RSomesearche authors agree on the positive role of RL ^[1], and high-ratio red (RL):far-red (FRL)^[2] on shoot proliferation ^[3]. RL significantly enhanced the adventitious bud formation and development in Gerbera jamesonii ^[4], in Lactuca sativa. ^[5], in Spathiphyllum cannifolium ^[6], in Stevia rebaudiana ^[7] and in Mirtus communis ^[8]. RL was effective for bud formation and outgrowth in Pseudotsuga menziesii embryo cultures ^[9]. In contrast, as compared to the cultivation under WL or combined RL with BL, under monochromatic RL or blue (BL), Bello-Bello et al. ^[10] observed a decrease in the proliferation ratio in Vanilla planifolia Andrews and Estrada et coll. ^[11] and Lotfi et al. ^[12] found the same decrease in Anthurium andreanum and in Pyrus communis L., respectively. Somatic embryo germination and conversion of three southern pine species ^[13] and Cydonia oblonga ^[14] were positively affected by application of RL.

Positive effects of RL illumination have been ascertained in many orchids. In Cymbidium Cymbidium Waltz ‘cv Idol’, the highest protocorm-like bodies (PLBs) formation rate (100%) was found in the culture media containing 0.01 and 0.1 mg L⁻¹ N- acetylglucosamine (NAG) under RL, although a promotive role was observed under green (GL), but at 1 mg L⁻¹ NAG ^[15]. In a study of Mengxi et al. ^[16], the highest PLBs induction rate, propagation coefficient and fresh weight of Oncidium Gower Ramsey were observed under RL treatment, which agrees with observations on the Cattleya hybrid ^[17]. However, in this last species, monochromatic RL resulted in an impaired leaf growth and chlorophyll content. Moreover, in Oncidium Gower Oncidium Gower RamseyRamsey, even if R-LEDs promoted PLB induction, it was observed that BL emitted by LEDs promoted a differentiation of PLBs ^[16]. Hamada et al. ^[18] found that R fluorescent lamps increased the PLB proliferation of Cymbidium finlaysonianum, even if used only during the early stage of the culture. The R spectrum was effective for Cymbidium callus proliferation ^[19] but not for the successive propagation. The combination of RL and FRL wavelengths determined the highest number of somatic embryos in Doritaenopsis ‘Happy Valentine’ ^[20].

The action mechanisms promoted by RL has been investigated by different researcheuthors. In Vitis vinifera, the axillary shoot development could be due to the release of apical dominance caused by BL, as suggested by Chée ^[21] and Chée and Pool ^[22]. Similarly, Burritt and Leung ^[23] observed that the inhibitory influence of FRL on shoot proliferation is reversible, whereas exposure to BL permanently reduces explant’s competence for new shoot formation. They suggested that PHY and an independent BL photoreceptor, probably CRY, regulate shoot production from Begonia × erythrophylla petiole explants. RL has been shown to exert effects on plants proliferation through the PHY, which, in the active form, would alter the endogenous hormonal balance increasing in the quantity of cytokinin (CK) in tissue, counteracting the action of auxin and thus determining an increase in the development of lateral shoots ^[24][25].

Moreover, research on the effects of PHY on in vitro multiplication of shoots of the Prunus domestica rootstock GF655-2 ^[26] demonstrated that the actions of WL, BL and FRL on shoot proliferation were fluence-rate dependent, while RL was effective both at 37 μmol m⁻² s⁻¹ and at 9 μmol m⁻² s⁻¹. The increase in light intensity had, instead, a positive effect on the production of axillary shoots in a Prunus domestica Mr.S.2/5 shoot exposed to RL and BL. However, if the number of shoots produced was expressed as a percentage of the total number of axillary buds, the rate of bud outgrowth for each shoot under RL was significantly higher than that detected under BL ^[27].

The effects of RL on proliferation are also largely dependent on the growth regulators, mainly cytokinins (CKs) applied to the culture medium, and they were found to be indispensable in the outgrowth of lateral buds in Prunus domestica rootstock shoots ^[27]. The same was true for Spiraea nipponica where the interaction between CKs and RL resulted in an enhancement of the shoot proliferation rate ^[1]. Plantlets of this species exposed to RL and FRL resulted in more marked growth than under WL ^[1]. Interesting interactions resulted from the growth of this species under low RL:FRL photon fluence followed by high-fluence WL and the benzyl aminopurine (BA) levels ^[1].

Stem elongation, leaf growth and chlorophyll reduction are frequently observed under RL and are all supposed to be associated with shade-avoidance syndrome (SAS) ^[28].

Shoot and internode elongation: It is mostly reported that RL enhances the elongation of primary and axillary shoots when there is an actively growing apex ^[29][30], and it determines changes in the plant anatomies ^[31] of multiple species ^[32]. The RL effect on stem elongation is species dependent. RL increases shoots and internode lengths in Pelargonium × hortorum ^[33], Vitis vinifera ^[34][35], Rehmannia glutinosa ^[36][37], Gerbera jamesonii ^[38], Abeliophyllum distichum ^[39], VacciniumVaccinium ashei Reade ashei Reade ^[40][41], Ficus Ficus benjamina benjamina ^[42], Cymbidium spp. ^[43], Plectranthus amboinicus ^[44] and Fragaria × ananassa plantlets ^[45]. The promotive effect of RL was also found on the elongation of secondary and tertiary shoots of Malus domestica rootstock MM106 ^[46], and on in vitro zygotic embryo germination and seedling growth in chestnut, whereas BL suppresses them ^[47]. In Populus americana, cultivar ‘I-476′, shoot length and leaf area of in vitro plants were greatest when exposed to RL, whereas on the other poplar cultivar, ‘Dorskamp’, BL plus RL were more effective ^[48]. An increase in the shoot elongation caused by internode elongation under red LEDs may result in stem fragility because of excessive elongation of the internode, as occurred in the third internode from the apex of Dendranthema grandiflorum Kitam cv.Cheonsu ^[49] and in Rehmannia glutinosa ^[37]. Following these results, it is required to adjust the ratio of RL when mixed with BL or Fl. In Fragaria × ananassa under R-LEDs, leaf petioles were elongated but the leaves turned yellowish green, revealing an irregular in vitro growth ^[45].

