Seed Propagation Methods for Ratibida columnifera (Nutt.) Wooton & Standl.

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Kaitlin A. HopkinsSchool of Agricultural Sciences, Sam Houston State University, 1019 Bowers Boulevard, Huntsville, TX 77340

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Michael A. ArnoldThe Gardens at TAMU, Texas A&M University, 2133 TAMU, College Station, TX 77843-2133

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Charles R. HallDepartment of Horticultural Sciences, Texas A&M University, 2133 TAMU, College Station, TX 77843-2133

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H. Brent PembertonTexas A&M Agricultural Research and Extension Center, P.O. Box 200, Overton, TX 75684

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Marco A. PalmaDepartment of Agricultural Economics, Texas A&M University, 2124 TAMU, College Station, TX 77843-2124

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Variation in floral characteristics and growth habits within the native range of the North American wildflower Ratibida columnifera (Nutt.) Wooton & Standl. suggests potential for breeding and selection efforts to develop improved cultivars for commercial and residential landscapes. Experiments in seed propagation were performed to enable perpetuation of unique germplasms. Overnight hydration, storage condition variations, stratification and scarification, and seed maturation effects were assessed to determine impacts on viability and percent germination. Overnight hydration had no impact on percent germination. Germplasm had a significant effect on germination for all remaining experiments. Seed maintained viability at the same rate through 18 months, when slight reductions were noted. Cold storage at 3 °C had no effect on viability or percent germination of dry seed compared with storage at 23 °C. All three germplasms exhibited increased percent germination with some stratification period, and declined significantly in percent germination with all acid scarification treatments. Experiments indicated that most germplasms benefit from between 30 to 60 days of cold, moist stratification. There was a significant interaction effect among germplasms, location on the inflorescences, and maturity stages for R. columnifera. Data suggest that seed should be harvested as close as possible to when natural dispersal would occur for optimum germination. The degree of improvement in viability and percent germination associated with harvesting at various developmental stages, seed pretreatments, and storage conditions suggests that to achieve germination success, pretreatments should be used for propagation of seed from mature inflorescences and that variation can be expected within different genotypes of this species.

Abstract

Variation in floral characteristics and growth habits within the native range of the North American wildflower Ratibida columnifera (Nutt.) Wooton & Standl. suggests potential for breeding and selection efforts to develop improved cultivars for commercial and residential landscapes. Experiments in seed propagation were performed to enable perpetuation of unique germplasms. Overnight hydration, storage condition variations, stratification and scarification, and seed maturation effects were assessed to determine impacts on viability and percent germination. Overnight hydration had no impact on percent germination. Germplasm had a significant effect on germination for all remaining experiments. Seed maintained viability at the same rate through 18 months, when slight reductions were noted. Cold storage at 3 °C had no effect on viability or percent germination of dry seed compared with storage at 23 °C. All three germplasms exhibited increased percent germination with some stratification period, and declined significantly in percent germination with all acid scarification treatments. Experiments indicated that most germplasms benefit from between 30 to 60 days of cold, moist stratification. There was a significant interaction effect among germplasms, location on the inflorescences, and maturity stages for R. columnifera. Data suggest that seed should be harvested as close as possible to when natural dispersal would occur for optimum germination. The degree of improvement in viability and percent germination associated with harvesting at various developmental stages, seed pretreatments, and storage conditions suggests that to achieve germination success, pretreatments should be used for propagation of seed from mature inflorescences and that variation can be expected within different genotypes of this species.

