Response of CO2 Exchange Pattern and Chlorophyll Fluorescence Property in Dendrobium Officinale to Drought Stress and Rewatering

Dendrobium officinale is a facultative crassulacean acid metabolism (CAM) plant showing the photosynthetic pattern in CAM, C3 and concomitance of CAM and C3. The specific environmental stress is known to change photosynthetic pathway from CAM to C3 pattern or from C3 to CAM pattern in some facultative CAM plants but is not clear for D. officinale. In this study, influences of drought stress on CO₂ exchange pattern and chlorophyll fluorescence characteristics of D. officinale were studied for photosynthetic pathway switching. After plants were well-watered, irrigation was withheld for 12 days before rewatered and measurements were continued for another 6 days. The net CO₂ exchange rates of D. officinale plants were continuously measured by a continuous photosynthesis measurement system for 18 days and the chlorophyll fluorescence parameters were measured by steady-state chlorophyll fluorescence measurement system on day 1, day 12, and day 18. The percentage of CO₂ exchanges observed during the dark period in the total daily CO₂ exchanges increased as the drought stress increased, but decreased again as the plants were rewatered. PI_ABS in photosynthetic performance index of D. officinale leaves under drought stress on day 12 decreased 31% comparing to that without drought stress. These results indicate that the CO₂ exchange pattern of D. officinale changed from concomitance of CAM and C3 to CAM pattern by drought stress and reversed from CAM pattern to concomitance of CAM and C3 by rewatering. The drought stress can induce a recoverable damage to photosystem Ⅱ and the photosynthetic pathway switch to CAM pattern in D. officinale.