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3 School of Biological Sciences, University of Kentucky, Lexington, Kentucky 40506-0225 USA; and 4 Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546-0091 USA
Received for publication July 24, 1999. Accepted for publication February 1, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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60% fully developed (elongated) at seed maturity. The embryo has to extend to the full length of the seed to germinate. Embryos in freshly matured seeds of S. canadensis and in those of S. pubens grew better at 25°/15°C than at 5°C, whereas the rate of embryo growth in S. racemosa was higher at 5°C than at 25°/15°C. Seeds of all three species germinated to significantly higher percentages in light (14-h photoperiod) than in darkness. Fresh seeds of neither species germinated during 2 wk of incubation over a range of thermoperiods. Warm followed by cold stratification broke dormancy in seeds of S. canadensis and in those of S. pubens. Thus, seeds of these two North American species have deep simple morphophysiological dormancy (MPD). In comparison, seeds of the European species S. racemosa required a cold stratification period only for dormancy break, and thus they have intermediate complex MPD. GA3 was much more effective in breaking dormancy in seeds of S. racemosa than it was in those of S. canadensis or S. pubens.
Key Words: Caprifoliaceae • embryo growth • germination phenology • imbibition • morphophysiological seed dormancy • Sambucus spp. • underdeveloped spatulate embryo
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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classified seeds into 12 types based on size and shape of the embryo. The spatulate seed is one of these 12 types, and it has an erect to slightly expanded to broad embryo. Seeds of Sambucus have spatulate-shaped embryos. However, unlike spatulate embryos in many other genera, those in Sambucus do not extend the full length of the seed (Martin, 1946
). Sambucus belongs to Caprifoliaceae, a family with both rudimentary and linear underdeveloped embryos that must grow inside the seed before germination occurs. Thus, the question arises as to whether growth of spatulate embryos must precede germination (radicle emergence) in seeds of Sambucus. A pregermination growth requirement heretofore has been reported only in seeds with a rudimentary embryo and in those with a linear underdeveloped embryo (Baskin and Baskin, 1998
). Both rudimentary and underdeveloped linear embryos are fully differentiated, i.e., a radicle and cotyledon(s) can be distinguished, but considerable growth (elongation) must occur before radicle emergence takes place.
If embryo growth and radicle emergence are completed at suitable temperatures in 30 d, without a dormancy-breaking pretreatment, seeds have morphological dormancy (MD) (Nikolaeva, 1977
; Baskin and Baskin, 1998
). However, if seeds with underdeveloped embryos require a dormancy-breaking pretreatment such as exposure to moist low temperature (5°C) conditions before they can germinate, they have both morphological and physiological dormancy; this is called morphophysiological dormancy (MPD) (Nikolaeva, 1969, 1977
). Thus, one purpose of this study was to determine whether seeds of Sambucus have MD or MPD, and if the latter, which one of the eight known types of MPD (see Baskin and Baskin, 1998
) do they have.
Sambucus occurs in Africa, the Asian-Malesian Region, Australia, the Caribbean Islands, Europe, and North America 
South America, although it primarily is a genus of the Northern Hemisphere (Ferguson, 1966
; Wood, 1970
; Hara, 1983
; Bolli, 1994
). In genera such as Sambucus with species on different continents, we have the opportunity to compare the requirements for embryo growth, seed dormancy-break, and germination in congeneric, intercontinental disjuncts. Information combined from several studies shows that the dormancy-breaking requirements of species in genera with disjunct distributions between eastern Asia and eastern North America essentially are identical (Baskin and Baskin, 1989a
; Grushvitzky, 1967
; Nikolaeva, 1969
; Stoltz and Snyder, 1985
; Terui and Okagami, 1993
). On the other hand, the type of dormancy in species of Erythronium and Osmorhiza from eastern North America differ from that in these two genera, respectively, from western North America (Baskin and Baskin, 1984a, 1991
; Baskin, Meyer, and Baskin, 1995
). Thus, a second purpose of our study was to compare the requirements for embryo growth, seed dormancy-break, and germination of two North American (S. canadensis L. and S. pubens Michx.) and one Eurasian (S. racemosa L.) species of Sambucus.
Prior to this study, relatively few data had been collected on the dormancy-breaking and germination requirements of Sambucus, and none of the studies could be described as definitive. For instance, Rose (1919)
mentioned that no satisfactory "forcing agent" has been found for seed germination of S. canadensis, and Brinkman (1974)
noted that seeds of Sambucus spp. are difficult to germinate because of their dormant embryo and hard seed coat. Clancy and Maguire (1979)
suggested that seeds of S. cerulea must overwinter at least one season before germination, and Clergeau (1992)
reported that two winters clearly appear to favor dormancy-break in S. nigra seeds. Thus, a third purpose of this study was to more fully characterize the germination biology of Sambucus than has been done heretofore.
The geographic range of S. canadensis mostly is from Nova Scotia to Manitoba, south to Florida and Texas. Sambucus pubens ("S. pubens" sensu Bolli, 1994
) occurs from Newfoundland to Ohio, south (in mountains) to Georgia (Pixler, 1950
; Ritter and McKee, 1964
; Ferguson, 1966
; Duncan and Kartesz, 1981
; Wofford, 1989
; Bolli, 1994
; Cooperrider, 1995
). The natural distribution of S. racemosa sensu stricto (sensu Bolli, 1994
) is from eastern Russia to the Netherlands, south to Japan and Spain. The species was introduced to Britain, Denmark, Norway, Sweden, and Finland, and from there its range was extended to 65° N (Ferguson, 1976
; Bolli, 1994
).
