| Reaumuria soongorica, Nitraria tangutorum and Calligomum mongolicum are the best plants to prevent wind and sand , to conservate soil and water at the arid and semi-arid areas because they are drought-resistant,salt-resistant and high temperature-resistant. They are wide-ranging distributed over desert especially over semi-arid area of NorthWest, and they have the drought-resistant ablities. So it is very important to study the drought-resistance of these plants. This paper presented the studies on the change of photosynthetic characteristic, chlorophyll fluorescence characteristic, water physiology, metabolic products, activities of enzyme and chlorophyll in Reaumuria soongorica, Nitraria tangutorum and Calligomum mongolicum from four area (LZJ, WWM, ZYL and JQJ) under natural environment. This studies were analyzed and discussed to elucidate further the mechanisms of drought-resistance in the three plants,and comprehensive evaluated their drought-resistance.The main results were summarized as follows:1. Diurnal changes of net photosynthetic rate and transpiration rate in leaves of Reaumuria soongorica, Nitraria tangutorum and Calligomum mongolicum could be classified tow types:(1) bimodal curve, including Reaumuria soongorica,(2)single curve, including Nitraria tangutorum and Calligomum mongolicum; Their phtosynthetic typy could be classified tow typies: (1)low net photosynthetic rate(Pn) and transpiration rate(Tr) , including Reaumuria soongorica,(2)high Pn and Tr, including Nitraria tangutorum,(3) low Pn and high Tr, including Calligomum mongolicum. Among the environmental factors, photosynthetically active radiation (PAR) had a leading effect on both Pn and Tr ,the high light intensity of noon caused photoinhibition on Nitraria tangutorum and Calligomum mongolicum,but did not produce on Reaumuria soongorica. The scope of the most suitable photosynthetic temperature of the three plants are 25~30℃(Reaumuria soongorica),28~32℃(Nitraria tangutorum)and 35~37℃(Calligomum mongolicum) respectively. Water condition was the main factor that caused the different Pn and Tr of different area, and relative humidity (RH)mainly effected on Tr. Among internal factors, stomatal conductance (Gs) was the major determinant of midday depression in Pn of Reaumuria soongorica and Pn before 10:00 of Calligomum mongolicum, and the water stress would increase the Gs ability of the tow plants. The Gs adjustment ability of Nitraria tangutorum was effected by many factors, but the ability effected on Pn more than on Tr. The results showed that under the water stress condition, Reaumuria soongorica increased water use efficiency(WUE) through adjusting its Gs and osmoregulation matters; WUE of Nitraria tangutorum changed with the enviromrntal condition,while WUE of Calligomum mongolicum was all over higher than other plants because its different photosynthetic way and transpiraion property.2. A decline in maximal photochemical efficiency of PSII (Fv/Fm) and actual photochemistry efficiency(ΦPSⅡ) but not photoinhibition of Reaumuria soongorica was caused by high light intensity. The decline of photochemical efficiency of PSII which was one of non-stomatal factors on midday depression in Pn of Reaumuria soongorica was caused by drought stress.High light intensity produced photoinhibition of Nitraria tangutorum and Calligomum mongolicum,and the decline of soil water aggravated this phenomenon.The photoinhibition of Calligomum mongolicum was not serious because it lagged behind that of Nitraria tangutorum. The major photoprotective mechanism which helped Reaumuria soongorica to adap the adverse circumstances was the xanthophyll cycle_dependent thermal energy dissipation but not the photoinhibition under extreme soil drought coupling with high temperature, strong solar radiation and low air humidity,while Nitraria tangutorum and Calligomum mongolicum were dependent on tow photoprotective mechanisms.3. The changes of water physiology in three desert plants from different area showed that the drought resistant capability of three desert plants decreased in order of Reaumuria soongorica, Nitraria tangutorum and Calligomum mongolicum from water potential(WP), but in order of Calligomum mongolicum, Nitraria tangutorum and Reaumuria soongorica from relative water content(RWC) and water saturation deficit (WSD).4.The results showed that it was useful to accumulate proline (Pro) and soluble sugar (SS) in short time under the water deficient in desert, it was the way that plants restrain drought stess,but the condition was different in different plants. The drought resistant capability of three desert shrub plants decreased in order of Reaumuria soongorica, Nitraria tangutorum and Calligomum mongolicum from Pro and SS. The content of malondialdehyde ( MDA) and O2-. production rate of three plants from different area were increased under the water deficient ,and it destroyed the cytomembrane. The drought resistant capability of three desert plants decreased in order of Calligomum mongolicum, Reaumuria soongorica and Nitraria tangutorum from MDA and O2-..The effect of Calligomum mongolicum to clean the active oxygen molecule by improving the activityof super oxide dismutase (SOD) , catalase (CAT) was much more remarkable than Reaumuria soongorica and Nitraria tangutorum. While the effect of peroxide enzyme (POD) was not remarkable to protect the plants under the drought stress. The drought resistant capability of three desert plants decreased in order of Reaumuria soongorica, Nitraria tangutorum and Calligomum mongolicum from SOD and CAT. Chlorophyll content of Reaumuria soongorica and Nitraria tangutorum were decreased because the soil drought stress.While chlorophyll content of Calligomum mongolicum was increased under the same condition .It showed that Calligomum mongolicum was tardied to the changing of water.5. The results showed that the drought resistant capability of Reaumuria soongorica from different area decreased in order of WWM﹥JQJ﹥ZYL﹥LZJ, the drought resistant capability of Nitraria tangutorum from different area decreased in order of JQJ﹥WWM﹥ZYL﹥LZJ,the drought resistant capability of Calligomum mongolicum from different area decreased in order of JQJ﹥WWM﹥ZYL.We could see that the more drought stress the more drought resistant capability of desert plants, it was the result that plants adapted to environmental condition through adjusting their physiological changing.6. Eighteen drought resistant indexes were restricted and relyed on each other in the drought resistant mechanisms of three plants, and they adjusted the inner drought resistant mechanisms comprehensively. While three mechanisms such as increasing WUE through declining Tr, the xanthophyll cycle-dependent thermal energy dissipation and osmoregulation matters relying on mainly on Pro occupied important place in the drought resistant mechanisms of Reaumuria soongorica. The xanthophyll cycle-dependent thermal energy dissipation and the photoprotective mechanism of photoinhibition played leading roles in the drought resistant mechanisms of Nitraria tangutorum. The drought resistant mechanisms of Calligomum mongolicum was complicated more than other plants,because chlorophyll adjusting and enzyme ability of SOD and CAT mechanisms which were not important in other plants played important roles in Calligomum mongolicum, at the same time, it adapted to the double drought stress of soil and air through higher ability of holding and saving water and WUE.
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