Abstract: We assessed how Neotyphodium infection influenced the photosynthetic characteristics of three natural populations of Achnatherum sibiricum, one of the main resources of forage in Inner Mongolia Steppe. We used the Licor6400 apparatus to measure the parameters including photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s) and intercellular CO₂ concentration (C_i). Some environmental parameters such as photosynthetic available radiation (PAR) and air temperature (T_a) were automatically recorded. We also obtained the light response curve under a gradient of controlled PARs. The results showed that in Leymus chinensis plot of the IMGERS-CAS population and Xi Ujimqin Qi population, the diurnal average photosynthetic of E+ plants were significantly higher than those of E- plants, but in Hulingol population the difference was not significant. The effect of Neotyphodium on photosynthetic might have some correlation ship with the water condition of the local habitat. In lower water availability habitats, the endophytes tended to increase the photosynthetic ability of the host plants, while in higher water availability habitats, the endophytes had no significant contribution. In each population, the T_r, G_s, C_i, LUE(light use efficiency) and WUE(water use efficiency) were not significantly different between E+ and E- plants. The decrease of mid-day P_n was caused by non-somatal factors, and has nothing to do with the endophytic infection.

Key words: Neotyphodium, Achnatherum sibiricum, photosynthetic characteristics, light response curve