Laboratory assays were conducted to determine the sensitivity of tomato and tobacco cell suspension cultures and tomato and pepper cotyledons to clomazone. A comparison of fresh weight and carotenoid content indicated up to a three-fold difference between the clomazone-tolerant tobacco and clomazone-susceptible tomato cell suspension cultures. In contrast, an approximate 60-fold difference between the tolerant pepper and susceptible tomato cotyledons was observed when total chlorophyll and carotenoid contents were measured. The effect of clomazone and its possible metabolites on in vivo and in vitro extractable IPP isomerase (EC 5.3.3.2) and prenyltransferase (EC 2.5.1.29) activity was investigated. There was no clear inhibitory effect of clomazone or possible clomazone metabolites upon enzyme activity in tomato or tobacco cell suspension cultures or on light or dark grown tomato or pepper cotyledons. No specific enzymatic target site of clomazone was identified in correlation with the reduction in total chlorophyll or carotenoid content.

Corn hybrids and inbreds were ranked for their relative tolerance to soil-incorporated clomazone, as assessed by the level of discoloration injury in the greenhouse. Inbred W117 was the most tolerant corn line tested. Some corn lines were affected similarly by clomazone. Inbred A619 was in the most susceptible group. Clomazone injury to A619 (susceptible) and W117 (tolerant) corn was similar when the clomazone rate was 10-fold greater on W117 than on A619. The distribution of corn lines on a sensitivity scale was of limited range; the distribution of hybrids on this scale was a single symmetrical peak. Changing the growth temperature or soil composition would change the absolute level of corn injury caused by a rate of clomazone but did not change the relative ranking of the corn lines in the test. A subset of the greenhouse-tested corn lines also was evaluated in several field locations. The tolerance of corn in a given field was highly (P<0.005) correlated with tolerance in the greenhouse; however, the absolute levels of injury differed among locations. The tolerance of- hybrids of known pedigree was highly (P<0.0002) correlated with the tolerance of the parent inbreds, indicating this trait was inherited.

A laboratory bioassay was conducted to determine the differential tolerance of cole crops to clomazone as measured by extractable total chlorophyll and carotenoids. Clomazone concentrations causing 50% inhibition (I50) in the biosynthesis of total chlorophyll in broccoli, cauliflower, and green and red cabbage cotyledons were 16, 11, 3, and 11 μM, respectively, while I50 values for carotenoid levels were 20, 10, 4, and 8 μM clomazone, respectively. Therefore, broccoli was the most tolerant to clomazone based upon extractable chlorophyll and carotenoid concentrations. Further laboratory studies were performed to investigate the basis for differential clomazone tolerance in 3-wk-old cole crop seedlings. No differences in total root uptake of 14C-clomazone were observed between these crops after 24 h. There were no differences in rate of metabolism of 14C-clomazone to methanol-soluble metabolites in roots of these crops. Percentage of polar metabolites in roots remained fairly constant over time. There were also no differences between crops in percentage of methanol-soluble 14C-clomazone metabolites formed in shoots between 24 and 96 h. In all crops, levels of 14C-clomazone decreased in a similar manner over time in methanolic extracts of roots and shoots while nonextractable 14C levels increased, indicating a conversion of clomazone to insoluble, nonextractable forms. Differential uptake, translocation, and metabolism do not appear to account for clomazone selectivity differences between cole crop seedlings.

Pronounced differences in the tolerance of tomatoes and bell peppers to clomazone observed in field studies were confirmed in a greenhouse experiment. In greenhouse studies, preemergence clomazone rates causing 50% visible injury on bell pepper and tomato seedlings 10 days after application were 9.4 and 0.1 kg/ha, respectively. Based on growth inhibition, bell peppers were 40-fold more tolerant of clomazone than tomatoes 20 days after clomazone application. In laboratory studies investigating the basis for differential clomazone tolerance, no differences in uptake of 14C-clomazone from nutrient solutions between tomato and bell pepper plants were observed after 24 h. Minor differences were observed in the distribution of 14C label within plants; a higher percentage of 14C was recovered in bell pepper roots than in tomato roots, while the opposite was true for the shoots. Clomazone was metabolized to two products in roots of both bell peppers and tomatoes within 48 h after treatment. Tomato shoots were more active in converting clomazone to these metabolites than were tomato roots. Bell pepper roots converted more clomazone to metabolites than did tomato roots 24 h after treatment. However, by 72 h, differences in clomazone metabolite levels between species were negligible in both roots and shoots. Enzymatic and acid hydrolysis of soluble, polaf clomazone metabolites indicated that these metabolites may be sugar conjugates of clomazone.

Experiments were conducted in 1985 to 1987 to evaluate the effects of conventional and no-tillage systems on the weed control provided by clomazone applied preemergence in soybeans. The persistence of clomazone in soil of the two tillage systems was also determined. Increasing the clomazone rate from 0.8 to 1.4 kg/ha did not increase weed control. Clomazone controlled 80% or more of jimsonweed, velvetleaf, and giant foxtail. Common cocklebur control ranged from about 50 to 70% in no-till and from 80 to 90% in conventional tillage. Generally, soybean pods/plant and yields were lower from clomazone treatments than from handweeded treatments due to inadequate common cocklebur control. Over 40% of the clomazone applied did not reach the soil surface; it was either intercepted by wheat straw, volatilized, or both. Clomazone persisted longer in conventional tillage than in no-tillage in. However, in 1986, clomazone was equally persistent in the two tillage systems. The half-life of clomazone was 34 and 6 days in 1985 in conventional and no-tillage, respectively, and in 1986, 18 and 16 days in conventional and no-tillage, respectively. Significant clomazone concentrations were not found below 10 cm in the soil profile. Corn planted without tillage (no-till) approximately 1 yr after clomazone application was not injured and yields were not reduced due to prior clomazone use.