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Ecology |
2Department of Biology, University of Kentucky, Lexington, Kentucky 40506 USA; 3Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546 USA; 4Center for Field Biology, Austin Peay State University, Clarksville, Tennessee 37044 USA; 5Department of Biological Sciences, Southern Illinois University, Edwardsville, Illinois 62026 USA
Received for publication July 11, 2002. Accepted for publication November 8, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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3 cm. Buried seeds of S. hallii exhibited an annual conditional dormancy/nondormancy cycle. Regardless of the month in which seeds were exhumed, they germinated to 59–100% in light in water with apple at daily alternating temperature regimes of 25°/15°, 30°/15°, and 35°/20°C, but germination at 20°/10°C (and to some extent at 15°/6°C) tended to peak in autumn to spring. Thus, seeds can germinate throughout the summer if flooded (ethylene production) and exposed to light. An ethylene cue for germination serves as a "flood-detecting" mechanism and may serve as an indirect signal that water is available for completion of the life cycle and competing species are absent.
Key Words: Cyperaceae • ethylene • flooding requirement • germination cue • light requirement • Schoenoplectus hallii • seed dormancy-breaking requirements • seed germination requirements
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). However, the species is rare and local throughout its range, being endangered in Illinois, Indiana, Kentucky, Michigan, Missouri, Oklahoma, and Wisconsin and possibly extirpated from Massachusetts and Iowa (Robertson et al., 1993
; McKenzie, 1998
). In Illinois (McClain et al., 1997
) and Kentucky (E. W. Chester, personal observations), S. hallii grows in depressions that fill with water during the growing season in years with above-average precipitation. These depressions are dry enough in years with normal or below-normal precipitation to be farmed; crops of soybeans or corn usually are planted in them. Consequently, S. hallii may grow at a given site for several consecutive years or be absent for 1–3 yr (E. W. Chester, personal observations from 1983 to 2002 in Christian County, Kentucky) to 20 yr (McClain et al., 1997
).
One of the mysteries about S. hallii is the presence of plants at a population site in some wet years and their absence in other wet years (J. Schwegman, Illinois Department of Conservation, personal communication, 1991). One possible reason for the absence of S. hallii in some wet years is that seeds germinate, but plants fail to become established (e.g., Robertson et al., 1993
). On the other hand, seeds of S. hallii may not germinate unless depressions fill with water before a certain date. That is, seeds of S. hallii in (or on) the soil may undergo an annual dormancy/nondormancy cycle, with seeds coming out of dormancy during winter and reentering it in summer (e.g., Baskin and Baskin, 1985
). Reentry of seeds into dormancy would explain why they did not germinate when a site was flooded. In an attempt to determine if buried seeds of S. hallii undergo annual dormancy/nondormancy cycles under natural temperature regimes, studies on buried seeds of the species were initiated in 1989.
On 29 September 1989, achenes (hereafter "seeds") were collected from S. hallii plants growing around the margins of a seasonal pool of water in a shallow limestone sinkhole in southern Christian County, Kentucky. After 9 mo of testing, however, no seeds had germinated under any condition. Consequently, from May 1990 to April 1998, we conducted many experiments in an attempt to break dormancy and promote germination. These included (1) cold and/or warm stratification under flooded vs. nonflooded conditions in light and in darkness; (2) wet and dry treatments in a nonheated greenhouse during summer; (3) boiled (to drive off oxygen) vs. nonboiled water for flooding; and (4) different water depths and substrate types at the various temperature regimes. Seeds on soil flooded to a depth of 5 or 10 cm in July 1994 germinated to 15% and 12%, respectively; otherwise, no seeds germinated in any of the experiments. In addition, in water (depth of 5 cm in glass jars with a lid) collected from a water-filled depression in Christian County on 3 June 1994, 5% of the seeds germinated.
On 1 April 1998, a bag of seeds buried under nonflooded conditions and exposed to natural seasonal temperature changes since 1989 was exhumed and poured into a jar of water (5 cm depth) and placed in light at a daily alternating temperature regime of 30°/15°C. Our plan was to flood the seeds for 1 wk at 30°/15°C and then test them under both nonflooded and flooded conditions at alternating temperature regimes of 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C. After 1 wk, essentially all of the seeds (about 2000) had germinated. We had discovered that a cue was required for germination of S. hallii seeds. Thus, the objectives of our study subsequently were to (1) identify the germination cue; (2) understand its effects on S. hallii seeds; and (3) to use the cue to investigate various aspects of the germination ecology of S. hallii seeds.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Germination tests were performed in light (14 h daily photoperiod of ca. 40 µmol · m–2 · s–1, 400–700 nm, cool white fluorescent light) or in continuous darkness. Incubators were set on a 12/12 h daily temperature regime, approximating mean daily maximum and minimum temperatures during the growing season in Christian County, Kentucky (Wallis, 1977
): March and November, 15°/6°C; April and October, 20°/10°C; May, 25°/15°C; June and September, 30°/15°C; and July and August, 35°/20°C. The photoperiod in each incubator extended from 1 h before to 1 h after the daily high-temperature period. Field observations indicated that S. hallii seeds can germinate in May or early June in Christian County. Thus, a temperature regime of 30°/15°C was chosen for many of the experiments because it approximates mean daily maximum and minimum temperatures in the region during late May and June (Wallis, 1977
).
