Light quality and adventitious rooting: a mini-review

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1 Light quality and adventitious rooting: a mini-review A. Christiaens 1,2, B. Gobin 1 and M.C. Van Labeke 2 1PCS-Ornamental Plant Research, Destelbergen, Belgium, 2 Ghent University, Gent, Belgium. Abstract Efficient adventitious rooting is a key process in the vegetative propagation of horticultural and woody species. A well-rooted cutting is essential for optimal growth and high quality plants. The use of spectral light quality to influence adventitious rooting has been studied for many years, but got a lot more attention since LEDs came on the market for horticultural practices. Contrasting results of the effect of light quality on adventitious rooting are reviewed in this paper. Even though more fundamental research is needed to easily determine which light quality can be used for a specific species, the use of LEDs for adventitious rooting is promising. For in vitro plantlets implementation can be done easily, for in vivo cuttings, the use of LEDs seems also promising in a multi-layered system. The controlled environment will lead to a year-round high quality rooted cutting production. Keywords: LED, light-emitting diode, auxin, adventitious rooting Abbreviations: R red; FR far-red; B blue; Y yellow; G green; W white; FL fluorescent lamps; HPS high pressure sodium lamps; FW fresh weight; DW dry weight; AR adventitious root; LED light-emitting diode INTRODUCTION Vegetative propagation is a common form of reproduction in plants. It is a fast method that keeps the genetic traits of the stock plant, creating clones with the same characteristics. In ornamental plants, woody plants with a long generation time or plants with a poor sexual reproduction, it is an efficient and economic method for producing large numbers of plants. During vegetative propagation, a stem or a leaf cutting is taken from the mother plant, and has to form adventitious roots (ARs), i.e., roots that initiate from non-root tissues such as stems. Adventitious rooting is a process that occurs naturally. However, not all species form ARs easily and the ability or competence to form ARs is species-dependent and highly related to the maturation of plant tissues. Recalcitrance to form ARs can be a major obstacle in vegetative propagation. AR formation is a complex process controlled by multiple endogenous and environmental factors such as auxin, light, temperature and mineral nutrition (da Costa et al., 2013; Pacurar et al., 2014). Among them, auxin plays a pivotal role. Environmental factors (light, temperature, wounding) affect AR through the interaction with auxin. Moreover, there is also a complex cross-regulatory interaction between auxin and many different phytohormones to regulate AR formation (reviewed in Pacurar et al., 2014). As auxin stimulates rooting, the use of exogenous auxin treatments has become routine in practice (Hartmann et al., 2002). For some species, exogenously applied auxins are necessary to form ARs, but in others it is a spontaneous threat. In these plants, endogenous auxin produced in the shoot apical meristem or young leaves is transported basipetally down the plant stem. This polar transport is directed by auxin efflux carriers, such as PIN-FORMED (PIN) proteins and P-glycoproteins of the ATP-Binding Cassette family B transporter family, at the base of the cell (Vanneste and Friml, 2009). This basipetal transport leads to the accumulation of auxins at the base of a cutting to initiate adventitious root formation. The removal of the apex reduces the level of endogenous auxin in the basal portion of a cutting leading to a reduction in the number of regenerated roots, which shows that auxin production in the shoot apex is essential (Liu and Reid, 1992). But also the polar auxin transport through auxin influx and efflux carriers is a key factor (Ford et al., 2002). Acta Hortic ISHS DOI /ActaHortic Proc. VIII Int. Symp. on Light in Horticulture Eds.: C.J. Currey et al. 385

