Abstract
Seed germination in an oxygenated water bath has been shown to be effective at reducing or eliminating dormancy barriers of some native western North American plant species; however, its full utility is not known. Particularly lacking is information on the effects of oxygenated water treatments on native forbs with physiological dormancy. Curlycup gumweed (Grindelia squarrosa (Pursh) Dunal [Asteraceae]) is a commonly occurring, early seral forb that is native to arid and semi-arid habitats in the Intermountain West and is used in wildlife habitat and other rangeland planting practices. We compared germination rates and final germination percentage of curlycup gumweed collections from 5 locations in the Intermountain Region exposed to 4 germination treatments: 1) oxygenated water bath; 2) non-oxygenated water bath; 3) 90-d cold-moist stratification; and 4) a non-treated control. Germination rates and final germination percentages after 35 d in oxygenated water bath treatments were equal to or greater than those from the 90-d stratification for all 5 accessions. Seed priming in oxygenated water treatments appears to be feasible for quickly and uniformly germinating seed in a laboratory environment with the possibility of expanding the technique for field settings.
Curlycup gumweed (Grindelia squarrosa) is a short-lived perennial forb native to western North America with potential for use in conservation and pollinator-friendly plantings in arid rangelands. High levels of dormancy, however, are an obstacle to uniform field and greenhouse germination.
Seed enhancement technologies have been viewed as a possibility to improve seedling germination success in rangeland seedings in the Intermountain Western sagebrush biome (Madsen and others 2016; Madsen and others 2018; Anderson and others 2019). Often, fall dormant seedings result in unintentional early germination because of unexpected warm spells in late fall (James and others 2011), resulting in high seedling mortality from frost damage. Seed priming could be used to initiate quick germination for planting during periods of suitable moisture and temperatures while allowing adequate time for seedlings to achieve the minimum required growth to survive through winter. Seed priming may be especially useful in post-fire situations where immediate establishment is necessary to prevent erosion during winter and the following spring (Beyers 2004; USDA NRCS 2013). Seed dormancy mechanisms, common among forb species in the Intermountain West, complicate the issue, as seeds require extended stratification periods prior to germination. Determining seed germination requirements and identifying dormancy traits can provide valuable information that leads to improved field establishment (Kildisheva and others 2018). Additionally, treatments to quickly and easily overcome seed dormancy mechanisms can facilitate laboratory germination and greenhouse propagation where uniform germination is desirable.
Researchers have shown that some species with significant stratification periods respond positively to submersion in oxygenated water (Young and Young 1986; Liu and others 2012; Tilley 2013). For example, Tilley (2013) showed that oxygenated water treatments were able to overcome physiological dormancy mechanisms and increase final germination percentages in Nebraska sedge (Carex nebrascensis Dewey [Cyperaceae]), a species of stream margins and wetlands, yet it is unclear how feasible this treatment is for use on upland forbs that may be maladapted to periods of saturation. One species that may serve as a good test subject for this idea is curlycup gumweed (Grindelia squarrosa (Pursh) Dunal [Asteraceae]).
Curlycup gumweed is a North American native, short-lived perennial forb adapted to disturbed upland areas throughout North America except for the southeastern states. It is currently under investigation for use in upland restoration plantings in the Great Basin where it occurs in disturbed areas and roadsides, competing with introduced Eurasian annual grasses and forbs (Welsh and others 2003; Tilley and Pickett 2019). Curlycup gumweed is highly attractive to native bees and has potential for inclusion in pollinator-friendly habitat plantings (Figure 1). Data from the USDA Agriculture Research Station Bee Research Laboratory in Logan, Utah, indicate visitations to curlycup gumweed by species from more than 40 genera of bees (Ikerd 2016), while Lee-Mäder and others (2016) stated it is mostly visited by leafcutter bees (Megachile spp. [Hymenoptera: Megachilidae]), long-horned bees (Melissodes spp. [Hymenoptera: Apidae]), and green metallic sweat bees (Agapostemon spp. [Hymenoptera: Halictidae]). Its drought tolerance and late-season flowering make it especially promising for use in the Conservation Reserve Program (CRP) and other range plantings in the arid West where late-blooming forbs are limited (Ogle and others 2011).
Curlycup gumweed is visited in late summer by numerous native and introduced insects for pollen and nectar.
Curlycup gumweed may also have value as a food source for at-risk upland birds. Peterson (1970) documented curlycup gumweed as a food source for Sage-grouse (Centrocercus urophasianus Bonaparte [Galliformes: Phasianidae]). In that study, 28% of monitored Sage-grouse chicks in central Montana between 5 and 8 wk of age were reported eating curlycup gumweed, which made up 3% of the crop contents by volume. Peterson (1970) also found it to be used by 39% of 9- to 12-wk-old chicks, making up 4% of crop contents of birds of that age group.