RL also caused thin elongated shoots and the formation of small leaves in Solanum tuberosum cv. Miranda, while BL produced short shoots with regular leaf development and many micro-tubers. The micro-tuber development was reversed when the IAA was added to the medium ^[50]. According to Kim et al. ^[49], synergistic interactions among CRYs and PHYs may promote or inhibit stem elongation in various ways in different species.

Differences in the response of the different species in the response to the RL:FRL ratios may be explained by the different habitats in which the species evolved. It has been proposed from studies on the elongation of shoots of Vitis vinifera ^[22], Disanthus cercidifolius and Crataegus oxyacantha axillary shoots ^[30] that this enhancement is PHY-mediated through the control of enzyme-affected auxin degradation, such that the extremely photolabile auxin would be conserved in cultures illuminated with RL and degraded in cultures under BL. In addition, other plant hormones may be modulated by light and by PHY directly (see paragraph 5).

Fresh and dry weight: The greatest mean fresh and dry weight of each cluster of the Malus domestica rootstock M9 was observed under RL and it was 83% greater than that observed under WL ^[3]. Gains in fresh weight were observed in Vaccinium ashei ^[40] and cattleya ^[17]. Dry weight was positively affected by RL in Myrtus communis L. ^[8], in Euphorbia milii and Spathiphyllum cannifolium ^[6] and in Plectranthus amboinicus ^[44]. Furthermore, increased growth of in vitro cultured plants provided by RL was also shown in Scrophularia takesimensis ^[51], Lippia gracilis ^[52] and Vitis vinifera ^[35]. Likewise, dry weight increased under RL, probably by the promotion of starch accumulation ^[53].

Chlorophyll content: R-LED increases chlorophyll content in Musa acuminata ^[54], Passiflora edulis ^[55] and Rehmannia glutinosa, although less than B-LED ^[36]. Most researcheuthors agree that RL, as compared to other light spectra, promoted leaf growth ^[29][48][56] but decreased the chlorophyll and carotenoids content of in vitro plantlets ^{[6][16][43][57][58]}. On the contrary, Cybularz-Urban et al. ^[17] found that in Cattleya plantlets grown in vitro RL caused the collapse of some of the mesophyll cells and a reduction of leaf blades, meaning that, in the absence of BL and/or WL/GL, the regular development of cells and leaf tissues is blocked. Similar results were found in cultures of birch ^[58] where the total content of chlorophyll under BL was twice that detected under RL. Smaller amounts of chlorophyll a and carotenoids were also detected in cultures of Azorina vidalii ^[29] under RL, FRL and RL:FRL. Other researcheuthors wrote that prolonged RL illumination may result in the ‘RL syndrome’, which is characterized by low photosynthetic capacity, low maximum quantum yield of chlorophyll fluorescence (Fv/Fm), carbohydrate accumulation and impaired growth. It was observed, also, that thylakoid disarrangement in the chloroplast is proportional to the increasing incidence of RL ^[59]. This damage may be reduced by adding BL to the light spectrum ^[60]. Regulation of carbohydrate metabolism by light quality has been well documented ^[61][62]. RL emitted by LED seemed to promote the accumulation of soluble sugar, starch and carbohydrate in upland Gossypium hirsutum L. and Brassica napus ^[53][63][64] and in Oncidium ^[65][66]. RL probably may inhibit the translocation of photosynthetic products, thereby increasing the accumulation of starch ^[53][58]. Moreover, Li et al. ^[53] suggested that plantlets with lower chlorophyll content utilize the chlorophyll more efficiently than plantlets with higher chlorophyll content under R-LEDs.

The effects of BL are often reported to be antagonistic of RL ones, although the studies reported in literature concerning the role played by BL on new meristem formation are not always consistent. The positive effects of BL on the stimulation of shoot production and growth of Nicotiana tabacum during in vitro culture were reported, but at a higher light intensity ^[67], and the researcheuthors hypothesized photoinactivation of IAA. Five weeks of exposure to BL induced the highest shoot production from Nicotiana tabacum callus ^[68]. Monochromatic BL increased shoot number in Ficus benjamina ^[42], the number of shoots and nodes in Vitis vinifera L. hybrid hybrid ^[21][22], the number of adventitious buds in Hyacinthus orientalis L. ^[69] and the percentage of organogenesis and the mean number of buds per explant in Curculigo orchioides ^[70]. Higher percentages of BL in the light spectrum were also effective on in vitro shoot induction and proliferation of Anthurium andreanum ^[71], Gerbera jamesonii ‘Rosalin’ ^[72], Remnania glutinosa ^[36] and Saintpaulia ionantha ^[73]. In various species, positive results on proliferation from adding different ratios of B to the R spectrum have been described and will be widely discussed in sub-paragraph 2.3.1. The proliferation rate was greater in Brassica napus plantlets when cultured under monocromatic BL and BL plus RL ^[63]. In lavandin, on a BA-free medium, shoot number was enhanced under BL, WL and RL at low photon fluence rates ^[74]. In Oryza sativa ^[75] under B-LED illumination, the time required for callus proliferation, differentiation and regeneration was the shortest and the frequency of plantlet initiation, differentiation and regeneration was the highest. Concerning orchids, in Dendrobium officinale, the monochromatic BL and RL:BL (1:2) emitted by LEDs determined a higher percentage of protocorm-like bodies (PLBs) producing a higher number (1.5 fold) of shoots ^[76], in Cattleya intermedia × C. aurantiaca the number of shoots regenerated from PLBs was enhanced by BL ^[77]. In Oncidium, RL promoted PLB induction from shoot apex and the higher content of carbohydrate but the lowest differentiation rate, while the highest differentiation rate and protein content were observed under B-LED ^[66]. BL increased node and total shoot number as compared to RL, FRL and dark in Prunus avium cv ‘Hedelfinger’ and one of its somaclones ^[78]. In contrast, on Begonia erythrophylla petiole explants, RL played a role in meristem initiation and BL and FRL were antagonistic to meristem formation, but BL was important for primordia development ^[23]. In Gerbera jamesonii ^[38], inhibition of shoot multiplication and a reduced plant height was observed under BL compared to what resulted from all other light treatments, and a decrease of lateral shoots number was observed on Malus domestica ^[3] as compared to RL. The same study demonstrated that BL inhibited the rate of proliferation, increasing the apical dominance. Inhibition of meristematic tissue proliferation by BL has also been observed for the embryogenic tissue of Norway spruce ^[79]. The conflicting reports found in the literature might not only be attributed to species effects, but also to the different types of explants and to the stage of the organogenic process. Hunter and Burritt ^[80], working on different Lactuca sativa L. genotypes, observed a significant decrease under monochromatic BL in shoot proliferation as compared to RL or WL. They argued that RL is required for the formation of shoot primordia, whereas BL is inhibitory to primordia initiation. The effects of RL and BL on this species depended on the stage of the organogenic process in which Lactuca sativa plantlets were exposed to the different lights. Exposure to BL during the critical first few days of culture, when meristems are being initiated, results in a significant reduction in the number of shoots produced as compared to exposure to RL and WL. Furthermore, this suppression of meristem initiation is permanent and not reversible afterward by culturing plants under RL. Observations with a scanning electron microscope (SEM) clarified that the lowest shoot development under BL was attributable to the production of much more callus as compared to those cultured under WL or RL, demonstrating that rapid cell division occurred, although the organized center of cell division required for primordia formation was reduced. Moreover, the same authors observed that explants exposed to continuous RL developed numerous small shoot primordia, which occurred more slowly than those detected on tissue exposed to WL. Based on the literature, they stated that the stimulatory effects of RL as compared to WL is genotype dependent, but the inhibitory effect of BL is more widely diffused. Callus formation as affected by continuous BL illumination was observed also in Pyrus communis, where callus weight doubled as compared to BL plus RL and BL plus FRL ^[12]. In Ficus benjamina, BL induced a huge formation of callus at the basal section of shoots ^[42]. Other studies have shown that the timing of exposure to different light regimes is also critical for shoot development in vitro. For example, at least 2 wks under RL were required to improve shoot numbers from Pseudotsuga menziesii callus, and the length of time in which RL promoted shoot production lasted only 2–3 wks ^[9]. It was suggested that PHY plays an inductive role in organogenesis of Lactuca sativa L., as suggested by Kadkade and Seibert ^[5], in contrast to antagonistic role of BL, probably via CRYs.