Although most taxonomic treatments consider S. canadensis a distinct species (e.g., Ritter and McKee, 1964
; Wofford, 1989
; Gleason and Cronquist, 1991
; Cooperrider, 1995
), in a recent monograph Bolli (1994)
designated it a subspecies of the near-cosmopolitan S. nigra complex [i.e., S. nigra L. ssp. canadensis (L.) R. Bolli]. Sambucus pubens also has been recognized as an infraspecific taxon of S. racemosa (e.g., Hultén, 1968
; Gleason and Cronquist, 1991
; also see Bolli, 1994
) or has been included in S. racemosa sensu lato (e.g., Bolli, 1994
). Bolli (1994)
regarded northeastern North America S. pubens as a morphogeographical race of S. racemosa and remarked that both northeastern North American S. pubens and European S. racemosa possibly belong to S. racemosa var. racemosa. In this study we refer to the three taxa as S. canadensis, S. pubens, and S. racemosa.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) in Kalmar, Småland, southern Sweden, on 24 August 1997. Ripe fruits of each species were collected from 
10 plants, and some of them already had been dispersed. When ripe, the fruit is soft and entirely bluish-black (S. canadensis) or red (S. pubens, S. racemosa). The exocarp and mesocarp were removed from the fruits of S. canadensis and of S. pubens, and then the seeds (true seed plus endocarp, hereafter seeds) were dried in the laboratory for 4–9 d before studies were initiated. Seeds of S. racemosa were cleaned (depulped) before they were mailed to Kentucky from Sweden, and experiments on them started 14 d after they were collected.
Imbibition
Rate of water uptake was monitored in the three Sambucus species. Three replications of 100 seeds each of S. canadensis, S. pubens, and S. racemosa were placed on Whatman Number 1 filter paper moistened with distilled water in 9-cm diameter glass petri dishes and kept in the laboratory at room temperature, 23°C. Initial (t0) seed mass was determined for air dry seeds that had been wetted for a brief period of time, blotted dry, and weighed to the nearest 0.1 mg. Thereafter, following this procedure seed mass was determined at 1-h intervals for the first 12 h and then every 24 h for the next 156 h. Percentage water uptake was calculated as actual increase in seed mass based on seed initial mass:
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Germination study
Five temperature- and light-controlled incubators and a refrigerator equipped with a light and time clock were used in these studies. The incubators were set at 12 h/12 h daily alternating thermoperiods of 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C and the refrigerator at a constant 5°C. Temperatures in the incubators approximate mean daily maximum and minimum monthly air temperatures in Kentucky and adjacent states (Wallis, 1977
) during the growing season: March and November, 15°/6°; April and October, 20°/10°; May, 25°/15°; June and September, 30°/15°; and July and August, 35°/20°C. In central and southern Sweden, 15°/6°C approximates the mean daily maximum and minimum monthly air temperatures for May and September, 20°/10°C for June and August, and 25°/15°C for July (Pearce and Smith, 1984
). Cool white fluorescent tubes, which produced a photon flux density (400–700 nm) at seed level of 40 µmol·m-2·sec-1, were used as the light source for incubation (20 W), warm stratification (20 W), and cold stratification (15 W). The daily photoperiod was 14 h in the incubators and in the refrigerator. At the alternating temperatures, the photoperiod extended from 1 h before the beginning of the high temperature period to 1 h after the beginning of the low temperature period.
Seeds were placed in 5.5-cm-diameter petri dishes on white quartz sand moistened with distilled water. All dishes were wrapped with plastic film to restrict water loss during incubation and stratification, and dishes of seeds incubated and stratified in darkness were wrapped additionally with aluminum foil.
Dormancy-breaking and germination requirements
Freshly matured seeds in all collections of each species were incubated in light and in darkness at 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C for 2 wk. Seeds incubated in darkness were checked only at the end of the 2-wk period.
The 25°/15°C thermoperiod was used for warm stratification and 5°C for cold stratification, since these temperatures are near optimal for breaking dormancy of many species whose seeds require warm or cold temperatures, respectively, to come out of dormancy (Stokes, 1965
; Nikolaeva, 1969
). Since seeds of S. canadensis and S. pubens are dispersed in late summer, they could experience several weeks of warm stratification (sensu Baskin and Baskin, 1998
) before they are exposed to cold-stratifying temperatures. Thus, seeds of S. canadensis were (1) warm-stratified in light or in darkness for 12 wk, (2) cold-stratified in light or in darkness for 12 wk, and (3) given the following warm (W)– cold (C), light (L)– dark (D) combinations: 6 wk WL–6 wk CL, 6 WL–12 CL, 12 WL–6 CL, 12 WL–12 CL, 6 WD–6 CD, 6 WD–12 CD, 12 WD–6 CD, and 12 WD–12 CD. Seeds of S. pubens were (1) warm-stratified in light or in darkness for 12 wk, (2) cold-stratified in light or in darkness for 12 wk, and (3) given the following warm (W)– cold (C), light (L)– dark (D) combinations: 6 wk WL–6 wk CL, 6 WL–12 CL, 12 WL–6 CL, 12 WL–12 CL, 6 WD–6 CD, 6 WD–12 CD, 12 WD–6 CD, and 12 WD–12 CD. Sambucus racemosa seeds are dispersed in autumn, and thus they probably receive little (if any) warm stratification after dispersal. Therefore, they were only cold-stratified, in light or in darkness for 12 wk. Following all stratification treatments for each species, seeds were incubated in light or in darkness at 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C for 2 wk. To avoid exposing them to any light during incubation (cf. Walck, Baskin, and Baskin, 1997
), seeds both stratified and incubated in darkness were not checked for germination until the end of the 2-wk incubation period.