Ratibida columnifera is readily available as seed and is a common component of many wildflower seed mixes (U.S. Department of Agriculture Natural Resources Conservation Service, 2006). To develop novel germplasm from native sources into a marketable product, seed and vegetative collections must be obtained for breeding. It is not enough just to collect seed; one must be able to store and germinate the seed. Some species have simple needs for germination, whereas others require treatments to break physiological dormancy and/or quiescence. Available literature stresses the need for cold, moist stratification of R. columnifera seed for successful germination (Niu et al., 2012; Romo and Eddleman, 1995). Most studies stratify from 3 to 8 weeks, at varying temperatures. An optimum yield of 60% total germination was obtained by stratifying at 15 °C for 28 d (Romo and Eddleman, 1995). Studies have examined optimal temperatures for stratification of R. columnifera seeds, but there is a lack of literature defining the number of days that allow maximum percent germination. In one study, R. columnifera seeds were germinated successfully with cold, moist stratification for 8 weeks in trays of sterile germination mix (Middleton et al., 2015). Another study did not produce sufficient germination until seed were stratified at 5 °C for 3 to 6 weeks (Niu et al., 2012). The seeds of R. columnifera do not require light to germinate (Romo and Eddleman, 1995). Other factors that can affect germination and therefore future use of collected seed are storage conditions and duration. Some species can store for years at room temperature, whereas others begin to have lowered germination rates over time and struggle to maintain viability without cold storage (Hong and Ellis, 1996). Germplasms within a species can vary drastically, especially when there are geographic differences. Genetic variation among populations along a geographic gradient can suggest the presence of clinal variation (Weber and Schmid, 1998). In an experiment on Solidago altissma L. and Solidago gigantea Aiton there were major differences in phenology, indicating that the populations were distinct in their physiological requirements for floral initiation (Weber and Schmid, 1998). Climate differences along latitudinal, longitudinal, and altitudinal transects are one of the most important abiotic factors regarding species distribution, and can lead to adaptation under natural selective pressures (Monty and Mahy, 2009). Selection of genotypes adapted to the region in which they intend to be sold may be necessary to achieve a high-performance, valued consumer product. Influences occurring in the maternal environment after formation of the embryo, but before seed dispersal takes place, can affect seed quality (Valencia-Díaz and Montaña, 2005). This phenomenon is known as the maternal effect (Valencia-Díaz and Montaña, 2005). Characteristics of the seed that can be affected by maternal plant stress include seed fill and seed size. These characteristics correlate strongly with viability and germination rates (Valencia-Díaz and Montaña, 2005). Because of the acropetal floral development in R. columnifera, there is concern that seed maturation and viability varies along the conical inflorescence and among inflorescence maturities as described for Erigeron (Harris et al., 1991). For these reasons, several germplasms were grown in a common environment. Seed was harvested from trial beds in a nursery crop setting, in which many environmental stresses that a plant in nature would experience were drastically reduced.

The following experiments sought to establish protocols for optimum germination and storage conditions necessary for maintenance of Ratibida columnifera seeds. Objectives that were examined included: 1) does soaking seed influence germination, 2) will fresh seed germinate, or 3) are treatments such as stratification or scarification needed, 4) seed storage duration and conditions were examined for effects on germination, and 5) to determine the impacts of stage of seed maturity on germination.

Materials and Methods

General conditions.

Seeds collected from five open-pollinated genotypes (TX RC 8, TX RC 12, TX RC 30, TX RC 44, and TX RC 48) grown in trial plots (Somerville, TX; lat. 30.522288N, long. –96.429397W) were used for germination studies. The trial plots were in a full-sun location in the nursery in a native soil of silty clay loam (0–6 inches) and silt loam (6–80 inches) (U.S. Department of Agriculture Natural Resources Conservation Service, 2022). Midday photosynthetically active radiation (PAR) averaged 1638 µmol⋅m–2⋅s–1 photosynthetic photon flux density (PPFD) during midday (Fieldscout Quantum Foot-Candle Meter; Spectrum Technologies, Inc., Aurora, IL). These plants were irrigated on an as-needed basis and were fertigated to the leaching point weekly with 20–20–20 water-soluble fertilizer (Peters Professional 20–20–20 General Purpose; Everris, ICL, Dublin, OH). Because of the nonuniform maturation of inflorescences on R. columnifera, seed was harvested from fully dried seed heads when mature. Seeds were harvested in the late summer and were placed into a paper envelope and fully sealed. The seed heads remained in the envelopes until dry, and were then placed into sealed plastic bags and put into storage at 3 °C. Seed for germination experiments was used within 6 months of harvest (excluding storage condition and seed maturity experimentation).