In a series of research projects carried out with different rootstocks of Malus domestica, Prunus domestica and Prunus persica, M9, MM106, Mr.S.2/5, and GF677, respectively ^{[27][46][81][82]}, it was demonstrated that BL induced, in the starting explant and in the developed shoots, a greater number of nodes with shorter internodes than those observed in RL and in dark. It should be noted that the percentage of nodes that formed lateral shoots was higher in the presence of RL as compared to the BL one. In the Malus domestica M9 rootstock, the percentage of sprouted buds under RL was double that under BL ^[3].

Based on these results, shoot multiplication can be defined as the result of two events: the induction and formation of new buds from the apical meristem and their sprouting through the reduction or the suppression of apical dominance ^[32][83]. BL would increase the number of axillary buds but, in contrast, it exerts an inhibitory action on buds sprouting (increase in apical dominance). RL, on the other hand, would reduce the apical dominance even though it reduces the formation of new axillary buds. The lower outgrowth of buds in the presence of BL compared to RL would indicate a role in a specific photoreceptor(s) of BL, which would act as an antagonist of the PHY. Photomorphogenetic events detected in the presence of RL and BL would agree with an antagonistic model of stem branching, modulated by light through the PHYs and the photoreceptors of BL, which would interact with each other according to a dynamic model. Moreover, Muleo et al. ^[27] also showed that the internode extension inhibition under BL exposure and the concomitant positive effect of BL in enhancing axillary bud formation (neoformed nodes) was dependent on the photon fluence rate, but not on PHY photoequilibrium or on concomitant exposure to RL. A quantitative BL threshold was found near 30 µmol m⁻² s⁻¹ (400–500 nm); up to this value, internode extension decreased ^[27].

Plants, thus, possess a complex and dynamic light response and memory system that involves reactive oxygen species and hormonal signaling, which are used to optimize light acclimation and immune defenses ^[84]. Thus, regulating the spectral quality, particularly by the B-LED, improves the antioxidant defense line and is directly correlated with the enhancement of phytochemicals in Rehmannia glutinosa ^[36]. Mengxi et al. ^[16] found higher values of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities in leaves under B-spectrum irradiance and concluded that B-LED may be more satisfactory for activating different defensive systems to reduce excessive amounts of reactive oxygen species. However, in two important Dianthus caryophyllus cultivars, ‘Green Beauty’ and ‘Purple Beauty’, RL treatment also increased the activities of antioxidant enzymes and nutrient contents ^[85]. The B-LED illumination also significantly increased the antioxidant enzyme activities in leaves and roots in Amaranthus tricolor and Brassica rapa L. subsp. oleifera ^[86]. In the in vitro cultured Pyrus communis plantlets, it was detected that the gene encoding the pathogenesis-related protein PR10 is regulated daily by the body clock of a plant, while PR1 was expressed without clear evidence of circadian regulation ^[87]. In the same studies, a specific function was played by PHYB and CRY1 photoreceptors, considering that in transgenic plants the first photoreceptor enhanced the gene expression of PR1 5- to 15-fold, and CRY1 enhanced plant resistance to the Erwinia amylovora bacterial infection ^[87]. Prunus avium rootstock plantlets, overexpressing the PHYA gene and grown in vitro, displayed a strong resistance to bacterial canker (Pseudomonas syringae pv. morsprunorum), highlighting a role of light quality and quantity in the regulation of plant resistance to bacterial disease ^[88]. Therefore, light quality through the regulative network of photoreceptors plays a relevant role in the endogenous rhythms of gene expression and pathogen attacks.

BL is mostly considered to be able to increase leaf growth, photosynthetic pigment synthesis, chloroplast development and stomatal opening, soluble proteins and carbohydrates and dry matter content and to inhibit stem and root elongation, while RL enhances stem growth and carbohydrate accumulation ^{[53][61][64][66]}. In Scrophularia kakudensis, BL imposed a stressful environment that resulted in the activation of several proteins related to stress tolerance, photosynthesis, gene regulation, post-translational modification and secondary metabolism ^[89]. The improvement in the leaf characteristics induced by the addition of BL to RL seem to indicate a better quality of micropropagated plantlets, which in turn may also improve acclimation ^[83][90].

