Mikania micrantha (Compositae) is one of the world's most aggressive weeds and is responsible for substantial damage caused to natural ecosystems and biodiversity in South china. A new virus infecting Mikania micrantha, designated Mikania micrantha wilt virus (MMWV), belongs to a third subgroup of the genus Fabavirus and represents a distinct new isolate of Gentian mosaic virus (GeMV). Although MMWV is not lethal to M. micrantha, MMWV efficiently inhibits the growth of its host under both laboratory and field conditions. The feasibility of using MMWV to control M. micrantha is therefore worth exploring further. To gain further understanding concerning the physiological and biochemical effects of MMWV infection on Mikania micrantha, the activities of superoxide dismutase (SOD), peroxidase (POD), phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), as well as chlorophyll a/b ratios, total chlorophyll content, and photosynthetic activity were evaluated in M. micrantha leaf tissues following MMWV infection. In comparison with uninfected control leaves, our results revealed that chlorophyll a, b and total chlorophyll contents, and the ratio of chlorophyll a/b decreased by 30.76%, 18.76%, 26.41% and 17.84%, respectively, by 32 d post-MMWV infection. SOD activity significantly increased and reached maximal levels within 16 d post-infection, however activities at 24 and 32 d were 13.28% and 25.37% lower than those of control (uninfected) plants, respectively. POD activities were significantly higher in leaves of inoculated plants relative to control plants throughout the experimental period, with maximal levels occurring by 24 d post-infection. POD activities at 16, 24 and 32 d post-infection were increased by 16.70%, 41.12% and 16.11%, respectively, relative to levels observed in control plants. The activity of PPO in infected M. micrantha leaves reached maximal levels at 8 d post-infection, increasing to 77.75% above control levels, however PPO levels were 14.27% below control levels by 32 d. PAL activities following MMWV infection exhibited a pattern similar to that observed for PPO, increasing to 23.58% above control levels, then subsequently declining to 20.53% below control levels, at 8 and 32 d day post-infection, respectively. Maximal net photosynthetic rate (P_max), dark respiration rate (R_d), light compensation point (LCP), light saturation point (LSP) and apparent quantum yield (AQY) were 11.76 μmol·m^-2·s^-1,-2.23 μmol·m^-2·s^-1, 42.56 μmol·m^-2·s^-1, 1063.92 μmol·m^-2·s^-1 and 0.04 μmol/μmol, respectively,in MMWV-infected leaves at 32 d post-infection. In contrast, P_max, R_d, LCP, LSP and AQY values were 17.37 μmol·m^-2·s^-1,-2.43μmol·m^-2·s^-1, 36.21 μmol·m^-2·s^-1, 1216.13 μmol·m^-2·s^-1 and 0.05 μmol/μmol, respectively, in uninfected leaves at 32 d. In comparison with control leaves, P_max and LSP values declined by 32.34 and 12.52% following MMWV infection, however, R_d, LCP and AQY values showed no significant change during the experimental period. Our results provide key information concerning the reduction of photosynthetic efficiency in leaves of M. micrantha associated with MMWV infection.