2 Interactions between light and auxin signaling have been reported (Halliday et al., 2009), and also links between light quality and auxin have been described in literature, mainly during photo-morphogenesis. For instance, a low red:far-red ratio, due to shading, results in an increased biosynthesis of auxin (Kurepin et al., 2007). This shade-avoidance response requires a rapid biosynthesis of auxin and its transport to promote elongation growth (Halliday et al., 2009; Hornitschek et al., 2012). Low-fluence red light has been shown to affect both auxin biosynthesis and auxin transport (Liu et al., 2011). Far-red light could reverse the response, suggesting the involvement of phytochrome. The phototropic response to blue light has been shown to be triggered by PIN3 localization. PIN3 is localized directly opposite to the unidirectional blue light, exporting auxin to the other side of the stem resulting in a bending towards the light (Ding et al., 2011). These findings indicate that there might be an effect of light quality on adventitious rooting through auxin signaling. ADVENTITIOUS ROOTING AND LIGHT Only a limited amount of research has been done on in vivo adventitious rooting under different light qualities (Table 1). As adventitious rooting of in vivo cuttings takes place in greenhouses, it is not a common practice to use supplemental lighting and the use of exclusively artificial lighting is even less a practice nowadays. A lot more research has been done on in vitro rooting of cuttings. A list of 18 selected papers is shown in Table 2. As in vitro culture of plants is done under artificial light, it is easy to replace the common white fluorescent lamps with an alternative such as colored fluorescent or LED lamps. Light quality as supplemental light Different light qualities applied as a supplemental lighting in day light conditions have been tested for in vivo cuttings. For Juniperus and Thuja, a supplemental light intensity of 20 µmol m -2 s -1 does not result in significant differences (Bielenin, 2000). However, when supplemental light intensity is higher (70 µmol m -2 s -1 ), there is a slightly higher root mass under a combination of 70% R + 30% B light for Petunia, one of three tested species by Currey and Lopez (2013). It is suggested that the increased rooting, is the result of enhanced photosynthate availability and preferential allocation into roots over stem and leaf growth. However, no differences in photosynthetic rate were measured. The main problem with supplemental light, is the intensity of the natural light, if the fraction of supplemental light is to low, only limited modifications of the spectrum can be achieved, which will lead to marginal effects on rooting and plant growth. Monochromatic light For in vivo cuttings, 3 studies used only monochromatic light. It is difficult to compare the experiments since light intensity, day length and species were different, which resulted in three different outcomes regarding to adventitious rooting. From the 18 selected papers on in vitro adventitious rooting, five compare only monochromatic light. Again different results are noted. In two papers, light intensities are not equal for the different light colors; this makes it impossible to compare them. Red light proves to be the best for Ficus (Gabryszewska and Rudnicki, 1997) and grapes (Poudel et al., 2008), while blue light is the best for Achillea (Alvarenga et al., 2015). Alvarenga et al. (2015) suggests that improved rooting by blue light is an effect of a higher photosynthetic efficiency as the ratio of chlorophyll b/a is high in the blue spectrum, indicating an energetic excess. However, it might also be a result of lower light intensity used in this paper compared to the other papers, 25 and 50 µmol m -2 s -1 respectively, as Fuernkranz et al. (1990) showed that monochromatic blue light inhibited rooting more at higher light intensities. The use of monochromatic light does not always result in a healthy looking plantlet and Nhut and Nam (2010) suggest that there is a minimum threshold level for blue light for optimal development under a red-based light source. 386