Plant height: A few papers report positive effects of BL on shoot length, while most studies agree on its inhibition of plantlet elongation. The blue spectrum was recognized to inhibit stem growth in Oncidium ^[16], in Pelargonium × hortorum ^[33], in Dendranthema grandiflorum ^[49] and in Zantedeschia jucunda ^[91], especially as compared to RL or RL:FRL. In different tree species, Prunus domestica Mr.S.2/5 and Malus domestica MM106 and M9, inhibition of internode elongation was also detected ^[3][27][46]. In contrast, BL (470 nm) and RL (660 nm) illumination were found effective for increasing shoot length in Achillea millefolium ^[92] and Dendrobium Sonia, where, however, BL significantly reduced multiplication as compared to YL ^[93].

In some cases, BL is necessary to contrast the excessive effects of RL on shoot length assuring good plantlet development. Nhut et al. ^[45] observed that Fragaria x ananassa plantlet growth was inhibited under BL, whereas an irregular plantlet growth and development was observed in the absence of BL. In the experiment of Jao et al. ^[91], a shorter stem of plant and a higher chlorophyll content was found in the RL plus BL treatment, highlighting that BL may be involved in the regulation of both plant height and chlorophyll development.

BL induces the production of short shoots with good leaf development and many micro-tubers in Solanum tuberosum. Under BL, kinetin not only strongly stimulated tuber formation, but also increased the total fresh weight and root(+stolons)/shoot ratio ^[50].

Fresh and dry weight: In Dendrobium officinale, compared to other light treatments (dark, Fl and R-LEDs), B-LEDs, alone or with R-LEDs (1:2), induced higher dry matter accumulations of PLBs and shoots ^[76]. Increased biomass production in cultures of A. millefolium ^[92] was noted under monochromatic B-LED or R + B-LEDs. Monochromatic BL determined higher fresh and dry weight and leaf number per plantlets in Euphorbia milii, Spathiphyllum cannifolium ^[6] and Rehmannia glutinosa ^[37].

It is noteworthy that monochromatic BL had a negative effect on the dry matter production of Lippia gracilis ^[52], Plectranthus amboinicus ^[44], Gossypium hirsutum ^[53] and Vanilla planifolia ^[10], as well as in the sensitive cv Dopey of Rhododendron where it also reduced leaf chlorophyll content ^[30]. In most cases, however, RL was the most effective in all these species.

Many researcheuthors, however, agree on the most positive effects obtained on fresh and/or dry weight of plantlets by adding different ratios of BL to RL as compared to only monochromatic BL (see the next chapter) ^{[16][36][94][95][96]}. Moreover, Kurilčik et al. ^[96] demonstrated that the influence on shoot length and weight of the BL component of a mixed light is tied to the photon flux density (PFD) of the FRL component. Once more, these results indicate the species-specific effects of BL on in vitro plantlet growth ^[63]. Cioć et al. ^[8] evidenced the relationship of BL and growth regulators. B-LED illumination and a high BA content in the substrates stimulated the growth of a greater number of Mirtus communis L. leaves leaves (BL and RL plus BL) and increased the fresh weight as compared to Fls, but did not affect the dry weight, whereas RL with low amount of BA enhanced both proliferation and shoot growth. Moreover, in Oncidium, the amounts of soluble protein in the PLBs and leaves were the highest in the BL treatment, which suggests that the B spectrum was advantageous for protein synthesis ^[16][66].

Leaf morphology and functionality: BL is considered an important regulator of leaf expansion; however, differences have been ascertained among the different species. BL induced the largest number of leaves per plant, and the largest leaf thickness and area in Altenanthera brasiliana ^[97] and Platycodon grandiflorum ^[64] and a similar response on leaf area was demonstrated in Gossypium hirsutum ^[53] and Brassica napus ^[63]. BL enhanced leaf chloroplast area and the translocation of carbohydrates from chloroplasts in Betula pendula ^[58]. In contrast, less leaf area was observed in Pyrus communis under monochromatic BL, as compared to RL, RL plus FRL and RL plus BL ^[12] and in Azorina vidalii ^[29], as compared to RL plus FRL. Furthermore, CRYs are known to regulate chloroplast development in response to BL ^[98].

Photosynthetic pigments accumulation: Several studies have reported that B irradiation resulted in higher chlorophyll contents and carotenoids in the in vitro plantlets as compared to RL and FL. Cultures of Euphorbia milii ^[99], Doritaenopsis ^[100], Oncidium ^[65][66], Stevia rebaudiana ^[7], Dendrobium officinale ^[76], Prunus avium cv ‘Hedelfinger’ and in its somatoclone ^[78], Zantedeschia jucunda ^[91], Tripterospermum japonicum ^[94], Chrysanthemum ^[96], Anthurium andreanum ^[11], Phalaenopsisis ^[101], Brassica napus ^[63] and Vaccinium ashei reade ^[41] exhibited higher total chlorophyll content under monochromatic B-LEDs or combinations of R- plus B-LEDs as compared to cultures exposed to R-LED or Fls treatments. The chlorophyll content, leaf and stomata number per explant were also highest on plants cultured under BL in Vitis vinifera ^[34] and in Gossypium hirsutum ^[53].

BL and UV irradiation enhanced chlorophyll content in Hyacinthus orientalis L. ^[69] and chlorophyll a+b content, but not the carotenoid content, in leaves of Pyrus communis ^[12]. Photosynthetic capacity was highest in Betula pendula Roth ^[58] and in chrysanthemum (Dendranthema grandiflorum) ^[49] when the plantlets were exposed to BL as compared to RL. In Dendrobium kingianum, the average number of PLBs and the chlorophyll content were highest under B-LEDs, in contrast to the explants cultured under R-LEDs where the highest shoot formation and fresh weight were observed ^[102]. Likewise, a study of Oncidium PLBs by Mengxi et al. ^[16] showed that chlorophyll a and b and carotenoid levels and the greatest growth were detected under B-LEDs. On the contrary, a reduction in chlorophyll levels in plants grown under BL was observed in Vanilla planifolia ^[10]. Thus, according to Li et al. ^[63], the chlorophyll content of in vitro plantlets grown under different light qualities varies within plant species or cultivars. Moreover, even if BL, as compared to RL or different RL:BL ratios, reduced leaf expansion and hence leaf area in Azorina vitalii, the chlorophyll and carotenoid content per unit leaf area was higher than RL:FRL ^[29].

Changes in chlorophyll biosynthesis induced by changes in spectral quality may provide advantages regarding plant growth ^[103]. The species-specific responses to the B spectrum, in terms of photosynthetic pigments, are probably tied to the different environments in which the different species developed and to the type of explant used for in vitro initiation. In Lippia gracilis, plantlets that originated from apical explants had higher pigment production under the BL spectrum, whereas those from nodal explants showed higher production under WL, followed by the BL conditions ^[52]. These studies indicate that BL provides important environmental information and mostly promotes higher photosynthetic efficiency.

