Three kinds of potted one-year-old pear rootstocks (Pyrus calleryana, P. pashia, and P. xerophila) and 15N trace technique were used to examine the effects of different phosphorus application rates (P0, P1, P2, P3 and P4 equivalent to 0, 50, 100, 150 and 200 kg·hm-2 P2O5, respectively) on plant