Controls for the stratification treatments were nonstratified seeds incubated in light at 15°/6°, 20°/10°, 25°/15°, 30°/15°, and 35°/20°C: 14 wk in experiments for S. racemosa, 26 wk for S. pubens, and 26 wk for S. canadensis. Seeds in the controls were examined every 2 wk, and seedlings were counted and removed from the petri dishes. Water was added to dishes in the controls as needed.
Three replications of 50 seeds each per dish were used per treatment. Emergence of the radicle was the criterion for germination. Viability of ungerminated seeds was determined by pinching them with forceps under a dissecting microscope to see if they contained firm, white (viable) embryos or soft, light brown (nonviable) ones. Tetrazolium tests (Grabe, 1970
) confirmed that white embryos were viable and that brown ones were not.
Requirements for embryo growth
Embryos were excised from seeds of S. canadensis (1997 collection), S. pubens, and S. racemosa using a razor blade, and their lengths measured under a dissecting microscope equipped with a micrometer. Seed and embryo lengths were determined for 50 freshly matured seeds of each species 24 h after they were placed on moist filter paper at room temperature (23°C). Embryo growth was monitored in seeds of all three species during warm and during cold stratification. Fifty seeds of each species were placed in each of 12 petri dishes: six dishes were placed at 25°/15°C and six at 5°C. After 2, 4, 6, 8, 10, and 12 wk of warm or of cold stratification, lengths of 50 embryos of each species were measured. If a seed had germinated, embryo length was recorded as the critical length required for germination, i.e., when embryos had grown enough to start splitting the seed coat.
Effects of GA3 on germination and embryo growth
Fifty seeds each of S. canadensis (1998 collection), S. pubens, and S. racemosa were placed on two sheets of Whatman Number 1 filter paper in 9-cm-diameter glass petri dishes. The paper was moistened with either distilled water (control) or with a solution of 10, 100, or 1000 mg/L of GA3 (K-GA3) dissolved in distilled water. Three replications (petri dishes) were used for the germination test and one dish for embryo growth. Seeds were incubated at 25°/15°C and at 5°C. The 25°/15°C thermoperiod was used since it is too high to be effective for cold stratification, and 5°C was used since it is too low to be effective for warm stratification (Stokes, 1965
; Nikolaeva, 1969
). Since warm plus cold stratification broke dormancy in seeds of S. canadensis and in those of S. pubens, dishes of these two species were placed in light at 5°C or at 25°/15°C to determine whether GA3 would substitute for warm or cold stratification, respectively. On the other hand, cold stratification only broke dormancy in seeds of S. racemosa, and thus dishes of this species were placed only at 25°/15°C in light. After 2, 6, and 12 wk of incubation, germination percentage was determined and embryo length measured for all three species.
Germination phenology
Three replications of 300 seeds each of S. canadensis (1997 collection), S. racemosa, and S. pubens were sown on soil in 20 x 30 x 9 cm deep metal flats on 14 September 1997, 14 September 1997, and 15 August 1998, respectively. Soil used was a 3:1 (v/v) mixture of limestone-derived topsoil and river sand, and it was covered with 5 cm of dead oak (Quercus) leaves. The flats were placed in a nontemperature-controlled greenhouse (no heating or air conditioning, windows open all year). At weekly intervals until 1 June 1999, leaves were lifted, and any germinated seeds counted and removed from the flats. Temperatures in this greenhouse are near those outdoors throughout the year in the Lexington, Kentucky, area (Baskin and Baskin, 1985a
). Continuous thermograph records were made inside a weatherhouse in the greenhouse, and mean maximum and minimum temperatures for each week of the studies were calculated from them. From 1 September to 30 April of each year, the soil was watered to field capacity daily (unless frozen in winter), and during the remainder of the year it was watered once each week. This watering regime simulates soil moisture conditions that might occur in the field, i.e., soil wet in autumn, winter, and early spring and alternately wet and dry in late spring and summer.
Statistical analyses
Germination data were transformed to germination percentages based on number of viable seeds. Means and standard errors were calculated for germination percentages and for embryo lengths. Means were compared by analyses of variance (ANOVAs) and by protected least significant difference tests (PLSDs, P = 0.05) (SAS, 1985
). A three-way ANOVA was used to test the effects and interactions of seed condition (fresh, control, stratified), light regime, and thermoperiod on germination percentages, and a two-way ANOVA the effects and interaction of GA3 concentration and length of incubation on germination and embryo growth. Year was included as a factor in the analysis if germination tests were done on two collections of seeds for a species. Species was not included as a factor since stratification treatments varied among the three Sambucus taxa. Variances for embryo length were heteroscedastic among species according to Fmax tests; log transformation standardized variances for embryo length.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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60% of the length of the seed (Fig. 2a). They grew to 2.51 ± 0.05 mm during 12 wk of warm stratification and to 2.75 ± 0.05 mm during 12 wk of cold stratification. Lengths of embryos were 3.19 ± 0.06 mm, when they had grown enough to start splitting the seed coat. Thus, embryo length increased 54% between seed maturity and germination.
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55%.