Many studies have been carried out on the effects of combining BL and RL. A mixture of photon quantity of BL plus RL may combine the advantages of monochromic RL and BL and may overcome the individual disadvantages of these lights. However, a large amount of research regarded the assessment of the best proportion of photon quantity of BL and RL, since different behaviors have been ascertained between species and varieties ^[53]. In some cases, the same ratio between RL and BL is effective (RL:BL = 1:1); in other cases, higher percentages of RL as compared to BL or vice versa are effective.

A large number of studies demonstrated the promoting role of R- plus B-LEDs in various combinations on shoot regeneration and the growth of the regenerated plants: BL:RL = 1:1 in Lilium oriental ^[104], RL:BL = 9:1 in the recovery of Solanum tuberosum plantlets after cryoconservation ^[105], RL:BL = 9:1 ^[106] and RL:BL = 7:3 in Fragaria x ananassa ^[45], RL:BL = 7:3 in Saccharum officinarum ^[107] and RL:BL = 1:1 in upland Gossypium hirsutum L. ^[53] and Abeliophyllum distichum ^[39]. In Gerbera jamesonii ^[38], the highest shoot multiplication rate (40% higher proliferation as compared to plantlets grown under Fls) was observed under RL:BL = 50:50 and RL:BL = 70:30. In Anthurium andreanum, shoot propagation was promoted by exposure to RL:BL illumination and higher growth under BL ^[11]. In the same species, following Budiarto ^[71], the number of regenerated shoots was greater when exposed to higher percentages of B than R-LEDs (RL:BL = 25:75). In Brassica napus L. as well, proliferation was greater under higher percentages of BL (BL:RL = 3:1 light, ^[63]. Good results on shoot proliferation have been also reported in Azorina vidalii using high RL and BL combinations (2,3; BL:RL, ^[29] or high RL:FRL ratios (1,1)). For Panax vietnamensis ^[108], the most effective plant formation was obtained when embryogenic calli were cultured under the combination of 60% RL and 40% BL and was reported to be two times higher than under Fl ^[108]. Concerning woody species, better results on proliferation were obtained on Phoenix dactylifera with an RL:BL ratio equal to 18:2 ^[109], on Pyrus communis with an RL:BL ratio equal to 1:1 ^[12] and on Populus x euramericana with an RL:BL combination of both 70:30 and 50:50 ^[48] as compared to monochromic lights and Fl.

Concerning orchids, it seems that higher RL percentages as compared to BL ones are effective. A combination of R:B = 9:1 gave the highest shoot proliferation in Phalaenopsis protocorms ^[110]. In Cymbidium, 100% R-LED was the most effective for callus induction, but callus proliferation was best under 75% R-LED plus 25% B-LED treatment. PLB formation from callus was obtained in 25% R-LED plus 75% B-LED ^[19].

The composite light of R- and FR-abundant G2 LEDs (8% BL, 2% GL, 65% RL and 25% FRL-Valoya Oy, Helsinki, Finland) resulted effective in C. grandiflorum, G. jamesonii, H. hybrida and Lamprocapnos spectabilis giving similar or higher propagation of the Fls. However, in this case, the influence of FRL and GL must be considered and will be discussed in the following chapters ^[2].

Many studies confirmed the effectiveness of R- and B-LEDs in enhancing growth and photosynthesis in many plant species. B- and R-LEDs were developed to grow in vitro plants because chlorophyll a and b show a maximum absorption at their respective wavelengths (460 and 660 nm). The same light ratios were effective on proliferation and in promoting the quality of plantlet characteristics.

Plantlet elongation: Various combinations of R- and B-LEDs proved to determine the best results for stem length and leaf growth for Saccharum officinarum ^[111], Stevia rebaudiana ^[7], Populus x euramericana cv ‘Dorskamp’ ^[48], Pyrus communis ^[12], Fragaria x ananassa ^[106] and Dendrobium officinale ^[76]. Sivakumar et al. ^[112] showed that continuous RL plus BL or intermittent BL significantly stimulated shoot elongation of sweet Solanum tuberosum plantlets in vitro. Hahn et al. ^[37], on Rehmannia glutinosa, found that shoot lengths under either B- or R-LEDs were greater than under mixed LED or Fls, but the plantlets overgrew and appeared fragile, whereas plantlets under mixed LED or Fls were healthy, with normal shoot lengths. Thus, normal plant growth was clearly related to the presence of monochromatic BL or RL. According to some authors, the synergistic interactions between CRY and PHY could either promote or inhibit the shoot elongation in different plant species.

Plantlet growth: The composite spectra of R- and B-LEDs positively regulated fresh and, in most cases, also dry matter accumulation. As compared to the cultures raised under Fls or monochromatic lights, in most cases LEDs supplying higher RL ratios (from 70–90%) as compared to the BL ones were effective in enhancing the in vitro growth of different species such as banana ^[113], grape ^[35], Fragaria x ananassa a ^[45], Vaccinium corymbosum ^[41], Tripterospermum japonicum ^[94], Eucalyptus citriodora ^[114], Phoenix dactylifera ^[109] and Lippia alba ^[115]. Highest growth was observed under Fl and under a mixture of BL and RL in Withania somnifera plantlets ^[116]. Highest fresh and dry weights were obtained when plantlets were cultured under an equal BL and RL combination (50:50) in different species such as Chrysanthemum ^[49], Lilium ^[104], Doritaenopsis ^[100], Pyrus communis ^[12], Saccharum officinarum (^[111], upland Gossypium hirsutum L. ^[53], Vanilla planifolia ^[10] and Solanum tuberosum ^[117]. As for proliferation, higher BL rates as compared to the other species are necessary to obtain the best growth in Brassica napus ^[63]. Similarly, to proliferation, higher RL ratios enhanced plant growth and the development of different orchids: Cymbidium Cymbidium ^[43] and Phalaenopsis ^[110]. RL plus BL and FRL or RL plus FRL light significantly enhanced the fresh and dry weights of Oncidium plantlets ^[118].

