The Study Of Transformation Mechanism Of Morphology And Physiology In The Switch From C3-photosynthesis To Crassulacean Acid Metabolism Of Dendrobium

Dendrobium owns1500species,and most of them are famous for their high ornamental or medicinal values. At present,on the basis of the study on the entire Dendrobium obligate C3, facultative and obligate CAM, the systematic research on the mechanism of the C3to CAM transition has not been reported.In this work, based on identification and classification of a large number of pre-Dendrobium species and wild specie, eight species of Dendrobium were chosen as materials.By comparing changes in opening of stomatas, content of malic acid, citric physiological indicators acids, carbohydrates and PEPC activity under sufficient water supply and drought stress, combined with the morphological characteristics, leaf thickness, mesophyll succulent index under sufficient water supply, their photosynthetic pathways and mechanisms of C3/CAM transition were characterized. The results were as follows:(1) Based on the morphology observation of pseudobulb and leaf in seven kinds of Dendrobium genus, the results showed that the pseudobulb and leaf of D. loddigesii was fleshy bulb, the thickness of its leaf was0.99mm, and Sm mesophyll succulent index (Sm) was2.55, suggesting it was an obligate CAM plant. The thickness of other six species were0.31～0.89mm, which implied that they were not CAM plants. The Sm was greater than1in Dendrobium mobile, drumstick Dendrobium, Dendrobium nobile ’V1’, and was0.91in Dendrobium nobile’V4’, whicn suggested that they might be C3/CAM intermediate plants.(2) Through observations of five kinds of Dendrobium leaves with scanning electron microscope, the results showed that D. loddigesii had the minimum stomatal length and density,20.17(urn) and45.44/mm2, respectively, and drumstick Dendrobium had the maximum stomatal density,137.37/mm2, then, in turn, the stomatal density of tassels Dendrobium, Dendrobium nobile, Dendrobium nobile ’V4’ were getting smaller and smaller. Among them, the stomatal length and density of Dendrobium nobile and Dendrobium nobile’V4’very close. Under sufficient water supply, the stomatas of, Dendrobium nobile, drumsticks and spring Dendrobium Dendrobium ’V4’ opened at noon, closed at night, while under drought stress, they closed at noon, and opened at night. Under both conditions, the stomata of D. loddigesii closed at noon, and opened at night, which was just opposite in D. fimbriatum.(3) Study on the physiological indices of eight kinds of Dendrobium under sufficient water supply and drought stress showed that in D. loddigesii and Dendrobium hybrida’Sonia’, the content of malic acid and carbohydrates in the morning were quite different from that in the evening, under both conditions, exhibiting characteristics of CAM pathway, namely malic acid was accumulated and carbohydrates were consumed at night, which was opposite at daytime. While in Dendrobium nobile, drumstick Dendrobium, Dendrobium nobile ’V1’, and Dendrobium nobile’V4’, the content of malic acid and carbohydrates in the morning was notably different from in the evening under both conditions, suggesting under drought stress, malic acid accumulated, and more carbohydrates were comsumed in the evening, which implied that this four kinds of Dendrobium were C3/CAM intermediate (facultative CAM or inducible CAM) plants. The content of citric acid was very low in8kinds Dendrobium, and there was no significant difference between the content in the morning and that in the evening, indicating that in these types of Dendrobium genus, malic acid played the major role in C3/CAM transition. Besides, In the8species, the activity of Rubisco was much greater than PEPC and PPDK. After the drought stress treatment, the activity of PEPC increased significantly in Dendrobium nobile, drumstick Dendrobium, Dendrobium nobile ’V1’, Dendrobium nobile ’V4’ and autumn dendrobium’Sonia’, while the activity of Rubisco and PPDK did not change significantly.In brief, through morphology observation, stomata movement, determination of physiological indicators and enzyme activity under sufficient water supply and drought stress, the results showed that D. loddigesii, Dendrobium hybrida ’Sonia’ were obligate CAM plants; D. crepidatum, D. fimbriatum, were obligate C3plants; D. nobile, D. chrysotoxum, Dendrobium nobile ’V1’ and Dendrobium nobile ’V4’ might be C3/CAM intermediate plants. This work found that there were three kinds of Photosynthetic pathways in Dendrobium, which was very important for the study on CAM revolution and ecological adaptability of plants.