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

Abstract

Many reports are available in the literature on the effects of 2,4-D on numerous physiological and biochemical processes. Among these are increased cell proliferation (5), increased respiration (20), increased carbohydrate depletion (17), decreased respiration of roots (27), decreased uptake of potassium by roots (18), decreased accumulation by the root system of KNO3 and KC1 (13), inhibition of lipase activity (9), inhibition of ascorbic acid oxidase (27), stimulation of phosphatase activity (15), stimulation of beta-amylase activity (22, 28), inhibition of alphaand beta-amylase activity (14), etc. Useful though these observations are for the understanding of auxin action, they do not tell us if these physiological changes are the specific reasons 2,4-D has such a high herbicidal activity. It would rather seem that these phenomena are not specific for 2,4-D but are caused as well by other auxins such as indoleacetic acid and naphthaleneacetic acid (for terminology see 22). Thus, an increased respiration, a stimulated proliferation, and a considerable loss of carbohydrate are caused in the Jerusalem artichoke tuber by indoleacetic acid (8). Increased phosphatase activity is found in Avena roots after treatment with indoleacetic acid and other auxins (4). Inhibition of amylase activity by indoleacetic acid and by a variety of other auxins has been demonstrated (26). Indoleacetic acid, though capable of affecting many reactions in a fashion similar to 2,4-D is definitely not an efficient herbicide. The clue to the solution of why 2,4-D displays such a high herbicidal activity appears to lie in a different direction. Rather than only asking: "What physiological changes does 2,4-D bring about?" it would appear to be more profitable to ask first: "In what fundamental aspect does 2,4-D differ from the other auxins?" The present paper will largely deal with this question.