Suspension cultures of proso millet cells were treated with haloxyfop at different phases of growth. Treatment of 1-d cultures with 1 μM haloxyfop completely inhibited growth within 48 h. In contrast, 1 mM haloxyfop was required to elicit a similar response in 4-, 7-, or 10-d cultures. Calculated IC50 values indicated a 300-fold decrease in haloxyfop sensitivity during the period from 1 to 4 d. The observed changes in sensitivity to haloxyfop could not be attributed to changes in cell concentration during culture growth. In both 1-d and 4-d cultures, an initial rapid uptake of radiolabel was followed by a slow loss of radiolabel from cells. Almost all radioactivity extracted from 1-d and 4-d cells was present as the parent acid. Several radiolabeled compounds in addition to the parent acid were present in media. No major differences in the amounts of these materials were found between 1-d and 4-d media. Our results indicate that a special aspect of metabolism expressed during cell division is particularly sensitive to the herbicide.
