Ecological floating-beds vegetated with  Vetiveria zizanioides

  Iris pseudacorus  and  Apium graveolens

as well as their combinations were constructed to evaluate their allelopathic effects on 6 phytoplankton species. The results showed that  V. zizanioides  floatingbed significantly inhibited the growth and Chl a  content of tested algae

while  I. pseudacorus  floating bed stimulated the growth and Chl. a  content of some phytoplankton species. Foatingbed vegetated with  V. zizanioides  +  I. pseudacorus  +  A. graveolens  also inhibited the growth and Chl. a  content of the tested algae

and the inhibitory rate was similar to  V. zizanioides  floatingbed  

but higher than that of  V. zizanioides  +  I. pseudacorus  and  V. zizanioides  +  A. graveolens . Study of MDA content in the tested algae showed that floating-beds of  V. zizanioides  and  V. zizanioides  +  I. pseudacorus  +  A. graveolens  caused a significant oxidative stress to the algae

inducing an increase in the lipid peroxidation in the algal cells. On the contrary

floating-beds vegetated with  I. pseudacorus

  A. graveolens  or  I. pseudacorus  +  A. graveolens  had no effects on MDA content of the algae. Our study indicated that  V. zizanioides  has significant allelopathic effects on phytoplankton

while the allelopathic effects of  A. graveolens  and  I. pseudacorus  are much weaker. Our results also suggested that planting the three species together might caused interspecific synergism

which increased the inhibitory efficiency of the floatingbeds to the phytoplankton. The results are of importance value in the design and plant selection of ecological floating bed.