Physiological adaptations of four dominant Caragana species in the desert region of the Inner Mongolia Plateau

The physiological characteristics of four dominant Caragana species (Caragana korshinskii, Caragana stenophylla, Caragana tibetica and Caragana roborovskyi) in the desert region of the Inner Mongolia Plateau were studied. These were compared with a semi-arid species of the same genus, Caragana microphylla, in order to understand physiological adaptation mechanisms of Caragana species to desert environments. Leaf osmotic potential of the four desert Caragana species was more negative than that of C. microphylla, and turgor potential was higher. The concentrations of soluble sugars and inorganic ions in the desert Caragana species were higher than those of the semi-arid species. Leaf water content and water potential of the four desert Caragana species were relatively low, as was the free water content, while bound water content and the ratio of bound water to free water were high. These characters favoured extracting and retaining water and enhanced resistance to drought and heat stresses. The four desert Caragana species had low stomatal conductance and transpiration rate, and high water-use efficiency (WUE), indicating a water-saving strategy. In addition, the four desert Caragana species had higher peroxidase (POD) and superoxide dismutase (SOD) activities. As a result, the free radical content was not higher, and sometimes lower, than that of C. microphylla in the semi-arid area. In conclusion, Caragana species adapt to the arid environment through effective osmotic adjustment, low transpiration, high WUE, and by efficient removal of free radicals.