Reproductive Biology of Common Lambsquarters

Common lambsquarters has an annual life cycle with peak seed germination and seedling emergence occurring in early to mid spring, but subsequent cohorts can emerge during summer and fall (Holm et al. 1977). Plants can grow to a height of 2 m or more and have ascending branches and simple, alternate leaves (Holm et al. 1977). Inflorescences are a spiked panicle in leaf axils or at the terminus of stems and branches, with small, dense flower clusters. Plants are monoecious (i.e., male and female flowers on the same plant); flowers are perfect, green, small, sessile, in irregular spikes and without petals (Bassett and Crompton 1978).

Common lambsquarters flowers at any day length, but short days hasten flowering and maturity (Holm et al. 1977). The time of anthesis varies from a few days to several weeks. Common lambsquarters is self-compatible and reproduces both by self- and cross-pollination (Bassett and Crompton 1978). Flowers are mainly wind pollinated, but insects may play a role in pollination. A mature plant produces approximately 20,000 pollen grains in a season (Bajwa et al. 2019). Average-sized plants have been reported to produce more than 70,000 seeds (Bassett and Crompton 1978), although large plants have been reported to produce 500,000 seeds (Holm et al. 1977). Because common lambsquarters does not have a natural seed dispersal mechanism, most seeds drop to the ground near the mother plant.

Herbicide Resistance in Common Lambsquarters

Common lambsquarters has evolved resistance to PS-II inhibitors, ALS inhibitors, and synthetic auxins (Heap 2020a; Warwick and Marriage 1982). Resistance to PS-II inhibitors (including atrazine, simazine, metribuzin, and metamitron) is the most widespread resistance (n = 42 cases globally). Resistance to ALS inhibitors in common lambsquarters has been confirmed in a total of six cases (three in Canada, two in the United States, and one in Finland). The only report of resistance to synthetic auxin herbicides (namely, aminopyralid, clopyralid, and dicamba) in common lambsquarters is from New Zealand in 2005.

Glyphosate-resistant (GR) common lambsquarters has not yet been reported; however, some populations have shown variable or inconsistent responses to glyphosate across a wide geographic range in the United States (Hite et al. 2008; Kniss et al. 2007; Sivesind et al. 2011; Westhoven et al. 2008). Glyphosate efficacy on common lambsquarters can be affected by plant size (DeGreeff et al. 2018; Schuster et al. 2007) and environmental conditions (DeGreeff et al. 2018; Sivesind et al. 2011), but in other instances, reduced efficacy has been attributed to reduced sensitivity (Hite et al. 2008; Kniss et al. 2007; Westhoven et al. 2008). A subsequent investigation showed that reduced translocation of glyphosate plays an important role in conferring reduced sensitivity (Yerka et al. 2013).

PMGF from HR Common Lambsquarters

The PMGF in common lambsquarters decreases rapidly with increasing distance from the pollen source. Gasquez (1985) estimated that PMGF was 50% between plants spaced 0.2 m apart, whereas it decreased logarithmically to 5% at a plant spacing of 2.0 m. The results were likely indicative of common lambsquarter's PMGF in sugar beet (Beta vulgaris L.), which is a relatively short species, whereas the expected PMGF among common lambsquarters plants in corn would likely be much lower because of the tall stature of crop plants impeding pollen movement by wind (Darmency and Gasquez 1990).

Yerka et al. (2012) quantified PMGF in common lambsquarters in field experiments using magenta spreen (Chenopodium giganteum D. Don; 2n = 54), which has a dominant magenta phenotypic marker, as the pollen source. A wild-type accession of common lambsquarters (2n = 54) served as pollen-receptor plants that were grown in a soybean stand and arranged in concentric circles 2 to 15 m from the center that contained 24 pollen-source plants. The concentric circles were divided into eight cardinal directions. Pollen movement was analyzed by determining the percentage of progeny with the magenta phenotype from the pollen-receptor plants. Average PMGF across directions was greatest (3.0%) at 2 m and decreased exponentially to low levels (0.16%) 15 m from the edge of pollen source. The PMGF was greatest in the south-southwest, west-southwest, and west-northwest directions, approximately 180° from the prevailing wind direction during the time of pollen shed (Figure 1). Because common lambsquarters does not have an active seed-dispersal mechanism, the results suggested PMGF may play an important role in the short-distance transfer and frequency of resistance alleles within and between populations.

Figure 1.