To estimate whether germination requirements varied among a population of seedlings, seed from open-pollinated parent plants with varied phenotypic morphology was tested. These were used for the following germination experiments.

Before each germination experiment, a percent viability test using a triphenyltetrazolium chloride (TTC; Sigma-Aldrich, St. Louis, MO) staining method (Riebkes et al., 2015) was conducted on the seed from each maternal genotype. Soaking in 1% TTC solution for 48 h caused the embryo within the seed to stain red if viable (Riebkes et al., 2015). This seed viability test was performed on a subsample of 100 seeds from each treatment combination at the beginning of each germination trial. Germination data were normalized to the proportion of seed that was staining viable for each maternal genotype.

Effect of overnight hydration.

The first germination experiment examined effects of soaking seed in water before germination (Dec. 2018). Nine hundred seeds consisting of 300 seeds for each of three genotypes (TX RC 30, TX RC 44, and TX RC 48) were soaked overnight in 50 mL double-distilled water and then surface-dried. Nine hundred additional seeds remained dry until placed into petri dishes (polystyrene disposable sterile, 100 × 15 mm, catalog no. 25384-302; VWR International, Radnor, PA). One hundred seeds were placed into each sterile petri dish on moist filter paper (9 cm, Qual. 415, catalog no. 28320-041; VWR International), for a total of 18 petri dishes. There were three replicates of each treatment. There were nine dishes with 100 soaked seeds in each, and nine dishes with 100 unsoaked seeds in each, for a total of 1800 seeds tested. Seeds were placed under grow lights, with 16-h on/8-h off photoperiod increments (60 W) in laboratory conditions (23.3 °C). PAR under the grow lights under laboratory conditions averaged 39.1 µmol⋅m–2⋅s–1 PPFD. Seed was checked for germination every 48 h for 4 weeks after treatment initiation. Germination was defined as the emergence of the radicle (>1 mm).

A completely random design was used for this experiment. Factorial combinations of three genotypes (TX RC 30, TX RC 44, and TX RC 48) and two hydration treatments (control and overnight hydration). Analysis of variance and Tukey’s honestly significant difference (hsd) were used for the interactions among treatments and genotypes, with P ≤ 0.05 for significance, using JMP Pro 15 for continuous variables.

Effects of storage conditions on viability.

To examine optimal storage temperatures for maintaining viability, a cold-storage vs. room-temperature storage experiment was conducted. This experiment was replicated every 6 months to express the maintenance or degradation of viability, with continuation until depletion of available seeds after 18 months (began Dec. 2018). Seed was harvested in late Summer 2018 (August), was placed into a paper envelope, and was fully sealed. The seed heads remained in the envelopes until dry, and were then placed into sealed plastic bags and put into storage at either 23 °C or 3 °C. Three genotypes were used (TX RC 30, TX RC 44, and TX RC 48), at two different storage temperatures, replicated three times for a total of 1800 seeds. There were 300 seeds in each treatment combination at storage temperatures of 23 °C and 3 °C. Seeds were stored away from light and were kept dry in sealed, semipermeable plastic bags. At the predetermined storage length, seeds were placed into sterile petri dishes (VWR International) on filter paper (VWR International) moistened with double-distilled water. Germination conditions and data collection were as described for the hydration experiment. Three replicates of each genotype were placed in culture conditions in a completely random design. Analysis of variance, a generalized linear model, and Tukey’s hsd were used for the interactions among treatments and genotypes, with P ≤ 0.05 for significance, using JMP Pro 15 for continuous variables.

Effects of scarification and cold stratification pretreatments.

