Weed Science, 49:732–737. 2001

Pollen morphological differences in Amaranthus species and interspeci®c hybrids

Aaron S. Franssen This study examined pollen morphological variation among Amaranthus species and Department of Agronomy, Kansas State University, interspecific hybrids. Ten weedy Amaranthus species, a cultivated grain species, and Manhattan, KS 66502 several putative hybrids resulting from interspecific mating between common water- hemp and Palmer were grown in a greenhouse. Mature pollen was col- Daniel Z. Skinner lected, viewed, and photographed with a scanning electron microscope (SEM). The USDA-ARS, Department of Agronomy, Kansas pollen grains were spherical shaped with polypantoporate, or golf ball-like, aperture State University, Manhattan, KS 66502 arrangement. Differences were observed between the monoecious and dioecious Am- aranthus species. Pollen grains of the dioecious species had a greater number of apertures on the visible surface. One exception to these trends was the dioecious Kassim Al-Khatib species, Palmer amaranth, whose pollen was similar to that of the monoecious species Corresponding author. Department of Agronomy, Kansas State University, Manhattan, KS 66502; spiny amaranth. However, pollen grain diameters did not differ between the mon- [email protected] oecious and dioecious . Significant differences also were noted between the pollen from the putative common waterhemp ϫ Palmer amaranth hybrids and the parental-type pollen grains. Pollen of the hybrids was similar in size to the maternal Michael J. Horak parent but had an aperture number that was intermediate between parents. This Monsanto Co., St. Louis, MO 63141 indicates that pollen characteristics may be controlled by the female and that hybrids may be more prevalent than originally thought.

Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; grain am- aranth, L. AMACR; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; Powell amaranth, Amaranthus powellii S. Wats. AMAPO; prostrate pigweed, Amaranthus blitoides S. Wats. AMABL; redroot pigweed, Amaranthus retro- flexus L. AMARE; sandhills amaranth, Amaranthus arenicola I. M. Johnst. AMAAR; smooth pigweed, L. AMACH; spiny amaranth, Amaranthus spinosus L. AMASP; tall waterhemp, (Moq.) J. D. Sauer AMATU; tumble pigweed, Amaranthus albus L. AMAAL.

Key words: Dioecious, monoecious, pigweeds.

The family includes some of the more is characterized by a large terminal thyrsus (inflorescence) prominent and aggressive annual weeds in the Midwest and an upright growth habit. Section amaranthus includes (Wax 1995). The aggressive growth habit and prolific seed smooth pigweed, Powell amaranth, redroot pigweed, and production enable Amaranthus species to compete well for spiny amaranth. Spiny amaranth has needle-like spines at light, moisture, and nutrients. Commonly found in culti- the axils and is easily distinguishable from all weedy vated fields, their presence can lower grain yield and quality species. The three other species in this group have similar as well as hinder mechanical harvest (Knezevic et al. 1997; morphology and are difficult to distinguish at all stages of Murphy et al. 1996; Wax 1995). Some of these species also growth. The species in this section compete very well in have been shown to possess allelopathic chemicals that re- most cropping situations. The section blitopsis of the sub- duce seedling vigor of several crop and weed species (Menges genus Amaranthus, also monoecious, is characterized as hav- 1987, 1988), while others may induce toxicosis and death ing flower placement in the axils of the . This section in dairy (Kerr and Kelch 1998). includes tumble pigweed and prostrate pigweed. These two Approximately 60 Amaranthus species are native to the species typically have a shorter growth habit than the other Americas, and an additional 25 species are present in the weedy species and do not compete very well in most crop- temperate and tropical regions of Africa, Asia, Australia, and ping situations. The monoecious species are typically self- Europe (Sauer 1967). Often collectively called ‘‘pigweeds,’’ pollinated with a small percentage of cross-pollination. at least 10 Amaranthus species are considered troublesome The subgenus Acnidia consists of those species that are weeds in the region (Horak et al. 1994; Wax dioecious (male and female flowers on separate plants) and 1995). include sandhills amaranth, Palmer amaranth, common wa- These 10 weedy Amaranthus species can be divided into terhemp, and tall waterhemp. Flowers are primarily con- two subgenera (Amaranthus and Acnidia) based on flowering tained on long slender inflorescences. These species have an morphology (GPFA 1986; Horak et al. 1994; Robertson upright growth habit, are somewhat branched, and are very 1981; Wax 1995). The subgenus Amaranthus consists of spe- competitive in most cropping situations (Sauer 1972). These cies that are monoecious (separate male and female flowers species are obligate cross-pollinators. on the same ) and can be further divided into two Several species of Amaranthus also are cultivated for grain sections based on flower placement. The section amaranthus production. Grain amaranth and other species are grown for