Differently from other cultures in which the same lights resulted in optimal proliferation and plantlet growth, according to Mengxi et al. ^[16], in Oncidium, the highest induction rate, propagation and fresh weight appeared in the RL treatment, whereas the largest dry weight per plantlet were obtained under B:R = 20%:80% and B:R = 30%:70%, respectively. Differently from other orchids, the in vitro growth of plantlets of the Calanthe hybrid was efficiently enhanced under a mixture of BL plus RL (0.7:1) and inhibited by RL plus FRL ^[119].

Leaf number and area: In Gerbera jamesonii ^[38], monochromatic RL and BL treatments resulted in a reduced leaf area, whereas leaf number was enhanced by exposure to RL:BL = 1:1.

R and B mixed LED treatments in various combinations improved leaf number and sometimes length of in vitro cultures of Fragaria x ananassa ^[45] and Doritaenopsis ^[100], leaf area of Populus x euramericana ^[48] and leaf growth of Stevia rebaudiana ^[7].

Photosynthetic pigment levels: Many studies showed that optimizing the RL:BL ratio may improve photosynthesis. The positive effect of the appropriate B-:R-LEDs combination on the synthesis of photosynthetic pigments was reported in several studies ^[63][76]. An appropriate mixture of B- and R-LEDs, compared with solely monochromatic BL or RL, is more effective to increase the chlorophyll a/b ratio and/or carotenoids content of the in vitro grown plants of Tripterospermum. japonicum ^[94], Lippia alba ^[115] and Staphylea pinnata ^[120]. On Fragaria x ananassa mixotrophic cultures, the chlorophyll content was the greatest under RL:BL = 70:30 and the least under 100% RL ^[45].

Plant growth and development caused by increasing the net photosynthetic rate was also observed in Chrysanthemum (Dendranthema grandiflorum) under mixed R-:B-LED treatments and has been attributed to the adjustment of the spectral energy distribution of RL:BL to chlorophyll absorption ^[49]. RL or BL plus RL treatments were found more effective in grape for net photosynthetic rates ^[35] as compared to BL alone. Differences in chlorophyll content in Artemisia and Nicotiana tabacum plants were ascertained. In plants grown under WL, significantly less chlorophyll content than plants growing in RL:BL (3:1) or RL:BL (1:1) was determined ^[121]. In Gossypium hirsutum L., chlorophyll content, leaf thickness and leaf and stomata area were higher in plantlets cultured under BL; however, the best growth was provided by BL:RL = 1:1 ^[53]. In addition, in the Colt rootstock of Prunus avium exposed to BL and BL plus RL dichromatic light, the leaves had a greater accumulation of chlorophyll ^[90].

A ratio of BL:RL = 1:1 emitted by LED light facilitated the growth and produced the highest chlorophyll, carotenoid contents and photosynthetic rates in Oryza sativa seedlings, but not callus proliferation, differentiation and regeneration, which were enhanced by BL ^[75].

Different from the other species, higher BL rates as compared to RL (3:1) are necessary in Brassica napus L. (cv Westar) to increase chlorophyll concentrations compared to the other LED treatments and Fl. Therefore, the response of chlorophyll content of in vitro plantlets to different light qualities may vary among plant species or cultivars ^[63].

In different orchid species, BL plus RL was reported as the most efficient treatment on the synthesis of photosynthetic pigments. Shin et al. ^[100], in Doritaenopsis, showed that mixtures of RL plus BL stimulated photosynthesis and chlorophyll accumulation. In Dendrobium officinale, chlorophyll a and b and carotenoid contents were the highest in protocorm-like bodies incubated under RL:BL LEDs = 66.6:33.3 ^[76]. Moreover, in Oncidium plantlets, it was demonstrated that the RL and BL combined with FRL or RL plus FRL radiation significantly enhanced chlorophyll content ^[118].

The use of monochromatic or combined R- or B- LEDs may determine a mismatch with the photosynthetic spectrum. The application of the broad band WL may overcome this problem ^[122].

Shoot number: The best proliferation in Vanilla planifolia Andrews ^[10] was obtained under WL and RL plus BL. Fls and WL increased the Gerbera jamesonii ‘Rosalin’ propagation ratio ^[72]. Similarly, W-LEDs (NS1 lamps of Valoya Oy, Helsinki, Finland) determined by the combination of 20% BL, 39% GL, 35% RL, 5% FRL and G2 LED lamps, enriched in RL and FRL, were as effective as Fls on shoot propagation of Gerbera jamesonii, Heuchera × hybrida, and Lamprocapnos spectabilis. In the same study, the propagation ratio for Ficus benjamina was significantly higher under Fls as compared to all tested LEDs. These positive results were attributed to the absence of UV or cool light in the LEDs ^[2]. Similarly, the most positive effects of Fls on propagation were observed in Saccharum officinarum ^[111] and in Spathiphyllum cannifolium, where, however, high citokinins (3 mg L⁻¹ BA) were applied ^[6]. White LED exposure improved the shoot proliferation as compared to Fls but also to RL or RL plus BL lamps in Musa spp. ^[123], Bacopa monnieri ^[124] and Malus domestica genotype MM106 ^[46]. An exposure to low-level WL after 10 days in the dark (to induce organogenesis) determined the regeneration of well-proportioned shoots within 3–4 weeks in transgenic Petunia x atkinsiana ^[125]. In Prunus domestica subsp. insititia, however, the effect of the light differed in relation to the concentration of CK applied. At the optimal BA concentration (2.7 mM), WL (66 μmol m⁻² s⁻¹) provided better responses on proliferation than RL, BL and FRL, if the CK concentration was below the optimal level, the production of axillary shoots was greater in the RL. The highest BA concentration (13.3 mM) decreased proliferation in monochromatic lights, as BL, RL and FRL, but not in WL ^[26].

The regeneration of buds from cotyledons of Lycopersicon esculentum was high under continuous RL and WL ^[73]. In Anthurium ^[11], proliferation obtained in WL was similar to Fl. Muleo and Thomas ^[81] working on Prunus cerasifera, obtained better effects on shoot proliferation in intact microcuttings (with apical bud) under WL. Although apical dominance was weakest in the RL and FRL treatments, the highest proliferation of new shoots was detected under WL because of the shorter internodes and high number of new nodes in that treatment as compared to RL, FRL and dark ^[81].

