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Water use efficiency (WUE) is an important trait associated with plant acclimation caused by water deficits, and delta C-13 is a good surrogate of WUE under conditions of water deficits. Water deficiency also enhances the accumulation of compatible solutes in the leaves. In this study, variations in foliar delta C-13 values and main osmotic solutes were investigated. Those included total soluble sugar (TSS), sucrose, free proline, glycine betaine (GB), and inorganic ionic (K+, Ca2+, and Cl-) content of Populus euphratica for different groundwater depths in a Ejina desert riparian forest, China. Results indicated that foliar delta C-13 values in the P. euphratica for different groundwater depths ranged from -29.14 +/- 0.06 to -25.84 +/- 0.04%. Foliar delta C-13 signatures became richer as groundwater levels declined. TSS, sucrose, free proline, GB, and K+ were accumulated in P. euphratica foliage with developing plant growth and increasing groundwater depth. Ca2+ and Cl- content increased under stronger P. euphratica transpiration rates for shallower groundwater depths (1-2.5 m) and decreased for deeper groundwater depths (greater than 3.0 m). Moreover, correlations between delta C-13, osmotic solutes, and groundwater depths showed that the primary osmotic solutes were TSS, sucrose, proline, GB, and K+. Correlations also showed that delta C-13 was not only a useful measure for P. euphratica-integrated WUE but also could be used as an indicator reflecting some physiological osmotic indexes.