Seed Dormancy and Germination Ecology of Calycanthus
floridus L., a Species with Threatened Status in Kentucky
Christopher A. Adams, Olamide C. Adejumo, Moondil Jahan, and
Kevin W. Montgomery
Southeastern Naturalist, Volume 16, Issue 4 (2017): 488–502
Full-text pdf (Accessible only to subscribers.To subscribe click here.)
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
488
2017 SOUTHEASTERN NATURALIST 16(4):488–502
Seed Dormancy and Germination Ecology of Calycanthus
floridus L., a Species with Threatened Status in Kentucky
Christopher A. Adams1,*, Olamide C. Adejumo1, Moondil Jahan1, and
Kevin W. Montgomery1
Abstract - The seed germination ecology of Calycanthus floridus (Eastern Sweetshrub)
has not been formally investigated. The purposes of this study were to determine the type
of seed dormancy found in the species, the most effective method of breaking this dormancy,
and the environmental conditions producing maximum germination. We employed
a variety of standard treatments to determine the specific type(s) of dormancy present, as
well as treatments to determine effective dormancy-breaking mechanisms. We determined
that seeds possessed physical dormancy (PY) imposed by a water-impermeable seed coat.
Mechanical scarification was the most effective method of breaking PY; submersion in acid
was also moderately effective. Seed germination following scarification can occur over a
wide range of temperatures. There is no light requirement for seed germination. This study
represents the first case of physical dormancy reported for the genus and only the second
for the family.
Introduction
The Calycanthaceae is a plant family of 2 (Johnson 1997, Weakley 2015) or 3
genera (Bingtao and Bartholemew 2008, Kubitzki, 1993) and ~9 species found in
temperate areas worldwide (Bingtao and Bartholemew 2008, Johnson 1997, Kubitzki
1993, Weakley 2015). In North America, the family is represented by the
single genus Calycanthus (Johnson 1997). Gleason and Cronquist (1991) recognized
1 species in North America, Calycanthus floridus L. (Eastern Sweetshrub).
Weakley (2015) recognized 3 species, Eastern Sweetshrub and C. brockianus Ferry
& Ferry (Georgia Sweetshrub), in the eastern US and C. occidentalis Hook. & Arn.
(Western Sweetshrub) in the western US. Nicely (1965) stated that as many as 6
species have been recognized in the east and southeast, but he concluded that these
should all be considered as varieties or ecological variants of Eastern Sweetshrub.
Johnson (1997) recognized only 2 species (Eastern Sweetshrub and Western Sweetshrub)
within the US, stating that Georgia Sweetshrub could represent a triploid
variation of particular Eastern Sweetshrub populations.
In Kentucky, where we conducted this study, the genus is represented only by
Eastern Sweetshrub (Kartesz 2015), a species of mesic forest slopes, hillsides,
and stream banks (Jones 2005, KSNPC 2015, Weakley 2015). Two varieties of
the species are recognized: Calycanthus floridus var. glaucus and C. floridus var.
floridus (Weakley 2015). Jones (2005), however, reported that only C. floridus
var. glaucus naturally occurs in the state. The species is ranked as threatened and
1Department of Biology, Berea College, Berea, KY 40404. *Corresponding author -
Christopher_Adams@berea.edu.
Manuscript Editor: Robert Carter
Southeastern Naturalist
489
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
occurs only in several southern counties (Campbell and Medley 2012, KSNPC
2015). C. floridus var. floridus may grow sporadically within the state, but these
records represent established, scattered populations of escaped cultivated plants
(Campbell and Medley 2012).
Very little information is available in the published literature on seed dormancy
and germination within the family. The Calycanthaceae is a very small family,
with only 3 genera (Calycnathus, Chimonanthus, and Idiospermum) and fewer
than 10 species (Bingtao and Bartholomew 2008, Kubitzki 1993); thus, our study
provides valuable information on the germination ecology of the family, primarily
to those interested in the evolution of seed dormancy and its different forms.
Idiospermum australiense (Diels) S.T. Blake (Ribbonwood) is a rare, tropical,
woody species with recalcitrant seeds (Franks and Drake 2003). Seeds of the species
have physiological dormancy at maturity (Baskin and Baskin 2014, Edwards et
al. 2001). Tang and Tian (2010) reported that seeds of Chimonanthus praecox (L.)
Link (Wintersweet) possess combinational dormancy, i.e., there are both endogenous
and exogenous components to dormancy: the embryos are physiologically
dormant—a period of cold stratification is required to break embryo dormancy
(Baskin and Baskin 2014, Tang and Tian 2010)—and the seed coat is impermeable
to water (Baskin and Baskin 2014). One of the 2 recognized Calycanthus species
(Johnson 1997) has been investigated. The seeds of Western Sweetshrub reportedly
possess non-deep physiological dormancy (Baskin and Baskin 2014) and require
90 d of cold stratification to break dormancy (Emery 1988). Thus, all species of the
Calycanthaceae examined thus far have some form of dormancy preventing mature
seeds from germinating.