Empirical model of pollen-mediated gene flow in common lambsquarters under field conditions (Yerka et al. 2012). The maximum observed cross-pollination percentage between wild-type Chenopodium album and C. giganteum was described by a negative exponential function using the two highest values at each distance from the south-southwest, west-southwest, and/or west-northwest sectors (generally 180° from the prevailing wind direction during the time of pollen shed). Mean values of the maximum cross-pollination percent (±SE) are shown.

Resistance to PS-II inhibitors is the most reported resistance in common lambsquarters (Heap 2020a). Consequently, research on gene flow of resistance alleles in this species has focused on PS-II inhibitors. Soon after the first report of triazine resistance in common lambsquarters (Bandeen and McLaren 1976), it was determined that the inheritance of atrazine resistance was uniparental (maternal) (Warwick and Black 1980). This discovery was critical for understanding the importance of seed-mediated gene flow of atrazine-resistance alleles in common lambsquarters.

In a study that assessed the spread of metamitron-resistant common lambsquarters patches in sugar beet and corn fields across four farms, using amplified fragment length polymorphism markers, Aper et al. (2012) found significant genetic variation among farm locations and among patches within farm locations. They also found a positive correlation between genetic distance and geographic distance, suggesting isolation by distance. However, the genetic similarity between patches from different fields indicated that seed transport by agricultural machinery and manure was likely to have played an important role in the spread of resistance.

Interspecific Hybridization and Gene Flow in Chenopodium Species

Common lambsquarters is a hexaploid (2n = 54) (Bassett and Crompton 1978). Recent research suggests that hexaploid Chenopodium species originated by hybridization between diploid (2n = 18) and tetraploid (2n = 36) Chenopodium species (Krak et al. 2016; Mandák et al. 2018). Common lambsquarters hybridizes with other hexaploid Chenopodium species, including the aforementioned magenta spreen (Yerka et al. 2012) and seaport goosefoot (C. opulifolium Shrad. ex W.D.J. Koch & Ziz) (CABI-ISC 2019; Mandák et al. 2018). Although experimental evidence suggests that Chenopodium species do not hybridize freely across ploidy levels (Mandák et al. 2012), common lambsquarters has been reported to hybridize with diploid and tetraploid species within the Chenopodium genus, including figleaf goosefoot (C. ficifolium Sm.; 2n = 18), Swedish goosefoot (C. suecicum Murr.; 2n = 18), and netseed lambsquarters or pitseed goosefoot (C. berlandieri Moq.; 2n = 36) (CABI-ISC 2019; Mandák et al. 2018).

Figure 2.

Giant ragweed inflorescence. (A) Initiation of male flowers and (B) seed clusters at the base of the flower heads.

Reproductive Biology of Giant Ragweed

Giant ragweed is a monoecious species with imperfect flowers; the main stem and branches of giant ragweed culminate in an inflorescence bearing male flowers; female flowers are usually present at the base of the male inflorescence or at the leaf axils in clusters of three or more sessile seed-bearing involucres (Figure 2) (Abul-Fatih and Bazaz 1979a; Bassett and Crompton 1982). Giant ragweed is a protogynous species, meaning stigmas in female flowers protrude and become receptive before the male flowers on the same plant start shedding pollen grains (Bassett and Crompton 1982). Giant ragweed is anemophilous (i.e., wind pollinated) and each plant can produce approximately 1 billion pollen grains during a normal growing season. It has been estimated that during peak flowering time, a single giant ragweed plant can release 1 million pollen grains day^–1 (Bassett and Crompton 1982). Ragweed pollen is relatively small (18–25 µm) and flat to nearly spherical, covered with miniature spine-like projections that likely support a low rate of deposition (0.02 to 0.06 m s^–1) (Barnes et al. 2001; Kanter et al. 2013). In addition, self-incompatibility has been documented in giant ragweed, which promotes PMGF despite being monoecious. Bassett and Crompton (1982) reported flower heads bagged before flowering produced viable seeds after self-pollination, although plants generated from self-pollinated seeds had reduced vigor compared with seeds resulting from cross-pollination.

