In order to investigate the allelopathic effect of walnut (Juglans regia) on crops,we conducted a pot experiment to study the impacts of naturally decomposing walnut leaf litter in soil on the resistance physiology,photosynthetic parameters and growth traits of lettuce (Lactuca sativa).The present experiment included a control (CK,without leaf litter addition in soil) and three treatments (with 30,60,90 g·pot^-1 leaf litter fragments,respectively,correspondingly denoted as T¹,T²,T³).In each treatment the corresponding amount of leaf litter fragments were mixed with pre-weighed soil (8 kg·pot^-1),and lettuce seeds were sowed.The indexes related to morphology,resistance and photosynthesis of lettuce were measured at 80,100,120,and 140 d after sowing.The results showed that:(1)The activities of superoxide dismutase (SOD),peroxidase (POD) and catalase (CAT) in leaves of lettuce treated with leaf litter were promoted at 80 d,but were restrained at 100 d,then recovered to the control level at 120 d.SOD was the most sensitive enzyme to the allelopathic effect of walnut leaf litter.At 100 d of sowing,the malondialdehyde (MDA) content in leaves of lettuce was significantly enhanced by each leaf litter treatment compared to CK;(2)Treated with the leaf litter,the content of soluble protein (SP) decreased,while soluble sugar (SS) increased obviously at 100 d and 120 d;(3)At 100 d and 120 d,the net photosynthetic rate (Pⁿ) was inhibited by each leaf litter treatment,accompanied with the decrease of stomatal conductance (G^s) and transpiration rate (T^r);(4)Height,aboveground biomass and its proportion in total biomass were significantly lower than that in CK after 120 d of treatment,while the suppression compromised at 140 d (without significant differences between any leaf litter treatment and CK).In the present research,the allelopathic effect of decomposing walnut leaf litter exhibited an up-down trend with the prolonged decomposing duration.In turn,lettuce responded to the allelochemicals by adjusting the activities of antioxidant enzymes and contents of osmolytes.Lettuce might be a candidate to be used to intercrop with walnut considering its good tolerance to allelopathic effect generated by decomposing walnut leaf litter.