Physiological Response of Two Typical Plant Species under Combined Pb and Cd Stress in Bioretention Facilities

Bioretention facilities reduce stormwater runoff and pollutants, but there is a concern that plants in bioretention facilities may absorb heavy metal pollutants from stormwater runoff, which might impair the growth of the plant species. To investigate this issue, stormwater runoff containing various amounts of Pb and Cd heavy metals was used as the irrigation water in a bioretention facility. The low concentrations of Pb and Cd were 0.08 and 0.04 mg/L, and the high values were 0.68 and 0.32 mg/L. The plant heavy metal content and physiological indicators were measured. The indicators were chlorophyll content (CC), net photosynthetic rate (NPR), and transpiration rate (TR). The results showed that the changes in plant chlorophyll content (CC) were highly correlated with changes in the plants&rsquo: Pb. Low concentrations of Pb and Cd slightly inhibited the Ginkgo biloba L. and Ligustrum &times: vicaryi NPRs, the effect was more obvious at high concentrations and the Ligustrum &times: vicaryi&rsquo:s NPR decreased from the initial 8.97 &mu:mol CO2/(m2s) to 5.77 &mu:mol CO2/(m2s) under high concentration conditions. Pb and Cd increased the Ginkgo biloba L. and Ligustrum &times: vicaryi&rsquo:s TRs, and the effect at low Pb and Cd concentrations was more significant. Under low concentrations of Pb and Cd stress, the Ginkgo biloba L.&rsquo:s TRs reached 0.63 mmol H2O/(m2s), Ligustrum &times: vicaryi&rsquo:s TRs reached 1.30 mmol H2O/(m2s). The TRs of the two plants in the experimental groups remained high throughout the experiment, and there was no significant inhibition. The study found that Pb and Cd in stormwater runoff did affect the physiological function of species to some extent. Different plant species behaved differently in bioretention facilities, but the stormwater runoff did not lead to the death of species. Our study may provide a better understanding of the development of typical plant species in bioretention facilities.