Herbicide Resistance in Giant Ragweed

Giant ragweed has evolved resistance to ALS-inhibiting herbicides (Heap 2020a; Marion et al. 2017; Patzoldt and Tranel 2002), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-inhibiting herbicide glyphosate (Harre et al. 2017; Regnier et al. 2016) and multiple resistance to ALS and EPSPS inhibitors (Regnier et al. 2016). Resistance to ALS inhibitors in giant ragweed is due to a tryptophan-to-leucine substitution at the 574 amino acid position (W574L position relative to Arabidopsis thaliana) (Marion et al. 2017; Patzoldt and Tranel 2002). The GR giant ragweed biotypes have two distinct phenotypes: a rapid response phenotype in which cell death or necrosis occurs within hours after glyphosate application in fully expanded mature leaves; and a nonrapid response phenotype that results in chlorosis in the meristematic tissues during the first week or cessation of normal growth with no visible symptoms (Moretti et al. 2018; Van Horn et al. 2018). The precise mechanism of glyphosate resistance in ragweed is not known (Ganie et al. 2017a; Wilson et al. 2020); however, in collaborative studies conducted with several GR populations across the United States and Canada, researchers concluded that the mechanism of glyphosate resistance in the rapid-response biotype appears to be different compared with the known target-site or non–target-site resistance mechanisms and involves production of reactive oxygen species (Moretti et al. 2018; Van Horn et al. 2018). A recent study revealed that a novel mechanism confers glyphosate resistance in non–rapid-response type giant ragweed from Wisconsin and more investigation is needed (Wilson et al. 2020).

PMGF from HR to Susceptible Giant Ragweed

PMGF has been reported in giant ragweed. Ganie and Jhala (2017) used glyphosate resistance (nonrapid-response type) as a selective marker to quantify the PMGF and the dissemination of resistance alleles from a GR population (a pollen source) to a glyphosate-susceptible population (a pollen receptor). In this 2-yr study, PMGF ranged from 43% to 60% at less than 0.5 m and 3% to 4% at 50 m from the edge of pollen source. In the same study, a double exponential-decay model predicted 90% reduction in PMGF at 49.5 m in year 1 and 106.6 m in year 2. Similarly, Brabham et al. (2011) reported 31% PMGF between GR (rapid-response type) and glyphosate-susceptible giant ragweed planted in rows at 0.76 m apart. These studies provided empirical gene flow rates and experimental evidence to support the hypotheses of spread of resistance alleles through PMGF. High rates of PMGF explain the high level of phenotypic plasticity and genetic diversity in this species and should be considered when implementing management plans before plants reach the reproductive stage.

With little fitness cost associated with cloransulam or glyphosate resistance in giant ragweed (Glettner and Stoltenberg 2015; Marion et al. 2017), herbicide-resistance traits may have a greater likelihood for wide dissemination due to PMGF and possible introgression (i.e., permanent fixation of the trait in the gene pool of natural, wild populations) over time (Ellstrand et al. 1999; Stewart et al. 2003). Surveys in Indiana showed that GR giant ragweed spread from 15% to 39% of Indiana counties from 2006 to 2014 (Harre et al. 2017). Furthermore, it is possible that PMGF between neighboring giant ragweed populations resistant to ALS inhibitors or glyphosate may have resulted in multiple HR populations (Regnier et al. 2016). Harre et al. (2017) reported that in Indiana, 43 of 53 giant ragweed populations were GR where five and 16 populations displayed rapid-response or nonrapid-response phenotypes, respectively; 22 populations had both rapid-response and nonrapid-response phenotypes. Therefore, it is possible that PMGF between rapid-response and nonrapid-response GR giant ragweed biotypes have integrated two mechanisms of glyphosate resistance into a single biotype.

Interspecific Hybridization and Gene Flow in Ambrosia species

Common ragweed (Ambrosia artemisiifolia L.; 2n = 36) is a closely related species of giant ragweed. These two species require somewhat similar climatic conditions and grow in common habitats but display some phenological differences such as plant height and leaf shape (Figure 3). They have overlapping flowering times and possess commonalities in their floral biology and breeding systems, both are monoecious and wind pollinated (Bassett and Crompton 1975). Interspecific gene flow between giant ragweed and common ragweed is possible, although it is rare in nature (Vincent and Cappadocia 1987; Vincent et al. 1988). Vincent and Cappadocia (1987) reported that common ragweed × giant ragweed hybrids were easily produced, compared with giant ragweed × common ragweed, due to the postzygotic barrier of interspecific incompatibility in the latter crosses that required embryo rescue to produce viable hybrids. Common ragweed has evolved resistance to several herbicide SOAs including ALS, EPSPS, PS II, and protoporphyrinogen oxidase (PPO)-inhibiting herbicides. Multiple HR biotypes include resistance to ALS + EPSPS, ALS + PPO, and ALS + EPSPS + PPO inhibitors (Heap 2020a). Consequently, common ragweed and giant ragweed growing in proximity in the same habitat create the potential for interspecific hybridization and possible transfer of herbicide-resistance alleles.

Figure 3.

Coexistence of Ambrosia species, common ragweed and giant ragweed, growing at the same height in a soybean field. Interspecific gene transfer is possible between the species.

