regulation. Relative humidity further modulates blue light responses. For example, tomato and cucumber plants exposed to blue light under high humidity (90%) exhibited greater stem elongation than those under moderate humidity (60%), likely due to improved stomatal conductance and chlorophyll biosynthesis [103]. Conversely, in Rehmannia glutinosa, blue light suppressed shoot elongation under high humidity in no-ventilation conditions, highlighting species- and system-specific differences [104].

In contrast to environmental conditions, other cultivation factors, such as planting density and growth medium, have relatively limited effects on plant elongation under blue light. As shown in Figure 2, the effects of blue versus red LED light on seedling growth remain consistent across various crops and cultivation setups. Plants grown in both peat-lite mix and rockwool cubes exhibited similar elongation responses to blue light, indicating that substrate type has minimal impact on blue light-mediated morphological changes. Likewise, comparable elongation trends were observed in arugula seedlings grown at both low and commercial planting densities. Even in red-leaf cultivars of mustard, blue light continued to promote stem elongation compared to red light. A more comprehensive discussion of the factors that influence plant elongation under blue light can be found in Kong and Zheng [13].

3.4.3. Night Interruption

Night interruption refers to breaking up the long dark period by briefly providing artificial lighting, thus creating modified long-day conditions for plants [105]. The quality of night interruption significantly affects morphogenesis across various plants [106,107,108,109,110,111,112,113,114]. Park et al. [112] reported that employing blue light during the night interruption period suppresses plant elongation. Additionally, they demonstrated that combining blue and red light promoted leaf expansion, whereas combining blue and far-red light suppressed this expansion. The enhanced relative growth rate observed under combined blue and red lights likely results from a synergistic interaction between these wavelengths. Moreover, night interruption techniques effectively simulate extended daylight hours by briefly illuminating plants during the dark phase, increasing plant growth and developmental responses associated with long-day conditions [107,115].

4. Photosynthetic Efficiency

Blue light serves as both a primary energy source and a regulatory signal that optimizes photosynthetic efficiency by modulating stomatal dynamics, chloroplast positioning, and gene expression [95,116]. Even low percentages of blue light during growth (as little as 7%, Figure 3) are sufficient to prevent the dysfunctional photosynthesis observed under monochromatic red light, indicating its qualitative necessity for normal photosynthetic function [117]. In cucumber, a red–blue light ratio of 50:50 has been shown to maximize photosynthetic rate, stomatal conductance, chlorophyll content, and photosynthetic nitrogen use efficiency (PNUE), outperforming monochromatic treatments [117]. Leaf photosynthetic capacity (A_max) grown under 7% blue light is twice that of those grown without blue light and continues to rise with increasing blue light percentage up to 50% (Figure 3a). Although A_max decreases under 100% blue light, photosynthetic functionality remains normal. The rise in A_max with increasing blue light percentage (0–50%) is correlated with increases in leaf mass per unit area (LMA) (Figure 3b), chlorophyll content per area (Figure 3c), nitrogen content per area, and stomatal conductance. Notably, A_max per leaf chlorophyll, A_max per leaf dry weight, and PNUE increase with blue light up to 50% but remain the same between 50 and 100% (Figure 3d–f). Above 15% blue light, parameters such as A_max, LMA, chlorophyll content, PNUE, and the chlorophyll-to-nitrogen ratio exhibit relationships similar to those observed in leaf responses to increased irradiance intensity. These results suggest that blue light during growth is not only qualitatively required for normal photosynthetic function but also quantitatively mediates leaf responses similar to those induced by higher light intensities.

However, excessive blue light exposure can lead to photoinhibition, particularly under high light intensities. Photoinhibition arises from the light-induced inactivation of photosystem II (PSII), which is often associated with oxidative stress through the overproduction of reactive oxygen species (ROS) and subsequent damage to the D1 protein of PSII [118,119]. This necessitates a constant repair cycle to maintain PSII efficiency [120]. Studies in Arabidopsis [121], cucumber [122], and rice [121] have shown that intense blue light rapidly disrupts PSII activity and overall photosynthetic performance.