In contrast, WL, which establishes a similar Pfr/Ptot ratio to RL, did not reduce apical dominance compared with dark. WL would also excite blue-absorbing photoreceptors and the effects of BL on apical dominance were similar to those of WL. It seems, therefore, that the cytokine ratio may be enhanced in woody species under WL to obtain higher proliferation; however, in some species, after a long cultivation time under WL the rate of newly formed sprouts was reduced regardless of the cytokinin concentration but increased when plantlets were exposed to RL ^[83]. Moreover, under a low BA addition to the substrate (0.5 mg L⁻¹), after one month permanence under an R-enriched light (12% BL, 19% GL, 61% RL and 8% FRL), significant enhancement in shoot proliferation in Ananas comosus was observed after it was transferred under WL (Cavallaro et al. unpublished data). More than one cycle permanence under the enriched RL, however, determined callus formation on the basis of the shoots, the loss of leaves and impaired growth in Euphorbia milii and in Ceratonia siliqua L. ^[126].

In Phalaenopsisis and Anthurium andreanum, treatments with Fls, W-LEDs (460 and 560 nm) and the combination of B- and R-LEDs showed the greatest plantlet length and number of leaves ^[101]. Shoot fresh and dry weight, plant height, number of leaves, number and length of roots were greater under Fls and W-LEDs in Vanilla planifolia ^[10].

Enhanced chlorophyll biosynthesis was also noted in Vanilla planifolia ^[10] and in different Saccharum officinarum varieties ^[107][111] under W-LED illumination. Exposure to WL was also beneficial for the accumulation of carotenoid pigments in Saccharum officinarum ^[111]. For the apical and nodal segments of Hyptis suaveolens, the best growth parameters were provided by W-LED light and RL:BL combinations ^[127].

GL has received less attention from the scientific community because it is a misconception that GL mainly plays a role in stomatal regulation, driving photosynthesis through chloroplast gene expression and so contributing to carbon gain. GL’s role in plant growth and development was controversial because it was supposed that, in conveying information, physiological responses were scarce. Since photons of the RL and BL spectrum are depleted by the absorption of plant tissues, the light reflected from and transmitted through the tissues is enriched in photons of the GL wavelength region that efficiently penetrate farther into the body of a plant ^[128]. Under this condition, GL carries signals for acclimation to irradiance on a whole plant, providing information for fine-tuning developmental acclimation to shade and acting as a secondary antagonistic regulator to the well-known RL:FRL and BL responses ^[129]. Unlike for RL and BL, a green-light-specific photoreceptor has yet to be discovered ^[130]. The most accredited GL sensor is the CRY-DASH, which reverts the physiological effect of CRY ^[131] because many physiological responses regulated by CRY are reversible by GL ^[132]. Tanada ^[133] hypnotized the existence of the heliochrome, an FRL:GL reversible receptor acting in complement to PHY. Therefore, GL effects share several attributes that are specific to the receptor antagonists of the physiological actions of RL or BL photoreceptors ^[3][46][134]. Consequently, GL penetration of the plant canopy potentially increases plant growth by increasing photosynthesis of the leaves in the lower canopy more efficiently than either BL or RL ^[135].

GL positively influenced shoot branching on the first- and second-order branches of Mr.S.2/5 Prunus domestica rootstock and determined a higher internode number and shoot elongation in GF677 Prunus persica rootstock ^[27]. Based on these results, Morini and Muleo ^[83] hypothesized that GL had a negative effect on apical dominance, similar to RL and YL.

Kim et al. ^[136] reported that adding 24% of GL to R- plus B-LEDs illumination increased Lactuca sativa L. biomass by 47%, even if the total PPFD was the same in both lighting treatments. They attributed the growth-stimulation effect of GL on its ability to penetrate deeper into leaves and canopies. In Achillea millefolium, the concentrations of chlorophyll a, chlorophyll b, b/a ratio and carotenoids were higher in plantlets under GL. The highest levels of pigments observed in the GL may indicate plant stress, which can be a way to compensate for the lack of photosynthetically active light ^[92].

In a study on the Cymbidium insigne orchid, the highest PLB formation, shoot formation rate (90%) and root formation rate (50%) were found among explants cultured in a medium supplemented with 0.1 mg L⁻¹ chitosan H under GL. After 11 weeks of culture, the fresh weight of PLBs was higher in the treatment with hyaluronic acid (0.1 mg/L) under GL ^[137]. GL and BL also enhanced in vitro PLB production in Cymbidium dayanum and Cymbidium finlaysonianum with the addition of chondroitin sulfate ^[138]. In Gerbera jamesonii, GL and RL illumination resulted in a highest number of axillary shoots and leaves number in the medium with 5 mg L⁻¹ kinetin. However, in the same medium, a high fresh weight was obtained in WL ^[4].

On Cymbidium Waltz ‘cv Idol’, the highest shoot formation (80%) was observed in the medium containing 0.1 mg L⁻¹ N- acetylglucosamine (NAG), under RL and 1 mg L⁻¹ under GL; the fresh weight of PLBs was highest at 0.01 mg L⁻¹ NAG under GL ^[15]. In the same orchid, six times of breaking the weekly light by 1 day of G-lighting during R-LED illumination showed optimal numbers and formation rates of PLBs. Optimal shoot formation was obtained by treatments of Fl+interval lighting of G-LED and B-LED+G interval lighting ^[139].

In combination with RL and BL, GL also positively affects plant growth, including leaf growth and early stem elongation ^[140][141], and is involved in the orientation of chloroplasts and in regulation of the stomatal opening ^[142].

In Solanum tuberosum plantlets in vitro, the addition of GL to the combined RL and BL increased stem diameter and leaf area, and the amounts of chlorophyll, soluble sugar, soluble protein and starch. The addition of GL to the combined RL and BL contributed to the growth and development of Solanum tuberosum plantlets more than the combined of RL and BL without GL ^[143].

Further research is necessary to understand the role of radiation oscillating around 550 nm, since the studies in this field are very limited and are mainly conducted in combination with other spectral wavelength radiations under in vivo conditions.

The reduction of apical dominance seems to be the main effect determined by YL and by the GL ^[3][46]. YL applied to cultures of Prunus domestica rootstocks Mr.S.2/5 and GF677 reduced apical dominance ^[144]; in Malus domestica rootstock M9, this light induced a production of axillary shoots greater than that detected under BL and FRL but still lower than that detected under RL ^[3]. Similar to the RL, the YL and GL induced a greater elongation of the internodes and outgrowth axillary shoots than the BL; in particular, the YL stimulated longer internodes in Prunus domestica rootstocks Mr.S.2/5 ^[27]. YL illumination induced higher proliferation in Populus alba × P. berolinensis ^[145].