Numerous reports indicate that curlycup gumweed exhibits physiological dormancy and requires 60 to 90 d of cold-moist stratification for adequate germination (Nuzzo 1978; Baskin and Baskin 2002; Luna 2008). Neupane and others (2016) observed uneven field germination of spring-seeded curlycup gumweed in studies for biofuel production. The authors reported that seed germination was accomplished only with extensive irrigation, causing field germination attempts to be abandoned and replaced with greenhouse-grown materials. However, fall-dormant seeding of curlycup gumweed in field trials under arid, non-irrigated conditions at Aberdeen, Idaho, in which seed could naturally stratify over winter, resulted in essentially 100% establishment of 25 accessions originating from across the Great Basin (Tilley and Pickett 2019).
MATERIALS AND METHODS
We tested germination of 5 accessions of curlycup gumweed, chosen to represent a broad spatial and ecological distribution within the semi-arid West (Figure 2), along with 4 seed treatments to determine if germination rates could be improved without the extended stratification period. Seed collections were made in San Juan County, Utah (UT1), Box Elder County, Utah (UT2), Park County, Wyoming (WY), Humboldt County, Nevada (NV1), and Elko County, Nevada (NV2) (Table 1). Treatments included: 1) oxygenated water bath (air); 2) non-aerated water bath (H2O); 3) 90-d cold-moist stratification (90-d); and 4) non-treated control (control). All seed was of the current season (<1 y old). Tetrazolium chloride (TZ) tests were performed by an independent seed laboratory on each collection to determine potential viability.
Collection site locations of the 5 tested accessions of curlycup gumweed.
Collection site location, elevation, soil, and associated plant community information for the 5 evaluated accessions of curlycup gumweed.
For the oxygenated water treatment, we placed 100 seeds in a fine mesh bag and submerged them into a Mason jar filled with distilled water and an aquarium bubbler for aeration. The non-aerated water treatment was the same, minus the bubbler. Aeration was performed using a Profile 1500 aquarium air pump fitted with a 2.5 cm (1 in) bubbling air stone (Figure 3). The cold-moist stratification was done by placing seeds in a fine mesh bag surrounded by moist potting soil in a cooler set for 1.1 °C (34 °F) for 90 d. For stratification and non-treated control treatments, 100 seeds were placed on blotter paper moistened with 5 ml distilled water in 90 mm Petri dishes, and the dishes were put in resealable plastic bags to retain moisture (Figure 4). All were placed in a Hoffman growth chamber (Hoffman Manufacturing, Corvallis, Oregon) with a 12-h light/dark cycle with 22 °C (72 °F) day and 15 °C (59 °F) night temperatures. Germination was recorded at 7, 14, 21, 28, and 35 d after initiation (DAI). We considered plants to have germinated if they were observed to have an emerging root or shoot greater than 2 mm (0.08 in) in length.
Curlycup gumweed seed germinating under oxygenated water conditions. The mesh bag used in the experimentation has been removed to allow viewing.
Curlycup gumweed seed germinated following 90-d cold-moist stratification and the non-treated control.
Germination rate was determined using the method described by Maguire (1962). The number of seedlings obtained at each counting (7, 14, 21, 28, and 35 DAI) was divided by the number of days after planting, and the values obtained at each count were summed at the end of the test as follows:
where N = the number of germinated seeds at each measurement, d = the number of days after initiation when a measurement is taken, and x is the total number of measurements taken.
Each treatment was replicated 4 times in a randomized complete block design with each shelf of the growth chamber serving as a block. Final germination at 35 DAI and germination rates were analyzed with the analysis of variance procedure after the data were determined to be normally distributed using the Shapiro-Wilk normality test with Statistix 10 (Analytical Software, Tallahassee, Florida). Means were separated using the least significant difference test at P < 0.05.
RESULTS
Germination Rates
Significant differences in germination rates were observed between treatments of each accession (NV1 P = 0.0000, NV2 P = 0.0001, UT1 P = 0.0000, UT2 P = 0.0000, WY P = 0.0000). Germination rates of the air treatment were significantly greater than the 90-d stratification treatment for NV1 and UT2 accessions; the air treatments resulted in as much as 2 times higher germination rates (NV1). Germination rates for the air treatment and 90-d stratification treatment were greater than the H2O and control treatments for all accessions and ranged from 3 to 10 times faster than H2O soaking treatments and 2 to 13 times faster than the non-treated control of all 5 accessions (Table 2). The lowest germination rates were obtained from the H2O treatment in 4 of 5 accessions, while the non-treated control produced the lowest germination rate from the Box Elder County, Utah, accession (UT2).