Unless otherwise stated, seeds were incubated at the 30°/15°C alternating temperature regime in light (a 14-h daily photoperiod) for 2 wk, then checked for germination. The criterion for germination was emergence of the cotyledon to a length of 1 mm; radicle emergence varied depending on test solution. All ungerminated seeds were pinched under a dissecting microscope to determine if the embryo was firm and white or soft and gray. Tetrazolium tests revealed that the former seeds were alive and the latter ones were dead. Germination percentages and standard errors were rounded to the nearest whole number. Glass jars used in experiments were round, 5.5 cm in diameter, and 12 cm deep. After treatments were prepared, jars were covered with a lid.
Effect of ethanol
Under flooded conditions, ethanol accumulates in living plant tissues as an end-product of anaerobic carbohydrate metabolism (see Jackson et al., 1982
). Depending on plant species and concentration of ethanol, this compound may stimulate germination of seeds (e.g., Smits et al., 1995
). Thus, the objective of this experiment was to determine if ethanol would promote germination of S. hallii seeds.
Seeds of S. hallii were exhumed (Table 1) and placed in glass jars in 125 mL (= a depth of 5 cm) of 0, 50, 100, 200, and 300 µmol/L solutions of ethanol (in distilled water). Three replications of 50 seeds each were used for each condition.
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). This compound has been shown to promote seed germination in some species (e.g., de Wilde and Darné, 1996
; Kepczy
ski and Kepczyska, 1997
; Baskin and Baskin, 1998
). Thus, the objective of the following three experiments was to determine if ethylene would promote germination of flooded S. hallii seeds.
Because ethylene is produced by ripening fruits (Salisbury and Ross, 1992
), pieces of apple and banana were added to jars of water. An unpeeled golden delicious apple was washed, cut into quarters, and each quarter was cut into three pieces of equal size. The banana was peeled and cut into 2-cm sections. One piece of apple was placed in each of three jars containing 125 mL of distilled water, and one piece of banana was placed in another set of three jars. The piece of apple and banana floated at the surface. Fifty exhumed seeds were placed in each of the six jars containing water and a piece of fruit. Three control jars had water and seeds but no apple or banana.
In a second experiment, ethephon was used as a source of ethylene (Warner and Leopold, 1969
). Fifty exhumed seeds were placed in each of 21 glass jars to which 125 mL of ethephon solution (in distilled water) were added. The concentrations of ethephon were 1, 10, 100, or 1000 µmol/L. One control had water but no apple or ethephon, and a second control had water and apple. Three replicates were used for each treatment and control.
In a third study, germination of seeds in 125 mL of 1 µmol/L ethephon was compared to that of seeds in jars with 125 mL of water either with or without apple. This experiment repeated part of the second experiment, but we wanted to know the effects of ethephon on seeds collected in a different year. On three dates, seeds were exhumed, and three replicates of 50 seeds each were used for each test condition.
Techniques for using apple
Because apple and 1 µmol/L ethephon were comparable in their ability to promote germination and yield healthy seedlings and because ethephon is toxic to humans (according to the label on the bottle) and relatively expensive, apple was used in all subsequent germination studies. Apple as opposed to banana was chosen because apple remained firm after 2 wk in water, whereas banana did not. In a preliminary experiment, either zero, one, two, or three pieces of apple were placed in each of three jars per treatment containing 125 mL of water, and 0, 84 ± 2, 35 ± 3, and 1 ± 1% (means ± 1 SE) of the seeds germinated, respectively. The apple pieces were obtained by cutting a washed golden delicious apple (6–7 cm in diameter) into quarters and then cutting each quarter into three pieces. The wedge-shaped piece of apple floated at the water surface, and the bulk of the piece extended down into the water about 2 cm. Because one piece of apple was sufficient for maximum germination, one piece was used in all subsequent experiments.