Seeds of Eastern Sweetshrub have a large embryo consisting of little to no endosperm
with the cotyledons folded around the hypocotyl (Eames 1961, Nicely 1965).
In the classification system of Baskin and Baskin (2014), Eastern Sweetshrub possesses
an embryo described as “folded”. In his monograph on the family, Nicely
(1965) reported germinating seeds within 2–3 weeks following scarification. This
result hints at the likelihood that the seeds possess at least physical dormancy; the
seed coat is impermeable to water, preventing the seed from making metabolic
advancement towards germination (Bewley and Black 1994). Seeds that display
physical dormancy alone simply require the seed coat to become permeable to
water for the embryo to begin growing, which results in germination (Baskin and
Baskin 2014). The observations of Nicely stand in contrast to Western Sweetbush,
which is reported to exhibiit physiological, but not physical, dormancy (Emery
1988). Thus, it is possible that seeds of Eastern Sweetbush could be physically
dormant, physiologically dormant, or possess a combination of both (i.e., combinational
dormancy).
Physical dormancy (PY) in seeds is typically broken through some form of
scarification, a process that produces a water-permeable seed coat, allowing the
embryo to imbibe (Baskin and Baskin 2014). Two of the most common forms of
scarification are abrasion of the seed coat (i.e., mechanical) and exposure to acid
(i.e., chemical) (Baskin and Baskin 2014). Mechanical and acid scarification both
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
490
correspond to natural phenomena that produce permeable seed coats, allowing for
the breaking of dormancy and subsequent germination. Nicely (1965) stated that
some bird species consume fruits and seeds of Calycanthus spp. Miller and Miller
(2005) referred to seeds of Calycanthus spp. as being “animal-dispersed”, which
could include consumption of seeds. It might also refer to animals chewing through
fruit coats, allowing seeds to be gravity dispersed. Scarification through abrasion
could occur following dispersal. C.A. Adams has observed fruits that have been
gnawed on, resulting in holes through which seeds easily can fall down to the soil
when the fruits are gently shaken. Kubitzki (1993) maintained that, due to toxicity,
seeds are not consumed by animals, but, rather, are gravity-dispersed only. If that
is correct, then abrasion following dispersal would be the more likely method of
breaking PY.
Physiological dormancy (PD) is present in seeds of many US species and is
the most common type among species in temperate climates throughout the world
(Baskin and Baskin 2014). Laboratory studies have repeatedly confirmed that
the processes of cold and/or warm stratification are usually effective in breaking
seed dormancy in most species, allowing for subsequent germination (Baskin and
Baskin 2014, Bewley 1997, Bewley and Black 1994). The particular stratification
process used to break physiological seed dormancy in seeds with fully developed
embryos, such as Eastern Sweetbush, depends upon the phenology of seed dispersal:
dormant seeds dispersed in late spring or early summer typically require warm
stratification (simulating summer environmental conditions), while dormant seeds
dispersed in autumn typically require cold stratification (simulating winter environmental
conditions) (Baskin and Baskin 2014). As seeds overcome dormancy, they
are able to germinate when ambient temperatures are favorable to support germination:
autumn for spring/summer-dispersed seeds or spring for autumn-dispersed
seeds (Baskin and Baskin 2014).
Baskin and Baskin (2004) define combinational dormancy as a combination of
both the presence of a water-impermeable seed coat and a physiologically dormant
embryo. The respective dormancies are broken via scarification and a treatment that
removes the physiological barrier to germination for the embryo, e.g., stratification
or dry storage (Baskin and Baskin 2014). For most species, the seed coat must
become permeable to water first, and only then can the physiological component
be broken (Baskin and Baskin 2014). It has been noted, however, that, at least for
a few species, physiological dormancy can be broken by dry storage prior to the
breaking of physical dormancy (Baskin and Baskin 2004).
Fruits of Eastern Sweetbush are mature in mid- to late autumn and most fruits
dehisce from the parent plant by late autumn/early winter. In these seasons, temperatures
are typically too low to promote germination for seeds of most species,
even if they are nondormant at maturity. Thus, most seeds are subjected to a winter
of cold stratification, which would most likely break physiological dormancy, if
present. In the case of the study species, however, it has been observed that a small
number of fruits (an average of 6 per plant) persist on the plant through the fall and
winter. They dehisce in the next year, often in the spring, when new flowers are
Southeastern Naturalist
491
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
produced. Seeds dispersed at this time experience the warm stratification regimes
of late spring and summer. Thus, the possibility of seeds requiring a warm-stratifi -
cation period to break physiological dormancy, if present, cannot be ruled out.
The purposes of this study were to determine: (1) the type of dormancy found
in seeds of Eastern Sweetbush, (2) the most effective method(s) of breaking this
dormancy, and (3) the germination conditions that produce maximum germination.
We propose the following hypotheses: (1) seeds of Eastern Sweetbush possess
physical dormancy, and (2) traditional methods of scarification should alleviate the
seeds of this germination inhibitor and allow embryos to emerge from the seed.