To mitigate this damage, plants rely on blue light-mediated activation of phototropins (Phot1 and Phot2), which regulate chloroplast movement to optimize light capture and minimize injury [123,124]. Under low blue light intensities (0.08–20 μmol m⁻² s⁻¹), chloroplasts position themselves perpendicularly to the light to maximize photon capture [125,126]. Under high intensities, chloroplasts relocate to align with cell walls parallel to the incident light, reducing photodamage by limiting direct exposure [125,126]. Additionally, blue light enhances non-photochemical quenching (NPQ), dissipating excess energy as heat and providing photoprotection under fluctuating light [125,126,127]. These responses are important under fluctuating light conditions, where rapid chloroplast repositioning and PSII protection maintain photosynthetic stability.

5. Flowering and Photoperiodic Responses

Blue light has a significant influence on flowering and photoperiodic responses in plants. Despite considerable advancements, inconsistencies persist in research findings due to variability among plant species and experimental conditions, such as light intensities, durations, spectral compositions, and interactions with environmental factors like temperature and irradiance [128]. Plants are broadly categorized as long-day, short-day, or day-neutral plants based on their flowering response to day length [129]. Long-day plants initiate flowering when the day length exceeds a critical threshold, typically 12–14 h of light per day. In contrast, short-day plants require a minimum duration of uninterrupted darkness (12–14 h) to initiate flowering [130]. Day-neutral plants flower independently of photoperiod and are unaffected by changes in day length [131,132]. Long-day and short-day plants are further categorized as qualitative (obligate) and quantitative (facultative). Qualitative long-day or short-day plants strictly require their respective photoperiods to flower, while quantitative plants show enhanced or accelerated flowering under favorable photoperiods but may still flower under non-inductive conditions [131].

At the molecular level, photoperiodic responses are mediated by light-activated enzymes, the circadian clock, and the FLOWERING LOCUS T (FT) gene, which produces florigen—a signal triggering flowering [129]. Blue light, perceived primarily by cryptochrome photoreceptors, stabilizes the CONSTANS (CO) protein, a key regulator of FT expression [109]. For example, in Arabidopsis, long-day conditions increase FT transcription, accelerating flowering compared with short-day environments [133]. This mechanism is conserved across species but modulated by the quality and intensity of light.

In commercial horticulture, growers strategically manipulate photoperiods via night interruption and day extension to control flowering. Effects of supplementary and night interruption of blue light are summarized on the flowering of ornamental plants in

Table 3. Effects of supplementary and night interruption of blue light on the flowering of ornamental plants.

Species	Photoperiodic Classification	Blue Light Intensity (μmol m−2 s−1)	Observed Responses
Chrysanthemum (Dianthus chinensis)	Qualitative long-day plant	1–2	Flowered
Rudbeckia (Rudbeckia hirta)	Qualitative long-day plant	1–2	Not flowered
		15	Flowered
Coreopsis (Coreopsis grandiflora)	Qualitative long-day plant	5	Flowered
Snapdragon (Antirrhinum majus)	Qualitative long-day plant	5	Flowered
Petunia (Petunia hybrida)	Qualitative long-day plant	15	Flowered
Chrysanthemum (Dendranthema grandiflorum)	Qualitative short-day plants	1–2	Flowered
		10	Flowered
Kalanchoe (Kalanchoe blossfeldiana ‘Spain’)	Qualitative short-day plants	10	Promoted flowering
Kalanchoe (Kalanchoe blossfeldiana ‘Lipstick’)	Qualitative short-day plants	10	Not flowered
Cosmos (Cosmos sulfureus)	Qualitative short-day plants	1–2	Flowered
Dahlia (Dahlia pinnata)	Qualitative short-day plants	1–2	Not flowered
Marigold (Tagetes erecta)	Qualitative short-day plants	1–2	Flowered

Note: Blue light intensity refers to photosynthetic photon flux density (PPFD).

