Abstract: A pot experiment was conducted to study effects of single and combined stress of Zn（30，400 mg•Kg^-1）and Cd （1，15 mg•Kg^-1）and simulated acid rain varying in pH on populations and compositions of rhizospheric microorganisms and content of total saponins in Gynostemma pentaphyllum. Results of the experiments show that growth of bacteria, actinomyces, and physiological groups in nitrogen cycling in the soil were inhibited by acid rain and their populations decreased with declining pH of the simulated acid rain. The content of total saponins in stem and leaf of Gynostemma pentaphyllum showed a similar trend. However, the effect of acid rain on fungi and cellulose-decomposing microbes differed. Their numbers ascended first and then descended with declining pH of the simulated acid rain. Under the stress of acid rain, the numbers of rhizospheric microorganisms, physiological groups in nitrogen cycling, and cellulose-decomposing microbes, and the content of total saponins in the treatments applied with Zn（30 mg•Kg^-1）or Cd （1 mg•Kg^-1）were all higher, but in the treatments with Zn concentration up to 400 mg•Kg^-1 or Cd up to 15 mg•Kg^-1, obviously lower than those in the treatment without addition of any heavy metal. However, when low in concentration (30 and 1 mg•Kg^-1, respectively), Zn and Cd were antagonistic in affecting populations of rhizospheric microorganisms, physiological groups in nitrogen cycling, cellulose-decomposing microbes, and the content of total saponins; but when high in concentration (400 and 15 mg•Kg^-1, respectively), they were synergidtic.

Key words: simulated acid rain, Zn, Cd, Gynostemma pentaphyllum, microorganisms, saponins, combined stress