The single and joint effects of cadmium(Cd) and benzo (a) pyrene (B[a]P) on the seedling growth and antioxidant enzyme activities of wheat (Triticum aestivum L.) were investigated after 4, 8, and 11 days of exposure under10 mg·L⁻¹ Cd and 54 mg·L⁻¹ B[a]P combined stress. In comparison with the control, the reductions under Cd + B[a]P stress treatment of Chl a, Chl b, Chl a+b and Chl a/b ratio were 53%, 44%, 49%, and 16% after 11-d exposure, the reductions under combination stress of Pn, Gs, Tr, and Ci were 61%, 72%, 67%, and 9%, the reductions of Fv/Fm, ΦPSII and qP under combination stress treatment were 24%, 23%, and 7% after 11 days, while the increases of WUE and qN under Cd + B[a]P were 19% and 81% after 11-d exposure. The nonstomatal limitation is the major reason for the decrease of Pn under the cadmium and B[a]P treatment in wheat leaves. The higher Tr value in wheat leaves is probably a positive adaptation response to the cadmium and B[a]P. This adaption response may play a protective role in the photosystem, resulting in a higher Pn. B[a]P may enhance the toxicity of the cadmium because they can penetrate into the perforated cells more easily. The toxicity of combined stress to photosynthesis and chlorophyll fluorescence parameters is stronger than the toxicity of single cadmium or single B[a]P, while cadmium had stronger toxic effects than B[a]P. The joint action of cadmium and B[a]P was a significant synergistic effect.
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