The vegetative organ structure and ecological adaptability of six orchid species in Karst area in the Beipan River of Guizhou Province

Abstract [Objective] The purpose of this study is to elucidate the adaptive traits of the vegetative organ structure of Orchidaceae to Karst habitats, in order to reveal the survival strategies of different orchid species in response to habitat heterogeneity, and to provide theoretical support and reference for the research, protection, and development of orchid species. [Methods] Investigation was conducted on the structure of leaves, pseudobulbs, and roots of six Orchidaceae species varying in life forms in the Karst region of the Beipan River, Guizhou, China. The paraffin section technique was employed to examine the structure and quantify relevant parameters. Descriptive and variance analysis, correlation analysis, and principal component analysis were employed for data analysis. [Results] (1) There were significant differences in structural characteristics among different orchid species. Cymbidium qiubeiense, Cymbidium cyperifolium , Cymbidium lancifolium , and Pholidota leveilleana had isofacial leaves with thin leaves. The leaves of Coelogyne flaccida and Bulbophyllum andersonii were heterofacial and thicker with palisade tissue and spongy tissue differentiation. (2) Pseudobulbs were mainly composed of epidermis, basic tissue, and vascular bundle. The cells of the basic tissue were small and the density of the vascular bundle was high. The basic tissue cells of the pseudobulbs of epiphytic orchid were larger and the water storage structure was more developed. (3) The root coat and cortex of the ground orchid were thick, the root coat cells were closely arranged, and the cortical cells had many small layers. The rhizome and cortex of epiphyte orchids were thin, and the cortical cells were large. (4) The thickness of the lower epidermis of the leaves was significantly positively correlated with leaf thickness, leaf vascular bundle diameter, leaf flesh thickness, and leaf vein thickness (P <0.01), while leaf flesh thickness was significantly negatively correlated with lower cuticle thickness and pseudobulb vascular bundle diameter (P <0.05). Pseudobulb diameter was significantly positively correlated with epidermis thickness, pseudobulb vascular bundle number, and pseudobulb cross-sectional area, while pseudobulb vascular bundle diameter ratio was significantly negatively correlated with pseudobulb diameter, epidermis thickness, pseudobulb vascular bundle number, and pseudobulb cross-sectional area. Root indicators were significantly positively correlated, and root diameter, root sheath thickness, and cortex thickness were significantly negatively correlated with root vascular bundle diameter ratio. [Conclusion] Orchids possess a notably convergent adaptability, as evidenced by their stable root structure. Leaf and pseudobulb structures of orchids are mutually reinforcing and harmonizing, constituting the primary mechanism in respond to the arid Karst environment.