| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Ecology |
2School of Biological Sciences, University of Kentucky, Lexington, Kentucky 40506-0225 USA; 3Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546-0091 USA; 4Department of Forest Vegetation Ecology, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden
Received for publication April 10, 2001. Accepted for publication September 7, 2001.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
15°C) stratification in breaking dormancy in seeds with stony endocarps that require warm-plus-cold (
0°–10°C) stratification for germination. This question was addressed using seeds (true seed + endocarp, hereafter called seeds) of Empetrum hermaphroditum. Only 2–5% of freshly matured seeds collected in September and October at five sites in Sweden germinated in light at daily alternating temperature regimes of 15°/6°, 20°/10°, and 25°/15°C. Dormancy was not due to impermeability of the stony endocarp surrounding each seed, and embryos did not grow prior to radicle emergence. Thus, seeds did not have physical, morphological, or morphophysiological dormancy. Long periods of either cold stratification (20 or 32 wk) or warm stratification (16 wk) resulted in a maximum of 22–38 and 10% germination, respectively, in light at 25°/15°C. After 12 wk warm stratification plus 20 wk cold stratification, 83–93% of the seeds germinated in light at the three temperature regimes. For a cold stratification period of 20 wk, germination increased with increase in length of the preceding warm stratification treatment. Gibberellic acid (GA3) promoted germination of 77–87% of the seeds. Based on dormancy-breaking requirements and response to GA3, 62–78% of the seeds had intermediate physiological dormancy; the others had nondeep physiological dormancy. Contrary to suggestions of several other investigators that warm stratification is required to make the endocarp permeable to water via its breakdown by microorganisms, our results with E. hermaphroditum show that this is not the case. In this species, warm stratification is part of the dormancy-breaking requirement of embryos in seeds with intermediate physiological dormancy.
Key Words: Empetraceae • Empetrum • imbibition • intermediate physiological dormancy • seed dormancy • stony endocarps • warm-plus-cold stratification
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
15°C) stratification in breaking dormancy of seeds covered by stony indehiscent endocarps that require both warm and cold (0°–10°C) stratification for germination? Stony endocarps occur in some (or all) members of several plant families, e.g., Anacardiaceae, Arecaceae, Cornaceae, Elaeagnaceae, Empetraceae, Juglandaceae, Meliaceae, Nyssaceae, Oleaceae, Rhamnaceae, and Rosaceae (Hill, 1933, 1937
; Nikolaeva, 1969
; Gleason and Cronquist, 1991
). A stony endocarp may be water-impermeable, thereby preventing imbibition and thus germination of the seed, e.g., Cotinus and Rhus (Anacardiaceae) (Baskin and Baskin, 1998
), or it may be water permeable, e.g., Prunus sp. (apricot, Rosaceae) (Nikolaeva, 1969
) and Sclerocarya (Anacardiaceae) (von Teichman, Small, and Robbertse, 1986
). However, tests for imbibition of water have been done for only a few species whose seeds are enclosed by a stony endocarp.
Some seeds with stony endocarps germinate well following cold stratification, e.g., Cornus spp. (90–120 d of cold stratification), Corylus (60–180 d), Menispermum (14–28 d), Morus (30–90 d), Nyssa (30–120 d), and Oemleria (120 d) (Young and Young, 1992
). Other seeds with stony endocarps, e.g., Halesia (Young and Young, 1992
), Rosa (Densmore and Zasada, 1977
), and Rubus (Young and Young, 1992
), require a period of warm stratification followed by a period of cold stratification before they will germinate. The reason some seeds with stony endocarps require a warm stratification treatment followed by a cold stratification treatment before they will germinate is not well understood. Crocker and Barton (1957)
concluded that Arctostaphylos, Cornus, Cotoneaster, Crataegus, Halesia, Rhodotypos, and Symphoricarpos had impermeable coats (endocarps) and dormant embryos. They suggested that soil microorganisms attacked the endocarps and broke them down while seeds were exposed to the warm, wet conditions during summer. Then, dormancy of the embryos was broken by cold stratification during winter, and seeds germinated in spring. Heit (1967)
recommended that seeds of Arctostaphylos, Elaeagnus, Rubus, Sambucus, and Shepherdia, which have stony endocarps, be acid scarified and then cold stratified to promote germination. The implication of using an acid scarification treatment is that the endocarps are impermeable; however, no data demonstrating endocarp impermeability were presented.
Acid scarification plus warm stratification plus cold stratification have been suggested as the best procedure to break dormancy in seeds (with intact stony endocarp) of Arctostaphylos spp., Elaeagnus angustifolia L., Cornus spp., Rubus idaeus L., Sambucus spp., and Symphoricarpos spp. (Crocker and Barton, 1957
; Heit, 1967
; Young and Young, 1992
). However, weighing Sambucus and Symphoricarpos seeds (with intact endocarp) after various periods of time on a wet substrate demonstrated that they were permeable to water (Hidayati, Baskin, and Baskin, 2000, 2001
). Warm followed by cold stratification treatments were required to break the morphophysiological dormancy of the small, underdeveloped embryos in seeds of these two genera. Thus, warm stratification was part of the regime of environmental conditions required to break embryo dormancy in seeds of Sambucus and Symphoricarpos and not to make the endocarp permeable.
