Photosynthetic physiology of Dendrobium nobile, Dendrobium pendulum, Dendrobium chrysotoxum, and Dendrobium densiflorum was studied. A bimodal diurnal variation of the net photosynthetic rate (Pn) was observed in the four Dendrobium species with the first peak [5.09 to 6.06 μmol (CO2) per m-2·s-1] ≈1100 hr and the second peak [3.83 to 4.58 μmol (CO2) per m-2·s-1] at 1500 HR. No CO2 fixation was observed at night. For all four Dendrobium species, the light compensation point (LCP) was 5 to 10 μmol·;m-2·s-1, light saturation point (LSP) ranged from 800 to 1000 μmol·m-2·s-1, apparent quantum yield (AQY) was 0.02, and CO2 compensation points (CCP) and saturation point (CSP) were 60 to 85 μmol·mol-1 and 800 to 1000 μmol·mol-1, respectively. Carboxylation efficiency (CE) values ranged from 0.011 to 0.020. The optimum temperature for photosynthesis was between 26 and 30 °C. The measurement of Pn seasonal variation indicated that July to August had the higher Pn for Dendrobium species. Additionally, the chlorophyll a/b (Chl a/b) ratios of the leaves were 2.77 to 2.89. Measurement of key enzymes in the photosynthetic pathway indicated relatively high Ribulose-1,5-bisphosphate carboxylase (RuBPCase) and glycolate oxidase (GO) activities but very low phosphoenolpyruvate carboxylase (PEPCase) activities. It suggested that these four Dendrobium species are typical semishade C3 plants.
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