Effect of Combined Pollution of Cd and BaP on Photosynthesis and Chlorophyll Fluorescence Characteristics of Wheat

The single and joint effects of cadmium(Cd) and benzo (a) pyrene (BaP) on the seedling growth and antioxidant enzyme activities of wheat (Triticum aestivum L.) were investigated after 4, 8, and 11 days of exposure under10 mgL -1 Cd and 54 mgL -1 BaP combined stress. In comparison with the control, the reductions under Cd + BaP 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, PSII and q P under combination stress treatment were 24%, 23%, and 7% after 11 days, while the increases of WUE and q N under Cd + BaP were 19% and 81% after 11-d exposure. The nonstomatal limitation is the major reason for the decrease of Pn under the cadmium and BaP treatment in wheat leaves. The higher Tr value in wheat leaves is probably a positive adaptation response to the cadmium and BaP. This adaption response may play a protective role in the photosystem, resulting in a higher Pn. BaP 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 BaP, while cadmium had stronger toxic effects than BaP. The joint action of cadmium and BaP was a significant synergistic effect.