An understanding of the role of warm and cold stratification treatments in promoting germination of Sambucus and Symphoricarpos seeds raises questions concerning germination of seeds that require warm plus cold stratification for germination and have stony endocarps with fully developed embryos. Is warm stratification necessary to make the endocarp permeable to water, or is it part of the dormancy-breaking requirements of the embryo? Thus, studies were conducted on seeds with fully developed embryos and stony endocarps that potentially had a warm plus cold requirement for germination.
The species chosen for study was Empetrum hermaphroditum Hagerup [= E. nigrum L. var. hermaphroditum (Hagerup) T. Soerensen]. Empetrum hermaphroditum (crowberry) is a low-growing (5–15 cm tall), evergreen shrub that is much-branched and has a spreading growth habit. This tetraploid species is circumboreal in distribution (Gleason and Cronquist, 1991
), and in northern Fennoscandia it dominates the ground-layer vegetation in coniferous forests with long fire-return intervals (Zackrisson et al., 1995
). The role of this species in the suppression of natural regeneration and seedling growth of conifers has received considerable research attention (Nilsson et al., 1993
; Nilsson, 1994
). Crowberry is a fire-sensitive species, but after a killing fire it reestablishes from seeds; this also occurs following anthropogenic disturbances such as clear-cutting (Wardle et al., 1998
). Once plants become established, they eventually dominate the ground-layer vegetation via clonal expansion (Wardle et al., 1998
).
Although initial establishment of E. hermaphroditum at a site depends on seed germination, little research has been done on this phase of the life cycle. Seeds (+ endocarp) of E. nigrum overwintering in the field in England germinated to higher percentages (only 16%) than those stratified at 3°C for 16 wk (percentage not given) (Bell and Tallis, 1973
). Seeds (+ endocarp) of E. nigrum sown under natural conditions in England (Dalby, 1961) and in Norway (Sanda, 1974
) required 2 yr for germination; however, a warm moist treatment for 5 mo followed by a cool moist treatment at 0°–3°C for 3 mo promoted germination (Sanda, 1974
). Although no data demonstrating endocarp impermeability to water were presented, Sanda (1974)
suggested that microorganisms broke down the endocarp during the warm treatment, seeds imbibed water, and dormancy was broken during the cool treatment. Thus, with regard to seeds (= true seed + endocarp) of E. hermaphroditum we asked: (1) Are freshly matured seeds dormant? (2) Are seeds permeable to water? (3) Will warm and/or cold stratification promote germination? (4) Do seeds placed in the natural habitat require >1 yr for germination?
If seeds of E. hermaphroditum are permeable to water but require warm and/or cold stratification for germination that would mean they have physiological dormancy. According to Nikolaeva (1977)
, there are three types of physiological dormancy: nondeep, intermediate, and deep. Nondeep physiological dormancy is broken by relatively short (1–8 wk) periods of warm or cold stratification, depending on the species, and gibberellic acid (GA3) is effective in promoting germination (Baskin and Baskin, 1998
). Intermediate physiological dormancy is broken by long (8–14 wk) periods of cold stratification; however, dry storage (afterripening) at room temperatures (Crocker and Barton, 1931
) or warm stratification (Baskin, Baskin, and Meyer, 1993
) may reduce the length of the cold stratification period needed to break dormancy. Also, GA3 can promote germination (Baskin and Baskin, 1998
). Deep physiological dormancy is broken by long periods of cold stratification, and dry storage does not decrease the length of the cold stratification period required to break dormancy; GA3 is not effective in promoting germination (Baskin and Baskin, 1998
). Since response of seeds to GA3 can be important in determining the type of physiological dormancy, the effect of GA3 on germination of E. hermaphroditum was determined.
Finally, preliminary observations revealed that seeds of E. hermaphroditum had a linear embryo, similar to that in seeds of E. nigrum (Martin, 1946
), but it was only 75% of the length of the endosperm. Since embryo growth prior to radicle emergence would indicate that seeds had an underdeveloped linear embryo and morphological dormancy, embryo measurements were made before and after seeds were subjected to dormancy-breaking treatments.
|
METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
|
40 µmol·m–2·s–1, 400–700 nm cool-white fluorescent light each day) at 12 : 12 h daily alternating temperature regimes of 15°/6°, 20°/10°, and 25°/15°C, which simulate mean daily maximum and minimum temperatures at the beginning and end (15°/6°C) and during the middle (20°/10°, 25°/15°C) of the growing season in Sweden (Müller, 1982
). The light period in each incubator extended from 1 h before the high temperature period began to 1 h after it ended. Each dish was wrapped with plastic film to reduce loss of water. Protrusion of the radicle was the criterion for germination. At the conclusion of each germination test, all ungerminated seeds were pinched open with forceps to determine whether they were filled and whether they contained a firm white embryo (viable as confirmed by tetrazolium test) vs. a soft gray (dead) embryo (nonviable as confirmed by tetrazolium test). Thus, all germination percentages reported in this paper are based on number of viable seeds.