732 • Weed Science 49, November–December 2001 their high nutritional value and drought tolerance. Grain height of 35 cm, when the photoperiod was changed to 12/ from cultivated species is about 50% higher in protein than 12 h (day/night) to induce flowering. that of other cereal crops and is a rich source of the essential Plants were monitored daily for flower initiation. At the amino acid lysine (Gupta and Gudu 1991). beginning of pollen shed, when the pollen grains were near Because of the great genetic and morphological diversity physiological maturity, a section of the inflorescence con- within Amaranthus species, identification is difficult. The taining unopened anther buds was clipped from the plants presence of flowers is required to identify most species, and and placed in a solution of 10% potassium hydroxide for even then, identification to the species level is sometimes 12 h. Unopened buds were used to reduce the risk of cross- difficult. In addition, interspecific hybridization between contamination by foreign pollen. The potassium hydroxide Amaranthus species causes further difficulties in identifica- solution oxidized the pollen and turned the anthers red. tion, as these hybrids can exhibit characteristics of both par- A revised pollen acetolysis protocol from Moore and ents (Horak et al. 1994). Identification of species is neces- Webb (1978) was used to remove organic materials and to sary for an effective weed management program (Mayo et fossilize the pollen grains. For each plant of each species, 12 al. 1998; Sweat et al. 1998). Molecular techniques based on to 24 red-colored anther buds were removed from each in- polymerase chain reaction (PCR) have been developed that florescence, placed in a 1.5-ml centrifuge tube, and washed can repeatedly provide identification of cultivated and weedy with distilled water. Samples were centrifuged at 23,000 ϫ Amaranthus species at early stages of development (Lanoue g for 3 min. The supernatant was discarded, and the anther et al. 1996; Transue et al. 1994; Wetzel et al. 1999a). How- buds were mashed with a blunt pipette tip to release the ever, the PCR technique is laborious, requiring the extrac- pollen grains. The pollen was dehydrated using sequential tion of DNA and sophisticated laboratory protocols. There- treatments of 50 and 95% ethanol. After addition of each fore, additional means are needed for identification of the ethanol treatment, the tubes were shaken briefly to resus- major Amaranthus species and interspecific hybrids. pend the pollen and centrifuged at 23,000 ϫ g for 3 min, The morphology of the pollen grain is generally a con- and the supernatant was discarded. After pollen dehydra- served characteristic, which is an excellent means for iden- tion, glacial acetic acid was added, and tubes were shaken tification of most species (Iwanami et al. 1988). Attempts briefly to resuspend the pollen and centrifuged at 23,000 ϫ to separate two genera, Betula (birch) and Pinus (pine), on g for 3 min. The supernatant was removed, and the glacial the basis of pollen size indicated consistent average differ- acetic acid step was repeated. An acetolysis solution con- ences between the species. However, considerable overlap- taining nine parts glacial acetic acid and one part hydrochlo- ping in pollen size ranges suggests that identification by size ric acid was prepared just prior to use and added to each alone would not be practical with individual pollen grains tube. Pollen samples were resuspended, tubes were placed in (Kapp 1969). Pollen grains contain a plasmalemma-encased a boiling water bath for 5 min, and then centrifuged at cytoplasm surrounded by a thin layer of cellulose called in- 23,000 ϫ g for 3 min. The supernatant was removed, and tine. Immediately surrounding the intine is a layer of exine. the pollen was resuspended in glacial acetic acid. Tubes were The exine surface often develops various forms of sculptur- centrifuged at 23,000 ϫ g for 3 min, and the supernatant ing and ornamentation, including various types and num- was removed. The pollen was resuspended in 75% ethanol, bers of apertures. The objectives of this study were to ex- and tubes were centrifuged at 23,000 x g for 3 min. The amine pollen from several Amaranthus species and putative supernatant was removed, and the final pollen pellet was hybrids to determine if differences exist among Amaranthus resuspended and stored in 75% ethanol at room tempera- species and interspecific hybrids. ture.