Reproductive Biology of Horseweed

Horseweed is a monoecious species with perfect flowers (Shields et al. 2006), and when the capitula are closed, self-pollination can occur (Huang et al. 2015). After the flowers are fully opened, horseweed releases pollen, largely influenced by its biological characteristics, ambient meteorological conditions, and geographic location (Huang et al. 2015; Ye et al. 2016). As a result, pollen production during the flowering season fluctuates day to day and year to year (Aboulaich et al. 2013). Horseweed pollen is lightweight and has a low settling speed (i.e., the downward-motion speed of a pollen particle in still air) and primarily carried by wind (Aylor 2002), although insect pollination has not been ruled out, because insects visit horseweed flower (Davis et al. 2010). The settling speed of horseweed pollen is 0.019 m s^–1; their density is 1,218 kg m^–3 (Wang et al. 2017). Horseweed pollen diameter ranges from 16 to 22 µm (Huang et al. 2015). The diameter may vary during the flight, because of dehydration (Aylor 2003), which affects the settling velocity of pollen (up to 30%) when moving in the air (Chamecki et al. 2011). After approximately 10 to 12 d of flowering, the seeds start to mature (Huang et al. 2015). A single plant can produce a substantial number of seeds, which subsequently become windborne with the aid of a pappus (Bhowmik and Bekech 1993; Weaver 2001).

Huang et al. (2015) in Illinois and Ye et al. (2016) in Tennessee reported that the total number of pollen grains generated from horseweed was 2.3 to 5.1 × 10⁶ plant^–1 during a pollination season. Diurnal pollen release started around 9 AM and the peak release was around 1:30 PM; little pollen was released after 7 PM. Approximately 79% of pollen was released between 9 AM and 7 PM (Huang et al. 2015; Ye et al. 2016). Seasonal pollen releases were multimodal (i.e., had multiple peaks) and lasted about approximately 60 d (Huang et al. 2015; Ye et al. 2016). A seasonal release pattern of horseweed pollen in a field study concluded that most pollen was deposited close to the source (Huang et al. 2015). At 25 m from the source, the pollen deposition rate (pollen grains m^–2) decreased to 18% of that at the horseweed source (148 m by 160 m source with a density of 9.5 plants m^–2). Because the pollen size was small and the settling speed was slow (1.7 cm s^–1), the pollen was lifted 80 to 100 m in the air in field experiments. Thus, pollen can be transported long distances. Smisek et al. (1998) found that an average of 96% of florets were self-pollinated and that the adjacent GR and susceptible horseweed plants had a PMGF of 4%. Within a population, PMGF was reported to range from 1.2% to 14.5% (Loux et al. 2006).

Herbicide Resistance in Horseweed

The first reported case of herbicide resistance in horseweed was to paraquat in Japan in 1980 (Matsunaka and Ito 1991). As of early 2020, there were 19 HR horseweed biotypes (Heap 2020a). GR horseweed population was confirmed in Delaware in the United States in 2001 (VanGessel 2001). Multiple HR horseweed biotypes, particularly those resistant to glyphosate and ALS inhibitors, have been confirmed and are widespread in corn-soybean cropping systems in the United States (Chahal and Jhala 2019; Davis et al. 2009; Heap 2020a).

PMGF from HR to Susceptible Horseweed

A field study conducted in Tennessee provided information about the release rate, release pattern, and pollen dispersal pattern of GR horseweed (Ye et al. 2016). Huang et al. (2015) conducted a field study in Illinois to measure horseweed pollen production, dispersal, and deposition, coupled with atmospheric data to reveal the release pattern and the factors that affect pollen release and dispersal. Wang et al. (2017) modeled the pollen dispersal on the basis of a Lagrangian statistical model. Like other weed species, pollen dispersal of horseweed is affected by many factors, including wind speed, atmospheric stability, size of the pollen source, pollen release rate, and presence of buffer plants. It has been estimated that a single horseweed plant can produce approximately 200,000 seeds (Smisek 1995; Wang et al. 2017; Weaver 2001) with a 4% outcrossing rate (Huang et al. 2015; Smisek et al. 1998). Results revealed that one horseweed plant might be sufficient to spread glyphosate-resistance alleles through pollen movement (Wang et al. 2017). Because of the rapid evolution of herbicide resistance in horseweed biotypes, the introgression to herbicide-susceptible plants might have happened relatively quickly (Chapman and Abbott 2010; Okada et al. 2015); however, little information is available concerning various stages of PMGF and introgression (Okada et al. 2015; Yuan et al. 2010).

