Effect of Haloxyfop and Haloxyfop-Methyl on Elongation and Respiration of Corn (Zea mays) and Soybean (Glycine max) Roots

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Abstract
The effects of haloxyfop {2-[4-[[3-chloro-5(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and haloxyfop-methyl on elongation and respiration of primary roots of corn [Zea mays L. ‘B37 X Oh43’] and soybean [Glycine max (L.) Merr. ‘Hodgson 78’] were examined. Intact roots of etiolated seedlings were exposed to both forms of the herbicide. At a concentration of 10-6 M, neither form of the herbicide had an effect on elongation or respiration of soybean roots. In contrast, elongation of corn roots was completely inhibited within 24 h of exposure to 10-6 M haloxyfop. This treatment reduced the respiration rate of corn root apices by 30% but had little effect on ATP content. After 72 h of exposure to haloxyfop [10-6 M], both ATP content and respiration rate of corn root apices had declined 35%. Haloxyfop-methyl produced equivalent effects on elongation and respiration of corn roots. Studies with isolated corn root mitochondria indicated that haloxyfop inhibits electron transport at relatively high concentrations (I50 = approximately 2 mM for various substrates). Because significant reductions in ATP content and respiration rate of corn root apices were not detected during the period when elongation was inhibited (initial 24 h of exposure), it is concluded that impairment of respiration is a secondary response to the herbicide.