Abstract: The biological yield is bound up with photosynthesis. Though the air temperature on Qinghai plateau is very low, yet the intense solar radiation on the plateau and of the long sunshine hours are favourable to photosynthesis. While the air temperature is very high, it may cause the photosynthesis to midday depression in the areas of low elevation. However, this does not occur on alpine meadow in Qinghai plateau. Diurnal variation of net photosynthetic rate(Pn) of plant leaves in alpine meadow under natural environmental conditions depends on the change of air temperature and solar radiation intensity. It gives a typical curve with one peak. None or little midday depression of net photosynthetic rate was observed. The apparent quantum requirement(AQR) of plant leaves in alpine meadow under natural environmental conditions remains constant in the morning and afternoon on fine days, which indicated that no significant photoinhibition occurred. However, diurnal variation of net photosynthetic rate of plateau plant still revealed as of double peak when soil is seriously lack of water and air temperature is 30℃ or over. Under general conditions, the light compensation point and dark respiration rate in plant of alpine meadow are lower. The light saturation point and the apparent quantum requirement are higher; Therefore the apparent quantum efficiency is lower. This is related to low atmospheric pressure in the areas of high elevation on Qinghai plateau.

Key words: Net photosynthetic rate, Apparent quantum requirement