Introduction
Kochia [Bassia scoparia (L.) A.J. Scott] is a summer annual, broadleaf weed in the Amaranthaceae family (formerly Chenopodiaceae), native to central and eastern Europe and western Asia (Georgia Reference Georgia1914; Whitson et al. Reference Whitson, Burrill, Dewey, Cudney, Nelson, Lee and Parker1991). Bassia scoparia is the most troublesome weed in arid and semiarid regions of the North American Great Plains (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019). Several unique biological characteristics such as early and rapid germination, significant outcrossing, high genetic variation, high seed production, and tumble mechanism of seed dispersal contribute to the weediness of this species in the region (Gressel and Segel Reference Gressel and Segel1978; Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019; Mengistu and Messersmith Reference Mengistu and Messersmith2002). Low temperature and osmotic potential requirements for germination are the most important characteristics that allow B. scoparia to compete with spring-planted crops in the region (Eberlein and Fore Reference Eberlein and Fore1984; Evetts and Burnside Reference Evetts and Burnside1972).
Bassia scoparia is often the first species to emerge in spring in the Northern Great Plains (Dyer et al. Reference Dyer, Chee and Fay1993; Schwinghamer and Van Acker Reference Schwinghamer and Van Acker2008). Seeds are either nondormant or exhibit very little (less than 5%) dormancy (Dyer et al. Reference Dyer, Chee and Fay1993). Therefore, mature seeds germinate as soon as germination requirements are met. Furthermore, seeds can germinate over a wide range of temperatures from 3.5 to 40 C (Eberlein and Fore Reference Eberlein and Fore1984). Alternate versus constant temperature regimes do not affect B. scoparia seed germination (Everitt et al. Reference Everitt, Alaniz and Lee1983).
Moisture is often a limiting factor for crop production in the semiarid U.S. Great Plains. Bassia scoparia can germinate at soil moisture levels at which other species fail to germinate (Everitt et al. Reference Everitt, Alaniz and Lee1983) or certain preemergence soil-residual herbicides are not biologically active (Sebastian et al. Reference Sebastian, Nissen, Westra, Shaner and Butters2017). Therefore, many preemergence herbicides do not provide consistent control of this species in this region. Early emergence in the spring enables B. scoparia to acquire limited soil moisture and provides a competitive advantage over crops and other weed species (Dyer et al. Reference Dyer, Chee and Fay1993). Additionally, B. scoparia is highly water-use efficient because of its C4 photosynthetic pathway (Chu and Sanderson Reference Chu and Sanderson2008).
Competitive dominance of weeds in crops is largely determined by their relative time of emergence (Cousens et al. Reference Cousens, Brain, O’Donovan and O’Sullivan1987). This is regulated primarily by soil temperature and water potential (Bradford Reference Bradford2002). In addition, timing of weed control practices and weed emergence should coincide to obtain the full potential of those weed control practices (Ogg and Dawson Reference Ogg and Dawson1984). Therefore, improved knowledge of temperature and osmotic potential requirements for B. scoparia seed germination is important to predict the timing and duration of weed emergence (Ogg and Dawson Reference Ogg and Dawson1984), which would ultimately aid in designing effective weed management programs.
Variable germination requirements and emergence patterns for B. scoparia have been reported in different geographic regions (Anderson and Nielsen Reference Anderson and Nielsen1996; Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss, Moechnig and Cole2017; Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018a; Schwinghamer and Van Acker Reference Schwinghamer and Van Acker2008). Dille et al. (Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss, Moechnig and Cole2017) reported that under field conditions, B. scoparia populations from Kansas required 690 growing degree days (GDD) to achieve 90% emergence, compared with only 230 GDD for Nebraska and Wyoming populations. Kumar et al. (Reference Kumar, Jha, Dille and Stahlman2018a) also observed a differential emergence pattern of B. scoparia populations collected from the U.S. Great Plains, suggesting the presence of different emergence “biotypes” among B. scoparia field populations. These differences in germination requirements or emergence patterns are not unusual, as there is a substantial genetic/phenotypic variation present among B. scoparia populations (Bell et al. Reference Bell, Nalewaja and Schooler1972; Mengistu and Messersmith Reference Mengistu and Messersmith2002).
