Abstract: The effect of root pruning and shoot pruning on net photosynthetic rate ( Pn) , transpiration rate (Tr) , stomatal conductance (Gs) , leaf water use efficiency (WUE) , and the reason causing the changes in WUE were explored in potted five2year old apple (Malus pumila Mill/ Malus hupenensis Rhed) trees. The results showed that root pruning increased WUE obviously from the second day to the 42th day after treatment . In the mean time ,
Tr and Gs decreased obviously , but Pn and carboxylation efficiency (CE) recovered rapidly and was higher than control on the seventh day of treatment . The total number of new roots and the concentration of zeatin riboside (ZR) in leaf restored to the control level after 28 days of treatment . Shoot pruning increased Pn、CE、Tr 、Gs and the concentration of ZR , but WUE heighten a little over control after 28 days of treatment . Pn、CE、Tr and Gs all decreased on the second day after both root and shoot pruning , then on the 21th day , Pn and CEwas higher than control , Tr and Gs went up to the control level , so the WUE had a little rise in the late stage of treatment . It is proposed that root pruning increased WUE by decreasing Gs significantly and shoot pruning increased WUE by increasing CE.

Key words: Apple, Root pruning, Shoot pruning, Water use efficiency, Root signal