Materials and Methods Pollen Viewing and Data Collection Plant Materials A few drops of the pollen/ethanol mixture were placed Ten common weedy Amaranthus species in Midwestern on a scanning electron microscope (SEM) stub, smeared, states, the cultivated grain amaranth, and several putative and allowed to dry. The pollen samples then were coated hybrids were obtained from known plants of each species with gold,2 viewed, and photographed with an SEM3 at that were identified by standard taxonomic procedures. Hy- ϫ3,500 to ϫ4,000 magnification. brid progeny were created by the interspecific mating be- Pollen diameter was determined using a reference scale tween inbred lines of common waterhemp as the maternal generated by the SEM computer software. The pollen grains parent and Palmer amaranth as the paternal parent. Progeny were not perfectly spherical. Therefore, measurements were from different common waterhemp females were screened, taken in the x-axis and y-axis, and the values were averaged and their hybrid nature was confirmed using molecular to estimate pollen diameter. The aperture number for each markers (Franssen et al. 2001). Seeds of each species were species was determined by counting the apertures on the sown in 35-cm-diam pots filled with 1,100 g of a 1 : 1 (v/ visible surface area of the pollen. The visible surface area v) mixture of soil and sand. The soil was a Morril loam was taken as one-half the surface area of the sphere. The (mesic typic Argiudolls) with pH 7.0 and 1.7% organic visible surface area was determined using the calculated ra- matter. Seedlings were grown at 30/25 Ϯ 3 C (day/night) dius of the pollen grains and the equation for the surface and 16/8 h (day/night) photoperiod. Supplemental light in- area of a sphere, A ϭ 4␲r2. Aperture density was determined tensity was 80 ␮mol mϪ2 sϪ1 photosynthetic photon flux. as a function of visible surface area and aperture number. Plants were watered as needed and fertilized weekly with a For each of the species, visible aperture number was divided fertilizer1 containing 300 ␮g/L N, 250 ␮g/L P, and 220 ␮g/ by the visible surface area to get aperture density (number L K. Plants were thinned to three per pot and grown to a ␮mϪ2).

Franssen et al.: Pollen differences in Amaranthus species • 733 FIGURE 1. Pollen of the monoecious Amaranthus species: (A) tumble pigweed, (B) prostrate pigweed, (C) grain amaranth, (D) smooth pigweed, (E) Powell amaranth, (F) redroot pigweed, and (G) spiny amaranth. Photographs A, B, C, D, F, and G were taken at ϫ4,000 magnification, and photograph E was taken at ϫ3,500 magnification.

Experimental Design and Data Analysis tative hybrids from different females were sampled. Each hybrid was treated as a replication, with five pollen grains Individual plants for each of the 10 weedy Amaranthus photographed for each. Means for the hybrids were averaged and cultivated grain amaranth were treated as one repetition across all the repetitions and compared to the parental for each species. Each species was replicated three times with means using standard error at the 95% confidence interval. five images captured for each repetition, for a total of 15 pollen grains for each species. The measured data were av- eraged for each species, and the means were analyzed using Results and Discussion analysis of variance. Duncan’s Multiple Range test was per- formed to test for significant differences between the species. Weedy and Cultivated Amaranthus Pollen For analysis of the interspecific hybrids, the parental data Images of the Amaranthus species pollen captured by the were obtained from the three repetitions of common water- SEM revealed spherical-shaped pollen grains with polypan- hemp and Palmer amaranth described above, and five pu- toporate, or golf ball-like, aperture arrangement. Differences

