The genus Dendrobium is one of the largest genera in Orchidaceae; there are ≈1500 species around the world. In China there are 74 species and two varieties (belonging to nine sections) and mainly distributed in the mountain ranges of southern and western China (Tsi et al., 1999). Most Dendrobium orchids are endangered species and are overexploited as a result of their ornamental and medicinal values, e.g., Dendrobium. nobile, D. pendulum, D. chrysotoxum, and D. densiflorum (Chen and Tsi, 1997). These four species all blossom in spring and can be cultivated as ornamental potted plants or used for extraction of polysaccharides and alkaloids from the stems. At present, Dendrobium is listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora. There have been considerable efforts in large-scale commercial cultivation of medicinal Dendrobium orchids. However, research on the physiology of wild Dendrobium species has been scanty, and only a few wild Dendrobium species was investigated (Chou et al., 2001; Su and Zhang, 2003a, 2003b; Zhu et al., 2013a, 2013b).
Generally, orchids can be divided into thick-leaved orchids and thin-leaved orchids according to their leaf thickness. The thick-leaved orchids belong to CAM plants, e.g., Phalaenopsis (Endo and Ikusima, 1989), Dendrobium Phalaenopsis (He and Woon, 2008), and Cattleya (Stancato et al., 2002); and the thin-leaved orchids are C3 plants, e.g., Dendrobiums (Zhu et al., 2013a, 2013b). There are fairly extensive studies on the photosynthetic physiology of thin-leaved Oriental Cymbidium (Pan et al., 1997; Pan and Ye, 2006). The photosynthetic characteristics of thin-leaved Oncidium have also been shown to be a C3 orchid (He et al., 2011; Li et al., 2002). No C4 plant was found in Orchidaceae. In our previous studies, photosynthetic characteristics of wild Dendrobium (D. williamsoii, D. longicornu, D. chrysanthum, and D. dixanthum) in China were reported to be semishade C3 orchids There is no conclusive evidence to indicate that the photosynthetic pathway of wild Dendrobium species endemic to China belong to the nobile type that blossom in spring, and they are different from the CAM orchid Dendrobium, which blossom in fall (Khoo et al., 1997). More works are needed to understand the ecophysiological and photosynthetic characteristics of the wild Chinese Dendrobium orchids.
The objective of the present investigation is to carry out a systemic study of the photosynthetic characteristics of these four well-known wild Dendrobium species (D. nobile, D. pendulum, D. chrysotoxum, and D. densiflorum) in China. This study provides useful information for the conservation and rational use of these four endangered orchids.
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