. Night interruption disrupts darkness with low-intensity light (1–10 μmol m⁻² s⁻¹), accelerating flowering in long-day plants but delaying flowering in short-day plants [131]. Day extension extends effective daylight exposure, primarily benefiting long day plants. While broad-spectrum light at 1–2 μmol m⁻² s⁻¹ is typically sufficient for regulating flowering [134], the efficacy of blue light varies significantly across species and intensities. For example, low-intensity blue light (1–2 μmol m⁻² s⁻¹) has a minimal effect on the flowering of both long-day plants, e.g., dianthus (Dianthus chinensis) and rudbeckia (Rudbeckia hirta), and short-day plants, e.g., chrysanthemum (Chrysanthemum × morifolium), cosmos (Cosmos sulfureus), dahlia (Dahlia pinnata), and marigold (Tagetes erecta) [135]. However, long day plants such as coreopsis (Coreopsis grandiflora) and snapdragon (Antirrhinum majus) respond to intensities as low as 5 μmol m⁻² s⁻¹, whereas rudbeckia (Rudbeckia hirta) and petunia (Petunia hybrida) require 15 μmol m⁻² s⁻¹ [136].

In contrast to earlier findings, low-intensity blue light (10 µmol m⁻² s⁻¹) supplementation during photoperiods promotes flowering in qualitative short-day plants such as chrysanthemum (Dendranthema grandiflorum) [107,108] and kalanchoe [137], even under noninductive long-day conditions. Shoot tip-targeted blue light night interruption reduces stem elongation by 30% and advances flowering by 14 days via CRY-mediated FT upregulation [109]. As shown in Figure 4, supplying low-intensity blue light under short-day or long-day conditions effectively promoted flowering in short-day plants, challenging traditional findings that prioritize uninterrupted darkness for floral induction. These results highlight the potential of blue light as a tool for commercial floriculture. However, the responses remain genotype-specific. For example, Kalanchoe ‘Spain’ flowers under short-day and blue light NI, but Kalanchoe ‘Lipstick’ remains unaffected [138]. The positioning of light used for night interruption further refines outcomes. Compared with leaf-targeted treatments, shoot tip-targeted blue light night interruption in chrysanthemum enhances FT expression by 50%, optimizing flowering induction [108]. Despite its advantages, challenges persist. High blue light intensity (>20 µmol m⁻² s⁻¹) delays flowering in some SDPs (e.g., Chrysanthemum morifolium), necessitating dynamic spectral adjustments [139].

The influence of blue light extends to diverse crops. In everbearing strawberry plants (Fragaria × ananassa), 24 h exposure to blue light accelerates flowering and increases yield, whereas supplemental blue light enhances flower cluster formation in greenhouse production [140]. The blue-dominant spectrum of blueberries (Vaccinium corymbosum) leads to improved fruit quality, although genotype-specific responses are observed [141]. Vegetables such as tomatoes and peppers benefit from blue-red LED combinations that balance compact growth and early flowering [142]. Specialty crops, such as saffron (Crocus sativus) and cannabis (Cannabis sativa), underscore the economic potential of blue light. High-intensity blue light increases saffron stigma yield and elevates cannabinoid concentrations in cannabis [143].

Mechanistic gaps, such as cryptochrome-phytochrome interactions under varying spectral ratios, require further investigation to clarify species-specific and stage-dependent photoreceptor crosstalk. Innovations such as smart lighting systems that adapt spectra in real-time could control light quality, intensity, and duration based on crop developmental stage, genotype, and energy use. Additionally, leveraging blue light for speed breeding, i.e., accelerating generational cycles in short-day crops, is promising for sustainable agriculture. By harmonizing spectral precision with plant physiology, horticulture can achieve year-round control of flowering, aligning production with global agricultural demands.

6. Plant Secondary Metabolism

Blue light is a key regulator of plant secondary metabolism, influencing the biosynthesis of a wide range of bioactive compounds that enhance plant defense, stress adaptation, and nutritional quality [6]. These metabolites, including flavonoids, anthocyanins, phenolics, carotenoids, glucosinolates, alkaloids, and terpenoids, enhance plant resilience against herbivores, pathogens, and oxidative stress, while also contributing to their nutritional and medicinal value [144]. The molecular regulation of these compounds involves photoreceptors, such as CRYs and PHYs, which activate transcription factors, including HY5, phytochrome interacting factor 4 (PIF4), and MYC2, to modulate biosynthetic pathways [145,146]. For example, blue light upregulates functional metabolites such as rutin and catechins in longan embryogenic calli through these regulatory pathways [145,146].

