Potential phytotoxicity, bioassay

Aliphatic acids such as butyric acid have been previously implicated as being allelopathic compounds (46, 47, 23). Chou and Patrick (23) isolated butyric acid from soil amended with rye and showed that it was phytotoxic. Hydroxy acids have also been shown to possess phytotoxic properties (48) but have not been implicated in any allelopathic associations. Since SHBA is a stereo isomer, and the enantiomer was not identified because of impurity, all bioassays were run using a racemic mixture. The D-(-) stereo isomer of SHBA has been isolated from both microorganisms and root nodules of legumes and is suspected to be a metabolic intermediate in these systems (49). It is likely that only one enantiomer was present in the extract therefore, the true phytotoxic potential of this compound awaits clarification of the phytotoxicity of the individual enantiomers. 

Berlin C, Harmon R, Akaogi M, Weidenhamer JD, Weston LA (2009) Assessment of the phytotoxic potential of m-tyrosine in laboratory soil bioassays. J Chem Ecol 35 1288-1294... 

Bertin C, Harmon R, Akaogi M, Weidenhamer ID, Weston LA (2009) Assessment of the phytotoxic potential of m-tyrosine in laboratory soil bioassays. J Chem Ecol 35 1288-1294 Blum U (1996) Allelopathic interactions involving phenolic acids. J Nematol 28 259-267 Blum U (1998) Effects of microbial utilization of phenolic acids and their phenolic add breakdown products on allelopathic interactions. J Chem Ecol 24 685-708... 

The bioassays showed the wide allelopathic potential of these plants as well as the phytotoxicity of some soils extracts and essential oils (Tables I, II, and III). 

Since the actual or potential phytotoxicity of a phenolic acid is determined by its physical and chemical properties and the susceptibility of the plant process involved, the actual or potential phytotoxicity of a given phenolic acid is best determined in nutrient culture in the absence of soil processes. The phytotoxicity observed in soil systems represents a realized or observed phytotoxicity, not the actual phytotoxicity, of a given phenolic acid. For example, the actual relative phytotoxicities (or potencies) for cucumber seedling leaf expansion were 1 for ferulic acid, 0.86 for p-coumaric acid, 0.74 for vanillic acid, 0.68 for sinapic acid, 0.67 for syringic acid, 0.65 for caffeic acid, 0.5 for p-hydroxybenzoic acid and 0.35 for protocatechuic acid in a pH 5.8 nutrient culture.5 In Portsmouth Bt-horizon soil (Typic Umbraquaalts, fine loamy, mixed, thermic pH 5.2), they were 1, 0.67, 0.67, 0.7, 0.59, 0.38, 0.35, and 0.13, respectively.19 The differences in phytotoxicity of the individual phenolic acids for nutrient culture and Portsmouth soil bioassays were due to various soil processes listed in the next paragraph and reduced contact (e.g., distribution and movement)36 of phenolic acids with roots in soils. 

Phytotoxins from nonpathogenic organisms also have potential as weed-control agents. Numerous phytotoxic compounds have been isolated and bioassayed for phytotoxic activity, and their chemical structures have been determined. Generally, these naturally occurring compounds have unique chemistries that differ from those of commercial synthetic herbicides. Some of these compounds—or others yet to be discovered—may be used directly as herbicides or provide templates for the development of new, efficacious weed control agents that are less persistent and more environmentally compatible than some compounds currently being used. 

Bioassays. The biological activities of the synthetic peptides were determined using assay methods that were adapted to require only small amounts of peptide. Antifungal and antibacterial bioassays were performed in 96-well microtiter plates. Potential phytotoxicity was routinely evaluated using an oxygen electrode to measure inhibition of photosynthesis or respiration. All tests included magainin 1 or magainin 2 as a standard reference. Assay conditions were established to compare the relative activities of peptides, the absolute values cannot be extrapolated directly to activities in the field. 

A sharp distinction must be drawn between bioassay methods used to screen for potentially herbicidal molecules and those used to study allelopathy. If one is interested only in identifying potential herbicides, standardized bioassays using selected crop and weed species under specified protocols are highly appropriate. Such assays are also useful in gauging the relative phytotoxicity of compounds. They are not adequate, however, for demonstration of allelopathy ... 

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