Abstract: The photosynthetic rate of four grass populations were measured under the natural condition. The results are as follows: The diurnal changes of the population photosynthesis of Bromus inermis, Agropyron cristatum, Elymus sibiricus belong to the noon descent form at the later of July fine day. It is correlated with lower relative humidity at noon. The photosynthetic rate in the morning is higher than in the afternoon. The diurnal changes of E. dahuricus population photosynthesis are single peak type without a reduction at noon. The diurnal net photosynthesis of B. inermis population (29.6 gCO2 · m-2 · d-1) is the highest among measureing grasses. It is closely correlated with more leaf area. Choosing the pasture with more leaf area and higher photosynthesis ability are effective ways for the rise production.

Key words: Grasses, Population, Photosynthetic rate