Abstract
Many Dendrobium species are both ornamental and medicinal plants in China. Several wild species have been exploited to near extinction, and facility cultivation has become an important way to meet the great market demand. Most Dendrobium species have evolved into crassulacean acid metabolism (CAM) pathways in adapting to harsh epiphytic environment, leading to low daily net CO2 absorption. Photosynthetic pathways of many facultative CAM plants are regulated by various environmental factors. Light/dark cycle plays an important role in regulating the photosynthetic pathway of several CAM species. The aims of this study were to investigate whether the photosynthetic pathway of Dendrobium species could be regulated between C3 and CAM by changing light/dark cycles and the daily net CO2 absorption could be enhanced by shortening light/dark cycle. In this study, net CO2 exchange rates of D. officinale and D. primulinum were monitored continuously during two different light/dark cycles conversion compared to Kalanchoe daigremontiana as an obligate CAM plant. The net CO2 exchange pattern and stomatal behavior of D. officinale and D. primulinum were switched from CAM to C3-like by changing the light/dark cycle from 12/12 h to 4/4 h. However, this switching was not completely reversible. Compared to the original 12/12 h light/dark cycle, the dark, light, and daily net CO2 exchange amount of D. officinale were significantly increased after the light/dark cycle was changed from 4/4 h to 12/12 h, but those in D. primulinum was opposite and those in K. daigremontiana was not affected. Daily net CO2 exchange amount of D. officinale increased by 47% after the light/dark cycle was changed from 12/12 h to 4/4 h, due to the sharp increase of light net CO2 exchange amount. However, the large decrease of dark net CO2 exchange amount could not be offset by increased light net CO2 exchange amount, leading to reduced daily net CO2 exchange amount of D. primulinum. In conclusion, the 4/4 h light/dark cycle can induce the photosynthetic pathway of D. officinale and D. primulinum to C3-like, and improve the daily CO2 absorption of D. officinale.
Highlights
Dendrobium is the second largest genus of Orchidaceae (Takamiya et al, 2011; Yan et al, 2015)
When switched to light period, the net CO2 exchange rate of K. daigremontiana first rose to the peak and fell to near zero at 0900 hours; that of D. officinale and D. primulinum first increased and decreased until 1600 hours before falling to near zero
The net CO2 exchange rates of D. officinale exhibited the shortest time of fluctuation around zero, followed by D. primulinum, that of K. daigremontiana exhibited the longest duration near zero
Summary
Dendrobium is the second largest genus of Orchidaceae (Takamiya et al, 2011; Yan et al, 2015). Many Dendrobium species have important ornamental values and possess high medicinal values in China (Ng et al, 2012; Yan et al, 2015; Teixeira Da Silva et al, 2016). Many wild species are endangered due to the exploitation, and facility cultivation has become an important way to meet the great market demand of Dendrobium. Su and Zhang (2003) measured the daily changes of net CO2 exchange rates of D. officinale under various weather conditions and found that it had a CAM pattern on sunny days, a C3 pattern on rainy days, and a pattern between CAM and C3 on cloudy days. Yang reported that diurnal net CO2 exchange rates in D. primulinum had four distinct phases of 24-h CAM cycle (Yang et al, 2011). More and more Dendrobium plants have been found to have CAM pathway to some extent
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