Bioassays with Higher Plant Species

Because plants are, in most cases, the primary target organisms for agricultural applications, the methods and influencing parameters of plant hiotests will he described in more detail than for all the other test organisms. 

OECD Method 208 [32] recommends the application of at least three tests in parallel using three different plant species. This seems to be justified, since plants are grouped into three categories with different inhibition mechanisms and sensitivities. Such tests could be varied for the determination of the germination rate or plant growth (biomass production), or for both effects at the same time. 

To determine the plant growth rate, seeds are germinated on a reference substrate and a number of young plants are then transferred to the prepared samples. This procedure requires some experience since the small roots must not be injured and the amounts of adhering reference substrate should be as small as possible. After a typical growth period, the plants are harvested and the biomass produced is evaluated. 

In all variants, the samples (150-250 g) are placed into trays made of inert plastic or glass. A thin layer of washed sand on the bottom forms a drain layer and a very thin layer on top (spread carefully over the seeds) prevents the seeds drifting during watering. 

Research results have shown that inhibition results obtained from soil samples after the degradation of polymers, using the three most commonly used species, i.e., cress, millet and rape, may differ greatly. In our group, we found that the typical physical and chemical parameters of such soil samples did not correlate strictly with the measured inhibition values. Some of the substances responsible for the effects may not be detected by conventional soil analysis and remain unknown. Following Chen and Inbar [66], and interpreting results obtained by our group [67] the most probable causes may be  

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