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Rhythmical Changes in the Sensitivity of Cotton Seedlings to Herbicides ¹

Plant Hormone Laboratory, Beltsville Agricultural Research Center (West), United States Department of Agriculture, Beltsville, Maryland 20705, Plant Stress Laboratory, Beltsville Agricultural Research Center (West), United States Department of Agriculture, Beltsville, Maryland 20705, Weed Science Laboratory, Beltsville Agricultural Research Center (West), United States Department of Agriculture, Beltsville, Maryland 20705, Department of Botany, University of Maryland, College Park, Maryland 20742

Cotton (Gossypium hirsutum L.) seedlings that were grown under a photoperiod of 12 hours darkness and 12 hours light showed oscillations in their sensitivity to the herbicides sodium 5-(2-chloro-4-trifluoromethyl)-phenoxy)-2-nitrobenzoate (acifluorfen), butyl 2-(4-((5-(trifluoromethyl)-2-pyridinyl)oxy)phenoxy)propanoate (fluazifop) and 3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide (bentazon). Sensitivity was expressed in appearance of necrotic areas on the cotyledons and in decreased growth of the shoot. The seedlings were least sensitive in the beginning and middle of the light period, then the sensitivity increased and reached its maximum during the beginning and middle of the dark period and then declined. Seedlings grown from germination under continuous light exhibited very small or no oscillations in sensitivity. The oscillations in sensitivity were entrained by one cycle of darkness and light. A cycle of 12 hours darkness and 12 hours light triggered the greatest oscillations while either increasing or decreasing the duration of the dark period resulted in smaller oscillations. Apparently, these oscillations in sensitivity to herbicides were endogenously controlled since after entrainment they continued irrespective of the light conditions.