Chin J Plan Ecolo ›› 2003, Vol. 27 ›› Issue (5): 631-637.DOI: 10.17521/cjpe.2003.0091

• Research Articles • Previous Articles     Next Articles

The Photosynthesis Pathway in Leaves of Dendrobium officinale

SU Wen-Hua, ZHANG Guang-Fei   

  • Published:2003-05-10
  • Contact: ZHU Li

Abstract:

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.