3 Table 1. Effect of light quality on ex vitro adventitious rooting. Plant material Light quality, quantity and photoperiod Effect on adventitious rooting Reference Olive cuttings FL, R, Y, G, B Y positive on rooting %, root length Morini et al., µmol m -2 s -1 Juniperus scopulorum and Thuja R, B, FL No differences Bielenin, 2000 occidentalis day light + 20 µmol m -2 s -1 for day length + 30 min Buxus, Platycladus, Rhododendron, Leucothoe cuttings B, R+B (75:25), R, R+B (75:25) +FR, R + FR 40 µmol m -2 s h + 15 µmol m -2 s -1 FR Best results for B and R+FR Van Dalfsen and Slingerland, 2012 Impatiens hawkeri, Pelargonium hortorum, Petunia hybrid cuttings Ocimum basilicum L. cuttings Hyacinthus orientalis L. leaf cuttings daylight + supplemental light from 2000 to 2200 h with HPS, R, R+B (85:15), R+B (70:30) 70 µmol m -2 s -1 natural sunlight FL, R (625 nm), B (460 nm) 30 µmol m -2 s h FL ( nm), B ( nm), G ( nm), Y ( nm), R ( nm) 25 µmol m -2 s h No differences for Impatiens and Pelargonium Petunia: R+B (70:30) higher root dry mass and root mass ratio Currey and Lopez, 2013 B stimulates root formation Lim and Eom, 2013 Rooting %, n of roots, root length highest under FL and R, lowest under G and Y Śmigielska and Jerzy,

4 388 Table 2. Effect of light quality on in vitro adventitious rooting. Plant material Light quality, quantity and photoperiod Effect on adventitious rooting Reference Prunus serotina W, R, Y, B Y highest rooting %, n of roots, DW Fuernkranz et al., 1990 in vitro shoots 4-5 µmol m -2 s -1 and µmol m -2 s -1 (measured as 1 and 10 W m -2 ) B inhibited rooting at µmol m -2 s -1 and reduced rooting at 4-5 µmol m -2 s -1 Ficus benjamina FL, B, G, R 16 h R stimulated n of roots Gabryszewska and Rudnicki, 1997 in vitro shoots FL highest root length and FW Cymbidium FL - 40 µmol m -2 s h FW and DW of roots: lowest under B and Tanaka et al., 1998 in vitro plantlets R, B, R+B (same n of LEDs) 45 µmol m -2 s h highest under FL Musa paradisiac FL, R (660 nm), R+B (9:1), R+B (8:2), R+B (7:3), higher root FW under R+B (8:2) Nhut et al., 2002 in vitro banana shoots Strawberry in vitro plantlets Zantedeschia albomaculata in vitro shoots Tripterospermum japonicum in vitro apical shoots Chrysanthemum x grandiflorum in vitro explants In vitro shoots of grapes Doritaenopsis hort. in vitro plantlets Chrysanthemum morifolium Ramat. Ellen in vitro plantlets B (450 nm) 45 µmol m -2 s h FL, R (660 nm), B (450 nm), R+B (9:1), R+B (8:2), R+B (7:3), R+B (same n of LEDs) 45 µmol m -2 s h FL, R+B, R, B 50 µmol m -2 s h FL, R (650 nm), B (440 nm), R+B (7:3), R+B (1:1) 40 µmol m -2 s h R 14 µmol m -2 s h Y 54 µmol m -2 s h G, B, W 85 µmol m -2 s h R (660 nm), B (480 nm), FL 50 µmol m -2 s h FL, R (660 nm), B (450 nm), R+B (1:1) 70 µmol m -2 s h 455 nm 640 nm 660 nm 735 nm total of 43 µmol m -2 s h B inhibited root formation Nhut et al., 2003 R+B (7:3) highest n of roots, root length, FW and DW Root DW highest under B; Chang et al., 2003 n of roots highest for FL; root length highest under R Rooting % was promoted by R, but inhibited by B Moon et al., 2006 rooting %, FW highest under G, B, W rooting % lowest under R For two genotypes, R improved rooting% and n of roots root DW highest under FL after 2, 5 weeks; and R+B and B after 8 weeks a fraction of B inhibits the formation of roots a small portion of FR increases rooting, but a higher percentage inhibits rooting Miler and Zalewska, 2006 Poudel et al., 2008 Shin et al., 2008 Kurilčik et al.,

5 Table 2. Continued. Plant material Light quality, quantity and photoperiod Effect on adventitious rooting Reference Prunus avium L. x R 35 µmol m -2 s -1 ; B 30 µmol m -2 s -1 ; B+R: highest n of roots Iacona and Muleo, 2010 Prunus cerasus L. in vitro microshoots B+R 45 µmol m -2 s -1 ; FL 32 µmol m -2 s -1 ; FR 30 µmol m -2 s h R more effective on root elongation than B Morinda citrifolia FL, R, B, R+B (1:1) 20 µmol m -2 s h n of AR highest under R; root DW highest for R+B; Baque et al., 2010 in vitro leaf explants FR 4.5 µmol m -2 s h root length highest under FL Protea cynariodes L. FL, R (630 nm), B (460 nm), R+B (1:1) rooting % highest under R, and poor under FL, Wu and Lin, 2012 in vitro plantlets 50 µmol m -2 s h n of roots highest under R Anthurium andraeanum FL, R (658 nm), B (460 nm), Y (585 nm), R+B (1:1) n of roots and DW highest under R+B Gu et al., 2012 in vitro microcuttings 40 µmol m -2 s h and W, followed by R, B, Y Jatropha curcas L. FL, B (450 nm), R (660 nm), R+B (1:1) R highest rooting %, n of roots Daud et al., 2013 in vitro shoots 45 µmol m -2 s h R+B prevent root formation Achillea millefolium L. FL, B, R, W, G Rooting %, root length, n of roots: highest Alvarenga et al., 2015 in vitro shoots Elegia capensis (Burm. f.) Schelpe in vitro shoot segments 25 µmol m -2 s h FL 65 µmol m -2 s h R (658 nm); B (466 nm); R+B (1:1) 54 µmol m -2 s h W (462 nm nm) 47 µmol m -2 s h under B and lowest under G R+B was most promotive for root formation Verstraeten and Geelen,