When one piece of apple was added to a jar containing 125 mL of water, water depth increased from 5 to 9 cm. An experiment was performed to determine if the increased water depth resulting from the addition of one piece of apple to a jar influenced germination. Seeds germinated to 86 ± 4 and 82 ± 2% in the presence of one piece of apple at water depths of 5 and 9 cm, respectively. Thus, the volume of water was kept constant (125 mL) in all control and treatment jars in the following experiments. That is, depth in controls was 5 cm, but depth in treatments was 9 cm after apple was added.
Minimum depth of flooding
The purpose of this experiment was to determine the minimum depth of flooding required for seeds of S. hallii to germinate. Apple was placed in 125 mL of distilled water in each of 21 jars, and after 1 d the piece of apple was removed. Water depth in the jars was adjusted to 0.1 cm (2 mL), 0.5 cm (10 mL), 1 cm (20.5 mL), 2 cm (47.5 mL), 3 cm (72 mL), 4 cm (100 mL), or 5 cm (125 mL); there were three replicates for each depth. Fifty exhumed seeds then were added to each jar.
In a second experiment, apple was floated on 125 mL of water in each of 12 jars, and after 3 d at room temperatures, water from all jars was combined in a large flask and apple pieces discarded. This leachate water then was added to each of three clean jars to a depth of 0 (moist filter paper in the bottom of the jars), 0.5, 1, 2, 3, 4, or 5 cm. Distilled water (only) was added to another set of 21 jars, with a water depth in each of three jars of 0, 0.5, 1, 2, 3, 4, or 5 cm. Fifty exhumed seeds then were added to each jar.
Constant vs. alternating temperatures
The amplitude of daily temperature fluctuations in bodies of water is reduced by an increase in depth, and sensitivity to daily temperature fluctuations can function as a depth-detecting mechanism (Pons and Schroder, 1986
; Baskin and Baskin, 1998
). That is, a decrease in amplitude of daily temperature fluctuations indicates that an area is flooded and plays a role in inhibiting germination of seeds of some species (Pons and Schroder, 1986
). Since seeds of S. hallii require flooding to germinate, would they germinate equally well at a constant temperature of 22°C and an alternating temperature regime of 30°/15°C? Seeds were exhumed on four dates, and 50 seeds were placed in each of 12 jars containing 125 mL of water; six of the jars had apple and six did not have any apple. Three jars of seeds with apple and three without apple were incubated in light at a constant temperature of 22°C and another six at an alternating temperature regime of 30°/15°C for 2 wk.
Addition of apple after various periods of flooding
The purpose of this experiment was to determine if the stimulatory factor from apple had to be present at the initiation of flooding for seeds to germinate. Fifty exhumed seeds were placed in each of 18 glass jars to which 125 mL of distilled water were added. After 0, 1, 2, 4, and 5 d of flooding, apple was added to each of three jars. Three jars of water received no apple (controls).
In a second experiment, 50 exhumed seeds were placed in each of 36 glass jars to which 125 mL of distilled water were added. After 0, 1, 2, 3, 4, 5, 6, and 7 d and 2, 3, and 4 wk (from beginning of experiment), apple was added to each of three jars. Germination percentages at 30°/15°C were determined 2 wk after the apple was added. Three jars of water received no apple (controls), and they were maintained at 30°/15°C for 6 wk.
Removal of apple after various periods of flooding
To determine if apple had to be present in the jars of water for the duration of incubation for seeds to germinate, 50 exhumed seeds were placed in each of 24 glass jars to which 125 mL of distilled water were added. One piece of apple also was added to all but three jars, which served as controls, and after 1, 2, 3, 4, 5, 6, and 7 d of incubation in light at 30°/15°C, the apple was removed from three different jars. All seeds were incubated in light at 30°/15°C for a total of 2 wk.
In a second experiment, 50 exhumed seeds were placed in each of 30 glass jars to which 125 mL of distilled water were added. Apple then was added to each jar, and after 1, 2, 3, 4, and 5 d of incubation in light at 30°/15°C, the apple was removed from six different jars. On each day that the apple was removed, seeds in three jars also were removed, washed (using a sieve), and reflooded with 125 mL of (fresh) distilled water. One control had three jars of water with 50 seeds each and no apple, and a second control had three jars of water with 50 seeds each and one piece of apple that had remained in the water for the 2-wk period.
