Factors Influencing the Phytotoxicity and Movement of Paraquat in Quackgrass

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Abstract
Quackgrass (Agropyron repens (L.) Beauv.) was defoliated for 25 to 30 days with applications of 1.0 lb/A of l,l′-dimethyl-4–4′-bipyridinium salt (paraquat). Applications were more effective made prior to darkness than at midday. Phytotoxicity was increased by adding surfactants in the field, and experiments utilizing 14C-paraquat under controlled environmental conditions indicated that the surfactant X-77 increased foliar absorption by 20%. Quackgrass plants grown in nutrient solutions exhibited chronic paraquat toxicity which increased as the nitrogen levels increased. Paraquat also inhibited the growth of lateral buds on quackgrass rhizome segments cultured in the dark. Greater movement of 14C-paraquat in quackgrass leaves occurred under 6 hr of light as compared to 6 hr of darkness following application. However, a 6-hr dark period after application prior to the light period further enhanced the movement from the treated leaf.