6.1. Flavonoids and Anthocyanins

Flavonoids, especially anthocyanins, are among the most extensively studied secondary metabolites influenced by blue light. Anthocyanin biosynthesis is tightly regulated by a photoreceptor-mediated transcriptional cascade involving CRY1/CRY2, which activates HY5, a central integrator of light signaling. The HY5, in turn, upregulates the expression of myeloblastosis (MYB) transcription factors, such as PRODUCTION OF ANTHOCYANIN PIGMENT 1 (PAP1)/MYB75, that activate late biosynthetic genes (LBGs) in the flavonoid pathway [147,148]. In Arabidopsis, blue light activates CRY1, leading to Constitutive Photomorphogenic 1 (COP1) inactivation and stabilization of HY5, which directly binds to the MYB75/PAP1 promotor, upregulating the transcription of anthocyanin pathway genes such as dihydroflavonol 4-reductase (DFR), leucoanthocyanidin dioxygenase (LDOX), and UDP-glucose: flavonoid 3-O-glucosyltransferas (UF3GT) [149]. In lettuce (Lactuca sativa), a clear wavelength hierarchy has been observed in anthocyanin induction (505 nm > 455 nm > 470 nm > 535 nm) [148]. In tomato, blue light-induced anthocyanin accumulation also requires PHY signaling, underscoring photoreceptor crosstalk [150]. Notably, green light suppresses this pathway by inhibiting CRY1 activity, leading to reduced anthocyanin content [151,152], while far-red light enhances anthocyanin levels in microgreens in a synergistic manner with blue light [147].

6.2. Phenolic Acids

Phenolic acids, including chlorogenic acid and rosmarinic acid, are also stimulated by blue light, although the transcriptional regulation remains less defined as compared to anthocyanins [153]. Nevertheless, studies in herbs such as coriander (Coriandrum sativum) and pea (Pisum sativum) sprouts have reported significant increases in total phenolic content and antioxidant activity under blue light [154,155]. The involvement of MYB family members in general phenylpropanoid pathway regulation suggests that these responses may also be downstream of the CRY–HY5–MYB axis, but further mechanistic studies are needed.

6.3. Carotenoids

Carotenoid metabolism, while typically associated with red/far-red light, also responds to blue light through indirect mechanisms [156]. In citrus fruits, blue light accelerates carotenoid accumulation and chlorophyll degradation, enhancing fruit coloration through transcriptional activation of structural genes such as phytoene synthase (PSY) and Lycopene Beta-Cyclase (LCYB) [157,158,159]. Although the specific photoreceptors remain to be fully elucidated, HY5 has been shown to influence carotenoid biosynthesis, suggesting a possible link to blue light signaling pathways [160].

7. Blue Light-Mediated Stress Resilience

Blue light enhances plant stress resilience to abiotic and biotic stresses through photoreceptor-mediated signaling networks that coordinate antioxidant defense, osmotic adjustment, and transcriptional reprogramming.

7.1. Drought Stress

Blue light enhances drought resilience primarily through stomatal regulation and reactive oxygen species (ROS) detoxification. Blue light activates phototropins (Phot1/2), initiating a signaling pathway that leads to the phosphorylation of plasma membrane H⁺-ATPases [161,162]. This activation drives K⁺ influx and promotes stomatal opening, facilitating CO₂ uptake under non-stress conditions. However, under drought stress, the hormone abscisic acid (ABA) is rapidly synthesized and plays a dominant role in guard cell behavior by overriding blue light-induced stomatal opening.

ABA triggers both stomatal closure in already open stomata and inhibits the initiation of further stomatal opening. These responses are mediated by the activation of S-type (SLAC1) and R-type anion channels, as well as outward-rectifying K⁺ channels, resulting in ion efflux, turgor reduction, and stomatal closure [163,164]. Simultaneously, ABA signaling through the PYR/PYL (pyrabactin resistance 1/PYR1-like)-PP2C (type 2C protein phosphatase)-SnRK2 (the serine/threonine protein kinase) core module [165] also interferes with blue light signaling by inhibiting plasma membrane H⁺-ATPases phosphorylation. This suppression is mediated by secondary messengers such as hydrogen peroxide (H₂O₂), hydrogen sulfide (H₂S), nitric oxide (NO), phosphatidic acid (PA), and cytosolic calcium (Ca²⁺), which collectively disrupt downstream phototropin signaling [166,167].