Cold stratification and scarification were examined in the same experiment (May 2019). Treatments included control (no stratification and no scarification), three moist stratification durations (30, 60, and 90 d at 3 °C stratification), and three sulfuric acid (51%, Macron Fine Chemicals, Center Valley, PA) scarification durations (5, 10, or 15 min acid scarification). Seed was harvested in Aug. 2018 and placed in semipermeable bags at 3 °C. Sufficient seeds from storage were placed into stratification at 30, 60, and 90 d before germination tests to allow scarification and stratification germination effects to be tested simultaneously with the control and scarification treatments. There were 300 seeds per treatment. Seeds were placed into sterile petri dishes on moist filter paper. Seeds were checked for germination every 48 h for 4 weeks after planting. Three replicates of 100 seeds per petri dish for each stratification and scarification treatment were arranged in a completely randomized design. Analysis of variance and Tukey’s hsd were used for the interactions among treatments and genotypes, with P ≤ 0.05 for significance, using JMP Pro 15 for continuous variables.

Effects of seed maturity on germination.

Requirements for seed maturity were examined by harvesting the columnar inflorescences at three stages of development: immature in appearance (green with ray flowers still attached), mid-age (ray flowers senesced, just beginning to change from green to gray-brown), and fully mature (ray flowers senesced, inflorescence brown to black-brown) but not shattered. Each seed head was divided into thirds and was named based on the region of the inflorescence: basal, middle, and apical. Seeds were aggregated for inflorescences of a given genotype and maturity stage. This was repeated with three different genotypes (TX RC 8, TX RC 12, and TX RC 30). Three hundred seed subsamples from each maturity stage × inflorescence segment × genotype were stained with TTC to estimate viability. This experiment took place in June 2020. Seed was harvested in June 2020 over the course of several weeks because of the nonuniform maturation of seed heads. Harvested seed was allowed to air-dry in paper envelopes, and was then transferred into semipermeable plastic bags for dry storage at 3 °C. Three petri dishes each contained 100 seeds that were placed on moistened filter paper for 24 h and placed under grow lights in laboratory conditions. Seeds were checked for germination every 48 h for 4 weeks after planting. Germination was defined as the emergence of the radicle (>1 mm). Factorial combinations of treatments were used in this completely randomized design. Analysis of variance and Tukey’s hsd were used for the interactions among treatments and genotypes, with P ≤ 0.05 for significance, using JMP Pro 15 for continuous variables.

Results and Discussion

Effect of overnight hydration.

Application of overnight hydration did not have a significant effect (P ≤ 0.05) on percent germination for the interaction among germplasm × soaking, nor for either of the main effects of soaking or germplasm (data not presented). This suggests that soaking R. columnifera seeds in double-distilled water before planting is not required.

Effects of storage conditions on viability.

No significant interactions among germplasm, duration of storage, or storage temperature were found. The main effect of storage temperature was also not significant (P ≤ 0.05). However, germplasm had a significant main effect on percent germination (Table 1). TX RC 30 had the highest percent germination at 82.5%, followed by TX RC 44 at 74.5%, with the lowest percent germination being TX RC 48 at 68.2%. All three germplasms differed from each other statistically (P ≤ 0.05). Storage duration also had a significant main effect on germination. The data show no significant difference in percent germination up to 12 months of storage (Table 1). By 18 months of storage, there was a slight but statistically significant reduction in percent germination, resulting in about a 10% loss in viability (Table 1). This minimal reduction in viability after 18 months suggests that seed can potentially be stored for longer periods if needed.

Table 1.

Effect of seed storage temperature and duration on percent germination of Ratibida columnifera for germplasms TX RC 30, TX RC 44, and TX RC 48.

Table 1.

