It is an effective way to save and take full advantage of limited water resource in China by breeding and applying water-saving cultivars of plant. Relative studies showed that water use efficiencies （WUE） of poplar were obviously different among clones, but its physiological mechanism was not very clear. Therefore, an experiment was conducted in field to probe it through surveying WUE of different age leaves, diurnal dynamics of leaf WUE of 4 clones with the portable photosynthesis system of LI-6400, and the experimental clones were XMH4 （Populus deltoides×P.nigra）, MH8 （P.trichocarpa×P.deltoides）、XMH12 （P.deltoides×P.nigra）、DMH3 （P. trichocarpa× P.nigra）. The results indicated that: 1) WUE was remarkably different between clones, and the WUE of XMH12 and DMH3 was higher than that of XMH4 and MH8. 2) In general, the leaves distributing at middle part of stem had higher WUE than those locating at two ends of stem. This reflected that WUE changed from low to high, then to low with leaf aging. 3) Diurnal dynamics of WUE generally declined from morning to evening. 4)The main physiological factors to influenc WUE were stomatal aperture, intercellular CO2 concentration, and activity of photosynthestic system. And the ecological factors included PAR intensity, temperature, air humidity,etc. It was clear that the regulation of stomatal aperture was central to the water use efficiency. Light, temperature, humidity and carbon dioxide concentration will all act in some way either directly or indirectly on the stomatal aperture, together with internal circadian rhythms, leaf water status and xylem borne signals.
