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In order to identify the relationship between diurnal changes in the net photosynthetic rate (Pn) of Pueraria lobata and environmental factors, diurnal changes in the Pn of leaves of two P. lobata cultivars were measured using a CIRAS-1 portable photosynthesis measurement system (PP-Systems, UK). The results show that diurnal changes in Pn of both cultivars could be interpreted as double-peak curves, indicating the occurrence of an obvious midday depression. Further analyses indicate that the correlation between Pn and stomatal conductance was positive and extremely significant (p ＜ 0.01). The correlations of Pn with intercellular CO2 concentration and transpiration rate were positive and significant (p ＜ 0.05), while the correlations of Pn with air and leaf temperatures were negative and significant (p ＜ 0.05). The results indicate that among the factors affecting photosynthetic properties, some can be grouped as stomatal limitations while others are non-stomatal limitations.