Characteristics of Photosynthesis and Water Use Efficiency of Platycladus orientalis Under Different Soil Moisture

In semi-arid region of Loess Plateau,the net photosynthetic rate,transpiration rate,stomatal conductance,intercellular CO_2 concentration of Platycladus orientalis and proportional factors were measured with the Li-6400 portable photosynthesis system under the condition of controlled soil moisture.The results showed that the diurnal course of net photosynthetic rate displayed a two-peak pattern and a typical midday depression pattern,net photosynthetic rate in forenoon was higher than that in afternoon;stomatal conductance was affected by the water,light and heat stress,which displayed a hollow pattern;intercellular CO_2 concentration was affected by the atmosphere CO_2 concentration and stomatal conductance,which displayed a hollow pattern;water use efficiency culminated in the early morning,water use efficiency in forenoon was higher than that in afternoon;the net photosynthetic rate and transpiration rate increased with an increase of the soil moisture,while water use efficiency declined.On the basis of two criteria of stomatal limitation of photosynthesis suggested by Farquhar and Sharkey,the re-dominant limiting factor of photosynthesis is stomatal conductance of stomatal limitation in the forenoon,but the midday depression of photosynthesis at noon and the decrease of photosynthesis in the afternoon are the results of low carboxylation efficiency of the mesophyll leaf.