Studies on the breakdown of 2,4-dichlorophenoxyacetic acid and some related compounds by soil micro-organisms

Modifications of the perfusion and cress-assay techniques of Audus were used to study breakdown in soil of 2,4-D and some related compounds. Attempts to produce active bacterial (pure) cultures from the enriched soils proved abortive. Mixed, crude cultures were partially successful though never energetic. A wide range of stimulants failed to induce pure culture activity. Enrichment was transferrable to fresh soil by treating it with perfusate from an active perfuser. The activating principle was thermolabile, probably consisting of adapted bacteria. The technique was used to quickly produce numbers of similar enriched perfusers without the necessity of lengthy adaptation processes. Compounds labile in soil followed the same general course, a characteristic lag phase being followed by a period of rapid breakdown. Further added substrate was usually decomposed with no further lag. Some control of the lag was obtainable by pre-treatment of the soil, or perfusion in mixture, with related compounds. Mixtures of certain phenoxyacetic acids appeared to be synergistic in their inhibition of root growth. 2,4-D adaptation proved very stable, persisting through prolonged perfusion with water, or storage for over a year, of Enriched soil. The optical isomers of the chlorophenoxypropionic acids appear to differ in resistance to attack as well as having different physiological activities. Relative Toxicity to cress and lability in soil appear to be controlled by the same molecular features and a close correlation exists between the two functions. Attachment to substrate ( enzyme? ) in each case is probably through the ?-H and carboxyl groups of the side-chain and the 3- or 5-nuclear positions. The effects of molecular substitution on stability and physiological activity are discussed as also are the possibilities of phenol formation and / or ring fission during breakdown. A possible relationship between the breakdown products and biotin metabolism is also indicated.