This paper focused on the water relations of two halophytes differing in photosynthetic pathway,phenotype,and life cycle:Karelinia caspica (Pall.) Less.(C3,deep-rooted perennial Asteraceae grass) and Atriplex tatarica L.(Ca,shallow-rooted annual Chenopodiaceae grass).Gas exchange,leaf water potential,and growth characteristics were investigated in two growing seasons in an arid area of Xinjiang to explore the physiological adaptability of the two halophytes.Both K.caspica and A.tatarica showed midday depression of transpiration,indicating that they were strong xerophytes and weak midday depression types.The roots of A.tatarica were concentrated mainly in the 0-60 cm soil layer,and the leaf water potential (ψL) increased sharply in the 0-20 cm layer due to high soil water content,suggesting that the upper soil was the main water source.On the other hand,K.caspica had a rooting depth of about 1.5 m and a larger root/shoot ratio,which confirmed that this species uptakes water mainly from deeper soil layer.Although A.tatarica had lower transpiration water consumption,higher water use efficiency (WUE),and less water demand at the same leaf water potential,it showed larger water stress impact than K.caspica,indicating that the growth of A.tatarica was restricted more than that of K.caspica when there was no rainfall recharge.As a shallow-rooted C4 species,A.tatarica displayed lower stomatal conductance,which could to some extent reduce transpiration water loss and maintain leaf water potential steadily.In contrast,the deep-rooted C3 species K.caspica had a larger root/shoot ratio that was in favor of exploiting groundwater.We concluded that C3 species (K.caspica) tapes water and C4 species (A.tatarica) reduces water loss to survive in the arid and saline conditions.The results provided a case for the phenotype theory of Schwinning and Ehleringer on halophytic plants.