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No freshly matured seeds germinated in light or in darkness at any thermoperiod (Table 3). Further, seeds were unable to germinate after 12 wk of warm stratification in light or in darkness unless they were subsequently exposed to cold stratification. On the other hand, seeds germinated to 16–30% in light at 15°/6° and 20°/10°, to 75–76% at 25°/15° and 30°/15 °C, and to 30% at 35°/20°C, following 6 wk of warm plus 6 wk of cold stratification in light; no seeds germinated in darkness. Moreover, seeds germinated to 79–91% in light over the range of thermoperiods following 6 wk of warm plus 12 wk of cold stratification in light and to 2–7% in darkness. However, following 12 wk of warm plus 6 wk of cold stratification seeds germinated to 4–19% and 2–5% in light and darkness, respectively. Only 2–15% of the seeds germinated following 12 wk of warm plus 12 wk of cold stratification in light, and none germinated in darkness. In addition, seeds germinated to 9–25% and 3–12% in light and darkness, respectively, over the range of thermoperiods following 12 wk of cold stratification in light or in darkness. None of the nonstratified (control) seeds germinated during 20 wk of incubation in light.
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60% of the length of the seed (Fig. 2b). They grew to 2.09 ± 0.06 mm in length during 12 wk of warm stratification and to 1.80 ± 0.05 mm during 12 wk of cold stratification. The average length of embryos was 2.59 ± 0.05 mm when they had grown enough to start splitting the seed coat. Thus, embryo length increased 60% between seed maturity and germination.
No seeds germinated after 2–12 wk of incubation at 25°/15°C (Table 2). On the other hand, seeds germinated to 11% in distilled water and to 15, 31, and 68% in solutions of 10, 100, and 1000 mg/L GA3, respectively, during 12 wk of incubation at 5°C. GA3 promoted growth of embryos. After 12 wk of incubation in 1000 mg/L GA3 at 25°/15°C, maximum length was 2.39 ± 0.03 mm. Thus, embryo length increased 50%.
A few of the seeds sown in a nonheated greenhouse in August 1998 germinated in late October and early November 1998 (Fig. 4). However, peak germination occurred between 21 and 28 March 1999, when mean maximum and minimum temperatures were 18.1° and 6.9°C, respectively. No additional seeds germinated after 1 June 1999.
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No freshly matured seeds germinated during 2 wk of incubation in light or in darkness at any thermoperiod; further, none had germinated in light after 14 wk of incubation (Table 4). However, seeds germinated to 77–100% in light over the range of thermoperiods following 12 wk of cold stratification in light.
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60% of the length of the seed (Fig. 2c). Embryos grew to 2.46 ± 0.06 mm in length during 12 wk of warm stratification and to 3.11 ± 0.05 mm during 12 wk of cold stratification. Average length of embryos was 3.32 ± 0.06 mm when they had grown enough to start splitting the seed coats. Thus, embryo length increased 50% between seed maturity and germination. None of the seeds incubated in distilled water or in solutions of 10, 100, or 1000 mg/L GA3 germinated during 2 wk of incubation (Table 2). However, seeds incubated in solutions of 10, 100, and 1000 mg/L GA3 germinated to 9, 49, and 83%, respectively, after 6 wk of incubation and to 19, 93, and 100%, respectively, after 12 wk. None of the control seeds germinated during 2, 6, or 12 wk of incubation.
A few seeds sown in a nonheated greenhouse in September 1997 germinated in late October 1997 (Fig. 5). However, peak germination occurred between 21 and 28 February 1998, when mean weekly maximum and minimum temperatures were 12.2° and 9.6°C, respectively. No seeds germinated after 4 April 1998.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), S. nigra, and S. racemosa, have been reported to be impermeable to water (Heit, 1967
; Clergeau, 1992
; Young and Young, 1992
), i.e., physical dormancy (sensu Baskin and Baskin, 1989b, 1998
); however, this is not the case. Air-dry seeds of S. nigra (Rose, 1919
), S. canadensis, S. pubens, and S. racemosa (Fig. 1) kept on a moist substrate for 7 d imbibed water equal to 40, 20, 33, and 31% of their initial mass, respectively. Thus, seeds of Sambucus do not require a scarification pretreatment to imbibe water, unlike those of species with hard (impermeable to water) seed coats (Baskin and Baskin, 1998
). Neither seeds of Sambucus nor those any other species of Caprifoliaceae are known to have physical dormancy (Baskin and Baskin, 1998
).
In most taxa with spatulate embryos, e.g., members of the families Apocynaceae, Boraginaceae, Cornaceae, Polemoniaceae, Rosaceae, Urticaceae, and Verbenaceae, the embryos are fully elongated at seed maturity (see Martin, 1946
). Thus, seeds with fully developed spatulate embryos either have physiological dormancy or no dormancy at all (Baskin and Baskin, 1998
). In contrast, seeds of the three Sambucus species have underdeveloped spatulate embryos and MPD. Seeds required either warm plus cold stratification (S. canadensis, S. pubens) or cold stratification only (S. racemosa) for dormancy break (Table 5). Embryos of S. canadensis and of S. pubens grew during warm stratification (Fig. 2a, b), but warm followed by cold stratification was required for germination (Tables 1, 3). A longer period of warm stratification was more effective for germination in S. canadensis seeds than for germination of those of S. pubens. On the other hand, only cold stratification was required for embryo growth and germination of seeds of S. racemosa (Fig. 2c, Table 4).
