To investigate the relationship between stable carbon isotope discrimination ([Delta]) of different organs and water use efficiency (WUE) under different water deficit levels, severe, moderate and low water deficit levels were treated at bud burst to leafing, flowering to fruit set, fruit growth and fruit maturation stages of field grown pear-jujube tree, and leaf stable carbon isotope discrimination ([Delta]L) at different growth stages and fruit stable carbon isotope discrimination ([Delta]F) at fruit maturation stage were measured. The results indicated that water deficit had significant effect on [Delta]L at different growth stages and [Delta]F at fruit maturation stage. As compared with full irrigation, the average [Delta]L at different growth stages and [Delta]F at fruit maturation stage were decreased by 1.23% and 2.67% for different water deficit levels, respectively. [Delta]L and [Delta]F among different water deficit treatments had significant difference at the same growth stage (P<0.05). Under different water deficit conditions, significant relationships between the [Delta]L and WUEi (photosynthesis rate/transpiration rate, Pn/Tr), WUEn (photosynthesis rate/stomatal conductance of CO2, Pn/gs), WUEy (yield/crop water consumption, Y/ETc) and yield, or between the [Delta]F and WUEy and yield were found, respectively. There were significantly negative correlations of [Delta]L with WUEi, WUEn, WUEy and yield (P<0.01) at the fruit maturation stage, or [Delta]L with WUEi and WUEn (P<0.01) over whole growth stage, respectively. [Delta]F was negatively correlated with WUEy, WUEn and yield (P<0.05), but positively correlated with ETc (P<0.01) over the whole growth stage. Thus [Delta]L or [Delta]F can compare WUEn and WUEy, so the stable carbon isotope discrimination method can be applied to evaluate the water use efficiency of pear-jujube tree under the regulated deficit irrigation.
