Laboratory and greenhouse studies were conducted to determine if the reported red morningglory control failures with atrazine in sugarcane are caused by triazine-resistant mutants. Plants were grown from seeds collected at 24 locations in Louisiana and Arkansas where atrazine had never been used, and where it had been used in sugarcane with poor results. Terminal fluorescence of leaf material from all locations increased after treatment with 10−3 M atrazine, indicating electron transport inhibition and, hence, triazine susceptibility. However, small differences in the magnitude of fluorescence increase were observed among populations, possibly indicating the existence of biotypes with slightly different inherent tolerances to atrazine. Some phenotypic differences were observed among the red morningglory populations. In a separate study, postemergence application of atrazine at 1.1 kg ai/ha plus nonionic surfactant controlled greenhouse-grown plants from all populations at least 99%, which supports the findings of the fluorescence assay. This research was unable to verify that reduced red morningglory control with atrazine was the result of an altered binding site mutation, even in populations exposed to atrazine annually for more than 10 yr. Other factors should be evaluated to determine their impact on atrazine performance.
Nomenclature: Atrazine; red morningglory, Ipomoea coccinea L. #3 IPOCC; sugarcane, Saccharum interspecific hybrids.
Additional index words: Chlorophyll fluorescence, herbicide resistance, IPOCC, photosynthesis inhibition, photosystem II, triazine resistance, weed biotypes.
Abbreviations: CRF, change in relative fluorescence; DAT, days after treatment; FT, terminal fluorescence; POST, postemergence; PRE, preemergence.
