The Physiological Action of 2,4-Dichlorophenoxyacetic Acid on Dandelion, Taraxacum officinale.

The recent introduction of the use of synthetic growth regulating substances as herbicides is giving new stimulus to weed control activities. Because such substances are effective in low concentrations, killing plants through direct effects within the plant system and not seriously damaging the soil, they are especially desirable herbicides. The compound 2,4-dichlorophenoxyacetic acid (2,4-D) is the most widely used growth substance for weed killing purposes at the present time, and it has been shown by several workers (6, 13, 14, 20) to have selective action, killing most broad leaf species, while grass species are more or less resistant. Plants which are susceptible to the action of 2,4-D show various morphological and histological responses (7, 23, 25) suggesting that physiological reactions of the cells are affected. Further evidence of effects on the physiological activity of the plant is indicated by the decrease in the dry weight of treated plants (5, 16, 21) which suggests the depletion of food reserves and possibly an increase in the respiration rate. A temporary increase followed by a decrease in the sugar content has been noted in annual morning glory [Ipomea lacuntosa (16) ] and in bindweed [Convolvulus arvensis (12)] as a result of 2,4-D applications. A similar response occurred in bean leaves (17) treated with alpha-naphthaleneacetic acid. The reserve carbohydrates of plants are rapidly depleted after treatment with 2,4-D (16, 21, 25) and the same effect may be caused by applications of other growth substances (1, 15, 17, 18, 22). Increased respiration has been shown also to follow applications of 2,4-D (5, 21) or indoleacetic acid (19), but none of these responses has been satisfactorily established as a direct cause of the death of treated plants. The common dandelion (Taraxacum officinale) is readily killed by 2,4-D, and has been used in this study in an attempt to determine the physiological reactions involved in the herbicidal action of the compound.

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