Using Scenedesmus acutus and Echinochloa utilis, a series of isomeric
compounds, namely thiadiazolidines and triazolidines, were assayed wit
h respect to decrease of chlorophyll contents, protoporphyrin-IX accum
ulation, light-induced ethane formation levels. The both types of phyt
otoxic compounds decreased chlorophyll contents, caused protoporphyrin
-IX accumulation and ethane evolution, and inhibited growth of Scenede
smus cells, just like the so-called peroxidizing herbicides such as p-
nitrodiphenyl ethers and cyclic imides. Correlations between the phyto
toxic parameters were highly significant. Our comparative data on diff
erent sets of the parameters suggest that both the thiadiazolidine and
triazolidine herbicides are classified as peroxidizing herbicides, af
fecting a crucial enzyme in the chlorophyll biosynthesis and inducing
ethane formation by the light induced radicals.