- 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
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.