Microbial degradation and phytotoxicity of picloram and other substituted pyridines
Attempts have been made to correlate the chemical structure of 38 pyridine derivatives, including two herbicides, picloram (Tordon) and Daxtron and a nitrification inhibitor (N-Serve), with their resistance to attack by soil microorganisms under aerobic and anaerobic conditions and with their herbicidal effect upon lucerne (Medicago sativa L., Hunter River). There was a rough correlation between the number of substituents in the pyridine ring and both recalcitrance and phytotoxicity, so that compounds with four to five substituents were mostly more persistent and herbicidal than those with one to two. Decomposition was often slower under anaerobic than aerobic conditions. Chlorination often conferred persistence and herbicidal properties, but some non-chlorinated pyridines also were markedly resistant to microorganisms and harmful to lucerne. The prospect of making picloram and Daxtron more biodegradable by altering their substituents but retaining their herbicidal properties, seems remote. Substituting -OH for -Cl in position six of picloram left its properties the same. Removal of the -COOH from picloram had no effect on persistence but reduced herbicidal activity more than one hundred-fold. Removal of -Cl from position five and -NH2 did not affect persistence and reduced phytotoxicity only 10-fold. Replacing -OH of Daxtron by -NH2 hardly affected resistance to microbial degradation but greatly lowered its herbicidal potency. Removal of -CCl3 from N-Serve or substituting -COOH in its place had no effect on biodegradation but increased phytotoxicity. Among mono- and disubstituted pyridines, comparisons of different substituents at positions two, three or four showed that -COOH at any position stimulated decomposition more often than any other substituent, while -Cl inhibited it most often; effects on phytotoxicity were more complicated. Substitution of -NH2 or -OH usually increased persistence but had inconsistent effects on phytotoxicity. Biodegradation was favoured by -CH3 in alpha or beta positions, but gamma -CH3 usually increased resistance to attack by soil microorganisms. Phytotoxicity was increased slightly by -CH3 groups. © 1972.
Naik, MN; Jackson, RB; Stokes, J; Swaby, RJ
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