734 • Weed Science 49, November–December 2001 FIGURE 2. Pollen of the dioecious Amaranthus species: (A) sandhills amaranth, (B) Palmer amaranth, (C) common waterhemp, and (D) tall waterhemp. All photographs were taken at ϫ4,000 magnification. between the monoecious species (Figure 1) and dioecious thus pollen grains ranged from 18.1 to 60.3. Aperture num- species (Figure 2) were observed in aperture number, with ber was greater in the pollen of dioecious species than in the exception of Palmer amaranth (dioecious) that resem- that of monoecious species, with the exception of Palmer bled spiny amaranth (monoecious). No similarities were amaranth (Table 1). Pollen of the dioecious species common found between the two modes of pollination (monoecious waterhemp had more apertures (60.3) than all other species. vs. dioecious) with regard to aperture number and aperture Pollen of sandhills amaranth and tall waterhemp had 55.5 density. and 56.1 visible apertures, respectively, which was less than The diameters of the pollen grains for the 10 weedy spe- common waterhemp pollen but more than pollen of all oth- cies and the cultivated species ranged from 17.8 to 22 ␮m, er species (Table 1). with a mean of 19.7 ␮m (Table 1). In general, the mean The only exception to the trend was the dioecious Palmer pollen diameter did not differ consistently between the amaranth with 25.6 visible apertures, which was not differ- monoecious and dioecious species. The largest and smallest ent from pollen of the monoecious species spiny amaranth, pollen grains were monoecious species. Powell amaranth and prostrate pigweed, and Powell amaranth (Table 1). Pollen prostrate pigweed had the largest pollen grains, with mean grains of the remaining monoecious species (tumble - diameters of 22 and 21.7 ␮m, respectively, whereas tumble weed, smooth pigweed, redroot pigweed, and grain ama- pigweed had the smallest pollen grains, with a mean di- ranth) all had similar mean aperture numbers (Table 1). ameter of 17.8 ␮m. The mean aperture density of pollen for the examined The overall number of visible apertures on the Amaran- species was 0.054 apertures ␮mϪ2. Aperture densities were

TABLE 1. Mean diameter, aperture number, and aperture density of pollen grains of 11 Amaranthus species. Reproductive Aperture Aperture Species biology Diameter number density ␮m No. visible No. ␮mϪ2 a Tumble pigweed Monoecious 17.75 E 19.60 D 0.040 C Prostrate pigweed Monoecious 21.68 A 24.20 C 0.033 D Grain amaranth Monoecious 20.35 B 18.07 D 0.028 D Smooth pigweed Monoecious 20.09 B 19.47 D 0.033 D Powell amaranth Monoecious 21.96 A 23.47 C 0.031 D Redroot pigweed Monoecious 19.52 BCD 18.60 D 0.031 D Spiny amaranth Monoecious 19.88 BC 25.13 C 0.041 C Sandhills amaranth Dioecious 18.77 CDE 55.53 B 0.100 B Palmer amaranth Dioecious 19.82 BC 25.60 C 0.042 C Common waterhemp Dioecious 18.45 DE 60.27 A 0.113 A Tall waterhemp Dioecious 18.58 DE 56.07 B 0.104 B a Means with the same letter are not significantly different according to Duncan’s Multiple Range Test (␣ϭ0.05).

Franssen et al.: Pollen differences in Amaranthus species • 735 FIGURE 3. Pollen of the Amaranthus species parental lines and interspecific hybrid: (A) common waterhemp, (B) Palmer amaranth, and (C) interspecific hybrid. All photographs were taken at ϫ4,000 magnification. higher in the dioecious species, with the exception of Palmer mediate nature of the hybrids. The mean density for pollen amaranth (Table 1). Pollen of common waterhemp had the from the hybrids was 0.06 apertures ␮mϪ2, which was al- highest aperture density, and pollen of tall waterhemp and most halfway between the mean densities of pollen from sandhills amaranth each had an aperture density greater than common waterhemp and Palmer amaranth parental lines, the average of all the species. The monoecious species and which had 0.1128 and 0.0416 apertures ␮mϪ2, respectively Palmer amaranth had pollen with lower aperture densities (Table 2). than the average for all species. Palmer amaranth pollen had Although aperture number and density were intermediate a mean aperture density of 0.042, similar to that of the in the hybrid between the parents, the diameter was not. pollen from the monoecious species spiny amaranth and The pollen diameters for the hybrids tended to be smaller tumble pigweed. Pollen from the remaining monoecious than for the maternal common waterhemp and paternal species (prostrate pigweed, smooth pigweed, redroot pig- Palmer amaranth (Table 2). weed, Powell amaranth, and grain amaranth) had similar The differences in pollen morphology between the mon- aperture densities. oecious and dioecious species may be related to their mode of pollination and can be explained using the aerodynamics Parental and Hybrid Pollen of a sphere. The addition of apertures on the surface creates a boundary layer of turbulent air surrounding the sphere. Pollen morphology of hybrids appeared to be intermedi- This boundary layer reduces the friction between the sphere ate between that of the two parental lines. The most obvious and the air as the sphere travels through air, thus increasing difference noted between the hybrids and the parents was the potential distance traveled (Jorgensen 1993). The mon- the number of visible apertures on the pollen surface (Figure oecious Amaranthus species typically are self-pollinated 3). The mean aperture number for the pollen of the hybrids plants. When both male and female flowers are on the same was 36.1 (Table 2). This was less than the aperture number plant, the pollen has a short distance to travel, in contrast for common waterhemp pollen (60.3) but greater than the to the dioecious species, which must disseminate pollen to aperture number for Palmer amaranth pollen (25.6). neighboring plants. This study revealed that dioecious spe- Aperture density of pollen also demonstrated the inter- cies pollen has evolved more surface apertures, which may