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). In contrast, GA3 substituted for cold stratification in seeds of S. racemosa, which is similar to what Baskin and Baskin (1984b)
found for seeds of Stylophorum diphyllum.
Eight types of MPD have been distinguished based on (1) temperatures required to break dormancy, (2) temperatures at time of embryo growth, and (3) whether GA3 overcomes dormancy (Baskin and Baskin, 1998
). These eight types of MPD can be divided into two categories: (1) relatively high temperatures (15°C) are required for embryo growth in seeds with simple MPD, and (2) low temperatures (0°–10°C) are required for growth in those with complex MPD. The MPD types can be subdivided further depending on the level of physiological dormancy: nondeep, intermediate, and deep. Since in seeds of S. canadensis and in those of S. pubens (1) GA3 only partially overcame dormancy (Table 2), and (2) warm plus cold stratification was required to break dormancy, seeds of these two species have deep simple MPD. According to Nikolaeva (1977)
, in seeds with this type of MPD a period of warm stratification is required for postdevelopment of the embryo after the seeds mature and a period of cold stratification is needed to prepare the seeds to germinate. In contrast, both GA3 and cold stratification overcame dormancy in seeds of S. racemosa, and thus seeds of this species have intermediate complex MPD.
The seed dormancy characteristics of S. canadensis are similar to those of its North American congener S. pubens. Seeds of both species required warm plus cold stratification to break dormancy. However, the length of warm stratification required to break dormancy differed between the two species. The optimum length of the warm stratification period for S. canadensis was 12 wk, while for S. pubens it was only 6 wk. On the other hand, seeds of the European species S. racemosa required only cold stratification for dormancy break. Further, the embryo of S. canadensis and of S. pubens grew better at warm than at cold temperatures, while rate of embryo growth of S. racemosa was higher at cold than at warm temperatures (Fig. 2).
Temperature (including temperature sequence) is a critical environmental factor for germination of Sambucus. Although seeds of S. canadensis and of S. pubens are dispersed in late summer, most of them germinate the following spring. The seeds are exposed to warm followed by cold temperatures between dispersal and germination. This sequence of a warm period followed by a cold period is required for embryo growth and dormancy break in both species. In contrast, seeds of S. racemosa required only cold stratification to come out of dormancy (Table 4).
For all three species, a high percentage of the seeds required light for germination. Maximum germination percentages for dark-stratified, dark-incubated seeds of S. canadensis, S. pubens, and S. racemosa were 6 ± 1, 12 ± 1, and 31 ± 1, respectively (Tables 1, 3, 4). However, in spite of this strong light requirement for germination, there are only a few reports of the occurrence of Sambucus seeds in the soil seed bank. Kramer and Johnson (1987) found 24 viable seeds (maximum density/m2 = 210) of S. cerulea + S. racemosa (= "S. microbotrys" sensu Bolli, 1994
) in mature conifer forests in central Idaho. Only two viable seeds of S. nigra were present in soil seed bank samples collected along a successional gradient in northeast England (Donelan and Thompson, 1980
), and only nine viable seeds (1.2 seeds/m2) of this species were found in seed bank samples collected in deciduous woodlands in southwest England (Warr, Kent, and Thompson, 1994
). Seed bank types for S. nigra and for S. racemosa in northwest Europe varied from transient to long persistent (Thompson, Bakker, and Bekker, 1997
). Recently, Halpern, Evans, and Nielson (1999) sampled the soil and litter seed banks in 40–60 yr old (following clear-cut logging) coniferous forests at three sites on the Olympic Peninsula of Washington. Frequency of S. racemosa seeds in litter ranged from 2.4–7.1% and density from 2.4–12.2/m2. In soil (0–10 cm deep), frequency ranged from 7.1–42.9% and density from 8.5–111.1/m2. The authors suggested that seeds of S. racemosa, along with those several other species, may have been deposited at these sites following origination of the present forest after clear-cut logging 40–60 yr earlier.
Seed dispersal in S. canadensis occurs from August to October, but the majority of seeds is dispersed between late August and mid September (Ritter and McKee, 1964
; Dirr and Heuser, 1987
). Fruits of S. pubens ripen in June, and seeds are dispersed between late June and early August (Bailey, 1906
). No data seem to exist on the dispersal phenology of S. racemosa. Since seeds of S. canadensis, S. pubens, and S. racemosa are dormant at maturity (Tables 1, 3, 4), it is expected that no seeds germinate in the field in autumn. However, although most seeds of the three species sown in the nonheated greenhouse germinated in late winter/early spring, a small percentages of them germinated in autumn (Figs. 3–5).
Animals, particularly birds, play an important role in dispersal of Sambucus seeds. Sambucus canadensis, S. pubens, and S. racemosa fruits are eaten by a variety of birds including rose-breasted grosbeak, downy woodpecker, red-eyed vireo, scarlet tanager, veery, and cedar waxwing (Ridley, 1930
; Degraaf and Witman, 1979
; Barnes, 1999
). In addition, several other bird species consume fruits of S. canadensis and of S. pubens (Degraaf and Witman, 1979
; Barnes, 1999
). Fox squirrel and white-footed mouse also are reported to eat fruits of S. canadensis and of S. pubens (Barnes, 1999
). However, we are aware of only two studies that determined the effect of passage through the digestive tract of animals on germination of Sambucus seeds. Traveset and Willson (1997) reported that seeds of S. racemosa (apparently "S. callicarpa"; Bolli, 1994
) ingested by varied thrushes, robins, and black bears germinated to 31, 19, and 14%, respectively, compared to 7% in the control. Clergeau (1992)
reported that seeds of S. nigra defecated or regurgitated by birds germinated to 43 and 36%, respectively, compared to 12% in the control.