Interspecific Hybridization and Gene Flow in Erigeron Species

Adaptive introgression of herbicide resistance from horseweed to closely related species can result from interspecific hybridization (Gaines et al. 2012; Okada et al. 2013; 2015). With the aid of micro-satellite markers, evidence was obtained of interspecific hybridization between horseweed and a polypoid Erigeron species (syn. Conyza sp.), hairy fleabane [C. bonariensis (L.) Cronq.] (Okada et al. 2013). However, it remains uncertain whether interspecific hybridization and introgression have occurred in nature between horseweed and hairy fleabane (Okada et al. 2015). Nevertheless, in the study of alleles between the two species, the low genetic structure and high value of genetic identity indicated there were exchanged alleles between hairy fleabane and horseweed. This provides additional evidence of the existence of PMGF between Erigeron species (Soares et al. 2015).

Reproductive Biology of Kochia

Kochia is self-compatible with moderate outcrossing; it produces protogynous flowers in which the stigmas emerge before anther development (Guttieri et al. 1995; Stallings et al. 1995; Thompson et al 1994). The stigmas usually emerge 1 wk before pollen is shed, are receptive to foreign pollen during that time, and deteriorate before anther dehiscence, thereby preventing self-pollination within the same flower. Pollen grains are spheroidal (20 to 40 µm diam) with a granular surface (Stallings et al. 1995). Pollen viability after dehiscence declines to approximately 30% after 1 d and 10% after 2 d under environmental conditions typically present during the pollination period: mid-day temperatures of 23 to 28 C and relative humidity of not more than 30% (Mulugeta et al. 1992; Stallings et al. 1995). Wind is the primary pollination vector, although bees (Colletidae and Halictidae) can contribute to PMGF in kochia (Blackwell and Powell 1981). Adult plants produce large amounts of pollen for extended periods (Mulugeta 1991), approximately 1 mo (Beckie et al. 2016).

Figure 4.

Pollen-mediated gene flow (PMGF) from a glyphosate-resistant kochia pollen source to varying distances of glyphosate-susceptible receptor plants at a field site at Saskatchewan, Canada, in (A) 2014 and (B) 2015. (Adapted from Beckie et al. 2016)

Herbicide-Resistance in Kochia

The PS-II inhibitor (triazine)-resistant kochia populations were discovered in Kansas and along railroads in Idaho and Iowa in 1976 in the United States (Heap 2020a). In 1987, the first kochia biotype resistant to ALS-inhibiting herbicides was confirmed in a wheat field in Kansas, only 5 yr after introduction of these herbicides (Primiani et al. 1990; Saari et al. 1990). In Canada, ALS inhibitor–resistant kochia was first reported in 1988 in Manitoba and Saskatchewan (Morrison and Devine 1994). Since the mid 2000s, most kochia populations are resistant to ALS inhibitors (Beckie et al. 2019b; Varanasi et al. 2015). Thus, area-wide incidence of ALS-inhibitor resistance in this species has progressed rapidly within 20 yr since first reported in the late 1980s (Heap 2020a). Kochia populations with multiple herbicide resistance (namely to PS II and ALS inhibitors) were first reported in biotypes from Illinois and Indiana in 1995 (Foes et al. 1999). The only reported case outside of North America is ALS inhibitor–resistant kochia from the Czech Republic (Chodova and Mikulka 2000; Holá et al. 2004).

Kochia populations resistant to auxinic herbicides were first discovered in 1995 in Montana and North Dakota (Nandula and Manthey 2002). To date, auxin-resistant biotypes have been reported in corn, wheat, or fallow fields in six states in the United States and two Canadian provinces (Beckie et al. 2019b; Kumar et al. 2019a). GR kochia was first reported in Kansas in 2007 and now occurs in 10 states in the United States and three Canadian provinces (Beckie et al. 2013, 2019b; Hall et al. 2014). The mechanism of glyphosate resistance in kochia is increased copy number and expression of the EPSPS enzyme, which is inhibited by glyphosate in susceptible plants (Wiersma et al. 2015). The EPSPS copy number correlates with level of glyphosate resistance, with four or more copies required for resistance to glyphosate when applied at labelled rates (Godar et al. 2015; Varanasi et al. 2015). Inheritance of increased copy number follows a single-locus Mendelian pattern, because the gene copies are in a tandem array on a single chromosome (Jugulam et al. 2014). Therefore, PMGF may contribute to an increase in EPSPS copy number and, therefore, levels of resistance to glyphosate. In Manitoba, where kochia is much less abundant, the incidence of glyphosate resistance in kochia increased from two fields (less than 1% of sites surveyed) in 2013 (Beckie et al. 2015) to 59% of fields in 2018 (Geddes et al. 2019). In the 2017 Alberta survey, 18% of kochia populations from nine counties were resistant to dicamba (Beckie et al. 2019b) compared with none in 2012 (Hall et al. 2014). Moreover, 10% of the populations in the 2017 survey were triple resistant to ALS inhibitors, glyphosate, and synthetic auxins. In surveys conducted in Colorado, 14% of fields in 2012, 15% in 2013, and 20% in 2014 had populations resistant to both glyphosate and dicamba (Westra et al. 2019).

