Effects of haloxyfop on sorghum (Sorghum bicolor (L.) Moench.) root meristems

Abstract

The effects of the grass-specific herbicide haloxyfop {2- [4- [[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]-phenoxy]propanoic acid} on the growth and ultrastructure of susceptible sorghum (Sorghum bicolor (L.) Moench.) root meristems were determined. Mitotic index analysis indicated that 1.0 μM haloxyfop inhibited cell division completely after 24 h, and arrested chromosomes in a state similar to prometaphase. Ultrastructural analysis indicated that after 4 h cells treated with 1.0 μM haloxyfop exhibited increased vacuolization, and by 24 h cells contained large vacuoles, some of which were autophagic. Complete cell destruction occurred at the 1.0 μM level by 72 h treatment time. The phytotoxicity of haloxyfop was not altered by the presence of either 2,4-DB [4-(2,4-dichlorophenoxy)butanoic acid] or TIBA (2,3,5-triicdobenzoic acid), though both compounds caused independent effects of their own, e.g. cortical cell expansion, extensive root hair development. These results suggest that a possible mechanism of action of haloxyfop is to cause premature cell differentiation, indicated by extensive vacuolization of meristematic cells, after which intracellular digestion, and subsequent death of the cells take place. Similar, although not identical results were obtained when sorghum roots were treated with the herbicides cinmethylin {exo-1-methyl-4-(1-methylethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo[2.2.1]heptane}, dalapon (2,2-dichloropropanoic acid), and sethoxydim {2-[1-(ethoxyimino)butyl]- 5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}. These results are also similar those found in research concerning bensulide {0,0- bis(1-methylethyl) S-[2[-(phenylsulfonyl)amino]ethyl]phosphorodithioate), bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4(1H,3H) pyrimidinedione}, isoxaben {N-[3-(1-ethyl-1-methylpropyl)-5-isoxazolyl]-2,6- dimethoxybenzamide}, and propham (1-methylethyl phenylcarbamate), herbicides which have different selectivity among plants and are not chemicallt related. Based upon the data from this research and research with herbicides producing similar effects, two possible mechanisms of action for haloxyfop are proposed.