TX RC 30 had the highest percent germination of the three selections, followed by TX RC 44, and then TX RC 48 (Table 1). This indicates that the ability to retain viability over time can depend on germplasm selection. Seed longevity for R. columnifera as shown in our study is in concurrence with the observation that seed longevity can vary among accessions within a species because of differences in genotype and provenance (Hong and Ellis, 1996). In addition to genetic influences, differences in germination among accessions can be the result of the cumulative effect of the environment during seed maturation, harvesting, drying, the time of seed harvest, duration of drying, and the period before the seed is placed in storage (Hong and Ellis, 1996). Each of the germplasms in our experiment had maximum germination percentages well over the 60% observed in the study by Romo and Eddleman (1995), in which their seed was collected from the University of Saskatchewan’s Matador Research Station, a native environment. Our seed was collected from well-fertilized and irrigated stock in a nursery setting. The differences in cultural protocols of the stock plant could be why our seed had greater germination percentages overall. This may also be why our seeds were able to maintain a percent germination of 71.2% after 18 months of storage regardless of the temperature of storage. Storage duration also had a significant effect on germination (Table 1). There was no significant difference in germination for the first year of storage. It was not until 18 months that we began to observe statistically significant reductions in germination. This implies that seed may not need to be subjected to cold storage to retain viability in the short term. Future studies should be performed to determine if temperature has significant effects on retaining viability in long-term, multiyear storage durations and what impact freezing seeds may have on viability.

Effects of scarification and cold stratification pretreatments.

Application of stratification affected significantly the percent germination of the three R. columnifera genotypes (Table 2). There was a significant interaction effect among the three genotypes and the pretreatments (Table 2). Thus, the effect that the seed pretreatment had on percent germination depended on which genotype was examined. Stratification proved to be an important factor in germination success, as the literature suggests (Romo and Eddleman, 1995). TX RC 30 had a significant decrease in percent germination compared with the control when exposed to a 90-d stratification. TX RC 44 and TX RC 48 had no statistical evidence of enhanced germination with any of the stratification treatments. Increased stratification to 90 d either reduced percent germination or had no benefit (Table 2). Sulfuric acid treatments of 5 to 15 min had substantial deleterious effects on germination of all three genotypes (data not presented).

Table 2.

Effect of 3 °C stratification on percent germination of Ratibida columnifera germplasms TX RC 30, TX RC 44, and TX RC 48.

Table 2.

None of the three genotypes exhibited a statistically significant increase in percent germination with stratification (Table 2). All three genotypes exhibited statistically significant negative effects from sulfuric acid scarification in this experiment. Our results indicate that cold stratification of up to 90 d failed to improve germination percentage, and sulfuric acid scarification was not beneficial (data not presented, no significant effects). Although this is true for these regional selections, stratification may be necessary for selections from other geographic locations. For this reason, we suggest a 30- to 60-d cold, moist stratification screening for R. columnifera selections. Future studies could examine additional stratifications times between 0 and 60 d to refine recommendations further for individual maternal genotypes. Of perhaps greater need is the investigation of potential variation in chilling requirements for germination across the large geographic range of R. columnifera. Although not promising from the results of our study, sulfuric acid scarification using a more dilute solution or shorter duration could be tested.

Effects of seed maturity on germination.

Developmental stage of the inflorescence and seed location on the inflorescence had a significant effect on percent germination (Table 3). There was a statistically significant two-way interaction for germination percentages among germplasms and developmental stage, and germplasms and region on inflorescence (Table 3). Mid-age and mature developmental stages had increased percent germination for all three germplasms. TX RC 12 had the lowest percent germination out of the three germplasms, except in reference to the mature inflorescences. Basal and middle portions of the inflorescence produced greater germination percentages than the apical portions, except for TX RC 30. Basal and middle portions of the inflorescence produced greater germination percentages than the apical portions, except for TX RC 30. In TX RC 12, the immature and mid-age developmental stages were not statistically different from one another. The mature developmental stage of TX RC 12 germinated more readily than the less mature stages and was comparable in germination percentages to the greater levels observed for TX RC 30 on mid-age and mature sections. There was no statistical evidence of differences in percent germination when looking at location on inflorescences of TX RC 12. TX RC 30 had increased percent germination in the mid-age and mature developmental stages, with the immature stage being significantly less, but greater than similar maturity stages on TX RC 12 or TX RC 8 (Table 3). The basal and mid-inflorescence seed locations outperformed the apical portions of the inflorescences for most stages of development. This was most pronounced on immature inflorescences, which is consistent with the apical portions containing largely immature seeds, whereas those in basal portions of the inflorescence would be more mature based on earlier chronological flowering as the inflorescence matured. Thus, for commercial seed production, harvest should be from fully mature inflorescences to encourage maximum and uniform germination. If collection cannot wait for full inflorescence maturity, then only the basal portions of the inflorescence should be retained.

