Stimulation of plant growth by sub-lethal concentrations of herbicides

Studies were conducted to determine the extent and nature of sub-lethal effects of herbicides on plant growth. At least one herbicide from each major chemical class of herbicides was evaluated under greenhouse conditions. Oat plants were grown in sand culture to eliminate soil interferences. Herbicides were applied pre-emergence in nutrient solution. No drainage of the herbicide solution was allowed. Plants were grown for two weeks, harvested by separating roots from shoots, and dry weights recorded. Roots were stimulated more often and to a greater degree than were shoots. Herbicides stimulating root growth were: amiben (3-amino-2, 5-dichlorobenzoic acid), barban (4-chloro-2-butynyl n-chlorocarbanilate), bromacil (5-bromo-3-uracil), brornoxynil (3, 5-dibromo-4-hydroxybenzonitrile), dalapon (2, 2-dichloropropionic acid), diuron (3-(3, 4-dichloro-phenyl)-1, 1-dimethylurea), EPTC (ethyl N, N-dipropylthiolcarbamate), MCPA (2-methyl-4-chlorophenoxyacetic acid), MSMA (monosodium methanearsonic acid), propachlor (2-chloro-N-isopropylacetanilide), simazine (2- chloro-4, 6-bis(ethylamino)-s-triazine), and terbacil (5-chloro-3-tert-butyl-6-methyluracil). Herbicides stimulating shoots were barban, dalapon, diuron, and MCPA. Chlorpropham (isopropyl N-(3-chlorophenyl) carbamate), picloram (4-amino-3, 5, 6-trichloropicolinic acid), and paraquat (1, 1-dimethyl-4, 4-bipyridinium) did not produce stimulation of roots or shoots at the dosages tested. Amiben, barban, and bromacil were selected for further laboratory studies. Stimulation of oat seedling respiration occurred at 10⁻⁴ ppm amiben at 72 hours and at 10⁻³ ppm barban at 48 hours. Other treatments did not increase seedling respiration. Three identical experiments were conducted using an infra-red gas analyzer to measure CO₂ exchange. Oat plants were treated with four rates of amiben, barban, or bromacil. Total photosynthesis and CO₂ evolved were measured. Bromacil reduced total photosynthesis at all rates. Total photosynthesis and CO₂ evolved were not affected by other treatments. Plants used for the infra-red study were taken to the laboratory to determine ethanol soluble protein, free amino acid, and total available carbohydrate contents of roots and shoots. Ethanol soluble protein content in shoots was increased by 10⁻⁵ ppm barban or bromacil. Free amino acids in shoots were increased by all three herbicides at 10⁻⁵ ppm of herbicide. Other treatments produced variable results. Total available carbohydrates in roots were reduced by 10⁻⁴ and 10⁻⁵ ppm barban and by all rates of bromacil.