TABLE 2. Mean diameter, aperture number, and aperture density of pollen grains of common waterhemp, Palmer amaranth, and hybrid progeny. Species Diametera Aperture number Aperture density ␮m No. visible No. ␮mϪ2 Common waterhemp 18.45 Ϯ 0.32 60.27 Ϯ 2.31 0.1128 Ϯ 0.003 Hybrids 18.18 Ϯ 0.38 36.08 Ϯ 1.12 0.0600 Ϯ 0.002 Palmer amaranth 19.82 Ϯ 0.13 25.60 Ϯ 0.66 0.0416 Ϯ 0.001 a Values following the mean indicate standard error of the mean.

736 • Weed Science 49, November–December 2001 facilitate dispersal over greater distances. This suggests that thank the Kansas Agricultural Experiment Station Scanning Elec- traits that offer a selective advantage (i.e., resis- tron Microscope Laboratory for use of their equipment. The au- tance) may spread more rapidly in dioecious than in mon- thors also thank the Kansas Agricultural Experiment Station editor oecious Amaranthus species. for reviewing the paper (contribution 01-199-J). Pollen of Palmer amaranth, a dioecious species, did not have the same morphology as pollen of the other dioecious Literature Cited species. Its pollen was similar to that of spiny amaranth in diameter, aperture number, and aperture density. Examina- Franssen, A. S., D. Z. Skinner, K. Al-Khatib, M. J. Horak, and P. A. tion of the genetic sequence of the internal transcribed spac- Kulakow. 2001. Interspecific hybridization and gene flow of ALS re- sistance in weedy Amaranthus species. Weed Sci. In press. er region of selected Amaranthus species by Kirkpatrick [GPFA] Great Plains Flora Association. 1986. Amaranthaceae, the pigweed (1995) demonstrated a high degree of homology between family. Pages 179–184 in T. M. Barkley, ed. Flora of the Great Plains. Palmer amaranth and spiny amaranth. The relation between Lawrence, KS: University Press of Kansas. these two species was closer than the relation between spiny Gupta, V. K. and S. Gudu. 1991. Interspecific hybrids and possible phy- amaranth and the other monoecious species examined. logenetic relations in grain . Euphytica 52:33–38. Horak, M. J., D. E. Peterson, D. J. Chessman, and L. M. Wax. 1994. These two species frequently share a common morphological Pigweed Identification: A Pictorial Guide to the Common Pigweeds characteristic—a chevron or v-mark on the leaves. These of the Great Plains. Manhattan, KS: Kansas State University Coop- relationships and the pollen similarities reported here suggest erative Extension Service Publ. S80. 12 p. that these two species share a relatively recent ancestor. Iwanami, Y., T. Sasakuma, and Y. Yamada. 1988. Pollen morphology of This study demonstrated differences in pollen character- flowering plants. Pages 10–122 in Pollen: Illustrations and Scanning Electromicrographs. Tokyo: Kodansha Press. istics among the Amaranthus species. With the exception of Jorgensen, T. P. 1993. The aerodynamics of golf. Pages 61–72 in The Phys- Palmer amaranth, the monoecious and dioecious reproduc- ics of Golf. New York: AIP Press. tive biology was correlated with specific pollen morphology. Kapp, R. O. 1969. Pollen and spore structure. Pages 3–10 in How to Know However, pollen morphology will have limited use in species Pollen and Spores. Dubuque, IA: W. C. Brown. identification because of similarities across species. To in- Kerr, L. A. and W. J. Kelch. 1998. Pigweed (Amaranthus retroflexus) toxi- cosis in cattle. Vet. Human Toxicol. 40:216–218. crease the usefulness of pollen morphology in species iden- Kirkpatrick, B. A. 1995. Interspecific Relationships Within the Genus Am- tification, additional analysis of naturally occurring popu- aranthus (Amaranthaceae). Ph.D. dissertation. Texas A & M University, lations as well as sampling from different geographic regions College Station, TX. 87 p. would be needed to account for variability not detected in Knezevic, S. Z., M. J. Horak, and R. L. Vanderlip. 