Dendrobium huoshanense and Dendrobium nobile are popular temperate orchids commonly marketed as traditional medicinal plants. Although the market demands are high, the resources of D. huoshanense and D. nobile are limited because seed propagation have been beset with problems of poor seed viability due to lack of endosperm. The objectives of this study were to investigate the use of T5 fluorescent light and with its supplementary wavelengths on the tissue culture seedlings of these two orchids. And, to find out suitable light wavelength or combinations which can enhance plant growth and phytochemical accumulation of these two Dendrobium species. Stem nodes of D. huoshanense and plantlets of D. nobile were grown under different light conditions in vitro, namely: T8 tubular white fluorescent lamps (T8), T5 tubular white fluorescent lamps (T5), T5 as the main light source and supplementary by Blue 440 nm (T5 + B440), Red 610 nm (T5 + R610), 658 nm (T5 + R658), Blue 440 nm plus Red 610 nm (T5 + B440 + R610), Blue 440 nm plus Red 658 nm (T5 + B440 + R658) and Red 610 nm plus Red 658 nm (T5 + R610 + R658). The effects of different supplementary light wavelengths on phytochemicals and growth were investigated. Growth characters, soluble sugar content, starch content and percentage of nitrogen were measured after 3 months of culture. Growth and phytochemical concentration of D. huoshanense and D. nobile were significantly affected by supplementary light wavelength treatments. The best results in terms of growth characters for these two Dendrobium species occurred on T5 white light supplemented with red 658 nm (T5+R658). The fresh weight, dry weight, number of nodes, number of leaves, number of roots and number of shoots of D. huoshanense was significantly increased with supplementary red 658 nm. Fresh weight, dry weight, number of roots, root length, number of leaves and plant height and of D. nobile also significantly increased with supplementary red 658 nm as compared to those in the T5 white light control. On other hand, when blue 440nm was supplemented single or combined wavelength, growth characters decreased. T5+B440+R610 treatment was more effective on dry matter accumulation of D. huoshanense and D. nobile. T8 and T5+B440+R658 showed higher plant color rating in D. huoshanense. Plant color rating of D. nobile was the greatest with T5 followed by T5 + R658. Percentage of nitrogen of D. huoshanense increased with supplementary blue 440 nm single or combined wavelength. D. nobile increased only with supplementary blue 440 nm wavelength. Starch content of these two Dendrobium species increased with supplementary blue 440 nm single wavelength as compared to the T5 white light. Soluble sugar content of D. huoshanense was significantly increased when grown under T5+B440+R610. But, soluble sugar content was not significantly affected with supplementary ligth wavelength treatment in D. nobile. This study suggests that T5 + B440 could significantly promote the starch content and percentage of nitrogen and T5 + R658 increased plant growth. Although the mechanisms of changes in phytochemicals under different supplementary light wavelength are not well known, the results demonstrated that supplementary light wavelength could be strategically used to enhance physiological responses and growth of D. huoshanense and D. nobile grown under white light in vitro.
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