利用LI-6400光合测定系统测定了不同天气条件下铁皮石斛（Dendrobium officinale）叶片24h CO2吸收的动态以及CO2吸收对光强和温度的响应。晴天的白天和夜间铁皮石斛都能吸收CO2，中午CO2吸收速率为负值, CO2的交换方式具景天酸代谢途径(CAM)的特点。阴雨天，只有白天吸收CO2,夜间表现为暗呼吸,光合作用碳代谢的途径为C3途径。在多云的天气条件下，白天吸收CO2，并持续至日落后。夜间21∶00仍有CO2吸收，23∶00以后至次日凌晨处于暗呼吸状态。在500 μmol·m-2·s-1光照件下，20℃出现最大CO2吸收值。在夜间，25℃时CO2的吸收速率最高。有光和无光条件下，低温或高温引起CO2吸收速率下降均为非气孔因素所致。晴天上午，铁皮石斛叶片的表观量子产额为0.035，光合补偿点为2.9μmol·m-2·s-1,饱和光强为500μmol·m-2·s-1，强光下出现光抑制现象。叶片受到强光预先照射后，即使光照减弱光抑制效应仍保持一段时间，致使光合补偿点升高，表观量子产额下降，相同光强下的CO2吸收效率降低。结果表明：铁皮石斛为兼性CAM植物，随着环境条件的变化，其光合作用在景天酸代谢途径(CAM)与C3途径间变化。
The patterns of CO2 exchange were studied in leaves of Dendrobium officinale during 24 hour periods in sunny, rainy and cloudy days to identify its photosynthetic pathway, and the response of CO2exchange rate to light intensity and temperature was studied. The results showed that there were differences in patterns of CO2 exchange among sunny, rainy and cloudy days. In sunny days CO2 was assimilated by D. officinale in daytime and at night, with the exception of midday, where no net CO 2 was taken up. The maximum rate of CO2 uptake in daytime and at night was at 9∶00 and 23∶00, respectively, which showed that the pattern of CO2 exchange was typical of Crassulacean Acid Metabolism (CAM) with four phases of CO2 assimilation during a 24 hour period. On rainy days CO2 uptake appeared only in daytime, and the CO2 exchange pattern was typical of C3 photosynthesis. On cloudy days, CO2 uptake occurred in daytime and continued to 21∶00, then CO2 was released from 23∶00 to dawn, showing a pattern intermediate between CAM and C3 photosynthesis. Under 500 μmol·m-2·s-1 light intensity CO2 uptake rate was the highest, (4.55±0.17) μmol·m-2·s-1, at 20 ℃; it decreased with temperature increase or decrease. The net rate of photosynthesis was near zero at 35 ℃. The stomatal conductance ecreased from 10 ℃ to 35 ℃, the lowest internal CO2 concentration and the highest stomatal limiting value both occurred at 25 ℃. The temperature-dependent reduction of CO2 uptake rate was due to stomatal limitation at 20-25 ℃, but not at the low temperature (<20 ℃) and high temperature (>25 ℃). However, the nocturnal CO2 uptake rate was the highest, (1.01±0.03) μmol·m-2·s-1, at a temperature of 25 ℃, and it showed a negative value at 35 ℃. Stomatal conductance was the highest at 25 ℃, and both the lowest internal CO2 concentration and the highest stomatal limiting value were at 30℃. The temperature-dependent reduction of nocturnal CO2 uptake rate was due to stomatal limitation at 20-30 ℃, but not at the low temperature (<20 ℃) and high temperature (>30 ℃) .Apparent quantum yield, light compensation point and saturated light intensity in the morning was 0.035, 2.9 μmol·m-2·s-1 and approximately 500 μmol·m-2·s-1, respectively. Photoinhibition of photosynthesis was induced by light intensity over 500 μmol·m-2·s-1. When leaves were pre-exposed to high-light for 20 min, light compensation point increased, while apparent quantum yield and CO2 uptake rate decreased. After leaves were pre-exposed to high-light of 2 000 μmol·m-2·s-1, apparent quantum yield decreased to 0.018, and the light compensation point was increased to 69 μmol·m-2·s-1. This indicated that photoinhibition remained while light intensity decreased. Based on the results above, D. officinale should be a facultative CAM plant. The pathway of photosynthesis could change between CAM and C3 pathway with variations of environmental factors. The difference of CO2 exchange patterns in D. officinale among sunny, cloudy and rainy days depended mainly on changes in light intensity, humidity and air temperature.
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