Germination of freshly matured seeds
Within 6–14 d after they were removed from fruits, seeds from each collection site were tested for germination in light at 15°/6°, 20°/10°, and 25°/15°C. Germination percentages were determined after 2, 4, and 6 wk.
Imbibition of water
Three replications of 100 seeds each of E. hermaphroditum from Salmisjärvi were placed on Whatman No. 1 filter paper moistened with distilled water in 9 cm diameter glass petri dishes and kept at room temperature (22°C). After 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, and 24 h on moist filter paper, seeds were blotted dry and weighed. Percentage of water uptake was determined as actual increase based on seed air-dry mass.
Cold stratification
Seeds collected at Rovågern (19 October 1997), Salmisjärvi (21 September 1998), Månsträsk (22 September 1998), and Månsträsk (14 October 1998) were cold stratified for 12 wk and then tested for germination in light at 15°/6°, 20°/10°, and 25°/15°C. Seeds from the first three collections were stratified at a 12 : 12 h daily temperature regime of 5°/1°C (day/night). Seeds were exposed to 40 µmol·m–2·s–1, 400–700 nm, of cool-white fluorescent light for 12 h each day, while the temperature was 5°C. Seeds from fourth collection were stratified in continuous darkness (dishes were wrapped with aluminum foil) at 1°C. Germination percentages were determined for all stratified seeds after 6 wk of incubation at the three alternating temperature regimes.
On the day seeds from each of the four collections were placed at 5°/1°C or at 1°C for stratification treatments, control (nonstratified) seeds were placed in light at the three alternating temperature regimes. Nonstratified seeds were kept in their respective incubator for 18 wk and checked for seedlings at 2-wk intervals; water was added as needed to keep the soil moist.
Cold-stratification temperature
To determine whether 5°/1° or 1°C is the most effective temperature regime for cold stratification, seeds of E. hermaphroditum from Månsträsk (collected 10 October 1998) were cold stratified in darkness at 5°/1°C and at 1°C for 12 wk and then incubated in light at 20°/10° and 25°/15°C for 6 wk. Germination at 20°/10° and at 25°/15°C was compared for seeds receiving cold stratification at 5°/1° and 1°C, using the Mann-Whitney U test at the 5% level of significance (Sokal and Rohlf, 1981
).
Warm stratification
To determine whether warm stratification would break dormancy, 12 dishes of E. hermaphroditum seeds from Månsträsk (10 October 1998) were placed in darkness at 20°/10°, 25°/15°, and 30°/15°C for 4, 8, 12, or 16 wk. After each period of time, three dishes of seeds from each regime were placed in light at 25°/15°C and checked for germination after 6 wk. Three dishes of seeds (controls) were incubated in light at each regime for 22 wk.
Warm stratification followed by cold stratification
Seeds of E. hermaphroditum from Månsträsk (22 September 1998) were used to determine the effect of warm stratification on germination of seeds that subsequently were cold stratified. The warm-stratification temperature regime was 25°/15°C, and the cold-stratification temperature was 1°C. Seeds were exposed to a 14 h daily photoperiod at 25°/15°C, but were in darkness at 1°C. Seeds were warm stratified at 25°/15°C for 12 wk, after which they were given a 20-wk period of cold stratification at 1°C. After 20 wk at 1°C, seeds were incubated in light at 15°/6°, 20°/10°, or 25°/15°C for 6 wk, and dishes were checked for seedlings at 2-wk intervals. There were two controls: (1) seeds receiving no warm stratification were given 20 wk of cold stratification and then incubated in light at the three alternating temperature regimes for the remainder of the experiment, and (2) seeds receiving neither warm nor cold stratification were incubated in light at the three temperature regimes for the duration of the study. A two-way analysis of variance (ANOVA) and comparison of means by protected least significant difference tests (PLSDs, P < 0.05) (SAS, 1985
) were based on the arcsine-transformed proportion of seeds that had germinated in each dish at the end of the experiment.
In a second warm-followed-by-cold-stratification experiment, seeds collected in Månsträsk (14 October 1998) were given a warm-stratification treatment of 4, 8, or 12 wk at 25°/15°C and then a 20-wk cold stratification period at 1°C. Seeds were incubated in light at 25°/15°C for 6 wk. Seeds receiving 20 wk of cold stratification but no warm stratification and those receiving neither warm nor cold stratification also were incubated in light at 25°/15°C.
