Abstract: In present work the p hydroxy phenylacetic acid (PHPA) and m hydroxy phenylacetic acid(MHPA)in rhizospheric soil solutions and decomposed root residues were detected by High Performance Liquid Chromatography(HPLC), and their effects on soybean germination at the level of 0,30,60,150,300 and 600 mg·kg^-1, on melting temperature of DNA at the level of 600 mg·kg^-1 and on cytoplasm protein patterns at the level of 0,30,150,300 mg·kg^-1 were also studied. PHPA and MHPA were detected in decomposed products of root residue and soil solutions collected from rhizosphere in which soil sickness was occurred, but not detected in soil on which soybean rotated with wheat. The standard samples of PHPA and MHPA were used for bioassay in Petri dish. The main root length and lateral root number of soybean were seriously reduced, even at the level of 30 mg·L^-1 . Soybean germination and root growth were retarded seriously by MHPA,in contrast with PHPA.At the level of 600 mg·kg^-1,the melting temperature (Tm) of soybean DNAwas increased sharply from 77℃ to 84.5 ℃ with PHPA,and from 77℃ to 88.5℃ with MHPA. Tm was affected markly by MHPA,compared to PHPA, and the result was consistent with that of bioassay. The cytoplasm protein patterns of soybean leaves were obtained from SDS PAGE,showing several low weight molecular proteins were affected, especially on 16.1kD and 20.2kD. The results implied that PHPA and MHPA had allelopathic effect on soybean,and might have a role in soil sickness of soybean.

Key words: Soybean (Glycine max L.), Allelopathy, Hydroxy phenylacetic acid