Toxicity, selectivity, uptake, distribution and site of action of EPTC in corn (Zea mays L.) as affected by a herbicide antidote

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Virginia Tech

Some factors affecting toxicity, selectivity, uptake distribution and site of action of EPTC (S-ethyl dipropydithiocarbamate) as affected by R-25788 (N,N-diallyl-2,2-dichloroacetamide) were investigated in field, greenhouse and laboratory experiments.

EPTC at 6.7 kg/ha was 20 to 30% more toxic to various corn cultivars when applied to sandy and lower organic matter (0.8%) soil than with fine sandy loam soil (0.M. 3.1%). The 'TXS114' cultivar was more tolerant than 'XL55' to EPTC treatment. The seeds of 'XL55' absorbed 20% more EPTC than the seeds of 'TXS114' after 12 hr of incubation in ¹⁴C-EPTC. Movement of ¹⁴C was toward the actively growing portions of root and shoot. Embryo of 'TXS114' accumulated 48% of total uptake compared to 60% in 'XL55' cultivar. Apparently, herbicide antidote R-25788 conjugated or otherwise delayed movement of ¹⁴C toward the shoot by concentrating ¹⁴C in seed of highly tolerant compared to shoot of 'XL55', a highly susceptible cultivar. Therefore, susceptibility of 'XL55' is apparently due to its relatively high uptake and site of accumulation.

The total nonstructural carbohydrate (TNC) in seeds of 'TXS114' was 18% more than 'XL55' and this differences strongly correlated with their injury responses to EPTC.

Lipid synthesis was inhibited by more than 50% in Chlorella cells when EPTC was used at 2 x 10⁻⁴ M concentration after 1 hr of incubation. Similar result was found in corn protoplasts which was inhibited by 23% after 2 hr of incubation with EPTC at 12 x 10⁻⁵ M, suggesting that primary site of action of EPTC is inhibition of lipid synthesis. The other metabolic processes investigated were possibly secondary effects resulting from lipid synthesis inhibition because their inhibitory actions were noticeable only after several hours of cell incubation.

The antidote R-25788 combined with EPTC caused a reduction in lipid synthesis inhibition by 31 and 20%, respectively in both Chlorella and corn protoplasts. It is then suggested that the primary action of antidote R-25788 in cells is reversal of the inhibitory effects caused by EPTC on metabolic processes in plants.