Prolonged periods of cold stratification
A 12-wk period of cold stratification is adequate to break dormancy in seeds of many species, but seeds of some species are known to require more than 12 wk before they will germinate (Baskin and Baskin, 1998
). Thus, seeds from Salmisjärvi (21 September 1998) and Månsträsk (22 September 1998) were given 12-, 24-, or 32-wk periods of cold stratification in light at 5°/1°C. After each cold-stratification treatment, seeds were incubated in light at 15°/6°, 20°/10°, and 25°/15°C for 6 wk.
After 12, 24, or 32 wk of cold stratification, germination percentages ranged from 1 to 27% (Tables 4, 5). Subsequently, all cold-stratified seeds were kept at 15°/6°, 20°/10°, or 25°/15°C for an additional 6 wk and then returned to 5°/1°C, where they received a 14-wk period of cold stratification. After 14 wk at 5°/1°C, seeds were returned to their respective 15°/6°, 20°/10°, or 25°/15°C incubator for 6 wk. The second cold-stratification period enhanced germination, thus we gave the ungerminated seeds a third warm-plus-cold-stratification treatment. Consequently, the warm- and cold-stratification treatments described above were repeated, and then seeds were returned to their respective 15°/6°, 20°/10°, or 25°/15°C incubator for 6 wk, at which time the experiment was terminated.
On the day seeds initially were placed at 5°/1°C for the first cold stratification treatment, control (nonstratified) seeds were placed in light at 15°/6°, 20°/10°, and 25°/15°C. These seeds were kept in their respective incubator for the duration of the experiment (100 wk). Dishes were checked for seedlings at 2-wk intervals, and water was added as needed to keep the soil moist. A two-way analysis of variance (ANOVA) and comparison of means by protected least significant difference tests (PLSDs, P < 0.05) (SAS, 1985
) were based on the arcsine-transformed proportion of seeds germinating in each dish at the end of the 6-wk incubation period following each of the three cold-stratification periods.
Seeds placed in the field
Fifty seeds of E. hermaphroditum from Månsträsk (16 October 1997) were placed in each of 90 fine-mesh polyester cloth bags, and on 21 November 1997 the bags were placed under snow in the natural habitat of E. hermaphroditum at Månsträsk. Half of the bags were placed in a 8 x 4 m plot, where snowcover was left intact all winter and the other half in a 8 x 4 m plot where snow depth was reduced by one-half every 2 wk during winter. Seeds under full snowcover would be subjected to the maximum period of cold stratification (temperatures near 0°C) that could occur in the habitat, while those under half snowcover potentially would have their cold-stratification period decreased by a few days or weeks, depending on rate of snow melt in spring. That is, the snow would melt earlier in the half snowcover plot than in the full snowcover plot. The full- and half-snowcover treatments would also simulate the natural variation in snow depth that could occur in a habitat due to snow drifting.
In May, June, and July 1998 and in May, June, and July 1999, five randomly chosen bags of seeds were removed from the full- and half-snowcover plots, and each was examined for presence of germinated seeds. Seeds in each bag were sown on moist soil and incubated in light at 25°/15°C for 6 wk. Many ungerminated seeds remained in the dishes after 6 wk of incubation at 25°/15°C, and these were given additional warm-plus-cold-stratification treatments. Thus, the ungerminated seeds were given an additional 6 wk of warm stratification at 25°/15°C and then transferred to darkness at 1°C for a 20-wk period of cold stratification. After 20 wk at 1°C, the dishes were returned to light at 25°/15°C, and germination monitored at 2-wk intervals for 6 wk. The warm-plus-cold-stratification treatments described above were repeated, and then seeds were incubated at 25°/15°C for 6 wk, at which time the experiment was terminated. A two-way analysis of variance (ANOVA) and comparison of means by protected least significant difference tests (PLSDs, P < 0.05) (SAS, 1985
) were based on the arcsine-transformed proportion of seeds germinating in each dish at the end of the 6-wk incubation period, when seeds were retrieved from the field, and at the end of the 6-wk incubation period following each of the two cold-stratification periods.
Effect of GA3
Seeds of E. hermaphroditum from Rovågern (19 October 1997) were placed in 9 cm diameter glass petri dishes on two sheets of Whatman No. 1 filter paper moistened with either distilled water (control) or with a solution of 10, 100, or 1000 mg/L GA3 (K-GA3) dissolved in distilled water. Three replications of 50 seeds each were used for each test condition. All seeds were incubated in light at 25°/15°C and checked for germination at 2 wk intervals for 12 wk.
The experiment was repeated with seeds from Månsträsk (14 October 1998), using distilled water and 1000 mg/L of GA3. Seeds were incubated in light at 25°/15°C and checked at 2-wk intervals for 8 wk.
Embryo measurements
Seeds from Rovågern (collected in 1997 and stored in a freezer at –20°C for 1 yr) were allowed to imbibe on filter paper moistened with 1000 mg/L GA3. After 24 h and after 2, 4, 6, and 8 wk in light at 25°/15°C, embryos were excised from 15 seeds with a razor blade and measured using a microscope equipped with a micrometer.
