Influence of Light Regime on the Toxicity and Physiological Activity of Herbicides

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Abstract
The response of plants grown under various light regimes to 2,4-dinitrophenol and 2,4-dichlorophenoxyacetic acid was studied under conditions of controlled light and temperature. Plants receiving a dark period after treatment with 2,4-dinitrophenol were significantly more injured than plants given light after treatment. Darkness prior to the application of 2,4-dinitrophenol also increased the phyto-toxicity of the compound. The pH at which the chemical was applied and the quality of light plants received after treatment influenced the differential toxicity of the compound in relation to the light and darkness. The action spectrum for prevention of dinitrophenol injury was similar to the action spectrum for photophosphorylation. It is postulated that the differential toxicity of dinitrophenol in relation to light and dark is a function of the adenosine triphosphate level in the leaves under the 2 conditions; plants receiving light after dinitrophenol treatment maintain a high adenosine triphosphate level through photophosphorylation along pathways insensitive to dinitrophenol, whereas plants in the dark are injured through the effect of dinitrophenol as an oxidative phosphorylation uncoupler. Bean plants subjected to short periods of ultraviolet radiation failed to exhibit the usual epinastic responses to 2,4-dichlorophenoxyacetic acid if placed in the dark subsequent to treatment, but would exhibit epinastic responses if given light after ultraviolet irradiation and chemical treatment. Visible light reversed the influence of ultraviolet light on the physiological activity of 2,4-dichlorophenoxyacetic acid in irradiated bean tissues.