Furthermore, the experimental design also allowed the detection of combinational
dormancy, if present in the seeds.
Methods
Seed collection
We collected mature seeds from 8 stems of Eastern Sweetbush growing in a
mesic deciduous forest in southern Kentucky. We did not determine whether each
stem represented a unique plant or was part of a largely clonal colony (i.e., had the
same genotype as other stems). Our collection included ~60 mature fruits from the
8 stems. We collected mature fruits (each containing an average of 22 seeds) in mid-
November 2015, ~1 month after the first frost of the autumn season. At the time of
collection, the fibrous hypanthium of each fruit had turned a dark brown, indicating
fruit (and thus seed) maturity (Baskin and Baskin 2014). Seeds were removed from
each hypanthium (average seed weight = 0.21 g), mixed together in a plastic pan,
and allowed seeds to air-dry for 5 days. We commenced seed germination studies
in early December 2015 and seed dormancy-breaking studies in early May 2016.
We stored the seeds used in the dormancy-breaking studies dry in a paper bag on
a laboratory shelf. Preliminary research conducted a year prior confirmed that dry
storage of seeds over an 8-month period did not result in any change in dormancy
state (C.A. Adams, unpubl. data).
Seed imbibition
We conducted the imbibition study 1 week after air-drying was complete. We
prepared 2 treatments: non-scarified and scarified seeds. We randomly chose 30
seeds for each treatment and nicked each seed in the scarified treatment with a razor
blade to ensure that the seed coat had become permeable to water; scarification
occurred prior to the exposing of seeds to the moist sand. For both treatments, we
placed seeds on moist quartz-sand in plastic Petri dishes and added 3 mL of distilled
water to each dish. Seeds remained on the moist substrate for 12 h, which was the
duration of the study.
Prior to scarification, we used an electronic balance to determine the initial
weight of each seed in both treatments, then calculated an average seed weight
for each treatment and placed the seeds on the moist sand in each of the 2 Petri
dishes. Every hour, we removed all seeds from the Petri dishes, dried their surfaces
with paper towels, and recorded their weight using an electronic balance. After
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
492
weighing, we returned the seeds to their respective treatments; the same procedure
was followed each hour for a total of 12 consecutive hours. We calculated average
seed weight every hour and we used the percentage of mass increase for each hour
to construct an imbibition curve.
Seed dormancy-breaking studies
A wide variety of laboratory methods have been used to break PY in seeds of
various species (Baskin and Baskin 2014). To determine which mechanism(s)
might be most effective in breaking physical dormancy, we exposed seeds to various
scarification methods. All treatments, except for the fire treatments, consisted
of 3 replicates of 30 seeds each. Fire treatments consisted of 1 replicate of 90 seeds
each. We placed all seeds subjected to scarification treatments on moist quartz-sand
in plastic Petri dishes and wrapped them in clear plastic film to retard water loss
during incubation and/or stratification. We incubated the seeds in a 14-h photoperiod
of around 40 μmol m-2s-1, 400–700 nm, of cool white fluorescent light at a 24-h
constant temperature of 25 °C. We chose this temperature as the general incubation
temperature because it was observed that scarified seeds stored on a moist substrate
at room temperature germinated at relatively high temperatures (C.A. Adams, unpubl.
data) This temperature is also optimal for the germination of seeds of many
species in a temperate climate (Baskin and Baskin 2014). Non-scarified seeds
served as controls for each treatment.
Mechanical scarification. We employed 2 treatments to mechanically scarify
seeds. In 1 treatment, we nicked the coat of each seed in every replicate with a
razor blade near its chalazal end. In the other treatment, we rubbed coarse-grade
sandpaper on the upper and lower surfaces of the seeds simultaneously for 15–
20 seconds.
Freeze/thaw cycles. We cycled seeds placed on moist quartz-sand in Petri dishes
between 0 °C and 25 °C over the course of 60 d; seeds remained at each temperature,
in light, for 5 days before being moved to the other regime (0°C → 25°C →
0°C → 25°C ...). At the conclusion of the 60-d treatment, we moved seeds to 25
°C to incubate for 30 d to determine if dormancy had been broken.
Dry storage. Fresh seeds that had dried on a lab bench for 5 days were transferred
to a brown paper bag, stored on an office shelf for 6 months, and placed into
incubation. This was a new treatment, distinct from a similar procedure conducted
a year prior in a preliminary study, as described in the seed collection methodology.
Fire. We employed 2 separate treatments to test the possibility that fire would
break physical dormancy. In the first treatment, we placed 90 seeds on dry soil in
a metal tray, scattered ~5 cm of dry leaves over the surface of the soil to cover the
seeds, ignited the leaves with a match, and allowed them to burn. We removed
the seeds and placed them on moist quartz-sand in Petri dishes for evaluation of
breaking dormancy. In the second treatment, we buried 90 seeds ~2 cm under the
soil surface in a metal tray, scattered ~5 cm of dry leaves over the soil surface, and
burned the leaves. We exhumed the seeds and placed them on moist sand in Petri
dishes for evaluation.