Although some studies quantified thermal requirements for B. scoparia germination (Kumar and Jha Reference Kumar and Jha2017; Kumar et al. Reference Kumar, Jha, Lim and Stahlman2018b), it is unclear whether observed differences in B. scoparia emergence patterns across the geographic sites are due to differential thermal requirements or differential osmotic potential requirements among populations. Additionally, information on germination requirements of weed populations collected across a wide geographic area may help in developing robust models to predict weed emergence patterns (Myers et al. Reference Myers, Curran, Vangessel, Calvin, Mortensen, Majek, Karsten and Roth2004). Therefore, the objectives of this research were to (1) quantify the temperature and osmotic potential requirements of B. scoparia populations collected from 44 locations across three states (Montana, Wyoming, and Nebraska) in the U.S. Northern Great Plains and (2) compare thermal or osmotic potential requirements for germination of B. scoparia populations between northern and southern parts of the three-state region.
Materials and Methods
Seed Collection
Mature seeds of B. scoparia plants growing in fields were collected in fall 2016 from three states of the U.S. Northern Great Plains. Eleven kochia populations each were collected from sites surrounding Huntley, MT, and Powell, WY, in the northern region, and Lingle, WY, and Scottsbluff, NE, in the southern region (Figure 1). Ten different crop field locations (approximately 10 km apart from one another) and one rangeland or non-crop collection site were located in each of the four areas. Therefore, a total of 44 B. scoparia seed samples were collected and considered to be 44 separate populations. To quantify the effect of geographic regions (across a latitudinal transect) on the germination requirements, populations were divided into two groups, northern and southern regions. The Huntley and Powell sites were included in the northern region, whereas the Lingle and Scottsbluff sites were included in the southern region. All B. scoparia seed samples collected were dried for 4 wk at room temperature (25 C), hand threshed, and then cleaned using mesh sieves. Cleaned seed samples were stored at 4 C until being used for the germination experiments.
Figure 1. Geographic map of four sites where 44 Bassia scoparia populations were collected in 2016 across a three-state region in the U.S. Great Plains.
Osmotic Potential Experiment
Laboratory experiments were conducted at the Montana State University Southern Agricultural Research Center (MSU SARC), Huntley, MT, in fall 2017 to quantify osmotic potential requirements for germination of B. scoparia populations. Ten osmotic potential treatments ranging from 0 to −2.1 MPa were created by using polyethylene glycol (PEG 8000, Fisher Scientific, One Reagent Lane, Fair Lawn, NJ 07410) based on the methods described by Michel (Reference Michel1983). The ten treatments included 0, −0.1, −0.3, −0.5, −0.7, −0.9, −1.2, −1.6, −1.8, and −2.1 MPa. Each treatment was replicated three times. For each experimental unit, 50 seeds from each population were counted out and placed between two layers of filter paper (Whatman® Grade 2, Sigma-Aldrich, St Louis, MO 68178) in a 10-cm-diameter petri dish (Fisher Scientific). Filter paper in each petri dish was moistened with 7 ml of PEG solution, except in the 0 MPa treatment, in which 7 ml of distilled water was used. Petri dishes were sealed with a thermoplastic wrapper (Parafilm™ M, Fisher Scientific) to prevent water loss through evaporation. Because light is not required for B. scoparia seed germination (Everitt et al. Reference Everitt, Alaniz and Lee1983), petri dishes were placed in the dark in an incubator (VWR® Signature™, VWR, 100 Matsonford Road, Radnor, PA 19087) set to a constant temperature of 20 C. The 20 C temperature was selected because PEG solution was prepared for this temperature and it is the optimum temperature for germination of B. scoparia seeds (Eberlein and Fore Reference Eberlein and Fore1984; Everitt et al. Reference Everitt, Alaniz and Lee1983; Kumar and Jha Reference Kumar and Jha2017). Treatments were arranged in a completely randomized design.
Temperature Experiment
Temperature requirements for germination of B. scoparia populations were quantified in laboratory experiments conducted at the MSU SARC, Huntley, MT, in fall 2018. Eight constant temperature treatments ranging from 4 to 26 C were used. The treatments included 4, 8, 12, 15, 18, 21, 24, and 26 C. Separate growth chambers (VWR® Signature™) were assigned for each temperature treatment. Petri dishes were prepared and maintained as described for the osmotic potential experiment. Petri dishes in all treatments were watered with 7 ml of distilled water.