Table 3.

Effect of inflorescence developmental stage and location on the inflorescence on percent germination of Ratibida columnifera germplasms TX RC 8, TX RC 12, and TX RC 30.

Table 3.

Developmental stage of the inflorescence and seed location on the inflorescence had a significant effect on percent germination (Table 3). There was a statistically significant interaction effect between the germplasm and developmental stage, and germplasms and region on the inflorescence. In all germplasms, mid-age to mature inflorescences had an increase in percent germination when compared with the immature inflorescences. Because of this, it is recommended to plan seed harvests for when a majority of the inflorescences have dropped their ray petals and turned brown. Two of the three germplasms had significantly higher percent germination when looking at the basal and middle portions of the seed head, with the third germplasm having no differences among locations. This makes sense, because R. columnifera disk florets open first on the basal portion of the inflorescence, and finish opening at the apical portion. This would mean that seed at the basal portion of the inflorescence has had a longer time to mature than apical seed. This phenomena happens in other Asteraceae species as well, such as Artemisia annua L., where among genotypes there were differences ranging from those having capitula with open ray and disk florets, to those with capitula that are tightly enclosed within involucral bracts (Wetzstein et al., 2014). For this reason, knowledge of the timing of flower development in key genotypes, as well as the relative developmental timing of ray and disk florets, are of critical importance for successful crossing (Wetzstein et al., 2014). All of these data imply that R. columnifera is consistent with the more generalized recommendation for wildflowers that seed should be harvested near a stage when they would disperse naturally for optimal germination yields (Hong and Ellis, 1996). The impact of genotype on total germination percentage was evident in our experiment (Table 3) as well as those discussed earlier (Tables 1 and 2). In our experiment, TX RC 30 had the greatest germination percentage on middle and fully mature seeds just as it had in earlier studies, whereas TX RC 12 had comparable germination only on basal portions of fully mature inflorescences, and TX RC 8 had much reduced germination even on basal portions of fully mature inflorescences.

Conclusion

This study provides insight into pretreatments, storage conditions, and harvest protocols that have proved valuable to optimizing the percent germination of R. columnifera seeds. Much of the data gathered from these experiments yielded higher percentages of germination than previous literature stated (Romo and Eddleman, 1995). This may be because of conditions that the stock plants from which seeds were collected were grown. Added stresses in the development of the seed can influence the longevity of the mature seed (Harrington, 1972). These stresses may lead to the maturity of a seed without it being fully developed or while it still lacks essentials (Harrington, 1972). Stresses to the mother plant that can reduce seed longevity may also include water stress, temperature stress, high salinity, disease, insect infestation, and frost damage (Harrington, 1972). The stock plants from which seed was gathered for our experiments were well fertilized, irrigated, and protected from many of the stresses that an R. columnifera plant may face in nature. This may be why our germination percentages outperformed those in previous literature. The data also reflected the general sentiment that, in many species, there can be differences in germination responses in different accessions because of germplasm and provenance differences (Hong and Ellis, 1996). Seed pretreatments and storage conditions also reflected the ideas brought forth in the literature (Hong and Ellis, 1996). Improvement in percent germination for R. columnifera seeds can be made by using 30- to 60-d cold, moist stratification pretreatments and harvesting seed in the correct developmental stages, which can vary by genotype. Sulfuric acid scarification pretreatment and extended stratification caused negative effects on percent germination, and therefore are not recommended. Hydration of seeds before planting had no significant impact on percent germination. Percent germination is not changed significantly by storage temperatures in the short term, but may be in longer terms. Germplasm effects were found to be significant throughout the series of experiments. Germplasm accession location could prove to be an important factor in how the plants respond to physiological stresses (Weber and Schmid, 1998). Future studies are needed to fine-tune propagation protocols, such as investigating frozen seed storage, optimizing germination temperatures, optimizing germination substrates, determining provenance impacts on germination requirements and seed storage protocols, and determining whether any residual impacts of germination treatments correlate to the vigor of seedling growth beyond germination.

