Can simultaneous inhibition of seedling growth and stimulation of rhizosphere bacterial populations provide evidence for phytotoxin transfer from plant residues in the bulk soil to the rhizosphere of sensitive species?

In order to demonstrate that allelopathic interactions are occurring, one m ust, among other things, demonstrate that putative phytotoxins move from pl ant residues on or in the soil, the source, through the bulk soil to the ro ot surface, a sink, by way of the rhizosphere. We hypothesized that the inc orporation of phytotoxic plant residues into the soil would result in a sim ultaneous inhibition of seedling growth and a stimulation of the rhizospher e bacterial community that could utilize the putative phytotoxins as a sole cart,on source. If true and consistently expressed, such a relationship wo uld provide a means of establishing the transfer of phytotoxins from residu e in the soil to the rhizosphere of a sensitive species under field conditi ons. presently, direct evidence for such transfer is lacking. To test this hypothesis, cucumber seedlings were grown in soil containing various concen trations of wheat or sunflower tissue. Both tissue types contain phenolic a cids, which have been implicated as allelopathic phytotoxins. The level of phytotoxicity of the plant tissues was determined by the inhibition of pigw eed seedling emergence and cucumber seedling leaf area expansion. The stimu lation of cucumber seedling rhizosphere bacterial communities was determine d by the plate dilution frequency technique using a medium containing pheno lic acids as the sole carbon source. When sunflower tissue was incorporated into autoclaved (to reduce the initial microbial populations) soil, a simu ltaneous inhibition of cucumber seedling growth and stimulation of the comm unity of phenolic acid utilizing rhizosphere bacteria occurred. Thus, it wa s possible to observe simultaneous inhibition of cucumber seedlings and sti mulation of phenolic acid utilizing rhizosphere bacteria. and therefore pro vide indirect evidence of phenolic acid transfer from plant residues in the soil to the root surface. However, the simultaneous responses were not suf ficiently consistent to be used as a field screening tool but were dependen t upon the levels of phenolic acids and the bulk soil and rhizosphere micro bial populations present in the soil. It is possible that this screening pr ocedure may be useful for phytotoxins that are more unique than phenolic ac ids.