Weed Science, 49:628±634. 2001 Herbicide ef®cacy, leaf structure, and spray droplet contact angle among Ipomoea species and small¯ower morningglory Demosthenis Chachalis Greenhouse and laboratory studies were conducted to evaluate responses of ivyleaf Southern Weed Science Research Unit, USDA-ARS, morningglory, pitted morningglory, palmleaf morningglory, and small¯ower mor- P.O. Box 350, Stoneville, MS 38776 ningglory to several herbicides in relation to leaf structure, epicuticular wax, and Current Address: Greek National Agricultural spray droplet behavior. Two- to four-leaf stage plants of each species were highly Research Foundation (NAGREF), Plant Protection susceptible to aci¯uorfen, bentazon, bromoxynil, glufosinate, and glyphosate. How- Institute of Volos, P.O. Box 303, Volos 380 01, ever, at the ®ve- to eight-leaf stage, these species were less susceptible, and control Greece was herbicide speci®c. Spray droplets of these ®ve herbicides had a higher contact angle on ivyleaf morningglory than the other three species with a few exceptions. Krishna N. Reddy Stomata and glands were present on both adaxial and abaxial leaf surfaces of all Corresponding author. Southern Weed Science species, and palmleaf morningglory and small¯ower morningglory had more of these Research Unit, USDA-ARS, P.O. Box 350, than did the other two species. Trichomes were present on all species except palmleaf Stoneville, MS 38776; [email protected] morningglory. Epicuticular wax mass was highest in ivyleaf morningglory (57 mg cm22) and lowest in small¯ower morningglory (14 mgcm22). Wax consisted of C. Dennis Elmore homologous short-chain (, C18) or long-chain (. C20) hydrocarbons, alcohols, ac- Application and Production Technology Research ids, and triterpenes. Small¯ower morningglory waxes lacked short-chain length com- Unit, USDA-ARS, P.O. Box 36, Stoneville, MS ponents. Triterpenes were absent in palmleaf morningglory and small¯ower mor- 38776 ningglory epicuticular waxes. Untriacontane (C31 hydrocarbon) and tridecanol (C30 alcohol) were common major long-chain components in waxes of all four species. Marcus L. Steele Heptadecane (C17 hydrocarbon) and octanoic acid (C18) were common major short- Delta State University, Department of Physical chain length wax components in pitted, ivyleaf, and palmleaf morningglory. In spite Sciences, P.O. Box 3255, Cleveland, MS 38733 of some differences in leaf surface structures, wax mass, and wax components among the four species, there was no clear relationship between these parameters and her- bicide ef®cacy. Nomenclature: Aci¯uorfen; bentazon; bromoxynil; glufosinate; glyphosate; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE; palmleaf morningglory, Ipomoea wrightii Gray IPOWR; pitted morningglory, Ipomoea lacunosa L. IPOLA; small¯ower morningglory, Jacquemontia tamnifolia (L.) Griseb. IAQTA. Key words: Spray droplet spread area. Ivyleaf morningglory, pitted morningglory, palmleaf mor- chomes (Hess 1985; Hull et al. 1982; McWhorter 1985; ningglory, and small¯ower morningglory are listed among Wanamarta and Penner 1989). Herbicide absorption is fa- the 10 most common or troublesome weeds in the southern cilitated with either cuticular or stomatal in®ltration, but no United States (Dowler 1998). These species cause interfer- conclusive evidence indicates the comparative level of each ence with crops, increased lodging, reduced ef®ciency of me- route of penetration (Hess 1985; Wanamarta and Penner chanical harvest, and reduced yields (Barker et al. 1984). 1989). These species are among the ®ve weeds that cause 66% of The epicuticular wax appears to be an effective barrier to all cotton (Gossypium hirsutum L.) weed losses (Elmore et herbicide absorption. Removal of epicuticular wax with al. 1990). Full-season interference of annual Ipomoea species chloroform greatly increased glyphosate absorption in Coca reduced yields of soybean (Glycine max L.) up to 70% (East- [Erythroxylum coca var. coca (Lam.)] compared to plants with man and Coble 1977; Oliver et al. 1976). leaf epicuticular wax (Ferreira and Reddy 2000). Epicutic- Leaf morphology varies considerably among morningglo- ular leaf wax commonly occurs as a wax crystal deposition ry species. It is generally known that ivyleaf morningglory (Baker 1982; Hess 1985). Leaf wax contains primarily a has extensive trichomes on both leaf and stem, whereas variety of short- and long-chain hydrocarbons, alcohols, ac- palmleaf morningglory is smooth (Elmore et al. 1990). ids, esters, aldehydes, and triterpenes (Baker 1982). Wax is However, the effect of leaf surface characteristics of these considered to be largely nonpolar and therefore hydropho- species on herbicide ef®cacy has not been investigated. bic, but variation exists among species. Hydrocarbons are Leaf surface structures affect the wetting and penetration considered highly hydrophobic, whereas alcohols and acids behavior of foliar-applied herbicides (Hess 1985; Hull et al. are relatively hydrophilic (Baker 1982). Information is lack- 1982; McWhorter 1985; Wanamarta and Penner 1989). ing on the characterization