Photosynthetic Carbon Assimilation Pathway Of Dendrobium Officinale And Its Physiological Response To Environmental Changes

The rare and endangered Dendrobium officinale Kimura et Migo who is called Plant Gold has been promoted to be cultivated under controlled environment due to its high economic value and extreme shortage in the wild. Although the cultivated area in D. officinale gradually expanded, the yield of D. officinale can not meet the market demand because of the slow growth rate. According to available literatures, D. officinale is a facultative crassulacean acid metabolism (CAM) plant and its photosynthetic pathway shifts between C3and CAM under sunny day and rainy day. However, little information is available on its physiological response to environmental changes. The objectives of this study were to determine the photosynthetic carbon assimilation pathway of D. officinale and its physiological response to environmental changes. The above research is in order to bring into play the photosynthetic ability and promote the growth and development of D. officinale, which provides theoretical basis and practical guidance for D. officinale cultivation.The commonly used methodology for measuring photosynthesis on a single leaf using the portable photosynthesis measurement device cannot accurately determine the true CO2assimilation rate due to the fact of stomatal movement, and the chlorophyll fluorescence measurement is also limited by dark adaptation. Therefore, two methods mentioned above are not suitable for CAM plant and for D. officinale. In this study, CO2exchange rates of D. offcinale were measured continuously on a whole plant and the electron transport efficiency of PSâ…¡ and PSI was determined based on steady-state chlorophyll fluorescence kinetics under light adaptation in order to understand the photosynthetic carbon assimilation characteristics. The photosynthetic characteristics including dark net CO2exchange percentages (ratio of net CO2exchange amount during the dark period to daily net CO2exchange amount), stomata opening percentage (ratio of opening stomata number to total stomata number in a microscope field), and fluorescence decreasing rate (decreasing percentage of max fluorescence intensity and fluorescence intensity in tenth seconds) were used for determing the photosynthetic pathway of D. officinale. The continuous photosynthesis measurement and steady-state chlorophyll fluorescence measurement are suitable for quantitative evaluation of gas exchange characteristics and electron transport efficiency in C3and C4plants, especially for CAM plants. Therefore, the two methods are suitable for photosynthesis measurements of D. officinale.Photosynthetic carbon assimilation pathway of D. officinale was determined by CO2exchange pattern, titratable acidity, stomatal movement and electron transport. The dark net CO2exchange percentage were6%and5%in growth chamber,70%and66%in Beijing greenhouse, and31%and29%in Jinhua greenhouse. The leaf titratable acidity amount of D. officinale was increased during dark period and decreased during light period under Beijing greenhouse and growth chamber conditions. The Beijing greenhouse grown plants had more1.1times titratable acid accumulation in the dark period compared to the plants grown in the growth chamber. The stomata opening percentage of D. officinale was68%in photoperiod and81%in dark period. The daily change of fluorescence decreasing rate expressed CAM pathway characteristics. Therefore, D. officinale had concomitance of C3and CAM photosynthesis patterns under conventional conditions, which change to CAM dominant pathway under stress condition and change to C3dominant pathway under non-stress condition. With decreasing substrate water content, the dark net CO2exchange percentage of D. officinale was51%on day12and the percentage again decreased gradually in12%after being rewatered on day18. The maximal photochemical efficiency of PS II (FV/FM) and performance index (PIABS) of D. officinale leaves under drought stress on day12have decreased to2.4%and34.1%comparing to that without drought stress. The drought stress can induce a recoverable damage to PSâ…¡, and the photosynthetic pathway with concomitance of C3and CAM switch from C3dominant to CAM dominant in D. officinale. The dark CO2exchange percentages of D. officinale were4%and6%under8h (4h light and4h dark) and16h (8h light and8h dark) light-dark cycles, respectively. The daily changes of leaf titratable acidity and fluorescence decreasing rate have showed similar C3pathway characteristics. A shorter light-dark cycle led D. officinale to a C3pathway alone. The drought stress and intermittent lighting can induce CAM dominant or C3dominant pathway in D. officinale.