Additionally, OST1 (OPEN STOMATA 1), a SnRK2 kinase activated by ABA, inhibits blue light signaling by directly phosphorylating and suppressing inward-rectifying K⁺ channels (e.g., KAT1), thereby preventing K⁺ uptake [168]. ABA further reduces the transcription of K⁺ channel genes by inactivating transcription factors like ABA-RESPONSIVE KINASE SUBSTRATEs (AKS) [169], and promotes endocytosis-mediated internalization of KAT1 proteins from the plasma membrane, limiting their function [170].

Interestingly, under well-watered conditions, blue light signaling can counteract ABA-mediated closure by stimulating H⁺-ATPase activity and suppressing SLAC1 activation via phototropin- and cryptochrome-mediated mechanisms [171]. However, under drought conditions, the ABA response predominates to minimize water loss through a tightly regulated stomatal response.

Beyond stomatal control, blue light also modulates antioxidant defenses to mitigate drought-induced oxidative stress. For example, in Arabidopsis, blue light upregulates CYCLIN H;1, a gene that suppresses ROS accumulation under drought, thereby enhancing the activity of ROS-scavenging enzymes such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) [172]. In Chinese cabbage (Brassica rapa), extended blue light exposure increases β-carotene production, contributing to protection against oxidative damage [173].

Pre-acclimation with blue light has shown significant benefits across various species. Lemon balm (Melissa officinalis) treated with blue-red light presented increased root hydraulic conductivity and proline accumulation, both of which are vital for osmotic adjustment [174]. Recent studies comparing Eruca sativa (rocket) and Lactuca sativa (lettuce) highlight species-specific responses to drought stress following pre-exposure to varying red–blue LED light ratios [175]. While both showed improved morphological traits and antioxidant accumulation under combined red and blue light, lettuce demonstrated stronger residual drought resilience after re-irrigation. In contrast, rocket exhibited greater water loss, particularly under high stomatal aperture density, suggesting a tighter link between stomatal architecture and drought sensitivity in rocket. These findings underscore the need for tailored blue light protocols based on species-specific drought response mechanisms.

Blue light also stimulates the secondary metabolic such as flavonoid glycosides and hydroxycinnamates, which contribute to membrane stabilization and ROS scavenging during drought. For example, wild privet (Ligustrum vulgare) accumulates ${O_2}^{-}$ scavengers, such as luteolin-7-O-glucoside, quercetin 3-O-rutinoside, and echinacoside, under combined light and drought stress [176].

Additional species-specific adaptations further highlight blue light’s potential in drought resilience. Chinese cabbage (Brassica rapa) exhibits enhanced carotenoid biosynthesis and ROS scavenging under prolonged blue light exposure [173]. Sweet pepper (Capsicum annuum) benefits from blue light-enriched spectra through improved leaf physiology and water retention [177], and rose (Rosa × hybrida) grown under high humidity develops improved stomatal function upon blue light exposure [178]. Likewise, tall fescue (Festuca arundinacea) adjusts leaf elongation rates and stomatal transpiration in response to blue light during drought, illustrating how calibrated blue light application can fine-tune plant responses for improved water conservation and stress tolerance [179]. These findings highlight the complex role of blue light in enhancing water conservation and stress resilience while emphasizing the necessity of calibrated application.

7.2. Salt Stress

Salinity stress, caused by elevated soil salinity or saline irrigation water, poses a significant threat to global crop productivity by triggering oxidative stress, osmotic imbalance, and ionic toxicity [180]. These disruptions lead to reduced photosynthetic activity, premature senescence, and programmed cell death, which threaten food security [181]. Recent studies highlight blue light as a critical regulator of plant salt tolerance, acting as a signaling cue that activates photomorphogenic and physiological adaptations to mitigate stress [182].

Blue light enhances plant salt tolerance by regulating ionic balance, osmotic homeostasis, and ROS detoxification. Under salt stress, maintaining a favorable K⁺/Na⁺ ratio is essential for cellular homeostasis. Blue light influences these processes by modulating specific ion transporters and increasing ABA sensitivity. For instance, in wheat, blue light activates G-BOX BINDING FACTOR 1 (TaGBF1), a transcription factor that enhances ABA signaling and increases the K⁺/Na⁺ ratio under salinity [183].