Nonflooding after apple treatment
The purpose of this experiment was to determine if S. hallii seeds can germinate under nonflooded conditions after they have been flooded to a depth of 5 cm in presence of apple for various periods of time. Fifty exhumed seeds were placed in each of 30 glass jars containing 125 mL of distilled water and apple. After 1, 2, 3, 4, and 5 d, the apple was removed from six different jars. When apple was removed, seeds from three of the jars were washed on a sieve and reflooded with 125 mL of (fresh) distilled water, while those from the other three jars were washed and placed on filter paper moistened with distilled water in 10-cm glass Petri dishes. Seeds in one control were flooded without apple for 2 wk, and those in a second control were placed on moist filter paper without flooding or apple.
Detection of ethylene
An apple leachate was prepared by floating apple in 125 mL of water in a closed jar (as previously described) for 3 d and submitted to a private, commercial laboratory in Illinois for analysis/detection of ethylene.
The Dräger Tube Technique (SKC, Eight Four, Pennsylvania, USA) was used to sample the air space in a jar about a floating piece of apple for presence of ethylene. This test involves a color change (from CH2=CH2 + PD-molybdate complex, a blue reaction product) if the concentration of ethylene in air is 0.2 ppm.
Germination of freshly matured seeds and after various periods of burial
The purpose of this experiment was to determine if freshly matured seeds of S. hallii would germinate under flooded conditions in the presence of apple. Further, if fresh seeds are dormant, when does dormancy break occur during burial under natural temperature regimes?
In September–October 2000, seeds were collected from plants of S. hallii growing in Scott County, Missouri, and from plants of S. hallii grown in Edwardsville, Illinois, from seeds originally collected in Cass and Mason counties, Illinois. These seeds from the Missouri and the Illinois collections then were buried in soil on 20 October 2000 and placed in the nonheated greenhouse, where they were watered daily except on some winter days when the soil was frozen. Fresh seeds (20 October 2000) and those exhumed on the first day of each month until 1 June 2001 and 1 April 2001, for Missouri and Illinois seeds, respectively, were tested in light at 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C under flooded conditions (jars with 125 mL of water), with or without apple in the water for 2 wk. Three replications of 50 seeds each were used for each test condition.
In summer 2001, plants were grown in the nonheated greenhouse from seeds of S. hallii collected in Christian County, Kentucky, on 27 August 1998. Seeds then were collected from the plants on 10 October 2001 and 3 d later placed under flooded conditions (jars with 125 mL of water) with or without one piece of apple in light at 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C for 2 wk, before checking germination.
Effect of flooding during winter
The purpose of this study was to determine if seeds flooded during winter would break dormancy. Seeds collected in Christian County, Kentucky, on 27 August 1998 were buried in each of 12 pots of soil on 7 September 1998. The soil in six pots was watered regularly to keep it moist (but nonflooded), while soil in the other six pots was continuously in a water-filled pool (1.4 m in diameter and 0.25 m deep) on the floor under a bench in the nonheated greenhouse. Both the nonflooded and flooded pots of seeds were exposed to natural temperature conditions from burial until exhumation. One arbitrarily chosen pot each of flooded and nonflooded seeds was exhumed in February, March, April, May, and July 1999. The seeds then were placed in jars with 125 mL of water both with or without apple. Three replicates of 50 seeds each were used for each test condition.
Light requirement for germination
Preliminary studies showed that seeds of S. hallii required light for germination, even if they were flooded and apple was present in the water. The purpose of this experiment was to determine how many days of exposure to light were required before seeds could germinate in darkness.
Fifty exhumed seeds were placed in each of 15 jars containing 125 mL of distilled water. Apple was added to each jar, and after 0, 1, 2, 3, and 4 d of incubation in light at 30°/15°C, three different jars were wrapped with aluminum foil (= darkness). On the day the experiment was initiated, 50 seeds also were placed in each of six jars of water to which no apple was added (controls); three controls with no apple were in light and three in darkness.
Responses to seasonal temperature cycles
The purpose of this study was to determine if buried seeds of S. hallii exposed to natural temperature regimes reenter dormancy in summer, and if so, whether they come out of dormancy the following winter. On 22 October 1999, seeds collected in 1998 were removed from the freezer, and about 1000 seeds were placed in each of 30 fine-mesh polyester cloth bags; each bag was buried to a depth of 7 cm in soil. The pots with buried seeds were placed on the floor under a bench in the nonheated greenhouse and watered daily except when frozen on some winter days. Beginning on 1 December 1999, seeds in one arbitrarily chosen pot were exhumed on the first day of each month until 1 April 2002. Seeds were placed in jars with 125 mL of water with or without apple in light at 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C for 2 wk, before checking for germination. Three replicates of 50 seeds each were used for each test condition.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Effect of ethylene
Flooded seeds of S. hallii in water with no fruit, in water with apple, and in water with banana germinated to 0, 77 ± 3, and 82 ± 1%, respectively.
