The herbicidal action of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} on corn (Zea mays L. ‘Goldencrossbantam’) was investigated in preemergence and postemergence experiments and hydroponic culture. Soil-applied sethoxydim did not inhibit corn germination. Leaves failed to emerge through the coleoptile with a high herbicide rate (1.6 kg ai/ha), but chlorotic leaves emerged at a lower rate (0.8 kg ai/ha). Growth of corn seedlings treated with a foliar application of 0.2 kg ai/ha was inhibited within 1 day after treatment, but at a lower application rate (0.05 kg ai/ha) growth continued with chlorotic zones on newly expanding leaves. Over the concentration range 3 × 10-5 to 1 × 10-6 M, sethoxydim inhibited the growth of primary roots of corn in hydroponic culture within 24 h. Cytological investigation showed that sethoxydim inhibited cell division but did not interfere with mitosis.

The selectivity mechanisms of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} were investigated by examining absorption of 14C-sethoxydim into tissues of corn (Zea mays L. ‘Goldencrossbantam′) and pea (Pisum sativum L. ‘Alaska′). Greater absorption of 14C-sethoxydim was observed only into the proliferative 2-mm terminals of the excised roots of susceptible corn, and the same result was obtained with corn seedlings in hydroponic culture. Absorption was not affected by 1 × 10−4 M 2,4-dinitrophenol (DNP), which indicated that the absorption of 14C-sethoxydim is probably mediated by a passive transport system rather than an active transport system. The selective herbicidal action of sethoxydim appeared to depend largely on the differential rates of absorption into meristematic regions by the susceptible and tolerant plants.

The effects of haloxyfop on elongation, mitotic index, and morphology of sorghum and unicorn-plant primary roots were examined. Elongation of sorghum roots was completely inhibited by haloxyfop concentrations of 10–6 M or greater 24 h after treatment, whereas unicorn-plant root elongation was unaffected by the same concentrations 72 h after treatment. Mitotic indices of sorghum roots were reduced by both 10–6 and 10–8 M haloxyfop, with the higher concentration reducing the index to near zero by 24 h of exposure. The mitotic indices of unicorn-plant roots were unaffected by the same levels of the herbicide after both 24- and 48-h treatment. Histological analyses showed that after 24 h exposure to 10–6 M haloxyfop, large vacuoles were present in cells at the root apex that normally did not exhibit these organelles. After 48 h of exposure many cells in the apical region appeared to lack visible cytoplasm and/or nuclei, and by 72 h only cell walls remained visibly evident, and many cells had collapsed. These changes are similar to those that occur in tissues undergoing senescence. Treatment of sorghum roots with 10–8 M haloxyfop did not cause discernible changes after 72 h. Unicorn-plant roots treated with 10–6 M haloxyfop appeared unaffected after 72 h.