Southeastern Naturalist
493
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
High heat. We conducted 6 separate treatments to test the efficacy of heat in
breaking physical dormancy. In the first 3 treatments, we placed seeds on dry sand
in metal trays in drying ovens at 60 °C, 70 °C, and 80 °C, respectively, for 30 mins.
Seeds were then transferred to the previously described incubation regime. In the
second 3 treatments, we placed seeds on moist sand in metal trays in drying ovens
at 60 °C, 70 °C, and 80 °C, respectively, for 30 min. Seeds were then moved to the
previously described incubation regime.
Acid Scarification. We submerged seeds in concentrated (95–97%) sulfuric acid
for treatment-time intervals of 15 sec, 30 sec, 1min, and 2 min. We placed the 3
replicates for each of the 4 treatments individually into a loose-leaf tea strainer,
immersed it in the acid for the particular treatment time, and transferred the seeds
to the previously described incubation regime.
Percussion. Following the protocol of Hamly (1932), we placed seeds, in 3 replicates
of 30 seeds each, into a glass bottle, shook it 3 times per second for 5 minutes
and transferred seeds to the previously described incubation regime.
Germination studies
Germination temperature. All treatments consisted of 3 replicates of 30 seeds
each. We placed seeds on moist quartz-sand in plastic Petri dishes wrapped in clear
plastic film to retard water loss during incubation and/or stratification and incubated
them in a 14-h photoperiod of around 40 μmol m-2s-1, 400–700 nm, of cool-white
fluorescent light at 24-h constant temperatures of 35 °C, 30 °C, 25 °C, 20 °C, 15 °C,
or 5 °C, depending upon the specific treatment, for 30 d. We chose the temperatures
that correspond to monthly mean maximum temperatures in Kentucky (Wallis 1977).
During incubation, we assessed seeds every 5 d for radicle emergence, at which
time water was added to the dishes to keep the sand moist. Our criterion for germination
was emergence of the radicle from the seed. We tested for viability all seeds
that remained ungerminated at the end of each treatment period by examining them
under a dissecting microscope to determine if the embryo was firm and white (indicating
they were viable) or soft and brown (indicating they were nonviable). Very
few (less than 10%) ungerminated seeds contained viable embryos.
Testing for physiological dormancy. We exposed seeds to either a warm- or
cold-stratification treatment to determine if Eastern Sweetbush has combinational
dormancy. Cold-stratified seeds were incubated, in light, at 5 °C for 4, 8, and 12
weeks, to simulate the winter season in Kentucky, then moved to 25 °C in light for
30 d. Warm-stratified seeds were incubated, in light, at 35 °C for 4, 8, and 12 weeks,
to simulate the summer season in Kentucky, then moved to 25 °C in light incubator
for 30 d. We assessed seeds every 5 days for the presence of radicle emergence. In
contrast to the cold-stratification treatment, we monitored seeds exposed to warm
stratification for germination during the stratification treatment. Scarified seeds can
germinate at 35 °C, so it was necessary to monitor germination at regular intervals
before moving seeds to the cooler temperature following stratifi cation.
To further confirm the presence of combinational dormancy, we also exposed
seeds to gibberellic acid treatments. Gibberellic acid-3 (GA3) promotes seed germination
and can allow seeds to overcome a physiological component to dormancy
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
494
(Bewley and Black 1994). We placed scarified seeds into Petri dishes on filter paper
and added 4-mL aliquots of 1000 ppm, 750 ppm, or 500 ppm GA3 to 3 replicate
dishes for each treatment. Dishes were wrapped in clear plastic film, placed on a
laboratory bench for 30 h, then washed with distilled water, dried with towels, transferred
to Petri dishes with moist sand, and incubated at 25 °C, in light, for 30 d.
Testing for a light requirement. We employed 2 treatments to determine whether
light was necessary for germination: (1) scarified seeds incubated in light, and
(2) scarified seeds incubated in darkness at 25 °C. We wrapped the Petri dishes containing
seeds for dark incubation in 2 layers of aluminum foil. We checked seeds
every 5 d in the dark using a green “safe” light (Walck et al. 2000) to prevent exposure
to any photosynthetically active radiation. We placed the “light” treatment
replicates into the incubator with only a clear plastic wrap covering.
Statistical analyses
We converted germination data to percentages based on number of viable seeds
and calculated means and standard errors for each percentage. A Shapiro–Wilk
test confirmed that the data were normally distributed; thus, no transformation was
required, and we compared means by 1-way analyses of variance (ANOVA) and
Fisher’s least-significant–difference tests (LSD, P < 0.5).
Results
Seed imbibition
Non-scarified seeds did not take up water; average mass increase was less than 1% after
12 h (Fig. 1). In contrast, for scarified seeds, average mass increase was 24%
mid-way (i.e., 6 h) through the experiment, and 36% at the conclusion of the study,
indicating that seeds were taking up water. Scarification clearly produced an opening
for water to enter the seed that was not present prior to the abrasion of the seed
coat. Seeds did not take up water without scarification, indicating that their seed
coats were impermeable to water.