Data Collection and Statistical Analysis
Bassia scoparia seed germination was observed on a daily basis for 2 wk. Germinated seeds were counted and removed from petri dishes at each observation time. A seed was considered germinated when the tip of the protruding radicle uncoiled (Dyer et al. Reference Dyer, Chee and Fay1993; Young et al. Reference Young, Evans, Stevens and Everett1981). Bassia scoparia germination data from each experiment were analyzed in the R statistical environment (R Core Team 2019) using the R extension package drc (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015). Data from each observation period were arranged in an event-time format (Ritz et al. Reference Ritz, Pipper and Streibig2013), then a three-parameter log-logistic model was fit (Equation 1; Ritz et al. Reference Ritz, Pipper and Streibig2013).
$$F\left( t \right) = {d \over {1 + {\rm{exp}}\left\{ {b\left[ {\log \left( t \right) - \log \left( {{t_{50}}} \right)} \right]} \right\}}}$$
In Equation 1, F(t) denotes the proportion of seeds germinated between time 0 (start of the experiment) and time t; d denotes the upper limit (expected maximum germination at very large t); t 50 denotes the time required to observe 50% germination (relative to the upper limit, d); and b denotes the slope of germination curve at time t 50. In the osmotic potential experiment, overall seed germination from all populations decreased to less than 30% at osmotic potentials of −1.6 and −1.8 MPa and ceased completely at −2.1 MPa. The lower germination proportions at −1.6 and −1.8 MPa did not allow model fit and parameter estimations. Therefore, only seven osmotic potential treatments ranging from 0 to −1.2 MPa were used to fit the model and generate germination curves. However, in the temperature experiment, B. scoparia seeds germinated in high proportions at all temperatures; therefore, all eight treatments were used to fit the model and generate germination curves. The accuracy of model fit was tested using the lack of fit test in the drc package (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015).
Additionally, a second three-parameter log-logistic model was fit using Equation 2 (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015) to quantify germination response of B. scoparia populations to osmotic potential treatments.
$$y = {d \over {1 + {\rm{exp}}\left\{ {b\left[ {{\rm{log\;}}x\; - \;\log \;e} \right]} \right\}}}$$
In Equation 2, y denotes the percent reduction in germination (relative to an osmotic potential of 0 MPa); x denotes the osmotic potential; d denotes the upper limit; e denotes the ψ50 (osmotic potential required to reduce the germination by 50%); and b denotes the relative slope around ψ50. Values of ψ10 and ψ90 were calculated using the ED function of the drc package.
To compare northern B. scoparia populations with southern populations for germination requirements, a two-step procedure described by Jensen et al. (Reference Jensen, Andreasen, Streibig, Keshtkar and Ritz2017) was used. In the first step, parameters of interest—t 50 and duration of germination (t 95–t 5)—were obtained for each population using Equation 1. Then, in the second step, these parameters were analyzed using a mixed-effects model in the lmer function of the lme4 package in R (Bates et al. Reference Bates, Maechler, Bolker and Walker2015). In the model, populations were considered to be random effects, whereas treatments and regions were considered to be fixed effects. Results were visualized in graphs using the ggplot2 package in R (Wickham Reference Wickham2016).
Results and Discussion
Effect of Osmotic Potential
Bassia scoparia populations germinated in high proportions (>60%) at osmotic potentials of −1.2 MPa or higher (Table 1; Figure 2). At an osmotic potential of 0 MPa, almost all B. scoparia seeds (>98%) from each site germinated during the observation period. Bassia scoparia exhibits rapid and high germination percentages when optimum conditions are met. Dyer et al. (Reference Dyer, Chee and Fay1993) and Thompson et al. (Reference Thompson, Thill, Mallory-Smith and Shafii1994) also reported greater than 95% germination of B. scoparia in less than 3 d under optimum seed germination conditions. At osmotic potentials of −0.9 MPa or higher, B. scoparia populations from all four sites achieved 50% of the maximum germination within 2 d. Similarly, at an osmotic potential of −1.2 MPa, B. scoparia populations took 4 to 8 d to achieve 50% germination; populations from northern Wyoming (near Powell) took the shortest time (4 d) and populations from Nebraska (near Scottsbluff) took the longest time (8 d). In addition, populations from Powell achieved 15% higher germination than populations from Scottsbluff at an osmotic potential of −1.2 MPa.
