Two greenhouse studies were conducted to investigate the variability in tolerance to a sublethal dose of glyphosate among accessions of pitted morningglory, hybrid morningglory (a fertile hybrid between pitted and sharppod morningglory), and sharppod morningglory, collected from several states in the southern United States. The first study was conducted to evaluate the variability in tolerance to glyphosate among accessions. Glyphosate at 420 g ae/ha was applied to plants at the four- to five-leaf stage, and control (percent shoot fresh weight reduction) was determined 2 wk after treatment (WAT). Pitted morningglory response ranged from −9% (indicating no response to glyphosate) to 39% control. A similar trend was observed in hybrid morningglory. Control of two related species, cypressvine morningglory and red morningglory, averaged 40 and 29%, respectively, and was similar to control of the most susceptible pitted morningglory and hybrid morningglory accessions. Ivyleaf morningglory control was 9%. Sharppod morningglory control was highest (48%) among the morningglories studied. A second study was conducted to determine levels of tolerance to glyphosate based on GR50 (dose required to cause a 50% reduction in plant growth) in 10 accessions that were least to most sensitive to glyphosate (7 pitted, 2 hybrid, and 1 sharppod morningglory). Glyphosate GR50 doses ranged from 0.65 to 1.23 kg/ha, a two-fold variability in tolerance to glyphosate among the 7 pitted morningglory accessions. Increasing levels of tolerance were associated with the absence of a leaf notch. These results indicate the existence of variable tolerance to a sublethal dose of glyphosate among accessions of pitted morningglory.

Greenhouse studies were conducted in 2003 at the Stine–Haskell Research Center to determine whether herbicide inhibitors of six specific sites in the carotenoid biosynthesis pathway would elicit synergistic responses when applied postemergence (POST) in combination with the photosystem II (PSII) inhibitor atrazine. Based on data analysis with the Isobole method, synergistic responses were observed on red morningglory, common cocklebur, and giant foxtail when atrazine was applied in mixtures with the deoxy-D-xylulose-5-phosphate reductoisomerase (DOXP reductoisomerase) inhibitor fosmidomycin, the p-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor mesotrione, and the DuPont proprietary zeta-carotene desaturase (ZDS) inhibitor DFPC. Clomazone (its metabolite ketoclomazone is the actual enzyme inhibitor), an inhibitor of 1-deoxy-D-xylulose-5-phosphate synthatase (DOXP synthase), provided synergistic responses on red morningglory, but antagonistic responses on both common cocklebur and giant foxtail when applied in mixtures with atrazine. Combinations of the lycopene cyclase (LC) inhibitor, CPTA, with atrazine produced synergistic responses on both common cocklebur and giant foxtail but were antagonistic on red morningglory. Norflurazon, a phytoene desaturase (PDS) inhibitor, applied in mixtures with atrazine provided synergistic responses on red morningglory, antagonistic responses on giant foxtail, and independent responses on common cocklebur. Because carotenoids have been determined to play a key role in quenching singlet oxygen species in the chloroplast and also assist in the maintenance of the D1 protein in PSII, this might help explain the synergistic responses with atrazine observed in our studies.