In the second experiment, seeds in 1, 10, 100, and 1000 µmol/L ethephon germinated to 71 ± 2, 65 ± 3, 72 ± 4, and 77 ± 2%, respectively. No seeds in the water control germinated, and 92 ± 3% of the seeds flooded in the presence of apple germinated. Seedlings in the 1 µmol/L ethephon solution and in water + apple had three green leaves and three or four roots that were up to 5 mm long. Seedlings in 10 µmol/L ethephon had three green leaves and three roots that were somewhat stunted. Seedlings in 100 µmol/L ethephon had three green leaves, but many of them had no roots; if roots were present, they were stunted. Seedlings in 1000 µmol/L ethephon had one white leaf and no roots.
In the third study, seeds exhumed on 25 March 1999 and incubated in water, water + apple, or in 1 µmol/L ethephon germinated to 1 ± 1, 86 ± 4, and 80 ± 3%, respectively. Seeds exhumed on 19 October 1999 germinated to 0, 84 ± 2, and 75 ± 5%, respectively, and those exhumed on 3 April 2002 germinated to 3 ± 1, 100, and 98 ± 2%, respectively.
Minimum depth of flooding
Seeds at depths of 0.1, 0.5, 1, 2, 3, 4, and 5 cm in apple leachate germinated to 14 ± 4, 16 ± 1, 19 ± 2, 33 ± 8, 56 ± 4, 54 ± 6, and 53 ± 2%, respectively. Thus, the minimum depth for optimum germination was 3 cm.
In the second experiment, seeds at depths of 0, 0.5, 1, 2, 3, 4, and 5 cm in apple leachate germinated to 1 ± 1, 5 ± 1, 3 ± 1, 11 ± 1, 43 ± 3, 58 ± 4, 59 ± 1, and 76 ± 4%, respectively, while those in distilled water (with no apple) germinated to 1 ± 1, 1 ± 1, 4 ± 2, 3 ± 1, 1 ± 1, 0, 1 ± 1, and 2 ± 1%, respectively.
Constant vs. alternating temperatures
Regardless of when seeds were exhumed, they required the presence of apple in the water for germination, and they germinated to significantly higher percentages at 30°/15°C than at 22°C (Table 2).
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In the second experiment, apple added to the water after 0, 1, 2, 3, 4, 5, 6, and 7 d and after 2, 3, and 4 wk resulted in 87 ± 6, 87 ± 2, 73 ± 4, 76 ± 6, 89 ± 5, 88 ± 1, 85 ± 4, 96 ± 3, 76 ± 4, 91 ± 4, and 88 ± 3% germination, respectively. After 6 wk of incubation in light at 30°/15°C, only 2 ± 0% of the seeds had germinated in the jars without apple.
Removal of apple after various periods of flooding
Seeds in jars from which apple was removed after 1, 2, 3, 4, 5, 6, and 7 d germinated to 72 ± 3, 86 ± 3, 82 ± 1, 92 ± 5, 91 ± 3, 91 ± 2, and 90 ± 1%, respectively. No seeds germinated in the control without apple.
In the second experiment, all seeds germinated in jars from which apple was removed after 1, 2, 3, 4, or 5 d when seeds were not washed and placed in fresh water. Germination of seeds washed after 1, 2, 3, 4, or 5 d and placed in fresh water, however, was 3 ± 1, 21 ± 4, 97 ± 3, 100, and 100%, respectively. None of the seeds in jars without apple germinated, while all seeds germinated in jars in which the apple was not removed. Thus, if seeds of S. hallii were transferred to fresh water after flooding for 3 d in the presence of apple, they germinated to 97–100%.
Nonflooding after apple treatment
Seeds washed after 1, 2, 3, 4, or 5 d of flooding in water with apple and then reflooded in distilled water germinated to 8 ± 2, 23 ± 5, 100, 100, and 100%, respectively, while those washed after 1, 2, 3, 4, or 5 d and placed on moist filter paper germinated to 3 ± 2, 8 ± 4, 35 ± 3, 77 ± 3, and 90 ± 2%, respectively. Thus, germination of seeds flooded in the presence of apple and subsequently placed on moist filter paper increased with an increase flooding duration. However, regardless of flooding duration, seeds germinated to a higher percentage in fresh water than on moist filter paper.
Detection of ethylene
Technicians at the private laboratory in Illinois were not able to detect ethylene in the water in which apple had been floating for 3 d. However, the Dräger Tube Technique gave a positive test for ethylene in the air space above the floating piece of apple.