Figure 1. Imbibition
curve for
30 non-scarified
(■) and 30 mechanically
scarified
(●) Eastern
Sweetbush
seeds. One hundred
percent of
the mechanically
scarified seeds
imbibed, but
only 3% of nonscarified
seeds
imbibed.
Southeastern Naturalist
495
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
Seed dormancy-breaking studies
We tested a variety of scarification methods to determine which method was
most effective in producing water-permeable seed coats and obtained mixed results
(Table 1). Mechanical scarification was the most effective treatment—90% of
seeds scarified by razor blade and 81% by sandpaper germinated. The only other
moderately effective treatment was immersion in acid, but of the 4 treatments, none
produced higher than a 42% seed-germination rate; germination varied from 23%
to 42%. Although the germination rates for acid-scarified seeds were not high, all
treatments produced significantly more germination than the non-scarified control.
The freeze/thaw treatment and the percussion treatment yielded 8% and 17%
germination, respectively. These rates were very low but statistically significantly
different from the control group.
All other treatments apparently were ineffective in producing the seed-coat
permeability that allows germination. No other treatment produced a significantly
higher germination percentage compared to the control. The highest germination
rate among this group of treatments was for the buried seeds over which we lit a
fire on the soil surface (5%), which was not statistically significantly different from
the control. No more than 5 seeds germinated in any of these treatments.
Germination studies
Mechanically scarified seeds germinated over a broad range of incubation temperatures
(Fig. 2). Seeds incubated at 35 °C, 30 °C, 25 °C, and 20 °C following
scarification germinated to 76%, 82%, 92%, and 86%, respectively. The 15 °C regime
was only moderately effective, with 38% germination.
Cold or warm stratification treatments did not increase overall germination
percentages, except for 1 treatment (Table 2). Scarified seeds that received
a warm-stratification treatment prior to incubation did not germinate to
Table 1. Mean germination percentages (± SE) of Eastern Sweetbush seeds exposed to various physical
dormancy-breaking mechanisms. Following the treatment, seeds were incubated, in light, at 25
°C for 30 days. Three replicates of 30 seeds each were used for each treatment. Reported means were
recorded on the 30th day of incubation. Means followed by the same letter are not significantly different
based on Fisher’s protected LSD at P < 0.05.
Treatment Germination %
Control (no treatment) 3.0 ± 0.2a
Mechanical Scarification
Razor blade 90.2 ± 3b
Sandpaper 81.4 ± 4c
Acid Submersion
15 seconds 35.5 ± 4e
30 seconds 42.3 ± 3e
1 minute 30.6 ± 4e
2 minute 22.8 ± 2d
Freeze/thaw cycle 8.3 ± 2d
Fire (seeds on soil surface) 0.0
Fire (seeds buried in soil) 5.3 ± 1a
Treatment Germination %
High Heat
With water
60 °C 0.0
70 °C 2.2 ± 0.2a
80 °C 0.0
Without water
60 °C 0.0
70 °C 0.0
80 °C 3.3 ± 0.4a
Dry storage 4.2 ± 0.8a
Percussion 16.8 ± 4d
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
496
statistically higher percentages than seeds that received no stratification. None
of the specific treatments produced higher than 88% germination; the control
group (no stratification) produced ~87% germination. Cold stratification yielded
Table 2. Mean germination percentages (± SE) of Eastern Sweetbush seeds incubated at 25 °C following
mechanical scarification and various periods of cold (5 °C) or warm stratification (35 °C): 0, 4, 8,
and 12 weeks. Reported means were recorded on the 30th day of incubation. Means followed by the
same letter are not significantly different based on Fisher ’s protected LSD at P < 0.05.
Stratification regime Percent germination
0 weeks
Cold 88.4 ± 2a
Warm 86.6 ± 4a
4 weeks
Cold 89.4 ± 3a
Warm 88.4 ± 2a
8 weeks
Cold 94.4 ± 3b
Warm 86.2 ± 4a
12 weeks
Cold 87.6 ± 4a
Warm 87.2 ± 3a
Figure 2. Mean (± SE) percent germination of freshly-matured Eastern Sweetbush seeds
mechanically scarified (using a razor blade) and incubated, in light, at various temperatures:
35 °C (○), 30 °C (■), 25 °C (□), 20 °C (●), 15 °C (Δ), and 5 °C (▲). Three replicates of 30
seeds per Petri dish were used for each treatment.
Southeastern Naturalist
497
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
similar results. Only 1 treatment (8 weeks at 5 °C) produced a significantly different
result from the control group (94% vs. 88%), a difference of only 5 seeds
out of 90 (85 seeds vs. 80 seeds). The other cold stratification treatments did not
produce significantly different germination percentages compared to the control
group.