PMGF from HR to Susceptible Kochia

The PMGF in kochia contributes to short-distance (less than 1 km) propagule dispersal. Studies in the western United States using ALS-inhibitor resistance as a marker have indicated that kochia is primarily self-pollinated, although moderate outcrossing can occur. The PMGF between ALS inhibitor–resistant (donor) and susceptible (receptor) plants occurred at a maximum rate of 13% at a distance of 1.5 m and declined to 1.4% at 29 m, the farthest distance sampled (Mallory-Smith et al. 1993; Stallings et al. 1995). In another PMGF study, 4% of progeny of susceptible plants transplanted within an ALS inhibitor–resistant kochia field population were resistant (Mulugeta et al. 1992).

Although western Canadian kochia populations have greater than expected levels of homozygosity (Martin et al. 2018), moderate levels of PMGF in the species were confirmed. Using glyphosate resistance as a marker, PMGF from resistant to susceptible plants averaged 6.4% at 0-m distance and exponentially declined to 0.25% at 96 m (farthest distance sampled; Figure 4) (Beckie et al. 2016). The predicted distance for 50% and 90% reduction in PMGF was at 3.3 m and 13.9 m, respectively. However, PMGF was markedly influenced by prevailing wind direction during the pollination period (over 1 mo), with a maximum PMGF of 17% downwind. The PMGF study in the United States was conducted in a spring barley (Hordeum vulgare L.) field (Stallings et al. 1995), whereas the Canadian study was conducted in a fallow field (Beckie et al. 2016) that favored PMGF because of lack of potential crop-pollen competition or the crop canopy acting as a wind break. Nevertheless, PMGF in kochia, using different herbicide resistance markers under cropped and noncropped conditions, were relatively similar in both studies.

Figure 5.

Palmer amaranth. (left) Anthers and (right) female flowers.

Interspecific Hybridization and Gene Flow in Kochia

Interspecific hybridization has not been reported in kochia. Both green molly [Bassia americana S. Watson) A.J. Scott] and rusty molly [B. californica S. Watson) A.J. Scott] are native perennial subshrubs, whereas forage kochia [B. prostrata (L.) A.J. Scott] is a recently naturalized species found in the western United States (Mosyakin 2003). No hybrids were obtained in artificial crosses between B. americana × B. scoparia (female × male) and B. prostrata × B. scoparia (female × male) (Lee et al. 2005).

Reproductive Biology of Palmer Amaranth

Palmer amaranth is dioecious; male (pistillate) and female (staminate) flowers are produced on separate plants (Figure 5A and 5B) in long, terminal inflorescences (Figure 6) that may be either solitary or branched and may droop with age (Mosyakin and Robertson 2008). On the basis of flowering morphology, Amaranthus species are placed in two subgenera: Amaranthus and Acnidia (Sauer 1957). Palmer amaranth and waterhemp belong to the subgenus Acnida (L.) Aellen ex K. R. Robertson (Borsch 1998). Male inflorescences can be distinguished from the female flower spikes by touch; male flowers lack the sharp, stiff bracts that subtend female flowers and thus feel softer (Mosyakin and Robertson 2008). Flowering in Palmer amaranth usually starts in summer and continues into fall, although it may also occur during the spring and winter in the southernmost part of the species' native range (Mosyakin and Robertson 2008). Keeley et al. (1987) demonstrated that Palmer amaranth flowering responses are influenced by day length; shorter days hasten flowering, which, in turn, reduces plant size and seed set.

