Microcystins produced by toxic cyanobacteria in freshwater ecosystems can present a harmful effect on growth and development of terrestrial plants through irrigation with contaminated water, but its phytotoxic effects on photosynthesis of plants are seldom reported.In this study, the phytotoxic effects and possible action mechanism of microcystin-LR (MC-LR) on the photosynthesis rice (Oryza sativa L.japonica) was studied.The growth and development, photosynthetic indicators including the net photosynthesis rate, stomatal conductance, intercellular carbon dioxide concentration and transpiration rate, as well as the ultrastuctural alteration of rice leaves were investigated when rice were exposed to 0, 0.1, 1, 10, 100, 500 μg·L-1 of MC-LR for 21 d.Results show that the boveground biomass and plant height of rice were reduced significantly under the exposure of 1 μg· L-1 MC-LR.The stomatal conductance, intercellular carbon dioxide concentration and transpiration rate were significantly inhibited by 46.6%, 8.44% and 44.0%, respectively, in the group of 0.1 μg· L-1 MC-LR, while the net photosynthesis rate was significantly inhibited by 39.9% in 100 μg· L-1 MC-LR.Following higher MC-LR exposure, a significant decrease of Chl a content accompanied by a increase of Chl b content were observed revealed that MC-LR may cause a change in the pigment pattern.The subcellular damages such as condensed chromatin and swollen or even disrupted thylakoids of chloroplast could be observed in mesophyll cells due to MC-LR exposure (> 1 μg· L-1).These results implied that the inhibition of photosynthesis might be an important action mechanism of MC-LR on terrestrial plants, inducing retardance in the growth and development of plants.