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Tetraena mongolica Maxim.belongs to a monotypic genus in the Zygophyllaceae and is a super xerophyte and a relic subshrub of the Tethys.This species is endemic to a very limited area of West Erods region in the western part of the Ordos Plateau in China.The limited distribution of this subshrub makes it an important conservation target.It is ranked as a second-class national endangered plant species as defined by China′s Red Book(endangered species list) and has been considered unique from both a plant taxonomical and regional systematics points of view.Also,from a practical standpoint,it plays an important role as a windbreaker and stabilizer of sand and contributes to soil conservation and the regulation of hydrological regimes.Populations of T.mongolica serve as an important element in ecosystem functioning and buffer local ecosystems against desertification.Deterioration of the environment and human activities in recent years have resulted in a high degree of landscape fragmentation and habitat loss in the area.As a result,population size and density of T.mongolica have been decreasing dramatically and populations are gradually being replaced by the closely related Zygophyllum xanthoxylon(Bunge) Maxim..T.mongolica has become severely endangered.So research on the conservation biology of this species has attracted attention.A comparative analysis in T.mongolica and its relative Z.xanthoxylon are ecologically important and will help document the threats to T.mongolica and offer a scientific basis for establishing protective measures for this species.One way to show the differentiation of adaptability in different plants is to study the characteristics of photosynthesis and water metabolism in these species.In this paper,seven water relation parameters were measured: osmotic potential at saturated point(Ψsats),osmotic potential at turgor loss point(Ψstlp),relative osmotic water content at turgor loss point(ROWCtlp),relative water content at turgor loss point(RWCtlp),relative content of apoplastic water(AWC),bound water/ free water(Va/V0),and maximum bulk modulus of elasticity(emax).The press-volume technique was used and photosynthetic characteristics were measured with a Li-6400 portable photosynthesis system.The goal was to compare the endangered plant T.mongolica to its relative Z.xanthoxylon to investigate differences in their viability and adaptability.The results show,first,T.mongolica had a higher Ψsats,Ψtlps,AWC,Va /V0 and lower emax,ROWCtlp,RWCtlp than Z.xanthoxylon.The two plants maintained their turgor pressure in different ways.T.mongolica maintains turgor pressure by elastic adjustment with small changes in cell volume and a strong ability to retain water.Z.xanthoxylon uses osmotic adjustment,which had a weak adjustment ability.T.mongolica is less able to maintain turgor pressure and has a lower water potential,which gives it a lower level of drought resistance than Z.xanthoxylon.Second,the curves of the diurnal change in the net photosynthetic rates(Pn) and transpiration rates(Tr) of these two plants had double peaks.The first peak appeared at 11:00 and the second at 15:00 with declines in Pn and Tr observed at midday in these two plants.When compared with Z.xanthoxylon,T.mongolica had lower a Pn and water use efficiency,which demonstrates its weaker photosynthetic capability and lower adaptability to a drought prone environment.The results revealed the physio-ecological viability and drought adaptability of T.mongolica were weaker than Z.xanthoxylon.