Key words: Soy bean (Glycine max); UV-B ra di ation; quercetin; ox i da tive stress Ab stractThe pos si ble ameliorative ef fects of quercetin on soy bean [Glycine max (L.) Merr.] leaves ex posed to UV-B ra di a tion were con ducted in green house. The sym met ri cal leaves supplied with quercetin so lu tion (0.2 %, 1 %) were ex posed to UV-B ra di a tion (0, 3.5, 6.5 kJ·m -2 ·d -1 ). 0.2 % quercetin amelio rated leaf pho to syn the sis, im proved leaf wa ter con tent (LWC), and de creased lipid ox i da tion. The un fa vor able ef fect on photosynthetic pa ram e ter was dis played in 1 % quercetin treat ment. The ef fect of quercetin on phenylalanine am mo nia lyase (PAL) ac tiv ity var ied with the quercetin con cen tra tion, UV-B ra di a tion in ten sity and leaf de vel op ment. In the later devel op ment polyphenol oxidase (PPO) ac tiv ity was in creased sig nif i cantly by quercetin treat ments. We sug gested that quercetin with suit able con cen tra tion could serve as UV-B pro tec tive agent partly due to its an ti ox i dant ca pac ity. List of ab bre vi a tions: UV-B ul tra vi o let-B; NPRNet photosynthetic rate; MDA malondialdehyde; PAL phenylalanine ammonialyase; SOD superoxide dismutase; PPO polyphenol oxidase; LWC leaf wa ter con tent; ROS re ac tive ox y gen spe cies In tro duc tionThe in creased lev els of ul tra vi o let-B (UV-B) ra di ation reach ing the Earth sur face due to the de ple tion of the strato spheric ozone layer (Madronich et al. 1998) prompted ac tive re search on how plant organs were pro tected by dif fer ent mech a nisms (Roxema et al. 1997, Gotz et al. 1999. The plant an ti ox i dant en zyme ac tiv i ties was in creased to elim i nate re ac tive ox y gen spe cies (ROS), the leaf ca rot en oids con tents were in creased to pro tect photosynthetic chain, and leaf flavonoid con tent was also in duced to screen the UV-B ra di a tion. In re cent years, a se ries of ex per i ments pro vided convinc ing ev i dence that plants sub jected to UV-B radi a tion re sponded to changes in the con tent and ratios of dif fer ent flavonoid in leaf epi der mal cells, wax, hairs (Harborne et al. 2000). Some flavonoid in creased much more than oth ers (Noriaki et al. 2000), es pe cially the flavonoid with ortho-hydroxy struc ture in B-ring such as quercetin and quercetin glycoside in Bras sica napus and Trifolium repens