In commercial micropropagation laboratories, light source is one of the most important factors controlling plant development. Light quality (spectral quality), quantity (photon flux) and photoperiod have a profound influence on the morphogenesis and growth of plant cell, tissue and organ cultures (Reuveni and Evenor, 2007). Fluorescent lamps (FLs) are generally used to increase photosynthetic photon flux (PPF) level for in vitro culture. However, FLs contains unnecessary wavelengths (350 to 750 nm) that are of low quality for promoting growth and morphogenesis (Gupta and Jatothu, 2013). Recently, the use of light-emitting diodes (LEDs) as a radiation source for plants has attracted considerable interest for commercial micropropagation. The most attractive fea-tures of LEDs are their small mass, volume, wavelength specificity, long life, minimum heating and photon output that is linear with electrical input current (Brown et al., 1995; Massa et al., 2008; Gupta and Jatothu, 2013). The flexibility of matching wavelengths of LEDs to plant photoreceptors may provide more optimal production, influencing plant morphology and metabolism (Kim et al., 2004; Massa et al., 2008; Morrow, 2008). Although, previous reports have confirmed physiological and mor-phological effects of LED light quality on morphogenesis and growth of several plantlets in vitro (Hahn et al., 2000; Jao et al., 2005; Shin et al., 2008; Lin et al., 2011; Li et al., 2012; 2013), these study results showed that LED light is more suitable for plant morphogenesis and growth than FL (fluorescent lamp). However, the responses vary according to plant species.

Effect of light quality on shoot proliferation

The shoot proliferation and growth of V. planifolia were significantly affected by different light treatments in vitro. The results indicated a significant increase in the number of shoots per explant under FL, W LED and B+R LED (1:1) light; these treatments produced more than 10 shoots. Explants under B and R LED conditions produced less than six shoots (Table 1). Shoot length was more than 3 cm in cultures under B, R and B+R (1:1) LEDs, and less than 3 cm in FL and W LEDs. Our results also showed that fresh weight, dry weight and dry matter were greatest in shoots under B+R (1:1) LED light, followed by FL, W, B and R LEDs (Figure 1).

Effect of light quality on the growth of plantlets

Different light qualities had variable effects on the growth of V. planifolia (Figure 2). Plant height, number of leaves, number and length of roots, fresh weight, dry weight, dry matter and chlorophyll content were greater under FL and W LED than under B LED, R LED and B+R LED (1:1) (Table 2). The growth variables were not significantly affected by FL and W LED light treatments. These treatments were beneficial for plantlet growth. Other studies appear to confirm that R LED retards development of plants in vitro (Lin et al., 2011; Edesi et al., 2014; Waman et al., 2015). Similarly, our results confirm that R LED affects the growth of V. planifolia plantlets in vitro. The present study showed that W LED light may be used as an alternative to FL for in vitro growth of V. planifolia in culture systems. After 60 days of in vitro culture, plantlets in all treatments were transferred to the greenhouse for acclimatization. Plant survival was 95% after 30 d growth under greenhouse conditions. The acclimatized in vitro plantlets were vigorous in aspect (Figure 3a) and developed normally when transplanted to soil (Figure 3b). Effect of LEDs on the micropropagation of orchid species has been studied. Fukai et al. (1997) reported in Calanthe Satsuma that the most vigorous protocorm development was obtained in the dark rather than by the irradiation of R, B, mixed light of R+B, and FL. In Oncidium, R LEDs significantly promoted the growth of PLBs, while B LEDs had the opposite effect (Xu et al., 2009). In Dendrobium officinale, R+B (1:2) LEDs are an effective light source for shoot growth (Lin et al., 2011). Godo et al. (2011) reported rhizoid formation in orange LED (590 nm) and R LED (625 nm) light in Bletilla ochracea. Our results do not coincide with those of Xu et al. (2009), Lin et al. (2011) and Godo et al. (2011). According to Kim et al. (2004), stem elongation could be promoted or inhibited by different synergistic interactions between blue or red light receptors and phytochrome depending on the species. Light quality also plays an important role in photosynthesis, influencing the way in which light is absorbed by chlorophyll (Tripathy and Brown, 1995; Topchiy et al., 2005). In our results, chlorophyll concentrations were greater in plantlets grown under FL and W LED light treatments, which is inconsistent with the idea that B light plays an important role in the synthesis of chlorophyll (Kurilcik et al., 2008; Li et al., 2010; 2012). According to Li et al. (2013), the chlorophyll content of in vitro plantlets grown under different light qualities may be correlated with the plant species or cultivar. In its natural habitat, V. planifolia grows on a shady forest floor, while other members of the Orquidacea family grow on open grassy or stony slopes.

Our results clearly demonstrate that the spectral quality of light has an influence on the morphogenesis and growth responses of V. planifolia. According to Chung et al. (2010), light induces plant morphogenesis. Light is a signal that is received by a photoreceptor that then regulates the differentiation and growth of plants (Wang et al., 2001; Muleo and Morini, 2006). The light source generally used for in vitro culture is fluorescent lamps. However, LEDs have a long life and changing the light source is therefore less frequent, resulting in reduced labor costs. Also, LEDs generate very little heat, thereby minimizing the need for an extensive cooling system in plant growth facilities. This is the first report on measuring the effects of LED light on V. planifolia in vitro; LED light is a novel illumination system for micropropagation of this important species.