The effects of varying concentrations and duration of alachlor [2-chloro-2′,6′diethyl-N-(methoxymethyl)acetanilide] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] treatment on root growth, cell division, and cell enlargement were studied. Peas (Pisum sativum L. ‘Alaska’) and oats (Avena sativa L. ‘Victory’) were treated from 0 to 48 h with concentrations ranging from 1 × 10-8 to 1 × 10-3 M of each herbicide. After 48 h, average growth rates were significantly inhibited at concentrations of 1 × 10-7 M alachlor and 5 × 10−8 M metolachlor, and 5 × 10−7 M alachlor and 1 × 10-6 M metolachlor for peas and oats, respectively. When growth inhibitions were examined across time at concentrations greater than these, the degree of growth inhibition was a function of both concentration and duration of treatment. Often the greatest decrease in growth occurred between 0 and 12 h. Mitotic indices of root tip squashes from pea roots and paraffin sections from oat roots were determined. There was a significant reduction in the mitotic indices of pea roots treated for 48 h with 5 × 10−6 M alachlor or 1 × 10-5 M metolachlor. After a 30-h treatment, the mitotic indices of oat roots were significantly reduced by 1 × 10−7 M metolachlor and 1 × 10−6 M alachlor. Significant inhibition of elongation of etiolated oat coleoptiles were observed at 5 × 10−6 M alachlor (27%) and 5 × 10−5 M metolachlor (30%). Inhibition of pea hypocotyl elongation did not occur at concentrations below 5 × 10−4 M. It was concluded that the growth inhibition of plants caused by alachlor and metolachlor results from both an inhibition of cell division and cell enlargement.