6 Dichromatic light Monochromatic and different combinations of red and blue light are investigated in one paper on in vivo rooting and thirteen papers on in vitro rooting. Only in three papers, monochromatic red light is considered to be beneficial for adventitious rooting. A clear reason is not provided, only Wu and Lin (2012) provide a hypothesis based on measurements of endogenous phenolic compounds. They suggest that the low endogenous concentrations of 3,4-dihydroxybenzoic acid and ferulic acid under red LEDs stimulate root formation as there is an inverse correlation between phenolic compounds and root growth. In the majority of papers (7) R+B gives the best results for adventitious rooting. Gu et al. (2012) suggests an influence of shoot growth (shown by a high correlation between total DW and root number) as the root formation relies substantially on photosynthetic products of leaves. Also Shin et al. (2008) argues this point showing the highest levels of starch and carbohydrates under dichromatic R+B. This theory of a higher supply of sugars is acceptable for root growth, however, for root initiation high levels of sugars are inhibitory and auxin plays the major role (Agullo -Anto n et al., 2010). Iacona and Muleo (2010) suggest that a better performance of dichromatic irradiation might indicate the presence of a co-action amongst phytochromes and blue photoreceptors signalization pathways. Far-red light Out of the fourteen papers considered in the paragraph on dichromatic light, 4 also included far-red light. Far-red light has been tested in two papers as such and inhibit in vitro adventitious rooting, suggesting an active role of phytochrome (Iacona and Muleo, 2010). When FR is used as a supplement to R, it stimulates adventitious rooting for in vivo cuttings of four ornamental species (Van Dalfsen and Slingerland, 2012) and in vitro Chrysanthemum (Kurilc ik et al., 2008). When FR is used as a supplement to R+B, it increases adventitious rooting as well, but only when the fraction of FR is not too high (Kurilc ik et al., 2008). The authors suggest that the complex influence of simultaneous illumination in blue and red/farred regions can be interpreted by synergistic interactions between cryptochromes and phytochromes, which was also suggested by Iacona and Muleo (2010). Light intensity and interaction with light quality Light intensity can also play a role in adventitious rooting as shown by many authors, including some presented in Table 2 (Alvarenga et al., 2015; Fuernkranz et al., 1990; Kurilc ik et al., 2008). It is shown under one light spectrum, that there is an optimum light intensity for adventitious rooting. However, there might also be an interaction between light quantity and quality (Cope and Bugbee, 2013), which is species-dependent and can lead to different conclusions. Comparing different papers In some papers it is not clear what the best light quality is for adventitious rooting, as the different quality factors (rooting percentage, number of roots, root dry weight, root length, etc.) are influenced in a different way by light quality (Baque et al., 2010; Chang et al., 2003). Also timing of these measurements can be crucial, as Shin et al. (2008) showed different results after 2 and 5 weeks compared to 8 weeks of rooting. This might indicate that light quality influences different phases of the rooting process in a different manner. The induction phase of adventitious rooting requires high amounts of auxin, while during the formation of roots high amounts of auxin can be inhibitory (da Costa et al., 2013). Assessing root quality at the start of the rooting phase under a light quality that promotes high amounts of auxin can give different results comparing to an assessment after several weeks of rooting under this light quality. An extra difficulty for comparing studies on in vitro rooting is the different culture media with/without hormones used. This in itself can lead to different conclusions, as for example blue light can enhance rooting more in NAA treated cuttings compared to nontreated cuttings (Lim and Eom, 2013). A common explanation for contrasting results is the species-dependent nature of 390