Table 1. Effect of osmotic potential on germination characteristics of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains.
a Parameter estimates were obtained using the log-logistic model (Equation 1). b, relative slope around t 50; t 50, time (days) taken to achieve 50% of the maximum germination; d, maximum germination (%) at the end of the observation period.
Figure 2. Germination response of Bassia scoparia populations to different levels of osmotic potential. Populations were collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains. Curves were generated using a three-parameter log-logistic model (Equation 1). Each curve represents germination response (cumulative proportion) at a given osmotic potential (−MPa) over time (days). Symbols on the curves are the observed means of 11 populations.
A proportion of B. scoparia seeds from each site were able to germinate within the range of osmotic potentials from 0 to −1.8 MPa (Table 2). However, decreases in the osmotic potential (more negative) significantly reduced B. scoparia germination rate (Figure 2) and cumulative germination (Figure 3) for all populations. Cumulative germination of B. scoparia populations from all the sites started declining rapidly at an osmotic potential of −0.9 MPa and declined to less than 20% at −1.8 MPa (Figure 3). A −0.85 MPa osmotic potential or lower reduced B. scoparia cumulative germination by 10%. These results agree with Everitt et al. (Reference Everitt, Alaniz and Lee1983), who previously reported that B. scoparia germination did not decline until osmotic potential reached −0.8 MPa. In the current experiment, a −1.9 MPa osmotic potential or lower reduced B. scoparia germination by 90%. Populations from Scottsbluff were more sensitive to the osmotic potential (ψ50 of −1.32 MPa) than the populations from Powell (ψ50 of −1.46 MPa). However, no differences were observed between these two sites for ψ90 values (Table 2).
Table 2. Effect of osmotic potential on the maximum germination of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains.
a Parameter estimates were obtained using the log-logistic model (Equation 2). b, relative slope around ψ50; ψ10, ψ50, and ψ90, osmotic potential (−MPa) required to reduce the germination by 10%, 50%, and 90%, respectively; d, maximum germination (%) at the end of the observation period.
Figure 3. Effect of osmotic potential on the cumulative germination of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains. Curves were generated using a three-parameter log-logistic model (Equation 2). Each curve represents percent maximum germination over a range of osmotic potentials. Symbols on the curves are the observed means of 11 populations.
Time to achieve 50% of the maximum germination (t 50) increased with decreasing osmotic potential, but did not differ between B. scoparia populations from northern versus southern regions based on the mixed-effects analysis (Figure 4). Similarly, the duration of germination (t 95–t 5) increased with decreasing osmotic potential, but no differences were observed between B. scoparia populations from northern versus southern regions. On average, populations from northern and southern regions completed germination in 25 d at an osmotic potential of −1.2 MPa. Individual populations within a site or region had a greater variability in rate and duration of germination than the variability between the regions.
Figure 4. Effect of osmotic potential on the germination rate (left) and the germination duration (right) of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains. Response lines were fit with a mixed-effects model. The Huntley and Powell sites were included in the northern region, whereas the Lingle and Scottsbluff sites were included in the southern region. Colored round symbols along the lines represent parameter values of individual populations. Shaded gray bands along the lines represent 95% confidence intervals.
Effect of Temperature
Populations from all four sites had a high germination percentage (≥88%) across the temperatures tested (Table 3). The germination rate (1/t 50) was lowest at 4 C for all populations (Figure 5); however, 50% of the maximum germination was achieved in 3 d at this temperature. This indicates that temperatures above 4 C are not likely to reduce germination rate and cumulative germination of B. scoparia seeds. At 15 C or above, all populations achieved 50% germination in less than 1 d. Therefore, germination rate was highest at temperatures of 15 to 26 C for all populations, indicating a wide range of temperatures favorable for B. scoparia germination. All populations had a cumulative germination of at least 90% over the range of temperatures tested. Dyer et al. (Reference Dyer, Chee and Fay1993) reported greater than 99% cumulative germination by B. scoparia at 17 C in a 2-d period.
Table 3. Effect of temperature on germination characteristics of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains.
a Parameter estimates were obtained using the log-logistic model (Equation 1). b, relative slope around t 50; t 50, time (days) taken to achieve 50% of the maximum germination; d, maximum germination (%) at the end of the observation period.