In another experiment, 15 seeds were incubated on filter paper moistened with distilled water for (1) 24 h in light at 25°/15°C, (2) 12 wk in light at 25°/15°C, (3) 12 wk in darkness at 1°C, or (4) 12 wk in light at 25°/15°C and then 12 wk in darkness at 1°C. After each treatment, embryos were excised and length measured.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Imbibition of water
Seeds of E. hermaphroditum imbibed water rapidly, reaching 94% of their final mass after 1 h (Fig. 1); thus, the stony endocarp enclosing seeds of E. hermaphroditum does not prevent imbibition of water.
|
Cold-stratification temperature
Seeds cold stratified at 5°/1°C and at 1°C germinated to 9 ± 1% (mean ± 1 SE) and 9 ± 4%, respectively, at 20°/10°C and to 16 ± 7% and 22 ± 3%, respectively, at 25°/15°C. Cold-stratification temperature had no significant effect on germination.
Warm stratification
Maximum germination was 10 ± 1% (mean ± 1 SE) for seeds warm stratified at 20°/10°, 25°/15°, and 30°/15°C for 4, 8, 12, or 16 wk and then transferred to light at 25°/15°C. Thus, warm stratification did not increase germination. None of the control seeds had germinated in light after 22 wk at the three temperature regimes.
Warm stratification followed by cold stratification
After 12 wk of warm stratification at 25°/15°C plus 20-wk of cold stratification at 1°C, 91 ± 2 (mean ± 1 SE), 93 ± 2, and 83 ± 4% of the seeds germinated at 15°/6°, 20°/10°, and 25°/15°C, respectively. Control seeds receiving no warm or cold stratification germinated to 7 ± 3, 1 ± 1, and 12 ± 3% at 15°/6°, 20°/10°, and 25°/15°C, respectively, whereas those receiving 0 wk warm stratification plus 20 wk cold stratification germinated to 13 ± 2, 15 ± 5, and 38 ± 4%, respectively. In the second experiment, seeds receiving 0, 4, 8, and 12 wk warm stratification plus 20 wk cold stratification germinated to 28 ± 6 (mean ± 1 SE), 59 ± 6, 88 ± 6, and 89 ± 2%, respectively, in light at 25°/15°C. Seeds receiving neither warm nor cold stratification germinated to only 9 ± 1%.
Prolonged periods of cold stratification
An increase in length of the first cold stratification period of E. hermaphroditum seeds from Salmisjärvi did not increase germination significantly at any temperature regime, but significantly more of the seeds receiving 32 wk of cold stratification germinated at 25°/15°C than at 15°/6° or 20°/10°C (Table 2). Thirty-two weeks of cold stratification significantly increased germination of seeds from Månsträsk incubated at 25°/15°C but not at 15°/6° or 20°/10°C, and more seeds germinated at 25°/15°C than at the other two temperatures (Table 3).
|
|
Seeds placed in the field
No seeds germinated while the bags were on the soil surface in the field. With one exception, exposure to full or half snowcover did not significantly affect germination of seeds when they first were retrieved from the field and tested in light at 25°/15°C for 6 wk (Table 4). Seeds retrieved on 23 July 1998 that had been in the half-snowcover plot germinated to a significantly higher percentage than those from the full-snowcover plot. Regardless of retrieval data or snowcover treatment, the first 20-wk cold-stratification period at 1°C significantly increased germination; however, the second cold-stratification period generally did not result in an increase in germination.
|
Embryo measurements
Embryos did not grow prior to the time of radicle emergence. Mean (±1 SE) length of embryos after 24 h of imbibition was 1.10 ± 0.04 mm; 2 wk, 1.10 ± 0.05 mm; 4 wk, 1.11 ± 0.04 mm; and 6 wk, 1.12 ± 0.05 mm. After 6 wk of imbibition in GA3, the endocarp had split on 7 of the 15 seeds, but the radicle had not elongated past the edge of the endocarp. Embryos were not measured after 8 wk of imbibition because eight of the seeds had germinated.
In the second embryo growth study, mean (±1 SE) embryo length after 24 h of imbibition at 25°/15°C was 1.07 ± 0.03 mm. After 12 wk incubation at 25°/15°C and at 1°C, embryo lengths were 1.07 ± 0.05 mm and 1.06 ± 0.03 mm, respectively, and after 12 wk at 25°/15° plus 12 wk at 1°C embryo length was 1.03 ± 0.04 mm.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Long periods (20 or 32 wk) of cold stratification resulted in a maximum of 22% (Table 2), 27% (Table 3), and 38% (data in text) germination of E. hermaphroditum seeds incubated at 25°/15°C, while warm stratification for 16 wk resulted in only 10% germination at 25°/15°C. Because cold or warm stratification given separately was mostly ineffective in breaking dormancy of E. hermaphroditum seeds, we concluded that only 22–38% of the seeds had nondeep physiological dormancy. On the other hand, warm followed by cold stratification was highly effective in breaking dormancy of E. hermaphroditum seeds, and 1000 mg/L of GA3 stimulated 77 and 87% of the seeds from Rovågern and Månsträsk, respectively, to germinate. Thus, based on dormancy-breaking requirements and ability of GA3 to promote germination, 62–78% of the seeds of E. hermaphroditum had intermediate physiological dormancy.
