Abstract: Photosynthetic characteristics of Ottelia alismoides and Hydrilla verticillata were studied after the two species were incubated under low CO₂ conditions.The PEPC/Rubisco ratio of O.alismoides increased from 0.45 to 4.17 and of H.verticillata increased from 0.47 to 4.17 after low CO₂ induction.The activity of PEPC and NAD-ME increased more than 10-fold and other C₄ enzymes such as NAD-MDH and PPDK also increased significantly after low CO₂ induction.Other photosynthetic characteristics also changed significantly.The net photosynthetic rate of O.alismoides and H.verticillata increased by 50.8% and 74.1%,respectively,and the oxygen inhibition on photosynthetic rate of the two species decreased to 35% and 60%,respectively,after low CO₂ induction.The chlorophyll fluorescence parameters also changed after low CO₂ induction.All these facts indicated that photosynthetic carbon assimilation of O.alismoides and H.verticillata might shift from C₃ to C₄ after low CO₂ induction.