In a study by Nichols (1934)
, seeds of S. canadensis given 83 d of refrigeration (presumably at 3°–5°C) on moist sterilized soil germinated to 57%, while nonstratified seeds germinated to only 5%. Further, seeds of S. canadensis stratified at 3°C for 16 wk and then incubated for 1 wk at 15°/5°C germinated to 49%, while nonstratified seeds germinated to only 1% (Cunningham and Farmer, 1982
). Fresh seeds of S. racemosa spp. pubens (Michx.) House var. microbotrys (Rydb.) Kearney and Peebles ("S. microbotrys"; Bolli, 1994
) failed to germinate during 30 d of incubation at 15°/5°, 25°/15°, or 30°/20°C (8 h light/16 h dark) (Conrad and McDonough, 1972
). However, seeds of this species stratified for 5 mo at 2°C germinated to 37–51%, at 2°C. In the present study, significantly higher germination percentages of S. canadensis and of S. pubens were obtained in seeds given warm plus cold stratification than in those given cold stratification only. Thus, we suggest that if a high temperature pretreatment had been given to seeds of these two species before they were cold stratified in the studies cited above, a higher percentage of the seeds would have germinated.
Adams (1927)
reported that planting dates affect germination percentages in S. canadensis. Seeds planted out of doors on 6 September 1924 had germinated to 95% by 2 July 1925, while those planted on 3 March 1925 had germinated to only 17% by 2 July 1925. In this study, seeds sown in September received enough exposure to warm temperatures to complete the first part of the dormancy-breaking process before they were exposed to cold winter temperatures. The exposure to this sequence of temperatures resulted in high percentages of germination. However, March-planted seeds did not receive exposure to the warm-cold temperature sequence required to break dormancy, and thus only a few S. canadensis seeds sown in March germinated.
Seeds of S. cerulea Raf. (= S. nigra ssp. cerulea (Raf.) R. Bolli sensu Bolli, 1994)
stratified in moist sand at 5°C for 100 d germinated to 30% after 7 wk at 20°C, while nonstratified seeds failed to germinate (Clancy and Maguire, 1979
). Further, seeds of this species stratified in a 1000 mg/L solution of GA3 or in water (control) for 100 d at 5°C and then incubated for 6 wk at 25°C germinated to 75 and 39%, respectively. Seeds of S. caerulea Raf. incubated in a 1000 mg/L solution of GA3 at 4°C for 30 d germinated to 55%, while those incubated in distilled water (control) failed to germinate (Norton, 1986
). Clergeau (1992)
reported that two winter periods favored germination of S. nigra seeds. Treatments in Clergeau's study were given in the following order: 8 wk at 15°C (simulated first autumn following dispersal), 10 wk at 0°C (first winter), 18 wk at 13°C (first spring), and 8 wk at 18°C (first summer). However, only 6% of the seeds germinated until he repeated the winter (10 wk at 0°C) and spring (16 wk at 13°C) temperature regimes; then, 62% of the seeds germinated.
The two North American species, S. canadensis and S. pubens, have deep simple MPD, while the European species, S. racemosa, has intermediate complex MPD. We know of only one other study that has compared seed germination of intercontinental disjuncts (Terui and Okagami, 1993
). In this study, seeds of Tertiary relict species of Dioscorea from both eastern Asia and eastern North America required cold stratification to break dormancy. Dioscorea seeds have small capitate embryos (sensu Martin, 1946
). As such, it seems likely that the embryo has to grow before germination can occur; however, Terui and Okagami did not investigate this aspect of seed dormancy in Dioscorea. Two other comparisons of dormancy-breaking requirements of eastern Asian–eastern North American species can be made from separate studies of individual taxa. Seeds of an eastern North American Panax (Stoltz and Snyder, 1985
) and of Jeffersonia (Baskin and Baskin, 1989a
) have nondeep simple MPD, which is the same type of MPD found in an eastern Asian species of Panax (Nikolaeva, 1969
) and of Jeffersonia (Grushvitzky, 1967
), respectively. Thus, unlike other closely related intercontinental disjunct species, which exhibit the same type of seed dormancy, the closely related S. pubens of eastern North America and S. racemosa of Europe (Bolli, 1994
; Eriksson and Donoghue, 1997
) differ in their dormancy-breaking requirements. Further, the two North American species (S. canadensis and S. pubens) that are not so closely related (Bolli, 1994
; Eriksson and Donoghue, 1997
) have the same type of MPD.
Results of a study on the dormancy-breaking requirements of the two genera Osmorhiza and Erythronium with disjunct distributions between eastern and western North America are similar to those obtained for S. pubens and S. racemosa in that the disjunct congeneric species have different types of MPD. Seeds of Osmorhiza longistylis (Baskin and Baskin, 1984a
), O. claytonii, (Baskin and Baskin, 1991
), and Erythronium albidum (Baskin and Baskin, 1985b
) from eastern North America have nondeep complex MPD, while Osmorhiza chilensis, O. occidentalis, and Erythronium grandiflorum (Baskin, Meyer, and Baskin, 1995
) from western North America have deep complex MPD. The relationships within Osmorhiza are mostly unresolved (Downie, Katz-Downie, and Spalik, 2000)
, and we are not aware of any studies on relationships within Erythronium.