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Contributor Notes

This work was supported in part by funds from Texas A&M AgriLife Research and the U.S. Department of Agriculture National Institute of Food and Agriculture. This study was included as part of dissertation written in partial fulfillment of the requirements for the Ph.D. degree by K.A. Hopkins. This work was supported in part by funds from Texas A&M AgriLife Research and the USDA National Institute of Food and Agriculture. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the authors, Texas A&M University, or Texas A&M AgriLife Research and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

K.A.H. is Assistant Professor.

M.A.A. is Professor and Director of The Gardens at TAMU.

C.R.H. is Professor and Ellison Chair.

H.B.P. is Professor and Regents Fellow.

M.A.P. is Professor.

K.A.H. is the corresponding author. E-mail: kah147@shsu.edu.

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  • Harrington, J.F. 1972 Seed storage and longevity Seed Biol. 3 145 245

  • Harris, E.M., Tucker, S.C. & Urbatsch, L.E. 1991 Floral initiation and early development in Erigeron philadelphicus (Asteraceae) Amer. J. Bot. 78 1 108 121 https://doi.org/10.1002/j.1537-2197.1991.tb12577.x

    • Search Google Scholar
    • Export Citation
  • Hong, T.D. & Ellis, R.H. 1996 A protocol to determine seed storage behaviour 2 62 Engels, J.M.M. & Toll, J. IPGRI technical bulletin no. 1. International Plant Genetic Resources Institute Rome, Italy

    • Search Google Scholar
    • Export Citation
  • Middleton, E.L., Richardson, S., Koziol, L., Palmer, C.E., Yermakov, Z., Henning, J.A., Schultz, P.A. & Bever, J.D. 2015 Locally adapted arbuscular mycorrhizal fungi improve vigor and resistance to herbivory of native prairie plant species Ecosphere 6 12 1 16 https://doi.org/10.1890/ES15-00152.1

    • Search Google Scholar
    • Export Citation
  • Monty, A. & Mahy, G. 2009 Clinal differentiation during invasion: Senecio inaequidens (Asteraceae) along altitudinal gradients in Europe Oecologia 159 2 305 315 https://doi.org/10.1007/s00442-008-1228-2

    • Search Google Scholar
    • Export Citation
  • Niu, G., Rodriguez, D.S. & McKenney, C. 2012 Response of selected wildflower species to saline water irrigation HortScience 47 1351 1355 https://doi.org/10.21273/HORTSCI.47.9.1351

    • Search Google Scholar
    • Export Citation
  • Riebkes, J.L., Barak, R.S. & Kramer, A.T. 2015 Evaluating seed viability in prairie forbs: A test of three methods Native Plants J. 16 2 96 106 https://doi.org/10.3368/npj.16.2.96

    • Search Google Scholar
    • Export Citation
  • Romo, J.T. & Eddleman, L.E. 1995 Use of degree-days in multiple-temperature experiments J. Range Mgt. 48 5 410 416 https://doi.org/10.2307/4002244

    • Search Google Scholar
    • Export Citation
  • U.S. Department of Agriculture Natural Resources Conservation Service 2006 Plants profile for Ratibida columnifera (upright prairie coneflower) 29 Aug. 2020. <https://plants.usda.gov/core/profile?symbol=RACO3>

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