Abstract: Desert poplar(Populus euphratica) is known as a heteroblastic species,with the leaves of mature individuals being roughly classified into three types,namely broad ovate leaf(BO),lanceolate leaf(L) and ovate leaf(O).Previous studies have shown that polymorphic leaves of Populus euphratica differs in their anatomical and physiological properties.The leaf level carbon balance which is determined by both photosynthesis and respiration,however,remains is unclear.Gas exchange characteristics,morphological and biochemical traits of polymorphic leaves of Populus euphratica were studied.Our results showed:(a) Specific leaf area differed(SLA) among polymorphic leaves,with the SLA being significantly higher for ovate leaf(SLA=(7.36±0.09)m2·kg-1) and lanceolate leaf(SLA=(8.09±0.16)m2·kg-1) than for broad ovate leaf(SLA=(6.69±0.24)m2·kg-1).Leaf nitrogen content marginally varied among leaves of different shapes but was not statistically significant;(b) Area-based leaf carbon assimilation rate(Aarea) of broad ovate leaf(Aarea=(11.92±1.22)μmol·m-2·s-1) was significantly higher than that of ovate leaf(Aarea=(10.21±1.43)μmol·m-2·s-1) and lanceolate leaf(Aarea=(9.98±1.04)μmol·m-2·s-1).However,mass-based(Amass) and nitrogen-based carbon assimilation rate(AN) did not differ among polymorphic leaves,with the Amass and AN even being slightly higher for lanceolate leaf(Amass=(80.37±7.07)μmol·kg-1·DW·s-1 and AN=(80.37±7.07)μmol·kg-1·DW·s-1) than for other leaf types;(c) Broad ovate leaf showed a significantly greater dark respiration rate on both area and mass basis(Rn-area=(1.90±0.17)μmol·m-2·s-1 and Rn-mass=(12.58±0.77)μmol·kg-1·DW·s-1) than that of ovate leaf(Rn-area=(1.60±0.19)μmol·m-2·s-1 and Rn-mass=(11.85±0.15)μmol·kg-1·DW·s-1)and lanceolate leaf(Rn-area=(1.57±0.29)μmol·m-2·s-1 and Rn-mass=(11.25±0.92)μmol·kg-1·DW·s-1).Furthermore,mitochondrial respiration taking place under daylight(RL) was consistently lower than Rn,reflecting the occurrence of light inhibition of dark respiration.Moreover,the inhibitory effect of light on dark respiration varied among leaf types,spanning from 50.37% in lanceolate leaf to 80.67% in broad ovate leaf;(d) Leaf level carbon balance(Ag/R) did not differe among leaf types,with Ag/R ratio being higher for lanceolate leaf(Ag/R=7.90±1.39) than for broad ovate leaf(Ag/R=6.19±0.87) and ovate leaf(Ag/R=7.65±1.71).Additionally,Ag/Rn was invariably lower than Ag/RL due to the inhibitory effect of light on dark respiration.It is concluded that broad ovate leaves with high physical rigidity and respiration rate may be of importance for individual maintenance,while lanceolate leaves with high carbon gain ability are predominant in supporting growth.