Abstract: In a study of grazed and ungrazed (fenced) plots in meadow steppe grassland located at Hulun Buir, Inner Mongolia, we investigated the diurnal pattern of ecophysiological behavior in Leymus chinensis, including water relations, gas exchange characteristics, light interception, osmotic adjustment and osmolytes, L. chinensis was sensitive to water deficit and able to cope with grazing disturbance by decreasing transpiration rate (T_r) and increasing water use efficiency (WUE). In response to grazing disturbance, L. chinensis showed a range of adaptations, including increased chlorophyll content, increased activity of Φ_PSⅡ and Rubisco, increased tissue concentrations of nitrogen and phosphorus, increased photosynthetic utilization efficiency (PNUE, PPUE) and assimilate accumulation, and changed electron flow (ETR). Photosynthetic rate (P_n) in both grazed and fenced plots displayed a mid-day depression, attributable to a combination of stomatal and nonstomatal factors. Similarly, the maximum photochemical quantum yield (F_v/F_m) of PSⅡ in both grazed and fenced plots showed photoinhibition. L. chinensis in grazed plots displayed increased non-photochemical quenching (NPQ) and photochemical quenching (qP) with diurnal fluctuation linked to light stress. From these results it is deduced that L. chinensis in grazed plots could absorb more light energy in the photosynthetic reaction center, and respond more rapidly and sensitively. Thus, moderate grazing disturbance can promote growth of L. chinensis and maintain a higher productivity in meadow steppe grassland, than in ungrazed grassland. These findings therefore provide a scientific basis for planning productivity restoration in meadow steppe grassland in Inner Mongolia.

Key words: grazing, Leymus chinensis, gas exchange characteristics, chlorophyll fluorescence characteristics, Rubisco activity