One of the main objectives of this study was to determine whether intercontinental disjunct species of the genus Sambucus have different types of MPD, and results showed that they do. The North American Sambucus canadensis and S. pubens had deep simple MPD, whereas the European S. racemosa had intermediate complex MPD. However, the study also showed that type of seed dormancy is not correlated with taxonomic-relatedness in these three species. Thus, seeds of the not-so-closely related S. canadensis and S. pubens had the same type of dormancy, whereas dormancy type differed between S. pubens and S. racemosa, which are very closely related. A next logical step in understanding the ecological and evolutionary aspect of seed dormancy in S. racemosa sensu lato would be a study of seed dormancy-break and germination requirements in this complex from Asia. In addition, detailed studies need to be done on other species of Sambucus.
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FOOTNOTES |
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2 Author for correspondence (e-mail: snhida00{at}pop.uky.edu
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Bailey, L. H. 1906 Cyclopedia of American horticulture. Doubleday, Page and Company, New York, New York, USA.
Barnes, T. G. 1999 Gardening for the birds. University Press of Kentucky, Lexington, Kentucky, USA.
Baskin, C. C., and J. M. Baskin. 1998 Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press, San Diego, California, USA.
Baskin, J. M., and C. C. Baskin. 1984a Germination ecophysiology of the woodland herb Osmorhiza longistylis (Umbelliferae). American Journal of Botany 71: 687–692.[CrossRef][Web of Science]
———, and ———. 1984b Germination ecophysiology of an eastern deciduous forest herb Stylophorum diphyllum. American Midland Naturalist 111: 390–399.
———, and ———. 1985a Does seed dormancy play a role in the germination ecology of Rumex crispus? Weed Science 33: 340–343.[Web of Science]
———, and ———. 1985b Seed germination ecophysiology of the woodland spring geophyte Erythronium albidum. Botanical Gazette 146: 130–136.[CrossRef]
———, and ———. 1989a Seed germination ecophysiology of Jeffersonia diphylla, a perennial herb of mesic deciduous forests. American Journal of Botany 76: 1073–1080.[CrossRef][Web of Science]
———, and ———. 1989b Physiology of dormancy in relation to seed bank ecology. In M. A. Leck, V. T. Parker, and R. L. Simpson [eds.], Ecology of soil seed banks, 53–66. Academic Press, San Diego, California, USA.
———, and ———. 1991 Nondeep complex morphophysiological dormancy in seeds of Osmorhiza claytonii (Apiaceae). American Journal of Botany 78: 588–593.[CrossRef][Web of Science]
Baskin, C. C., S. E. Meyer, and J. M. Baskin. 1995 Two types of morphophysiological dormancy in seeds of two genera (Osmorhiza and Erythronium) with an Arcto-Tertiary distribution pattern. American Journal Botany 82: 293–298.[CrossRef][Web of Science]
Bolli, R. 1994 Revision of the genus Sambucus. Dissertationes Botanicæ 223: 1–227 + plates 1–29.
Brinkman, K. A. 1974 Sambucus L. In C. S. Schopmeyer (Technical Coordinator), Seeds of woody plants in the United States, 754–757. USDA Forest Service Agriculture Handbook Number 450.
Clancy, J. A., and J. D. Maguire. 1979 Seed dormancy in blue elderberry. Journal of Seed Technology 4: 24–33.
Clergeau, P. 1992 The effect of birds on seed germination of fleshy-fruited plants in temperate farmland. Acta Oecologica 13: 679–686.
Conrad, P. W., and W. T. McDonough. 1972 Growth and reproduction of red elderberry on subalpine rangeland in Utah. Northwest Science 46: 140–148.
Cooperrider, T. S. 1995 The Dicotyledoneae of Ohio. Part 2: Linaceae through Campanulaceae. Ohio State University Press, Columbus, Ohio, USA.
Cunningham, M., and R. E. Farmer, Jr. 1982 Stratification and temperature requirements for germination of Sambucus canadensis seed from eastern Tennessee. Plant Propagator 28(3): 4–6.
Degraaf, R. M., and G. M. Witman. 1979 Trees, shrubs, and vines for attracting birds. University of Massachusetts Press, Amherst, Massachusetts, USA.
Dirr, M. A., and C. W. Heuser. 1987 The reference manual of woody plant propagation from seed to tissue culture. Varsity Press Inc., Athens, Georgia, USA.
Donelan, M., and K. Thompson. 1980 Distribution of buried viable seeds along successional series. Biological Conservation 17: 297–311.[CrossRef][Web of Science]
Downie, S. R., D. S. Katz-Downie, and K. Spalik. 2000 A phylogeny of Apiaceae tribe Scandiceae: evidence from nuclear ribosomal DNA internal transcribed spacer sequences. American Journal of Botany 87: 76–95.
Duncan, W. H., and J. T. Kartesz. 1981 Vascular flora of Georgia. An annotated checklist. University of Georgia Press, Athens, Georgia, USA.
Eriksson, T., and M. J. Donoghue. 1997 Phylogenetic relationships of Sambucus and Adoxa (Adoxoideae, Adoxaceae) based on nuclear ribosomal ITS sequences and preliminary morphological data. Systematic Botany 22: 555–573.[CrossRef][Web of Science]
Ferguson, I. K. 1966 The genera of Caprifoliaceae in the southeastern United States. Journal of the Arnold Arboretum 47: 33–59.