Male Palmer amaranth plants can produce substantial amounts of wind-dispersed pollen grains that are spherical and covered in surface pores (Borsch 1998; Franssen et al. 2001a; Tsukada 1967; Walkington 1960). Mean pollen-grain diameter is reported to range from 20 to 31 µm depending on pollen collection timing (i.e., from unopened or opened anthers) and visualization or size determination methodology (i.e., using electron or light microscopy or an electronic particle sizer) (Durham 1946; Franssen et al. 2001a; Sosnoskie et al. 2009; Vaissière and Vinson 1994). One significant result from Franssen et al. (2001a) is the observation that pollen size and morphology differed with respect to reproductive biology, with the notable exception of Palmer amaranth, which was more like the monoecious amaranths in appearance (both size and pore density). Sosnoskie et al. (2009) estimated the mean density of fully hydrated Palmer amaranth pollen grains to be 1,218 kg m^–3. Using Stokes's law, the estimated settling velocity for individual Palmer amaranth pollen grains ranged from 0.015 to 0.05 m s^–1. Results from laboratory studies conducted using a settling chamber showed that most Palmer amaranth pollen grains fell at a rate of 0.05 m s^–1. The difference between the theoretical and observed estimates of settling velocity was suggested to be a function of changes in pollen density and shape, due to desiccation postanthesis, as well as the presence of pollen clusters, which may affect the movement of nearby solitary grains (Sosnoskie et al. 2009).

Figure 6.

Palmer amaranth inflorescence. (Left) male plants and (right) female plants.

Herbicide Resistance in Palmer Amaranth

As of 2020, there were 65 reported cases of Palmer amaranth with resistance to herbicides across eight SOAs (ALS, microtubule, PS II, EPSPS, PPO, long-chain fatty acid, 4-hydroxyphenylpyruvate dioxygenase inhibitors, and synthetic auxins) in Argentina, Brazil, Israel, and the United States (Heap 2020a). The first incidence of herbicide resistance in Palmer amaranth was to trifluralin and other dinitroanilines confirmed in 1989 in South Carolina (Gossett et al. 1992). The first GR Palmer amaranth was confirmed in Georgia (Culpepper et al. 2006). The most recent documented occurrence was to 2,4-D, with potential cross-resistance to other auxin herbicides in Kansas (Heap 2020a). There have been 18 confirmed cases of multiple herbicide resistance in Palmer amaranth, the majority (n = 16) of which are to two SOAs (Heap 2020a). Reports have documented multiple resistance to four herbicide SOAs in Kansas (ALS, EPSPS, and PS II inhibitors, and synthetic auxins) (Kumar et al. 2019b) and to five SOAs in Arkansas (ALS, EPSPS, long-chain fatty acid, microtubule, and PS-II inhibitors) (Heap 2020a).

PMGF from HR to Susceptible Palmer Amaranth

Field experiments were conducted in cotton and soybean fields in Georgia to evaluate PMGF from GR to susceptible Palmer amaranth using a source and receptor experimental design for 2 yr (Sosnoskie et al. 2012). The site was a 30-ha field planted alternately to cotton and soybean with a 0.01-ha center block planted to a GR Palmer amaranth biotype. Glyphosate-susceptible Palmer amaranth plants were planted at distances between 1 and 300 m from the edge of the GR source in the four cardinal and four ordinal directions to account for daily variability in wind flow patterns during peak pollination hours. Across all distances, the mean percentage of GR progeny ranged from 18% to 80%, with a trend toward fewer resistant offspring at the farthest distance from the source. The proportion of resistant individuals at each distance from the GR pollen source was greater in this trial compared with a similar study conducted in Nebraska in waterhemp, a closely related species of Palmer amaranth (Sarangi et al. 2017). It is unknown if apomixis (i.e., asexual seed formation) may have affected seed set and observed resistance patterns (Ribeiro et al. 2014; Trucco et al. 2007).

Reproductive Biology of Waterhemp

Waterhemp is a dioecious species with male and female inflorescences present on separate plants (Figure 7A and 7B). Pollen grains of waterhemp are spherical and very small (18.5 µm diam) with golf ball–like aperture arrangement (Franssen et al. 2001a). Franssen et al. (2001a) reported that pollen grains of waterhemp had more apertures on the visible surface compared with monoecious Amaranthus species. Flowers of waterhemp are contained on long, slender inflorescences (Figure 7A and 7B). The lower pollen settling velocity of waterhemp (0.0185 to 0.021 m s^–1) compared with major wind-pollinated crops such as corn (Costea et al. 2005) help pollen grains travel longer distances under pollen competition (Raynor et al. 1972).

