In the alpine region of Loess Plateau

light responses ofS.argenteaandH.rhamnoidesseedlings under different soil moisture con-ditions were analyzed in terms of gas exchange parameters using Li-6400 portable photosynthesis system.The results show that net photo-synth-esis rate(pn)

transpiration rate(Tr)and water use efficiency(WUE)of the two species were both increased with PAR in the beginning.With continued increase of PAR

pnand WUE ofS.argentea andH.rhamnoidesseedlings declined but Trstill increased.With increasing soil water contents(SWC)

pn

Tr

Stomatal conductance(Gs)and intercellular CO2concentration(Ci)of two species were both increased

while WUE increased first and then declined after reaching the maximum under the mild water-stress condition.Under similar soil moistureconditions

light compensation point(LCP)ofS.argenteawas clearly lower than that ofH.rhamnoides

light saturation point(LSP)and efficiency of light energy utilization ofS.argenteawere higher than those ofH.rhamnoides

and the apparent quantum yield(α)in the leaves ofS.argenteawas higher than that ofH.rhamnoides.Compared withH.rhamnoides

S.argenteaexhibited higher photosynth-etic capacity at low PAR.Contrastive analysis indicates thatH.rhamnoidesshowed stronger response to high PAR under the waterstress condition

whileS.argentea had higher utilization capacity of low PAR and WUE.Overall

S.argenteaseedlings displayed higher drought-stressed productivity thanH.rhamnoides seedlings.