Exposure to various concentrations of gibberellic acid did not produce significantly
higher germination percentages than the control (Table 3). Seeds exposed to
500 ppm and 750 ppm GA3 had germination rates of 87% and 90%, respectively,
while seeds in the control showed germination of 89%. The 1000-ppm treatment
actually showed decreased germination over the duration of the incubation regime
(80%), which was significantly different from the control.
In our trials to assess light requirement for germination, the overall germination
percentage for scarified seeds incubated in darkness was not sig nificantly different
from the those in the light-exposed control group (Fig. 3). The germination rate for
seeds incubated in light was 92% and 88% for those incubated in darkness.
Figure 3. Mean
(± SE) percent
g e r m i n a t i o n
of f r e s h l y -
matured Eastern
Sweetbush
seeds mechanically
scarified
(using a razor
blade) and incubated,
in light
(●) or darkness
(▲), at 25 °C
for 30 d. Three
replicates of 30
seeds per Petri
dish were used
for each treatment.
Table 3. Mean germination percentages (± SE) of Eastern Sweetbush seeds incubated at 25 °C following
mechanical scarification and imbibition in gibberellic acid (GA3) solution. Three replicates of 30
seeds each were used for each treatment. Reported means were recorded on the 30th day of incubation.
Means followed by the same letter are not significantly different based on Fisher’s protected LSD at
P < 0.05.
Treatment Percent germination
Scarification only 89.4 ± 3a
Scarification + 500 ppm GA3 86.6 ± 4a
Scarification + 750 ppm GA3 90.2 ± 3a
Scarification + 1000 ppm GA3 80.4 ± 3b
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
498
Discussion
Prior to this study, the only information in the published literature regarding
seed germination ecology for the genus Calycanthus is for the California endemic,
Western Sweetbush (Johnson 1997). Emery (1988) reported that the seeds possess
physiological dormancy, a mechanism that inhibits the embryo and prevents radicle
emergence (Bewley and Black 1994). Species within the same genus often have the
same type of dormancy (Baskin and Baskin 2014), so it is reasonable to expect that
seeds of the southeastern species, Eastern Sweetbush, might have PD as well.
Preliminary work revealed that mature seeds of Eastern Sweetbush have water-
impermeable seed coats, suggesting that these seeds are physically dormant
(Baskin and Baskin 2014). Seeds exposed to water, at room temperature, for a
period of 8–12 h with no increase in weight over this time frame are described as
having water-impermeable seed coats (Bansal et al. 1980). The imbibition curve
produced in this study clearly shows that non-scarified seeds do not take up water.
However, scarified seeds did take up water and demonstrated a substantial increase
in mass over the 12-h imbibition period; thus, demonstrating that seeds of Eastern
Sweetbush possess physical dormancy.
In terms of correlating embryo size/shape to dormancy, it might be expected that
Eastern Sweetbush seeds possess PY. Seeds of most plant families that contain taxa
with PY produce large embryos (Baskin and Baskin 2014), and the findings in this
study are consistent with general patterns. The embryo in Eastern Sweetbush seeds
is a large type referred to as “folded”, and the demonstrated PY is consistent with
general patterns. The seeds of the Western Sweetbush apparently do not possess PY.
Physical dormancy, however, is known within the Calycanthaceae: seeds of Wintersweet
possess PY (Tang and Tian 2010) in addition to physiological dormancy of
the embryo. Thus, despite the apparent absence in its closest relative, it should not
be surprising that PY is found in seeds of Eastern Sweetbush.
Despite the expression of physiological dormancy in the western congener
and combinational dormancy within the family, embryos in most seeds of Eastern
Sweetbush do not appear to be physiologically dormant. The vast majority of seeds
germinated without any kind of PD-breaking treatment, thus there is no physiological
component to dormancy for most propagules.
Approximately 2–3% of mature seeds will germinate without any kind of scarification
treatment. This result suggests 2 major possibilities: either the individual
seeds have seed coats that developed with permeability to water or that these seeds
were not fully mature when collected and PY had not developed. Baskin and Baskin
(2014) reported that, in many species, if a seed is collected before it completely
dries prior to full maturation, then the seed coat would have not developed water
impermeability. In either case, there seem to be only a very small number of seeds
in any population that do not possess PY.
Most Eastern Sweetbush seeds possess PY and not combinational dormancy. We
also examined mechanisms to break PY. The most effective laboratory method of
breaking PY was mechanical scarification. Either abrading seeds with sandpaper or
nicking the seed coat with a razor blade produced significantly higher germination
Southeastern Naturalist
499
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
percentages compared to the non-scarified control. In nature, some sort of abrasion
to the seed coat, following dispersal, should produce seed coats that are permeable
to water and able to germinate under favorable environmental conditions. Seeds
of Eastern Sweetbush are easily carried by water (Chambert and James 2009) following
dispersal; thus, abrasion could occur during transport by water. The limited
published literature available suggests that the most common natural means of
dispersal is via gravity, aided by the activity of animals, and the most likely cause
of dormancy break is abrasion due to water transport (Kubtizki 1993, Miller and
Miller 2005, Chambert and James 2009).
