Abstract: In early heading stage of Chinese cabbage, diurnal change and midday depression of photosynthetic rate (Pn) were in investigated with cultivar “Lubai No. 3” under field conditions. The results showed that diurnal change of Pn was double-peak curve, two peak at 10: 30–11:00 A. M. and 14: 00–14:30 P.M. respectively. There was a midday depression at 13: 00 P. M., and stomatal limitation was one of the most importance factors causing midday depression. Light intensity and CO2 concentration had an effect on midday depression too. No midday depression appeared under lower light intensity and higher CO2 concentration. Besides, vapour pressure deficiency, relative humidity, and leaf temperature had an effect on midday depression of photosynthetic rate to a certain extent.

Key words: Chinese cabbage, Midday depression phenomenon, Diurnal change of photosynthetic rate