Abstract: This paper studied the fatty acid composition, reactive oxygen species (ROS), lipoxygenase (LOX) activity, and malondialdehyde (MDA) content in the leaves and roots of Malus hupehensis seedlings under effects of cadmium (Cd) stress. Noticeable changes were observed in the kinds and relative contents of fatty acids after treated with CdCl2 for 7〖KG-*2〗-〖KG-*7〗12 hours. The relative contents of unsaturated fatty acids in leaves and roots reached the maximum after treated for 7 hours, being 8282% and 7243%, respectively. The kinds of fatty acids in leaves increased from 11 to 14 after treated for 12 hours, while those in roots increased from 4 to 6 after treated for 17 hours. The O2〖SX（B-*3〗-〖〗·〖SX）〗 generation rate and the H2O2 content reached the maximum after treated for 3 and 7 hours, respectively, and the MDA content and LOX activity increased with treating time. Cd stress altered the fatty acid composition of
Malus hupehensis via the inducement of reactive oxygen species and lipoxygenase, and induced lipid peroxidation, which was caused by both ROS and LOX within the first 12 hours of CdCl2 treatment and mainly by the increase of LOX activity since then.

Key words: cadmium, Malus hupehensis, fatty acid, lipoxygenase, Loess Plateau, semi-arid region, apple orchard, planting density, water productivity, WinEPIC model.