Diurnal Course of Gas Exchange and Water Use Efficiency of Platycladus orientalis in the Semiarid Region of the Loess Plateau

The daily gas exchange, stomatal conductance, and water use efficiency of Platycladus orientalis in the test field of Fangshan county of Shanxi Province in the semiarid region of the Loess Plateau were measured with portable Li-6200 gas analysis system in natural conditions. The results showed that the diurnal course of net photosynthetic rate displayed a two-peak pattern, that of stomatal conductance displayed a hollow pattern, and that of transpiration rate displayed a signal-peak pattern. Water use efficiency culminated in the early morning. On the basis of two criteria of stomatal limitation of photosynthesis suggested by Farquhar and Sharkey, the predominant limiting factor of photosynthesis was the stomatal conductance of stomatal limitation in the morning (10:00–12:00). However, the midday depression of photosynthesis at noon (12:00–14:00) and the decrease in photosynthesis in the afternoon (16:00–18:00) were the results of nonstomatal limitation, such as low carboxylation capacity of the leaf mesophyll.