Germination rates of 5 curlycup gumweed accessions using 4 germination treatments: 90-d cold-moist stratification (90-d), oxygenated water bath (Air), non-oxygenated water bath (H2O), and non-treated control (control).
Germination over Time
Germination responses to the H2O soaking treatment were very similar to the non-treated control, as both treatments resulted in low germination rates and final germination percentages (Figure 5). Oxygenated treatments of the 5 accessions produced significantly faster germination rates and final germination after 35 d than did the control and H2O. We first observed radicle emergence from the air treatment after only 24 h, and approximately 50 to 80% of the final germinants had emerged from the oxygenated treatments by 7 DAI. After 7 DAI, germination leveled to a much slower rate, and seeds continued to germinate but more sporadically through day 35. The 90-d cold-moist stratification treatment resulted in a quick burst of early germination, closely in line with the oxygenated treatment; however, after the initial germination very few additional germinants emerged, resulting in significantly lower final germination percentages.
Graphs indicate germination over time from 0 to 35 DAI of 5 curlycup gumweed accessions from semi-arid sites in the western US following 1 of 4 seed treatments: 1) oxygenated water bath (air); 2) non-aerated water bath (H2O); 3) 90-d cold-moist stratification (90-d); and 4) non-treated control (control). Error bars represent ± 1 standard error.
Final Germination
Laboratory TZ tests indicated viability of the 5 accessions ranging from 96% to 98%. Final germination percentages under our 4 treatments ranged from 5% to 72%, and significant differences were observed among all accessions (NV1 P = 0.0000, NV2 P = 0.0008, UT1 P = 0.0000, UT2 P = 0.0000, WY P = 0.0000) (Table 3). Seed subjected to the air treatment had significantly greater final germination percentages than those from all other treatments for all 5 accessions. Final germination percentages of the air treatment ranged from 1.3 to 2.4 times greater than those of the 90-d stratification, 2 to 7 times greater than those of the H2O soaking treatment, and 2 to 10 times greater than those in the non-treated control group.
Percentage of germinated seed after 35 d following 4 germination treatments: 90-d cold-moist stratification (90-d), oxygenated water bath (Air), non-oxygenated water bath (H2O), and non-treated control (control).
DISCUSSION
Although not adapted to flooded or saturated water conditions, curlycup gumweed collections from multiple locations in the Great Basin showed an improved germination response from soaking in oxygenated water compared to both the non-stratified control and the water-soaking treatments. Improvements were in both germination rates and final germination percentage. Faster germination rates may be useful in field plantings by quickly capturing available moisture and establishing an adequate taproot before the next dry period, resulting in greater final establishment. Treatment in oxygenated water significantly increased germination rates and percentages over the 90 d of cold-moist stratification, thus eliminating the need for extensive pretreatments to achieve desired germination. The oxygenated water treatment, however, failed to achieve full germination potential as determined from TZ tests after 35 d in any of the 5 tested accessions, indicating that dormancy was not fully overcome for at least some portion of the seed.
Screening of these techniques on additional upland forbs is needed to ascertain the full extent of its utility. Using oxygenated water treatments to overcome physiological dormancy may have application in laboratory and field settings. The techniques used in this experiment do not require specialized equipment or training and can be utilized by non-research-oriented restoration practitioners and technicians. Oxygenated baths could be a useful method to produce germinants for greenhouse production of certain species, especially in situations where production of plants of a similar age is desirable. Furthermore, field establishment via hydroseeding of seed primed in oxygenated water treatments could be a quick means of establishing plants (Figure 6), when waiting for natural breaking of dormancy mechanisms, such as after wildfire, would be prohibitive (Tilley and St John 2013). Variations in priming requirements of conservation plant species need to be better understood for this practice to be successful. For example, seed from this experiment appeared well primed in approximately 24 h, whereas seed of Nebraska sedge was optimally primed after 6 to 8 d (Tilley 2010). Further investigation into seed priming and development of priming protocols on an individual species basis would seem promising.
Primed curlycup gumweed seed from the experiment was hydroseeded by pouring the seed onto greenhouse flats of soil. More extended use of this technique may be feasible in rangeland seedings under the right conditions.
ACKNOWLEDGMENTS
We thank Heidi Anderson of Yellowstone National Park for providing the seed collection from Wyoming.
Footnotes
Photos by Derek Tilley
This article was prepared by a U.S.government employee as part of the employee’s official duties and is in the public domain in the United States.
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