Germination of freshly matured seeds and after various periods of burial
Because most seeds from Missouri and Illinois were dormant when the study was initiated (Table 3), they did not germinate under any temperature regime. By 1 December 2000, however, 86–92% of the seeds from both states germinated at 25°/15°, 30°/15°, and 35°/20°C in water with apple, but no seeds germinated in the absence of apple. Throughout the study, seeds germinated to 56–100% in the presence of apple at these three temperatures. At 20°/10°C, Missouri seeds reached peak germination (17%) in April 2001 and Illinois seeds (29%) in February 2001. By May 2001, 0 and 9% of the Missouri and Illinois seeds, respectively, had germinated at 20°/10°C. Few or none of the seeds germinated at 15°/6°C, regardless of seed origin or month tested.
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Effect of flooding during winter
Only 3% of the nonflooded seeds exhumed and tested in February 1999 germinated, while none of the flooded seeds germinated (Table 4). However, in March, April, May, and July, seeds that had been buried in nonflooded soil in the greenhouse germinated to 79–86% in the presence of apple, but only 1–2% of those that had been flooded during winter germinated, even in the presence of apple.
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2 d exposure to light.
Responses to seasonal temperature cycles
Seeds were dormant at the time they were buried in soil in October 1999. By December 1999, exhumed seeds germinated to 80–99% in the presence of apple in light at 20°/10°, 25°/15°, 30°/15°, and 35°/20°C but to only 11% at 15°/6°C (Fig. 1). From December 1999 through April 2002, exhumed seeds germinated to 75–100% in presence of apple in light at 30°/15° and 35°/20°C and to 59–95% at 25°/15°C, regardless of when they were exhumed and tested. During this time, however, germination at 15°/6°C peaked in spring or summer and at 20°/10°C in late autumn to midsummer. Thus, seeds exhibited conditional dormancy/nondormancy cycles.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, ethylene accumulates in anaerobic waterlogged soils from the enhanced stability and entrapment of ethylene in water. Further, ethylene is soluble in water (ca. 140 ppm, 25°C), and its diffusion is about 10 000 times slower in water than in air (Arshad and Frankenberger, 2002
). Thus, if ethylene was in the air above the floating apple, then it also was probably in the water, explaining why apple in water elicited a germination response in S. hallii seeds comparable to that of 1 µmol/L ethephon. Consequently, in the presence of apple or in the natural habitat of S. hallii ethylene is the probable germination cue.
We conclude that seeds of S. hallii produce a small amount of ethylene when they are flooded and that, when large numbers of them (about 2000) are flooded in a relatively small volume (125 mL) of water, enough ethylene accumulates to promote germination. Dormant as well as nondormant seeds produce ethylene, but production is higher in nondormant than in dormant seeds (Kepczyski and Kepczyska, 1997
). However, in jars with only 50 S. hallii seeds, or in water-filled depressions in nature, the concentration of ethylene resulting from S. hallii seeds is too small (or too diluted) to promote germination. In the field, however, ethylene production by flooded soils and organic material (Smith and Restall, 1971
) would increase ethylene concentrations to levels that promote germination. Perhaps lack of seed germination in flooded populations of S. hallii in some years is related to low ethylene production. Ethylene production is influenced by the availability of substrate (e.g., crop residues) for microbial activity, temperature, and concentration of oxygen (Smith and Dowdell, 1974
). Because ethylene production also is reduced by high levels of NO3– in the soil (Arshad and Frankenberger, 2002
), high levels of fertilizer in runoff water might inhibit production of ethylene and consequently prevent seed germination. At the molecular level, ethylene promotes germination by inhibiting signaling of the dormancy-inducing hormone abscisic acid (Beaudoin et al., 2000
).
Apple and flooding enhanced germination within a relatively short period of time. Seeds kept in apple leachate solution germinated to 72% when the apple had leached into the water for just 1 d. Further, seeds that were washed and placed in fresh water after the apple was removed germinated to 97% after only 2 d of exposure to apple. Additionally, seeds flooded for 5 d in the presence of apple subsequently germinated to 90% on filter paper moistened with distilled water. We cannot rule out the possibility that seeds may have begun to germinate before they were washed (after 5 d) and placed on the wet filter paper; however, no emergent cotyledons (indicating completion of germination) were observed. The implication of the results from our experiments involving removal of apple is that seeds of S. hallii might germinate if a depression is flooded to a depth of 3 cm for 5 d and the water then recedes, assuming that ethylene was produced during flooding and that seeds were exposed to light.
