Abstract: 【Objective】 The effect of Cd on photosystem activities of maize leaves, and the action site and ways of Cd to photosynthesis were studied in order to provide a theoretical foundation for understanding of the injury mechanisms of Cd to the photosynthetic apparatus.【Method】Two maize varieties were chosen as materials. One is the common variety mainly planted in northern China, Zhengdan 958 (ZD958). Another is a new variety with higher yielding potential, Denghai 661 (DH661). Maize seedling was continuously cultured with different Hoagland’s solution containing 0, 10, 30 and 60 μmol•L-1 of CdSO4 after 3-leaf-stage. After 40 days of treatment, the parameters of gas exchange, chlorophyll a fluorescence transient and light absorbance at 820 nm of maize leaves were measured for analying the injury reason of Cd to the photosynthetic apparatus. 【Result】 It was found that the declines of Pn were not mainly related to stomatic factors under Cd stress. After Cd treatment, both fluorescence at K-step (Wk) of PSⅡ electron donor side and the fluorescence at J-step (Vj) of acceptor side were increased. However, the decrease of activity of electron donor side was lager than the electron acceptor side, which caused the decline of Ψo. With the increase of Cd concentration, the activity of RUBP (Ribulose 1,5-Biphosphate) decreased significantly, the utilization rate of NADPH was also decreased. They caused the electron accumulation in PSⅠ increased and the 820 nm light absorption of PSⅠ (ΔIR/Io) decrease. And then, which caused the activity declined of PSⅡ was not in keeping with PSⅠ, thus depressed the coordination between PSⅡ and PSⅠ (Φ(PSI/PSⅡ)).【Conclusion】Under the Cd stress, the main reason of Pn declined was not caused by non-stomata limitation. Cd decreased the activity of PSⅡ electron donor side, which resulted in the performance of PSⅡ declined and reduced the amount of electron transport to PSⅠ. The most importantly, the increased amount of electron accumulation in PSⅠ fed back by the carboxylation system was more than the decreased amount of electron transport form PSⅡ that resulted in the PSⅠ performance declined sharply, which depressed the coordination of PSⅡ and PSⅠ, and caused Pn declining.

Key words: maize, Cd, chlorophyll fluorescence, photosynthetic performance, RuBPcase activity