Is there site-to-site variation in the same year, or year-to-year variation in a particular site, in the portion of seeds in a seed crop with intermediate physiological dormancy? Seeds of E. hermaphroditum were collected from Månsträsk in 1997 (Table 4) and in 1998 (Table 3) and from Salmisjärvi in 1998 (Table 2). Long-term (20 or 32 wk) cold stratification studies were not done on 1997 seeds from Månsträsk; however, seeds placed under snow at this site on 21 November 1997 would have received an extended period of cold stratification but no warm stratification. Maximum proportion of intermediate physiological dormancy in seeds from Månsträsk was 58–66% in 1997 and 62–73% in 1998, and it was 88% in 1998 in seeds from Salmisjärvi. Thus, there was more variation between seeds from Månsträsk and Salmisjärvi in 1998 than between seeds from Månsträsk in 1997 and in 1998, but regardless of site or year 60% or more of the seeds had intermediate physiological dormancy.
Because seeds mature in autumn after temperatures are too low for warm stratification (see Zackrisson et al., 1995
), the only seeds that can germinate the first spring are those with nondeep physiological dormancy, i.e., the seeds that can come out of dormancy in response to cold stratification. The implication of a warm-plus-cold-stratification requirement to break dormancy in seeds with intermediate physiological dormancy is that these seeds will not germinate until the second spring. That is, the warm stratification requirement would not be fulfilled until the following summer, and seeds would be cold stratified during the subsequent winter. Seeds removed from full- and half-snowcover plots in the field in May 1998 (after receiving only cold stratification) germinated to 42 and 34%, respectively, at 25°/15°C, whereas those removed from full- and half-snowcover plots in May 1999 (after receiving warm plus cold stratification) germinated to 79 and 82%, respectively, at 25°/15°C (Table 4). Thus, data for seeds placed in the natural habitat corroborate those obtained under simulated field conditions in the laboratory. One possible adaptive advantage of having both nondeep and intermediate physiological dormancy is that it would spread germination of a cohort of E. hermaphroditum seeds over time.
When the length of the cold-stratification period was held constant (i.e., 20 wk), germination increased with an increase in warm-stratification pretreatment. Thus, assuming that the cold-stratification period is adequate to break dormancy in the natural habitats of E. hermaphroditum, the number of seeds with intermediate physiological dormancy germinating in spring reflects the amount of warm stratification seeds received the previous summer. For example, seeds from Salmisjärvi (Table 2) and Månsträsk (Table 3) received 32 wk of cold stratification and then were incubated at 15°/6°, 20°/10°, and 25°/15°C for 12 wk, during which time they were exposed to 1008, 1008, and 2016 h of temperatures 15°C, respectively. After the second and third stratification periods, seeds exposed to 1008 h of warm stratification (15°C) at the 15°/6°C temperature regime germinated to a significantly lower percentage than those receiving 2016 h of warm stratification at 25°/15°C. Thus, relatively cool summers could help spread germination of a cohort of seeds with intermediate physiological dormancy over several years.
Our studies on E. hermaphroditum show that warm stratification is a part of the sequence of environmental conditions required to break physiological dormancy and not to make the endocarp permeable. As noted in the introduction, various species have seeds enclosed by stony endocarps, and they require warm-plus-cold stratification for germination (Crocker and Barton, 1957
; Young and Young, 1992
). It must be shown via imbibition studies that seeds (+ stony endocarp) of Arctostaphylos, Cornus, Cotoneaster, Crataegus, Elaeagnus, Halesia, Ostrya, and Rhodotypos are impermeable to water before it can be concluded that high summer temperatures play a role in making them permeable. We suggest that warm stratification may be involved in breaking physiological dormancy in seeds of many of these species, rather than making the surrounding stony endocarp permeable. Further, there is no taxonomic or anatomical basis for presuming that endocarps in these genera are water impermeable (Baskin, Baskin, and Li, 2000)
.
Seeds enclosed by stony endocarps are not the only ones with fully elongated (developed) embryos whose germination is promoted by warm-plus-cold stratification. Seeds of the hemiparasite Melampyrum lineare, which are not enclosed by a stony endocarp, require a period of dry storage at room temperatures followed by cold stratification before they will germinate under laboratory conditions (Curtis and Cantlon, 1963
). It is assumed that under field conditions, seeds of M. lineare require warm-plus-cold stratification for germination. Other species whose seeds lack stony endocarps but have intermediate physiological dormancy, and thus require warm- plus-cold stratification to germinate in spring, include Cardamine concatenata (Michx.) O. Schwarz. (Baskin and Baskin, 1995
), Floerkea proserpinacoides Willd. (Baskin, Baskin, and McCann, 1988
), and Mahonia fremontii (Torr.) Fedde (Baskin, Baskin, and Meyer, 1993
). Seeds of C. concatenata and F. proserpinacoides mature in spring, and those of M. fremontii mature in summer. Seeds of these three species do not have an absolute warm stratification requirement for dormancy break, but warm stratification significantly reduces the length of the cold-stratification period required to break dormancy. In fact, if seeds of these species did not receive a period of warm stratification in the natural habitat in summer and/or early autumn prior to receiving cold stratification in winter, they would not receive a long enough period of cold stratification during winter to be able to germinate to high percentages in spring.