Light and flooding
The light requirement for germination also may play a role in regulating germination in years when population sites are flooded. Specifically, if a site has been disturbed regularly (e.g., plowed each year for crop production) and seeds are covered with soil, darkness would prevent germination. Our studies show that apple (ethylene) does not substitute for the light requirement for germination; however, flooded seeds exposed to light for 2 d in the presence of apple subsequently germinated in darkness. Consequently, if seeds in the field were flooded and exposed to light for several days prior to the time that runoff water from a large precipitation event carried silt into a water-filled depression, seeds might be able to germinate in the reduced light (or even in darkness). However, burial in soil in nature also results in a decrease in daily temperature fluctuations, which could reduce or inhibit germination.
Although flooding per se is not the germination cue, seeds of S. hallii need to be flooded for a minimum period of time in the presence of apple before they can germinate, either under water or on a moist nonflooded substrate. In the presence of apple leachate solution, 50% germination required water depths of 3 cm. Thus, unlike seeds of the summer annual Schoenoplectus purshianus (Fern.) Strong, which germinate on moist sand (Baskin et al., 2000
), seeds of S. hallii require the presence of apple and flooding to germinate. The relatively shallow depth of the apple leachate solution required to promote germination of S. hallii is consistent with higher germination at the alternating temperature regime (30°/15°C), associated with shallow water, than at a constant temperature (22°C), associated with deep water.
Ecology of seed dormancy and germination
Identifying ethylene as the apparent germination cue for seeds of S. hallii in nature made it possible to investigate the ecological aspects of dormancy break and subsequent germination in this rare species. Freshly collected seeds of S. hallii from Missouri and Illinois (Table 3) and those from plants grown in the nonheated greenhouse in 2001 germinated to a maximum of only 6% at 35°/20°C, even in the presence of apple; that is, most seeds did not respond to flooding and apple and thus were dormant. Seeds of S. hallii buried under moist, but nonflooded conditions during winter in the nonheated greenhouse acquired the ability to germinate to high percentages (80%) in light in water with apple by the following spring (Table 3, Fig. 1). However, seeds buried under flooded conditions did not acquire the ability to germinate, even in water with apple added to it (Table 4). The implication of these results for habitat management is that the best water regime for dormancy break and germination of S. hallii seeds in the field is nonflooding during winter and flooding in spring. In contrast to S. hallii, seeds of S. purshianus that are flooded during winter do break dormancy, but the highest germination occurred in seeds that were not flooded in winter and then flooded in light at spring temperatures in spring (Baskin et al., 2000
).
The rapid loss of dormancy in nonflooded, buried seeds of S. hallii during winter was similar to the responses of seeds of S. purshianus (Baskin et al., 2000
), Cyperus erythrorhizos Muhl., C. flavicomis Michx., Fimbristylis vahlii (Lam.) Link, and F. autumnalis (L.) R. & S. buried under nonflooded conditions during winter in a nonheated greenhouse (Baskin et al., 1993
). Although the latter five Cyperaceae are summer annuals and grow on mudflats, seeds of these species exhumed in spring germinated to high percentages on moist sand in light but did not require flooding or ethylene to germinate. Like buried seeds of C. erythrorhizos, C. flavicomis, F. vahlii, and F. autumnalis (Baskin et al., 1993
), those of S. hallii never lost the ability to germinate at 30°/15° or 35°/20°C after the initial dormancy was broken. In contrast, buried seeds of S. purshianus lost the ability to germinate at all temperatures during summer (Baskin et al., 2000
). Lack of germination in some wet years at natural populations of S. hallii cannot be attributed to inability of seeds to germinate in light at high temperatures during late spring or summer, if flooding and ethylene are available.
Germination of exhumed S. hallii seeds at the March temperatures (15°/6°C) in March was 45% in 2000, 0% in 2001, and 37% in 2002, and at the April temperature (20°/10°C) in April it was 87% in 2000, 21% in 2001, and 26% in 2002 (Fig. 1). Because germination in March and in April varies with the year, we conclude that seeds of S. hallii may or may not germinate if natural populations are flooded in March or in April. Whereas, if flooding is delayed until May (25°/15°C) or June (30°/15°C), temperatures would be favorable for germination. Thus, another reason why flooded population sites of S. hallii may have plants in some wet years and not in others could be related to the temperatures at time of flooding. Significantly, ethylene production in flooded soil increases with an increase in temperature (Smith and Restall, 1971
).
Seeds of S. hallii do not come out of dormancy if they are flooded during winter, but if nondormant seeds are flooded in summer they remain nondormant. Seeds exhumed in May and flooded in light at 30°/15°C did not germinate, but they remained capable of responding to the germination cue (apple) for at least 4 wk. We do not know how long seeds could remain flooded and still be able to respond to apple. Thus, if seeds were flooded in the field in early spring when temperatures are too low for germination, perhaps they would still germinate when temperatures increased, resulting in an increase in ethylene production in the anaerobic soil.