Blue light also mitigates oxidative damage by enhancing ROS-scavenging enzymes. In lettuce exposed to a red–blue (1:3) light ratio, levels of antioxidant enzymes involved in the ascorbate–glutathione cycle—such as monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR)—are significantly elevated after 12 days of light treatment, with corresponding gene expression upregulated as early as day 3 [184]. Similarly, in alfalfa, blue or combined blue-red light treatments enhances the activities of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and GR, and induces the expression of genes like MsMDHAR, MsDHAR, MsAPX, and MsGR, which collectively reduce ROS levels [185].

These findings highlight the potential of spectral manipulation to alleviate salt stress. To translate these insights into practice, future work should prioritize (1) optimizing the light spectrum for crop-specific needs, (2) elucidating stress-response gene regulatory networks, and (3) validating field applications under realistic agronomic conditions.

8. Conclusions

The use of blue light in plant science and controlled environment agriculture represents a significant advancement in optimizing crop production, nutritional quality, and stress resilience. Through its interaction with photoreceptors such as cryptochromes (CRY1/2) and phototropins (Phot1/2), blue light essential physiological and developmental processes, including morphogenesis, photosynthesis, flowering time, secondary metabolism, and stress adaptation. These effects are medicated through complex downstream signaling networks involving key transcription factors (e.g., HY5, PIFs), hormones (e.g., auxin, gibberellins, ABA), and secondary messengers (e.g., ROS), collectively shaping plant architecture, metabolism, and adaptive capacity.

In CEA systems, blue light-enriched LED technologies have demonstrated strong potential in increasing crop yield, nutritional quality, and resilience to environmental stresses, which are key priorities for sustainable food systems. By precisely adjusting the intensity of blue light and exposure duration, growers can effectively manage flowering, optimize phytochemical profiles, and enhance growth efficiency. However, optimizing blue light application requires species-specific and stage-specific calibration to avoid potential trade-offs.

Importantly, blue light-mediated plant elongation and stress adaptation involve intricate and context-dependent molecular and physiological mechanisms that are not yet fully understood. As LED systems become more integrated into precision agriculture, future research must aim to disentangle these networks through both fundamental and applied approaches. Key areas of focus include: (1) Elucidating core signaling pathways activated by blue light, including receptor activation dynamics, transcriptional regulators, and hormone crosstalk (e.g., auxin-GA balance); (2) Utilizing multi-omics approaches, e.g., transcriptomics, proteomics, and metabolomics, to map gene expression patterns and protein–protein interactions under pure and mixed spectral conditions; and (3) Exploring interactions of blue light with other spectral bands, especially red and far-red light, to better understand synergistic or antagonistic effects across different developmental stages.

From an application standpoint, future work should focus on the following: (1) Optimizing delivery strategies (intensity, duration, day vs. night application) tailored to species and developmental stage; (2) Developing dynamic and intermittent lighting systems, simulating natural photoperiods while reducing energy consumption; and (3) Evaluating novel lighting technologies, such as laser diodes and plasma lighting, for improved spectral control.

To advance smart lighting systems for real-time spectrum modulation, several technical challenges must be addressed: (1) Spectral contamination between neighboring treatment zones can reduce specificity and compromise experimental outcomes; (2) Variable background lighting (e.g., greenhouse sunlight), which may obscure blue light effects depending on timing; and (3) Inconsistent photon flux across treatments can introduce confounding variables. To mitigate these issues, future experiments must implement well-isolated lighting zones, uniform background conditions, and carefully calibrated intensities.

Additionally, integrating artificial intelligence (AI)-powered spectral control platforms and sensor networks can enable real-time adjustment of lighting based on plant developmental status and environmental feedback. The intersection of blue light technologies with advances in salt stress physiology, nanobiotechnology, and microbiome engineering holds significant promise for developing resilient, high-performing crops that can thrive under suboptimal environmental conditions. As global food demands intensify and climate variability increases, precision photobiology—centered around the smart application of blue light—will be pivotal for building resilient, efficient, and sustainable agricultural systems.