1997. Relative time of this greenhouse study. redroot pigweed (Amaranthus retroflexus L.) emergence is critical in pigweed–sorghum [Sorghum bicolor (L.) Moench] competition. Weed We observed that the pollen of hybrids was similar in size Sci. 45:502–508. to the maternal parent but that the aperture number was Lanoue, K. Z., P. G. Wolf, S. Browning, and E. E. Hood. 1996. Phylo- intermediate between male and female parent lines. This genetic analysis of restriction-site variation in wild and cultivated Am- indicated that pollen characteristics may be controlled by aranthus species (Amaranthaceae). Theor. Appl. Genet. 93:722–732. the female parent in these species and that naturally occur- Mayo, C. M., M. J. Horak, D. E. Peterson, and J. E. Boyer. 1998. Dif- ferential control of four Amaranthus species by six postemergence her- ring hybrids may be more prevalent than originally thought. bicides in (Glycine max). Weed Technol. 9:141–147. Sauer (1972) noted that interspecific hybrids between these Menges, R. M. 1987. Allelopathic effects of Palmer amaranth (Amaranthus two Amaranthus species are highly sterile. Our study showed palmeri) and other plant residues in soil. Weed Sci. 35:339–347. that most of the hybrid pollens tested under SEM appeared Menges, R. M. 1988. Allelopathic effects of Palmer amaranth (Amaranthus shriveled and malformed (data not shown). These findings palmeri) on seedling growth. Weed Sci. 36:325–328. Moore, P. D. and J. A. Webb. 1978. The collection and treatment of may suggest that a large portion of hybrid pollen was not samples. Pages 16–29 in An Illustrated Guide to Pollen Analysis. New fertile. However, having pollen of some individual hybrids York: Halsted Press. larger than that of parents may suggest that fertility varies Murphy, S. D., Y. Yankubu, S. F. Weise, and C. J. Swanton. 1996. Effect among hybrids (data not shown). Wetzel et al. (1999b) on planting patterns and inter-row cultivation on competition between demonstrated through backcrossing that hybrid progeny corn (Zea mays) and late emerging weeds. Weed Sci. 44:856–870. Robertson, K. R. 1981. The genera of amaranthaceae in the southeastern were able to transfer herbicide resistance to the susceptible . J. Arnold Arbor. Harv. Univ. 62:267–314. parental line. Because of the significant differences found Sauer, J. D. 1967. The grain amaranths and their relatives: a revised tax- between common waterhemp and Palmer amaranth pollen, onomic and geographic survey. Ann. Mo. Bot. Gard. 54:101–113. analysis of wild-type pollen grains potentially could be used Sauer, J. D. 1972. The dioecious amaranths: a new species name and major to identify hybrid populations resulting from interspecific range extensions. Madrono 21:426–434. Sweat, J. K., M. J. Horak, D. E. Peterson, R. W. Lloyd, and J. E. Boyer. mating between these two species. 1998. Herbicide efficacy on four Amaranthus species in (Gly- cine max). Weed Technol. 12:315–321. Sources of Materials Transue, D. K., D. J. Fairbanks, L. R. Robinson, and W. R. Anderson. 1994. Species identification by RAPD analysis of grain amaranth ge- 1 Miracle-Gro soluble fertilizer, Scotts Miracle-Gro Products netic resources. Crop Sci. 34:1385–1389. Inc., Consumer Products Division, Port Washington, NY 11050. Wax, L. M. 1995. Pigweeds of the Midwest—distribution, importance and 2 Desk II Sputter/Etch unit, Denton Vacuum LCC, Moores- management. Proc. Int. Crop Mgmt. Conf. 7:239–242. town, NJ 08057. Wetzel, D. K., M. J. Horak, and D. Z. Skinner. 1999a. Use of PCR-based 3 S-3500N SEM, Hitachi Science Systems Ltd., Hitachinaka, molecular markers to identify weedy Amaranthus species. Weed Sci. Japan Ibaraki Pref 312-0033. 47:518–523. Wetzel, D. K., M. J. Horak, D. Z. Skinner, and P. A. Kulakow. 1999b. Transferral of herbicide resistance traits from Amaranthus palmeri to Acknowledgments Amaranthus rudis. Weed Sci. 47:538–543. This work was supported by grant 9700596 of the National Research Initiative Competitive Grants Program. The authors Received November 21, 2000, and approved June 12, 2001.

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