YL irradiation followed by the RL one induced higher shoot proliferation (98%), a higher number of shoots per explants and early PLB formation, differentiation and shoot initiation in Dendrobium sonia ^[93]. YL elicited response of callus multiplication in Vitis vinifera ^[146]. YL also determined a higher leaf area and fresh weight and a lower shoot length in Dendrobium sonia ^[93]. YL showed a smaller increase in mean fresh weight as compared to BL but less than RL ^[3].

The YL positively affected growth in Lactuca sativa ^[147]. Based on current knowledge, the behavior of in vitro cultures subjected to YL would not be attributable to the actions of PHYs and BL photoreceptors.

Sunlight emits almost as much FR radiation as R radiation. Leaves absorb most RL but reflect or transmit most FRL ^[148]. As stated before, plants under a canopy or the lower leaves of plants spaced close together receive a greater proportion of FRL than RL radiation, i.e., a reduced RL:FRL ratio. Plants perceive this filtering of light and, in response, redirect growth and development according to the survival strategies of shade avoidance, increasing apical dominance and typically elongating in an attempt to capture available light ^[149]. In contrast, once sunlight has been reached, PHY and UVR8 inhibit shade avoidance. Several studies suggest that multiple plant photoreceptors converge on a shared signaling network to regulate responses to shade ^[150]. PHYs are the receptors of RL and FRL and are mainly involved in this perception, but plants shaded within a canopy also perceive reduced BL and possibly enriched green light through CRYs ^[131]. The detection of canopy gaps may be further facilitated by BL sensing phototropins and the UV-B photoreceptor, UVR8. Moreover, Zhen and van Iersel ^[151] reported that adding FRL consistently increased net photosynthesis of Lactuca sativa L. as compared to RL and BL. They attributed this effect to the increased quantum yield of photosystem II (ΦPSII).

The commonly applied Fl but also the R:B LEDs usually lack FRL, which is important for plant development, stem elongation and PHY activity, whereas they are abundant in GL and YL, which are less efficient for plants ^[2].

PHY in its active form, as may occur under high RL or RL:FR ratio, seems to alter the endogenous hormonal balance, reducing the apical dominance and increasing the shoot proliferation rate through enhancing lateral shoot development. On the contrary, low RL:FRL ratio or FRL alone reduces in vitro proliferation ^[83][152].

FRL appeared to increase node formation and decrease internode extension (but to a less degree than BL) as compared to the effects of RL. With dichromatic BL plus FRL, the effects on these two variables induced by BL were found to be slightly modified, indicating that the active form of PHY was only partially able to influence CRY-regulated physiological functions. While the effects of RL and BL and the RL:FRL effects during in vitro phases have been extensively examined, the effects of FRL alone have been less studied ^[12]. A high RL:FRL ratio or a low BL:RL ratio stimulated the sprouting of axillary buds in Azorina vidalii ^[29] and Vaccinium corymbosum, where, however, the presence of UV in the lighting device influenced shoot length differently in different cultivars ^[153]. Even in Spirea nipponica, shoot proliferation was greater when explants were exposed to combinations of high-ratio RL and FRL ^[154]. In a study on Oncidium ^[118], the best results on PLB formation were obtained under R+B+FR LEDs. This study also indicated that this combined radiation or RL:FRL radiation significantly enhanced leaf expansion, number of leaves and roots, chlorophyll contents and fresh and dry weight. The highest propagation ratios for Chrysanthemum × morifolium, Heuchera × hybrida, Gerbera jamesonii and Lamprocapnos spectabilis were reported under light emitted by RL- and FRL-abundant G2 LEDs ^[2]. The G2 spectrum was favorable in most of the species tested, probably because of the high GL:BL and RL:FRL ratios, which provide a higher portion of active PHYs ^[155].

Under a constant fraction of RL and BL, root number, length of roots and stems and fresh weight of the plantlets was related to the FRL component of the total PPFD in the Chrysanthemum morifolium. At the higher intensity of FRL tested (9 μmol m⁻² s⁻¹ of the total 43 μmol m⁻² s⁻¹ of PPFD), a reduction of the previous morphogenic characters was observed ^[96].

On the Prunus domestica rootstock GF655-2 cultured in vitro in the presence of BA, at a photon fluence rate of 20 µmol m⁻¹ s⁻¹, FRL irradiation significantly promoted shoot proliferation as compared to the dark ^[26]. At a lower photon fluence rate of 9 µmol m⁻¹ s⁻¹ the response was lower than the other lights and similar to that detected in the dark. Based on the data obtained in their experiments, the authors concluded that the proliferation rate induced under BL, FRL and WL strongly depended on the photon fluence rate, while no statistically significant differences could be found in the effects of RL irradiation at different photon fluence rates. In Pyrus communis, FRL was advantageous for shoot number, but shoot quality was inferior because of low shoot weight, hyperhydricity and chlorosis as indicated by the low total chlorophyll and carotenoid content ^[12]. Werbrouck et al. ^[42] reported the negative effect of FRL on in vitro biomass production of F. benjamina showing a reduction in the total number of shoots and in both shoot cluster and callus weight.

A reduced RL:FRL ratio (1:1.1) had an inhibitory effect on the growth of two Calanthe hybrids ^[119].

In microcuttings of a Prunus cerasifera rootstock, BL and WL produced a higher number of nodes, with shorter internodes compared to RL or FRL or dark. Differently, the proportion of nodes producing outgrowing of lateral shoots was higher in RL followed by FRL than in WL, BL or dark because of the weakening of apical dominance induced by the former two lights ^[81]. However, the highest proliferation of new shoots was seen in WL because of the high number of new nodes. Even here, as evidenced also by Baraldi et al. ^[26], the effectiveness of FRL required prolonged exposures and was dependent on photon fluence rates ^[81]. On M9 rootstock of Malus domestica, the development of phytomers appeared to be primarily caused by the active form of PHY, with a marginal effect from BL. Shoot growth, which combines internode elongation, development of the phytomer and branching, was highest under RL and the lowest under BL and FRL, showing the largely positive role of PHY photoequilibrium. FRL was the most inhibiting light type, reducing the proliferation rate compared with BL. Under FRL, reduced stem elongation was due to the very small number of phytomers formed ^[3].