Seeds of S. hallii break dormancy under nonflooded conditions during winter, but they require flooding, ethylene, and light for germination (Fig. 2). An ethylene cue for germination of S. hallii seeds serves as a "flood-detecting" mechanism, and it indirectly indicates that the habitat is suitable for seedling establishment and growth of this relatively short (ca. 3–4 dm) summer annual. Thus, if there is adequate depth and duration of flooding for ethylene to be produced, the depression probably would hold enough water for successful seed germination, seedling establishment, and maturation of the plants. Also, because ethylene production is increased by an increase in organic material (Smith and Restall, 1971
), perhaps ethylene is an indirect signal that competing species are absent or dead. Further (as noted by one of the reviewers), microbial decomposition of organic material results in mineralization of nutrients; consequently, increased ethylene also may be a cue for increased available nutrients.
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FOOTNOTES |
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6 Author for reprint requests (ccbask0{at}uky.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Baskin C. C. J. M. Baskin 1998 Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press, San Diego, California, USA
Baskin C. C. J. M. Baskin E. W. Chester 1993 Seed germination ecophysiology of four summer annual mudflat species of Cyperaceae. Aquatic Botany 45: 41-52[CrossRef][ISI]
Baskin C. C. J. M. Baskin E. W. Chester 2000 Effect of flooding on the annual dormancy cycle and on germination of seeds of the summer annual Schoenoplectus purshianus (Cyperaceae). Aquatic Botany 67: 109-116
Baskin J. M. C. C. Baskin 1985 The annual dormancy cycle in buried weed seeds: a continuum. BioScience 35: 492-498[CrossRef][ISI]
Beaudoin N. C. Serizet F. Gosti J. Giraudat 2000 Interactions between abscisic acid and ethylene signaling cascades. Plant Cell 12: 1103-1115
de Wilde A. G. Darné 1996 Attempts to improve the grape seed germination. Journal International des Sciences de la Vigne et du Vin 30: 113-120
Jackson M. B. B. Herman A. Goodenough 1982 An examination of the importance of ethanol in causing injury to flooded plants. Plant, Cell and Environment 5: 163-172
Kepczyski J. E. Kepczyska 1997 Ethylene in seed dormancy and germination. Physiologia Plantarum 101: 720-726[CrossRef]
McClain W. E. R. D. McClain J. E. Ebinger 1997 Flora of temporary sand ponds in Cass and Mason counties, Illinois. Castanea 62: 65-73
McKenzie P. M. 1998 Hall's bulrush (Schoenoplectus hallii) status assessment. U.S. Fish and Wildlife Service, Columbia, Missouri, USA
Pons T. L. H. F. L. M. Schroder 1986 Significance of temperature fluctuation and oxygen concentration for germination of the rice field weeds Fimbristylis littoralis and Scirpus juncoides. Oecologia 68: 315-319[CrossRef][ISI]
Robertson K. R. L. R. Phillippe S. M. Gehlhausen 1993 The current status of Scirpus hallii A. Gray, Hall's bulrush, in Illinois. Project Completion Report, 1 June 1993 through 31 December 1993. Prepared for Illinois Department of Conservation, Springfield, Illinois, USA
Salisbury F. B. C. W. Ross 1992 Plant physiology, 4th ed. Wadsworth, Belmont, California, USA
Smith K. A. R. J. Dowdell 1974 Field studies of the soil atmosphere. I. Relationships between ethylene, oxygen, soil moisture content, and temperature. Journal of Soil Science 25: 217-230[CrossRef][ISI]
Smith K. A. S. W. F. Restall 1971 The occurrence of ethylene in anaerobic soil. Journal of Soil Science 22: 430-443[CrossRef][ISI]
Smits A. J. M. G. H. W. Schmitz G. Van Der Velde L. A. C. J. Voesenek 1995 Influence of ethanol and ethylene on the seed germination of three nymphaeid water plants. Freshwater Biology 34: 39-46[CrossRef][ISI]
Steyermark J. A. 1963 Flora of Missouri. Iowa State University Press, Ames, Iowa, USA
Wallis A. L., Jr. (compiler and editor) 1977 Comparative climatic data through 1976. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Environmental Data Service, National Climatic Center, Asheville, North Carolina, USA
Warner H. L. A. C. Leopold 1969 Ethylene evolution from 2-chloroethylphosphonic acid. Plant Physiology 44: 156-158
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