Herbicide Resistance in Waterhemp

The widespread occurrence of waterhemp in the midwestern United States is partly due to its rapid evolution of resistance to many herbicide SOAs in the past two decades. GR waterhemp in the United States was first reported in Missouri in 2006 (Legleiter and Bradley 2008) and has now been confirmed in 22 states (Heap 2020b) and in Ontario, Canada (Schryver et al. 2017). In addition, waterhemp biotypes resistant to ALS inhibitors (Horak and Peterson 1995; Sarangi et al. 2015), PS-II inhibitors (Anderson et al. 1996), PPO inhibitors (Sarangi et al. 2019; Shoup et al. 2003), 4-hydroxyphenylpyruvate dioxygenase inhibitors (Hausman et al. 2011; Oliveira et al. 2017), and 2,4-D (Bernards et al. 2012) have been confirmed. Multiple HR biotypes have been identified, including glyphosate, ALS-, and PPO-inhibitor resistance in Nebraska (Sarangi et al. 2019), atrazine, glyphosate, ALS-, and PPO-inhibitor resistance in Illinois (Bell et al. 2013), and a six-way SOA-resistant biotype (2,4-D, atrazine, chlorimuron, glyphosate, fomesafen, and mesotrione) in Missouri (Shergill et al. 2018).

Figure 7.

Waterhemp inflorescences of (A) male and (B) female plants.

PMGF from HR to Susceptible Waterhemp

The dioecy and wind-pollinated nature of waterhemp are believed to promote the rapid spread of herbicide-resistance alleles in an agricultural landscape via pollen migration. Liu et al. (2012) conducted greenhouse and field experiments to determine pollen biology and dispersal dynamics in waterhemp and revealed that waterhemp pollen can remain viable up to 120 h and pollen dispersal declined exponentially with distance, with most pollen fertilizing recipient plants within 50 m; however, a rare gene-flow event was observed at 800 m. A multiyear field study was conducted in Nebraska to evaluate PMGF from GR to susceptible waterhemp and role of wind direction (Sarangi et al. 2017). More than 130,000 waterhemp plants were screened and 26,199 plants were confirmed GR. The PMGF declined by 50% at less than 3 m from the pollen source. The greatest gene flow ranged from 54% to 38% at 0.1-m distance from the pollen source during both years of the study, with gene flow decreasing with increasing distance; however, 5% to 9% gene flow was detected at the farthest distance of 50 m (Sarangi et al. 2017). Mallory-Smith et al. (2015) outlined procedures for conducting a PMGF study and noted that the phenotypic screening of herbicide resistance can be considered an excellent marker, though it should be supplemented with a molecular marker to confirm the resultant hybrid. Therefore, EPSPS gene copy-number analysis was conducted and it was determined that the mechanism of glyphosate resistance was gene amplification in the GR waterhemp parent used in this study, with the mean relative copy number of 5.3 (Sarangi et al. 2017). The researchers concluded that EPSPS gene amplification is heritable in waterhemp and can be transferred via pollen.

Interspecific Hybridization and Gene Flow in Amaranthus species

Amaranthus is one of the largest groups of weed species native to the Americas, with approximately 60 species present, and additional 25 species are present globally (Sauer 1957). Herbicide-resistance alleles can transfer via pollen and successful hybridization occurs between Amaranthus species, although, the numbers of successful crosses are typically low. For example, Wetzel et al. (1999) documented the transfer of ALS-inhibitor resistance between Palmer amaranth and waterhemp, although only 15 potential hybrids resulted from 10 000 pollination events. Franssen et al. (2001b) reported 35 confirmed hybrid plants from 44 000 seeds generated by crossing 11 female ALS inhibitor–sensitive waterhemp plants with male ALS inhibitor–resistant Palmer amaranth. Oliveira et al. (2018) conducted field experiments in Nebraska to determine interspecific and intraspecific transfer of metabolism-based mesotrione resistance in dioecious Amaranthus species. Results showed that 0.1% hybridization between waterhemp and Palmer amaranth occurred under field conditions. Gaines et al. (2012) described the results of a study examining transfer of glyphosate resistance from Palmer amaranth pollen donor to green amaranth (A. hybridus L.), Powell amaranth (A. powellii S. Watson), redroot pigweed (A. retroflexus L.), spiny amaranth (A. spinosus L.), and waterhemp under field and greenhouse conditions. Hybridization was observed between Palmer amaranth and green amaranth (less than0.01% in the field), Palmer amaranth and waterhemp (0.2% in the field), and Palmer amaranth and spiny amaranth (less than 0.01% to 0.4% in the field; 1.4% in the greenhouse). Nandula et al. (2014) reported that spiny amaranth resistant to glyphosate had a portion of EPSPS amplicon derived from resistant Palmer amaranth, suggesting a naturally occurring hybridization event between the species. The transfer of resistance traits between Palmer amaranth and spiny amaranth is likely because they have the same chromosome number (2n = 34), similar genome size, and produce comparably sized pollen (Franssen et al. 2001b; Rayburn et al. 2005).