Abstract:

Caragana opulens belongs to the Caragana genus of legume, and it is found across a vast area of the Inner Mongolian Plateau from Daqing Mountain and Erlianhaote (112° E) in the east to Alashan (105° E) in the west. There have been many reports about its distribution, floristic composition, growth habits, anatomy and morphology. However, there have not been any reports about its physiological and biochemical characteristics. The adaptation of plants to their environment is determined by their genetic potential, but light energy and water metabolism are ready measurable indicators. The characteristics of photosynthesis and water metabolism of C. opulens populations which are found in Helinger (a semi-arid/partial humid region with lower temperature, lower light intensity and shorter day length) and Alashan (a very droughty region with higher temperature, higher light intensity and longer day length) were compared in this paper in order to understand the adaptative mechanisms of the species to its habitat. The results indicated that the light compensation point (<500 μmol proton·m－2·s－1), light saturation point (1 200 μmol proton·m－2·s－1) and optimum temperature for photosynthesis (26 ℃) in the Helinger population were all lower than those in the Alashan population (700-800 μmol proton·m－2·s－1, 1 500 μmol proton·m－2·s－1 and 28－29 ℃, respectively). The Helinger population exhibited a higher photosynthetic rate at lower temperature and light intensity; i.e. the Helinger population exhibited better adaptations to lower temperature and light radiation, while the Alashan population was better adapted to higher temperature and more intensive light radiation. The Helinger population needed higher relative humidity to maintain its higher net photosynthesis rate than the Alashan population. The Helinger population was characterized by higher transpiration rates, higher photosynthetic rates and lower WUE, whereas the Alashan population exhibited water-saving strategies with lower photosynthetic rates and lower transpiration rates. These results suggest that the different populations have adapted physiologically to local conditions of light, temperature and humidity allowing them to photosynthesize most efficiently in their native habitats. Considering that water is a key factor for plant growth and development in the Inner Mongolia Plateau and that there is a difference in annual precipitation, soil water content and plant water status between the Helinger and Alashan regions, it is suggested that water shortage was the key driving factor responsible for the physiological differences in the net photosynthetic rate, transpiration rate, light use efficiency, and water use efficiency between the two populations.