Figure 5. Germination response of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains to different temperature treatments. Curves were generated using a three-parameter log-logistic model (Equation 1). Each curve represents germination response (cumulative proportion) at a given temperature (C) over time (days). Symbols on the curves are the observed means of 11 populations.
Time taken to achieve 50% of the maximum germination (t 50) by B. scoparia populations decreased with increasing temperatures, but did not differ between populations from northern versus southern regions at temperatures of 8 C or above based on the mixed-effects analysis (Figure 6). Similarly, the duration of germination (t 95–t 5) decreased slightly with temperatures above 4 C, but did not differ between populations from northern versus southern regions. Regardless of temperature treatment, B. scoparia populations from both regions completed their germination in less than 10 d. These results are consistent with previous findings that B. scoparia can germinate as soon as the minimum soil temperature rises above 3 or 4 C (Everitt et al. Reference Everitt, Alaniz and Lee1983; Nussbaum et al. Reference Nussbaum, Wiese, Crutchfield, Chenault and Lavake1985). Regional differences in emergence patterns of B. scoparia populations are likely to occur at low temperatures. For example, at 4 C, populations from the northern region took 5 h fewer to achieve 50% germination than populations from the southern region of the U.S. Great Plains (Table 4; Figure 7). Although this is not a large difference, it is likely to increase with further reductions in temperatures below 4 C, which needs to be investigated.
Figure 6. Effect of temperature on the germination rate (left) and the germination duration (right) of Bassia scoparia populations collected in 2016 from four sites across a three-state region (MT, WY, NE) in the U.S. Great Plains. Response lines were fit with a mixed-effects model. The Huntley and Powell sites were included in the northern region, whereas the Lingle and Scottsbluff sites were included in the southern region. Colored round symbols along the lines represent parameter values of individual populations. Shaded gray bands along the lines represent 95% confidence intervals.
Table 4. Effect of low temperature (4 C) on the rate and maximum germination of Bassia scoparia populations collected in 2016 across a north–south transect (MT, WY, NE) in the U.S. Great Plains.
a Parameter estimates were obtained using the log-logistic model (Equation 1).
b, relative slope around t 50; t 10, t 50, and t 90, time (days) taken to achieve 10%, 50%, and 90% of the maximum germination, respectively; d, maximum germination (%) at the end of the observation period.
b Huntley, MT, and Powell, WY, sites were included in the northern region; Lingle, WY, and Scottsbluff, NE, sites were included in the southern region.
Figure 7. Effect of low temperature (4 C) on the rate and cumulative germination of Bassia scoparia populations collected in 2016 across a north–south transect (MT, WY, NE) in the U.S. Great Plains. Curves were generated using a three-parameter log-logistic model (Equation 1). Each curve represents germination response over time (days). Symbols on the curves are the observed means of 22 populations. The Huntley and Powell sites were included in the northern region, whereas the Lingle and Scottsbluff sites were included in the southern region.
Management Implications
The results of this research indicate a wide range of optimum temperatures and osmotic potentials requirements for B. scoparia germination. Mengistu and Messersmith (Reference Mengistu and Messersmith2002) previously reported higher levels of genetic diversity within a B. scoparia population than across populations, and this may contribute to the lack of differences in response to temperature or water potential attributable to the northern versus southern location. The ability of B. scoparia to germinate in high proportions in a short period of time at low temperatures reinforces its competitive advantage over other weed species and crops. For example, B. scoparia achieved 80% of its maximum emergence at the time when other weed species common to the Northern Great Plains started emerging (Bullied et al. Reference Bullied, Marginet and Van Acker2003; Schwinghamer and Van Acker Reference Schwinghamer and Van Acker2008). In these studies, a minimum of 530 GDD were required to achieve 50% emergence for wild oat (Avena fatua L.), wild buckwheat (Polygonum convolvulus L.), field pennycress (Thlaspi arvense L.), common lambsquarters (Chenopodium album L.), and redroot pigweed (Amaranthus retroflexus L.) compared with 175 GDD for B. scoparia. The ranges of optimum temperatures and osmotic potentials required for germination in the current study were consistent across geographic regions. However, B. scoparia from the southern region had lower germination than B. scoparia from the northern region, especially at low temperatures tested, indicating the need for site-specific management practices for this early-emerging weed.