The next most-effective method of breaking PY was exposure to sulfuric acid.
Seeds submerged in concentrated sulfuric acid for various time periods produced
significantly higher germination percentages than the control. In nature, an analogous
scenario would be passage of a seed through an animal’s digestive tract; thus,
if animals do consume Eastern Sweetbush fruits, as suggested by some (Miller and
Miller 2005, Nicely 1965), stomach acid seems to be a likely agent for producing
water-permeable seed coats.
The other scarification methods were only mildly effective or ineffective in
breaking PY. Percussion generated the 3rd-highest percentage of germination of all
treatments. Only 15 seeds experienced dormancy break using this method; however,
we tested only 1 form of treatment. It is possible that an increased period of
percussion or more vigorous percussion might result in the breaking of PY in a
higher number of seeds. The only other PY-breaking method that produced results
significantly different from the control was the freeze/thaw cycling treatment. The
overall percent germination, however, was still very low.
No other treatment produced significantly higher germination than the nonscarified
control. Some of these results, however, are inconclusive. For example,
exposure of seeds to high heat occurred at only 3 temperatures over the same time
period. For species where high heat can break PY, the effective temperature and
duration of exposure can vary greatly (Baskin and Baskin 2014). In the case of
Sida spp. (fanpetals), exposure to temperatures of 70–90 °C for either 12- or 24-h
intervals are required to break PY (Chawan 1971), while Trifolium pratense L. (Red
Clover) seeds become water permeable after only 4 min at 104 °C (Rincker 1954).
To conclusively state that dry heat does or does not break PY, a more extensive
study would need to be conducted using a broader range of temperatures and a
broader use of heat-exposure durations. Similarly, more expansive studies should
be conducted to determine whether high heat (wet), dry storage, and freeze/thaw
cycles are effective in breaking PY in seeds of Eastern Sweetbush.
Fire, which is known to create permeability in the seed coats of many species
(Baskin and Baskin 2014) was not effective in breaking dormancy, and, in fact,
was detrimental to seed health. We found that direct exposure to a surface fire was
lethal to seeds. No germination occurred following the burning of leaf litter on top
of exposed seeds. Most of the fire-exposed seeds began to decay within just a few
days, and we observed 100% seed mortality within 10 days (embryos were almost
completely liquefied); thus, direct exposure to fire results in rapid seed mortality.
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
500
Even seeds buried a few centimeters below the soil surface experienced high mortality
following fire; within 10 d of the burn, 75% of the buried seeds perished.
Unless seeds are buried relatively deeply within the soil, it is very likely that few
will survive a surface fire.
Once Eastern Sweetbush seeds have overcome physical dormancy, they will
germinate under a wide range of environmental conditions. There is no light
requirement for germination; therefore, nondormant seeds could germinate if completely
shaded or buried. Nondormant seeds will germinate over a broad range of
temperatures, provided that the embryo can imbibe water. The lower temperature
limit for germination appears to be around 15 °C, at which at least a few seeds germinate,
suggesting that, in nature, nondormant seeds could germinate during any
season of the year except winter. No scarified seeds germinated in temperatures less
than 15 °C.
Conclusions
This study confirms that seeds of Eastern Sweetbush possess physical dormancy.
Seeds that have broken PY can germinate over a broad range of temperatures, predominantly
those that occur in spring and summer. Seeds will germinate in light
or darkness. This is the first report on seeds of a species in the Calycanthaceae
where PY is the dormancy class solely possessed by the majority of seeds, as well
as the first report on the seed germination ecology of Eastern Sweetbush. Although
we conducted our study using seeds collected from a single location, there is currently
no evidence for changes in the dormancy state of seeds with PY within a
species’ range (Ferreras et al. 2017); thus, we expect that Eastern Sweetbush seeds
throughout the southeast possess PY. Our results provide information on the seeddormancy
status for Eastern Sweetbush, the only species in the genus Calycanthus
for which it was previously unknown.
Acknowledgments
We thank L. Ballou, A. Wentworth, J. Darling, S. Nilan, A. Morgan, J. Terrell, J. Mc-
Clain, and C. Krebs for providing technical assistance and support on various aspects of
the project. Funding for this research was provided by the Berea College Undergraduate
Research and Creative Projects Program.
Literature Cited
Bansal, R.P., P.R. Bhati, and D.N. Sen. 1980. Differential specificity in water imbibitions
of Indian arid-zone seeds. Biological Plantarum 22:327–331.
Baskin, C.C., and J.M. Baskin. 2014. Seeds: Ecology, Biogeography, and Evolution of
Dormancy, and Germination, 2nd Edition. Academic Press, San Diego, CA. 666 pp.
Baskin, J.M., and C.C. Baskin. 2004. A classification system of seed dormancy. Seed-
Science Research 14:1–16.
Bewley, J.D. 1997. Seed germination and dormancy. The Plant Cell 9:1055–1066.
Bewley, J.D., and M. Black. 1994. Seeds: Physiology of Development and Germination.
