Diurnal rhythm of sensitivity of cotton seedlings to herbicides.

The inhibition of growth of cotton seedlings (Gossypium hirsutum, var. Stardel) varied diurnally to applications of three herbicides [1,1-dimethyl-3-(alpha,alpha,alpha-trifluoro-m-tolyl) urea, 3',4'-dichloro-2-methacrylamide, and ethyl-N,N-dipropylthiocarbamate], but not to a fourth [3-(3,4-dichlorophenyl)-1, 1-dimethylurea]. Inhibition was strongest when the plants were treated at about daybreak. The rhythmic response was apparently not endogenously controlled, since most of the diurnal effect was lost under conditions of constant light and temperature.