Abstract:Armoracia rusticana leaves treated with lanthanum (Ⅲ) were determined in net photosynthetic rate (Pn) and chlorophyll (Chl) content. Changes in chloroplast ultrastructure were observed under transmission electronic microscopy. After 24-hour treatment with 30 mg/L La(Ⅲ), Pn and Chl content in leaves were increased by 0.82% and 3.69%, respectively, and the ultrastructure of chloroplast varied slightly. After 48-hour treatment with 30 mg L-1 La(Ⅲ) , photosynthesis was inhibited and granum-thylakoid structure tended to be thinner and loose and incompact. After 24 or 48-hour treatment with 60 mg L-1 La(Ⅲ) , Pn and Chl content were remarkably decreased and the chloroplast ultrastructure was effectively destroyed. Moreover, the ultrastructure got worse as the treatment time prolonged. The multifactors variance analysis showed that the length of treatment time had a slight effect on Chl content but a significant effect on Pn, whereas the lanthanum level singly or together with the treatment time significantly affected Chl content and Pn.