If seeds of E. hermaphroditum are warm- and cold-stratified for an adequate length of time to break dormancy, it does not appear that a cool summer would inhibit germination per se. For example, seeds given 12 wk warm stratification plus 20 wk cold stratification germinated to 83–93% at 15°/6°, 20°/10°, and 25°/15°C. Thus, after intermediate physiological dormancy is broken, seeds germinate to high percentages in light over a range of alternating temperature regimes. The light : dark requirements for germination were not investigated in this study. However, it should be noted that seeds cold stratified in darkness (after receiving warm stratification in light) germinated to high percentages in light. Thus, seeds do not have to be cold stratified in light to be able to germinate in light in spring.
There are various reports of persistent soil seeds banks for E. hermaphroditum. The mean number of E. hermaphroditum seeds germinating in 25 soil cores (100 mm diameter and the total depth of the organic layer) taken from 16-, 29-, and 50-yr-old coniferous forests in Sweden was 1, 3, and 6 seeds, respectively (Granström, 1982
). Further, seeds buried in 1-mm mesh nylon bags under the moss/litter plus 1 cm of the F-layer in Swedish coniferous forests never germinated to more than 25% when exhumed in late May or early June of five consecutive years and incubated in light at 20°C and then at 30°/20°C (Granström, 1987
). After 5 yr, about 26% of the ungerminated seeds had intact embryos, and Granström (1987)
judged them to be viable.
In the Abisko area in northernmost Swedish Lapland, soil samples collected at 700, 800, and 900 m (altitude) contained 1.38 x 105, 0.13 x 105, and 2.83 x 105 E. hermaphroditum seeds/m2, respectively (Molau and Larsson, 2000)
. 14C analysis of the E. hermaphroditum seeds taken from the bottom of the soil layer revealed that they were 230 ± 65 yr old (Molau and Larsson, 2000)
. In northern Finland, 241 ± 56 (mean ± 1 SE), 278 ± 87, and 233 ± 48 E. hermaphroditum seedlings/m2 germinated in soil samples collected from dry, medium, and mesic plots, respectively, without dwarf shrubs, and 466 ± 78, 568 ± 156, and 375 ± 153 seedlings/m2 in dry, medium, and mesic plots, respectively, with dwarf shrubs (Vieno, Komulainen, and Neuvonen, 1993
).
In view of the warm-plus-cold stratification requirement to break intermediate physiological dormancy in seeds of E. hermaphroditum, a lack of a sufficient number of days with temperatures 15°C may help explain the existence of persistent seed banks in this species. For example, most of the E. hermaphroditum seeds Granström (1987)
buried in the field in Sweden failed to germinate when exhumed in late May–early June of five consecutive years. He found that temperatures at seed depth during winter were close to 0°C, especially when covered with snow, and they were 8°–12°C during summer (Granström, 1987
). Thus, it appears that soil disturbance so that the insulating cover over seeds is removed during summer (thus allowing seeds to be warm stratified) may be one way to promote germination of E. hermaphroditum seeds with intermediate physiological dormancy in the natural habitat. Finally, summer temperatures play a role in determining the northern limits of the geographical distribution of E. hermaphroditum (Elvebakk and Spjelkavik, 1995
). Relatively high summer temperatures not only are required for production of flowers and seeds (Elvebakk and Spjelkavik, 1995
), but now we also know that summer temperatures must be high enough for warm stratification of seeds with intermediate physiological dormancy.