In osmotic potential experiments, 25% of B. scoparia seeds germinated at an osmotic potential of −1.6 MPa, an osmotic potential at which some weeds and crop species are unable to germinate (Guillemin et al. Reference Guillemin, Gardarin, Granger, Reibel, Munier-Jolain and Colbach2013; Hoveland and Buchanan Reference Hoveland and Buchanan1973). For example, downy brome (Bromus tectorum L.), a problem weed in the U.S. Great Plains (Stougaard et al. Reference Stougaard, Mallory-Smith and Mickelson2004; Thill et al. Reference Thill, Beck and Callihan1984), did not germinate when osmotic potential dropped below −1.5 MPa (Thill et al. Reference Thill, Schirman and Appleby1979). Similarly, wheat (Triticum aestivum L.), an important crop in this region, did not germinate at an osmotic potential of −1.5 MPa (Singh et al. Reference Singh, Abdou, Flury, Schillinger and Knappenberger2013). The ability of B. scoparia to germinate at such a low osmotic potential may provide a competitive advantage over other species that are not present at the time of B. scoparia germination (Bullied et al. Reference Bullied, Marginet and Van Acker2003; Schwinghamer and Van Acker Reference Schwinghamer and Van Acker2008). Additionally, the efficacy of certain preemergence soil-residual herbicides for B. scoparia control was reduced linearly with decreasing osmotic potentials (Sebastian et al. Reference Sebastian, Nissen, Westra, Shaner and Butters2017). These results indicate that B. scoparia may become difficult to control in dry years (Teasdale et al. Reference Teasdale, Mangum, Radhakrishnan and Cavigelli2003) given the reported (Wienhold et al. Reference Wienhold, Vigil, Hendrickson and Derner2018) trends of frequent droughts in the U.S. Northern Great Plains.
One of the practical ways to deplete soil seedbanks of troublesome weed species is to identify and manipulate the environmental factors that control their germination and emergence (Schonbeck and Egley Reference Schonbeck and Egley1980). Models have been used to predict weed emergence in a specific region or across regions. However, these models often rely solely on GDD to predict weed emergence (Myers et al. Reference Myers, Curran, Vangessel, Calvin, Mortensen, Majek, Karsten and Roth2004). Use of a hydrothermal time, which includes both soil temperature and osmotic potential parameters can improve the accuracy of predicting emergence of weed species in the field (Bradford Reference Bradford2002; Forcella Reference Forcella1998; King and Oliver Reference King and Oliver1994). Therefore, parameter estimates generated from this study could be used to develop B. scoparia emergence models and predict emergence patterns across the three-state region using historical climate data. Knowledge of the timing and duration of B. scoparia emergence in a particular geographic location can then be used to modify crop practices and develop ecological strategies to manage the weed seedbank. For example, seedbanks of early-emerging populations of this weed can be exhausted using a stale seedbed approach before planting of crops in irrigated regions of the U.S. Great Plains. Similarly, the late-emerging populations can be suppressed using competitive crops such as wheat and barley (Hordeum vulgare L.), planted in the fall or early spring in this region (Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018a). In conclusion, this research indicates that B. scoparia has rapid germination under a wide range of temperatures and osmotic potentials, which should be exploited using ecologically based strategies for its to control.
Osmotic potential requirements for B. scoparia seed germination in the current study were determined using PEG solutions (Michel Reference Michel1983). Although this has been the most widely used method to create different water potentials in seed germination experiments, it may not simulate soil water potentials accurately (Camacho et al. Reference Camacho, Heitman, Gannon, Amoozegar and Leon2021). For instance, 18% of Palmer amaranth (Amaranthus palmeri S. Watson) seeds germinated at water potential of −1.2 MPa in PEG solution, compared with 67% in a silty loam soil at the same water potential (Camacho et al. Reference Camacho, Heitman, Gannon, Amoozegar and Leon2021). Therefore, development of germination predictive models for field use based on the parameters generated in this study would require additional considerations, such as soil texture.
Acknowledgments
This work was supported by the USDA National Institute of Food and Agriculture (grant no. 2016-70006-25831). We thank the MSU SARC, Huntley, MT for providing resources and support to conduct this study. Vipan Kumar provided helpful comments on an earlier draft of this article. No conflicts of interest have been declared.
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