Plenum Press, New York, NY. 445 pp.
Southeastern Naturalist
501
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
Bingtao, L., and B. Bartholemew. 2008. eFloras. Missouri Botanical Garden, St. Louis,
MO and Harvard University Herbaria, Cambridge, MA. Available online at http://www.
efloras.org. Accessed 25 January 2017.
Campbell, J., and M. Medley 2012. The atlas of vascular plants in Kentucky. Draft of July
2012, with provisional listing of authors, Atlas introduction, and exclamation. Available
online at https://www.bluegrasswoodland.com. Accessed Accessed 25 January 2017.
Chambert, S., and C.S. James. 2009. Sorting of seeds by hydrochory. River Research and
Applications 25:48–61.
Chawan, D.D. 1971. Role of high-temperature pretreatments on seed germination of desert
species of Sida (Malvaceae). Oecologia 6:343–349.
Eames, A.J. 1961. Morphology of the Angiosperms. McGraw-Hill, London, UK. 400 pp.
Edwards, W., P. Gadek, E. Weber, and S. Worboys. 2001. Idiosyncratic phenomenon of
regeneration from cotyledons in the Idiot Fruit Tree, Idiospermum australiense. Austral
Ecology 26:254–258.
Emery, D.E. 1988. Seed propagation of native California plants. Santa Barbara Botanic
Garden, Santa Barbara, CA. 115 pp.
Ferreras, A.E., S.R. Zeballos, and G. Funes. 2017. Inter- and intra-population variability in
physical dormancy along a precipitation gradient. Acta Botanica Brasilica 31(1):141–146.
Franks, P.J., and P.L. Drake. 2003. Dessication-induced loss of seed viability is associated
with a 10-fold increase in CO2 evolution in seeds of the rare tropical rainforest tree Idiospermum
australiense. New Phytologist 159:253–261.
Gleason, H.A., and A. Cronquist. 1991. Manual of the Vascular Plants of Northeastern
North America and Adjacent Canada, 2nd Edition. New York Botanical Garden, Bronx,
NY. 910 pp.
Hamly, D.H. 1932. Softening of the seeds of Melilotus alba. Botanical Gazette 93:345–375.
Johnson, G.P. 1997. Calycanthaceae. Pps. 23–-25, In Flora of North America Editorial
Committee (Eds). 1993+. Flora of North America North of Mexico. 20+ vols. Volume 3.
Oxford University Press, New York, NY. 616 pp.
Jones, R.L. 2005. Plant life of Kentucky: An Illustrated Guide to the Vascular Flora. University
Press of Kentucky, Lexington, KY. 856 pp.
Kartesz, J.T. 2015. The Biota of North America Program (BONAP). North American Plant
Atlas. Chapel Hill, NC. Available online at http://bonap.net/MapGallery/County/Calycanthus%
20floridus.png. Accessed 2 February 2015.
Kentucky State Nature Preserves Commission (KSNPC). 2015. Rare and extirpated biota
of Kentucky. Journal of the Kentucky Academy of Science 61:115–132.
Kubitzki, K. 1993. Calycanthaceae. Pp. 197–200, In J.G. Kubitzki, G. Rowher, and V. Bittrich
(Eds.) The Families and Genera of Vascular Plants. Volume 2: Flowering Plants–
Dicotyledons. Springer-Verlag, Berlin, Germany. 653 pp.
Miller, J.H., and K.V. Miller. 2005. Forest Plants of the Southeast and Their Wildlife Uses.
University Press of Georgia, Athens, GA. 464 pp.
Nicely, K.A. 1965. A monographic study of the Calycanthaceae. Castanea 30:38–81.
Rincker, C.M. 1954. Effect of heat on impermeable seeds of Alfalfa, Sweet Clover, and Red
Clover. Agronomy Journal 46:247–250.
Tang, A.J., and M.H. Tian. 2010. Breaking combinational dormancy in seeds of Chimonanthus
praecox L. Seed-Science Technology 38:551–558.
Walck, J.L., C.C. Baskin, and J.M. Baskin. 2000. Increased sensitivity to green light during
transition from conditional dormancy to nondormancy in seeds of 3 species of Solidago
(Asteraceae). Seed-Science Research 10:495–499.
Southeastern Naturalist
C.A. Adams, O.C. Adejumo, M. Jahan, and K.W. Montgomery
2017 Vol. 16, No. 4
502
Wallis, A.L., Jr. (Ed.). 1977. Comparative climatic data through 1976. National Climatic
Data Center. Environmental Data Service, National Oceanic and Atmsopheric Administration,
US Department of Commerce, Asheville, NC.
Weakley, A.S. 2015. Flora of the southern and mid-Atlantic States. University of North
Carolina Herbarium (NCU), North Carolina Botanical Garden, and University of North
Carolina at Chapel Hill. Available online at www.herbarium.unc.edu/FloraArchives/
WeakleyFlora_2015-05-29.pdf. Accessed 2 February 2015..