|
FOOTNOTES |
|---|
5 Author for reprint requests (ccbask0{at}uky.edu
)
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
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 S. E. Meyer 1993 Seed dormancy in the Colorado Plateau shrub Mahonia fremontii (Berberidaceae) and its ecological and evolutionary implications. Southwestern Naturalist 38: 91-99[CrossRef][ISI]
Baskin J. M. C. C. Baskin X. Li 2000 Taxonomy, anatomy, and evolution of physical dormancy in seeds. Plant Species Biology 15: 139-152
Baskin J. M. C. C. Baskin M. T. McCann 1988 A contribution to the germination ecology of Floerkea proserpinacoides (Limnanthaceae). Botanical Gazette 149: 427-431[CrossRef]
Bell J. N. B. J. H. Tallis 1973 Biological flora of the British Isles. Empetrum nigrum L. Journal of Ecology 61: 289-305[CrossRef][ISI]
Curtis E. J. C. J. E. Cantlon 1963 Germination of Melampyrum lineare: interrelated effects of afterripening and gibberellic acid. Science 140: 406-408
Crocker W. L. V. Barton 1931 After-ripening, germination, and storage of certain rosaceous seeds. Contributions from the Boyce Thompson Institute 3: 385-404
Crocker W. L. V. Barton 1957 Physiology of seeds. Chronica Botanica, Waltham, Massachusetts, USA
Dalby M. [ed.] 1961 The ecology of crowberry (Empetrum nigrum) on Ilkley Moor 1959–1960. Naturalist 877: 37-40
Densmore R. J. C. Zasada 1977 Germination requirements of Alaskan Rosa acicularis. Canadian Field-Naturalist 91: 58-62
Elvebakk A. S. Spjelkavik 1995 The ecology and distribution of Empetrum nigrum ssp. hermaphroditum on Svalbard and Jan Mayen. Nordic Journal of Botany 15: 541-552[ISI]
Gleason H. A. A. Cronquist 1991 Manual of vascular plants of northeastern United States and adjacent Canada, 2nd ed. New York Botanical Garden, Bronx, New York, USA
Granström A. 1982 Seed banks in five boreal forest stands originating between 1810 and 1963. Canadian Journal of Botany 60: 1815-1821
Granström A. 1987 Seed viability of fourteen species during five years of storage in a forest soil. Journal of Ecology 75: 321-331[CrossRef]
Heit C. E. 1967 Propagation from seed, Part 6, Hardseededness—a critical factor. American Nurseryman 125: (10)10-12, 88–96
Hidayati S. N. J. M. Baskin C. C. Baskin 2000 Morphophysiological dormancy in seeds of two North American and one Eurasian species of Sambucus (Caprifoliaceae) with underdeveloped spatulate embryos. American Journal of Botany 87: 1669-1678
Hidayati S. N. J. M. Baskin C. C. Baskin 2001 Dormancy-breaking and germination requirements for seeds of Symphoricarpos orbiculatus (Caprifoliaceae). American Journal of Botany 88: 1444-1451
Hill A. W. 1933 The method of germination of seeds enclosed in a stony endocarp. Annals of Botany 47: 873-887
Hill A. W. 1937 The method of germination of seeds enclosed in a stony endocarp.II. Annals of Botany 51: 239-256
Martin A. C. 1946 The comparative internal morphology of seeds. American Midland Naturalist 36: 513-660[CrossRef][ISI]
Molau U. E.-L. Larsson 2000 Seed rain and seed bank along an alpine altitudinal gradient in Swedish Lapland. Canadian Journal of Botany 78: 728-747
Müller M. J. 1982 Selected climatic data for a global set of standard stations for vegetation science. Dr. W. Junk, The Hague, The Netherlands
Nikolaeva M. G. 1969 Physiology of deep dormancy in seeds. Izdatel'stvo "Nauka," Leningrad (translated from Russian by Z. Shapiro, National Science Foundation, Washington, D.C.)
Nikolaeva M. G. 1977 Factors controlling the seed dormancy pattern. In A. A. Khan [ed.], The physiology and biochemistry of seed dormancy and germination, 51–74. North-Holland, Amsterdam, The Netherlands
Nilsson M.-C. 1994 Separation of allelopathy and resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup. Oecologia 98: 1-7[CrossRef][ISI]
Nilsson M.-C. P. Högberg O. Zackrisson W. Fengyou 1993 Allelopathic effects by Empetrum hermaphroditum on development and nitrogen uptake by roots and mycorrhizae of Pinus silvestris. Canadian Journal of Botany 71: 620-628
Sanda J. E. 1974 Mjølbær og krekling. Norsk Havetidende 90: 304-306
SAS. 1985 SAS user's guide: statistics. SAS Institute, Cary, North Carolina, USA
Sokal R. R. F. J. Rohlf 1981 Biometry, 2nd ed. Freeman, New York, New York, USA
Vieno M. M. Komulainen S. Neuvonen 1993 Seed bank composition in a subarctic pine-birch forest in Finnish Lapland: natural variation and the effect of simulated acid rain. Canadian Journal of Botany 71: 379-384
von Teichman I. J. G. C. Small P. J. Robbertse 1986 A preliminary study on the germination of Sclerocarya birrea subsp. caffra. South African Journal of Botany 52: 145-148[ISI]
Wardle D. A. M.-C. Nilsson C. Gallet O. Zackrisson 1998 An ecosystem-level perspective of allelopathy. Biological Reviews 73: 305-319[CrossRef]
Young J. A. C. G. Young 1992 Seeds of woody plants in North America. Dioscorides, Portland, Oregon, USA
Zackrisson O. M-.C. Nilsson I. Steijlen G. Hörnberg 1995 Regeneration pulses and climate-vegetation interactions in nonpyrogenic boreal Scots pine stands. Journal of Ecology 83: 469-483[CrossRef][ISI]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