of epicuticular wax in these spe- These leaf surface characteristics include the cuticle (epicu- cies. The objective of this research was to determine the ticular wax, cutin, and pectin), leaf age and development, responses of ivyleaf, pitted, palmleaf, and small¯ower mor- leaf angle and position, and numbers of stomata and tri- ningglory to several herbicides in relation to leaf morphol- 628 x Weed Science 49, September±October 2001 ogy, epicuticular wax, and herbicide spray droplet charac- iance (ANOVA), and means were separated using Fisher's teristics. protected LSD test at P 5 0.05. Materials and Methods Spray Droplet Contact Angle and Spread Area Contact angle of herbicide spray droplets on leaves was General Greenhouse Procedures measured on the adaxial surface of the second to fourth fully Seeds were purchased locally from a commercial source1 expanded leaf from the apical meristem as described by and were stored at 4 C prior to scari®cation with HCl to Chachalis et al. (2001). The leaves were selected daily at promote germination. Scari®ed seeds were placed in rect- random just before measurement from ®ve- to eight-leaf angular 6 by 7-cm containers ®lled with a mixture of soil stage plants. The contact angles of both sides of a 1-ml droplet on a leaf surface were measured approximately 1 min (Bosket sandy loam, ®ne-loamy, mixed, thermic Mollic Ha- 4 pludalfs) and potting soil2 (1 : 1 by volume). Seedlings were after droplet application using a goniometer. Each value transplanted (one plant per pot) approximately 1 wk after was the mean of the contact angles of two sides of the drop- germination to 10-cm-diam pots containing the soil mixture let. For spread area measurements, the surface area of a 1- above. Plants were grown in a greenhouse at 32/24 C (6 3 ml droplet on the leaf surface was measured approximately 5 min after the droplet application. Spread area was calcu- C) day/night temperature. Natural light was supplemented 2 with sodium vapor lamps to provide a 14-h day length to lated using the formula pr , where r was the radius of the prevent ¯owering. The soil mixture was surface irrigated as droplet. Preliminary trials showed that the 1-ml droplet on needed. Small¯ower morningglory was transplanted 10 d the leaf surface was rarely circular. Therefore, the radius was before the other three species to synchronize plant size and estimated as the mean of horizontal and vertical dimensions herbicide application. of the droplet. In some cases, spread was so great that the boundaries of the droplet were not clearly visible, and mea- surements were not possible. Contact angle and spread area Herbicide Ef®cacy measurements were replicated ®ve times, and the experiment was repeated. Data were subjected to ANOVA, and means Herbicide rates were selected to represent the manufac- were separated using Fisher's Protected LSD test at P 5 turers suggested use (13) rates (Anonymous 2000). Com- 0.05. mercial formulations of the following herbicides were used: aci¯uorfen, bentazon, bromoxynil, glufosinate, and gly- phosate. A nonionic surfactant (X-77)3 at 0.25% (v/v) was Leaf and Stem Surface Structures added to all herbicide spray solutions for uniformity. Spray Five to eight leaves and stem were randomly sampled solutions were applied using an indoor spray chamber from ®ve- to eight-leaf stage ®eld-grown plants. Two leaf equipped with an air-pressurized system equipped with segments (approximately 20 mm2) from the center of the 8002E nozzles at a volume of 190 L ha21 at 140 kPa. Her- leaf and stem segments (1 cm) were ®xed for 12 h in 4% bicides were selected to represent those used in transgenic glutaraldehyde and rinsed three times with distilled water (bromoxynil, glufosinate, and glyphosate) and in nontrans- before dehydration in a graded ethanol series. Samples were genic (aci¯uorfen and bentazon) corn (Zea mays L.), cotton, critical point-dried5 and mounted on aluminum stubs. Sam- and soybean. The rates were as follows: aci¯uorfen 0.56 kg ples were gold-coated using a sputter coater6 and examined ai ha21, bentazon 1.1 kg ai ha21, bromoxynil 0.56 kg ai using a scanning electron microscope.7 Leaf and stem sur- ha21, glufosinate 0.41 kg ai ha21, and glyphosate 1.1 kg ai faces were photographed at the same magni®cation for all ha21. Herbicides were applied to plants at two- to four-leaf species, and individual structures were identi®ed and count- stage and ®ve- to eight-leaf stage. Bamboo sticks supported ed. From each sample, two photographs were examined, and vining plants of ®ve- to eight-leaf stage. Ivyleaf morningglo- the study was replicated four times. The numbers of sto- ry, pitted morningglory, and palmleaf morningglory plants mata, glands, and trichomes were counted as described pre- reached two- to four-leaf and ®ve- to eight-leaf stages ap- viously (Chachalis et al. 2001; Elmore and Paul 1998). proximately 10 and 15 d after transplanting, respectively. Small¯ower morningglory plants reached two- to four-leaf and ®ve- to eight-leaf stages within 20 and 25 d after trans- Wax Extraction and Analysis planting, respectively.