DIFFERENCE IN ACTIVITY BETWEEN 2,4-DICHLORO-PHENOXYACETIC ACID AND OTHER AUXINS, AND ITS SIGNIFICANCE IN HERBICIDAL ACTION

Abstract

The herbicidal action of 2,4-D is considered to consist of 2 steps: (1) the very high activity of 2,4-D as an auxin, and (2) the metabolic changes brought about by this auxin activity. It is in the first step that 2,4-D differs from the other, "non-herbicidal," auxins. It is a quantitative rather than a qualitative difference. It is in the 2d step that the phytocidal effects take place. The difference between 2,4-D and "non-herbicidal" auxins is on the basis of undissociated molecules: (1) a considerably greater auxin activity at all concns. and (2) an increasingly greater relative activity with increasing auxin concn. Thus in the pea stem section test an increase in length of about 10% over that of the control is brought about by a concn. of undissociated molecules of indoleacetic acid (IAA) which is about 5 times higher than that of 2,4-D. But in order to bring about a 35% increase in the length of the stem sections it takes a concn. of IAA which is 10,000 times greater than that of 2,4-D. With this in mind, it is conceivable that in the range of herbicidal effects 1 undissociated molecule of 2,4-D may have the activity equivalent to 1 million or more undissociated molecules of IAA. Because of its very high auxin activity 2, 4-D can be expected to bring about metabolic changes to a larger degree than "non-herbicidal" auxins. At the present state of knowledge coumarin derivatives suggest themselves as plant metabolites which accumulate in excessive quantities under influence of 2,4-D. Since some of these coumarin derivatives have been shown to be more toxic to broad-leaved plants than to grasses, the abnormal accumulation of coumarin derivatives under influence of 2,4-D would explain both the phytotoxicity and the selectivity of this auxin herbicide.