Study on the diurnal variations of photosynthetic characteristics and water use efficiency of Stipa bungeana Trin. under drought stress.

The diurnal variations of photosynthetic characteristics of Stipa bungeana Trin.were measured under pot planting and water controlling condition using the Li-6400 portable photosynthesis system.The results show that the diurnal course of net photosynthetic rate was displayed in a two-peak pattern with a typical midday depression pattern under different drought stress;the net photosynthetic rate in forenoon was higher than that in afternoon and increased along with the raising of soil moisture.The diurnal course of transpiration rate was also displayed in a two-peak course and the higher transpiration rate was corresponding to the higher soil moisture.The stomatal conductance was affected by soil moisture and stress of light and heat,and the diurnal dynamics was a concave form.The intercellular CO2 concentration was affected by the atmosphere CO2 concentration and stomatal conductance with higher value in morning and evening and lower in midday.The water use efficiency culminated in the early morning and decreased along with the raising of soil moisture.