7 reactions to light quality. This can be a valid reason, and a closer look at the different species might result in a better understanding. For instance, shade tolerant and intolerant species react differently to shifts in red:far-red ratio. As phytochromes seem to be involved in AR, this might explain some of the contrasting reactions of different species with respect to adventitious rooting as well. In-depth studies on mechanisms linking light quality and adventitious rooting Even though there are interactions between light and auxin (mentioned in the introduction), in depth studies regarding these factors affecting adventitious rooting are minimal. Nevertheless, there is already evidence that it is relevant as Gutierrez et al. (2009) has shown in a study on adventitious rooting in Arabidopsis that some AUXIN RESPONSE FACTORs have a differential response to far-red and red light and these ARFs play a role in adventitious rooting. Research linking light quality, auxin and rooting has been done more fundamentally for lateral roots (Meng et al., 2015). In tobacco seedlings the best rooting results were obtained under red light. This was attributed to a higher auxin transport from leaves to roots. Evidence was found in the higher IAA concentrations measured in roots and lower concentrations in leaves of plants treated with red light; reduction of lateral root number and density after application with NPA; direct IAA transport measurements indicating increased polar auxin transport from shoot to root; and higher PIN3 expression levels under red light. Despite the fact that lateral and adventitious rooting have specific factors to determine root identity, it is believed that there are common pathways (Verstraeten et al., 2014). CONCLUSIONS It is not surprising that contrasting results are found on the effects of light quality on adventitious rooting. Differences in species, culture environment, measured characteristics, light intensities, day lengths, timing of assessment, etc. can all result in different conclusions. As the responses to light quality (activity of photoreceptors) and adventitious rooting are both complex processes, a lot of direct and indirect links might be present not only between light and auxin, but other factors regulating adventitious rooting as well. This will make a complete overview of the regulation of adventitious rooting under light quality a very complex matter. However, the use of a specific light quality offers possibilities to regulate adventitious rooting in different phases of the process. One could argue that for induction, a high requirement for auxins can be obtained with a specific light spectrum, but that further formation of the root system requires a complete different light spectrum since then a high auxin concentration is inhibitory. Nonetheless, it remains a very complex matter. ACKNOWLEDGEMENTS The authors wish to thank the Belgian Agency for Promotion of Innovation through Science and Technology (Grant N ) for funding. Literature cited Agullo -Anto n, M.A., Sa nchez-bravo, J., Acosta, M., and Druege, U. (2010). Auxins or sugars: what makes the difference in the adventitious rooting of stored carnation cuttings? J. Plant Growth Regul. 30 (1), Alvarenga, I.C.A., Pacheco, F.V., Silva, S.T., Bertolucci, S.K.V., and Pinto, J.E.B.P. (2015). In vitro culture of Achillea millefolium L.: quality and intensity of light on growth and production of volatiles. Plant Cell Tissue Organ Cult. 122 (2), Baque, M.A., Hahn, E.J., and Paek, K.Y. (2010). Induction mechanism of adventitious root from leaf explants of Morinda citrifolia as affected by auxin and light quality. In Vitro Cell. Dev. Biol. Plant 46 (1), Bielenin, E. (2000). Effect of red or blue supplementary light on rooting of cuttings and growth of young plants of Juniperus scopulorum Skyrocket and Thuja occidentalis Smaragd. Gartenbauwissenschaft 65, Chang, H.S., Charkabarty, D., Hahn, E.J., and Paek, K.Y. (2003). Micropropagation of calla lily (Zantedeschia albomaculata) via in vitro shoot tip proliferation. In Vitro Cell. Dev. Biol. Plant 39 (2),

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