Introduction
Orchidaceae Juss. is one of the largest and most fascinating angiosperms, comprising more than 800 genera with more than 25,000 species worldwide. In nature, orchids grow very slowly. It takes years to get a flowering specimen. In the wild, the majority of flowers are not pollinated, so the ovules cannot be fertilised and as a result, seed pods with viable seeds are rarely formed. In addition, the growth and development of orchids is clearly influenced by climatic conditions as well as by the protective vegetation leaf crowns of local habitats, as is generally known for epiphytes or perching plants.
The role of light during in vitro propagation of orchids
Light is a significant influencing factor for every photosynthetic plant. Plants perceive the irradiance of the light, the light wavelength and the light duration. These various factors are also of considerable importance in the in vitro propagation of plants.orchids are frequently irradiated with light during most in vitro propagation phases. However, there are also cases where the tissue cultures have to be kept under darkness. The transition from darkness to light should be gradual.Selecting a suitable light source for orchid illumination is an interesting task: Among various types of light available, white fluorescent light is a commonly found light source. White fluorescent light covers a light wavelength bandwidth from 350 to 750 nm. However, the maximum light spectrum of fluorescent lamps does not meet the light needs of plants. These needs are determined by the light absorption capacity of plant light receptors, such as chlorophylls and carotenoids. In this case, important parameters for photosynthetic performance are the number of photosynthetic pigments and the intensity of photon irradiation. The optimal wavelength is in the range of 430 to 450 nm and 640 to 660 nm. For this reason, LED lamps are the best light source for orchids, especially red and blue light. LED lamps are increasingly used to influence the organogenesis and morphogenesis of various orchid species grown in tissue culture.
LED light applications for the improvement of orchid tissue cultures
A decisive advantage of LEDs is the emission of light of a specific wavelength. Modern LED plant lighting systems allow dynamic control of the spectral composition of the emitted light. Some of these lighting systems can mix light of specific wavelengths - from violet, blue, green, orange, red to dark red. Both the light spectrum and the photon flux density can be adjusted depending on physiological and morphological plant needs. For this reason, LED lamps can be used more efficiently than fluorescent lamps or sodium lamps. In addition, LED lamps generate and emit no or very little heat radiation. This makes it possible to place LED lamps close to plants without exposing the plants to possible heat damage. These are the main reasons why LED lamps are successfully used in various plant tissue cultures, including orchids. The following table shows that fluorescent lamps can be successfully substituted by LED lamps.
Species / Variety | LED irradiation | LED lighting conditions (PPFD) and comparison light | LED lighting conditions (PPFD) and comparison light |
|---|---|---|---|
"Bletilla ochracea" | (a) Blue (470 nm);(b) Green (525 nm);(c) Orange (590 nm);(d) Red (625 nm);(e) White (460 nm and 560 nm) | 40 μmol m-2 s-1,24 h light/0 h darkness(continuous light); darkness | Green and orange light achieved the highest seed germination frequency.Orange and red light stimulated root growth sustainably.Leaves were larger under white and blue LED light. |
"Calanthe" / "Christmas Orchid | (a) Red (660 nm) and blue light (450 nm); 0.7:1(b) Red (660 nm) and dark red light (730 nm); 1:1.1 | 20 μmol m-2 s-1, 16 h light / 8 h dark;White fluorescent light | Red and blue LED light improved the in vitro growth of plants, while red and dark red light had an inhibiting effect. |
"Cymbidium" | Red light (660 nm) and blue light (450 nm) in the ratio:(a) 1:0;(b) 0:1;(c) 1:1 | 45 μmol m-2 s-1,16 h light / 8 h darkness;White fluorescent light (40 μmol m-2 s-1PPFD) | Red light (1:0) stimulated leaf growth (especially leaf length) and blue light (0:1) promoted chlorophyll content. |
"Oncidium" | Red-R (660 nm), Blue-B (455 nm), and Dark Red- DR (730 nm) mixedor monochromatic:(a) RBFr;(b) RFr;(c) BFr;(d) RB;(e) R;(f) B | 50 μmol m-2 s-1,16 h light / 8 h darkness;White fluorescent light | Dark red mixed with blue light and red light significantly increased leaf growth, number of leaves and roots, chlorophyll content and fresh and dry weight of the plants. |
Red light is mostly used within the vegetative plant growth phase. Monochromatic red light has been found to increase the fresh and dry weight of plants.According to recent research, orchid propagation can be significantly increased when green LED light is used. Green light stimulates seed germination of, for example, 'Bletilla ochracea' and organogenesis of two 'Cymbidium' varieties. Furthermore, green light together with red light can improve the new formation of 'Cymbidium'. On the other hand, monochromatic red or blue light has adverse effects on plant growth. Embryogenesis of "Doritaneopsis" can be stimulated by dark red light and red light. The plant quality of "Oncidium" was increased by a mixture of dark red and red light as well as blue and dark red light. However, the growth of "Calanthe" plants was inhibited under dark red and red light. It has been found that the growth response of orchids to dark red light depends on the orchid variety and the ratio of dark red to other light wavelengths.