Ferguson, I. K. 1976 Sambucus. In T. G. Tutin, V. H. Heywood, N. A. Burges, D. M. Moore, D. H. Valentine, S. M. Walters, and D. A. Webb [eds.], Flora Europaea, vol. 4, 44–45. Cambridge University Press, Cambridge, UK.
Gleason, H. A., and A. Cronquist. 1991 Manual of the vascular plants of northeastern United States and adjacent Canada, 2nd ed. New York Botanical Garden, Bronx, New York, USA.
Grabe, D. F. 1970 Tetrazolium testing handbook for agricultural seeds. Contribution No. 29 to the Handbook on seed testing. Association of Official Seed Analysts, North Brunswick, New Jersey, USA.
Grushvitzky, I. V. 1967 After-ripening of seeds of primitive tribes of angiosperms, conditions and peculiarities. In H. Borris [ed.], Physiologie, okologie und biochemie der keimung, vol. I, 329–336. Ernst-Moritz-Arndt Universität. Greifswald, Germany.
Halpern, C. B., S. A. Evans, and S. Nielson. 1999 Soil seed banks in young, closed-canopy forests of the Olympic Peninsula, Washington: potential contributions to understory reinitiation. Canadian Journal of Botany 77: 923–935.
Hara, H. 1983 A revision of Caprifoliaceae of Japan. Ginkgoana Number 5. Academia Scientific Books, Tokyo, Japan.
Heit, C. E. 1967 Propagation from seed. Part 7: germinating six hardseeded groups. American Nurseryman 125(12): 10–12, 37, 43–44.
Hultén, E. 1968 Flora of Alaska and neighboring territories. Stanford University Press, Stanford, California, USA.
Kramer, N. B., and F. D. Johnson. 1987 Mature forest seed banks of three habitat types in central Idaho. Canadian Journal of Botany 65: 1961–1966.
Martin, A. C. 1946 The comparative internal morphology of seeds. American Midland Naturalist 36: 513–660.[CrossRef][Web of Science]
Nichols, G. E. 1934 The influence of winter temperatures upon seed germination in various native American plants. Ecology 15: 364–373.[CrossRef][Web of Science]
Nikolaeva, M. G. 1969 Physiology of deep dormancy in seeds. Izdatel'stvo "Nauka," Leningrad, Russia. (Translated from Russian by Z. Shapiro, NSF, Washington, D.C.)
———. 1977 Factors controlling the seed dormancy pattern. In A. A. Khan [ed.], The physiology and biochemistry of seed dormancy and germination, 54–74. North-Holland, Amsterdam, The Netherlands.
Norton, C. R. 1986 Low temperature and Gibberellic acid stimulation of germination in Sambucus caerulea Raf. Scientia Horticulturae 28: 323–329.[CrossRef]
Pearce, E. A., and C. G. Smith. 1984 Times books world weather guide. Times Books, New York, New York, USA.
Pixler, V. A. 1950 The Caprifoliaceae of West Virginia. Castanea 15: 80–91.
Ridley, H. N. 1930 The dispersal of plants throughout the world. L. Reeve, Ashford, Kent, UK.
Ritter, C. M., and G. W. McKee. 1964 The elderberry: history, classification, and culture. Pennsylvania State University Agricultural Experiment Station Bulletin Number 709.
Rose, R. C. 1919 After ripening and germination of the seeds of Tilia, Sambucus, and Rubus. Botanical Gazette 67: 281–308.[CrossRef]
SAS. 1985 SAS user's guide: statistics. SAS Institute Inc., Cary, North Carolina, USA.
Stokes, P. 1965 Temperature and seed dormancy. In W. Ruhland [ed.], Encyclopedia of plant physiology, vol. 15, part 2, 746–803. Springer-Verlag, Berlin, Germany.
Stoltz, L. P., and J. C. Snyder. 1985 Embryo growth and germination of American ginseng seed in response to stratification temperatures. HortScience 20: 261–262.[Web of Science]
Terui, K., and N. Okagami. 1993 Temperature effects on seed germination of East Asian and Tertiary relict species of Dioscorea (Dioscoreaceae). American Journal of Botany 80: 493–499.[CrossRef][Web of Science]
Thompson, K., J. P. Bakker, and R. M. Bekker. 1997 The soil seed banks of North West Europe: methodology, density and longevity. Cambridge University Press, Cambridge, UK.
Traveset, A., and M. F. Willson. 1997 Effect of birds and bears on seed germination of fleshy-fruited plants in temperate rainforests of southeast Alaska. Oikos 80: 89–95.[CrossRef][Web of Science]
Walck, J. L., J. M. Baskin, and C. C. Baskin. 1997 A comparative study of the seed germination biology of a narrow endemic and two geographycally-widespread species of Solidago (Asteraceae). 3. Photoecology of germination. Seed Science Research 7: 293–301.
Wallis, A. L., Jr. (compiler and editor). 1977 Comparative climatic data through 1976. National Climatic Data Center. Environmental Data Service. National Oceanic and Atmospheric Administration. U.S. Department of Commerce, Asheville, North Carolina, USA.
Warr, S. J., M. Kent, and K. Thompson. 1994 Seed bank composition and variability in five woodlands in southwest England. Journal of Biogeography 21: 151–168.[CrossRef][Web of Science]
Wofford, B. E. 1989 Guide to the vascular plants of the Blue Ridge. University of Georgia Press, Athens, Georgia, USA.
Wood, C. E., Jr. 1970 Some floristic relationships between the southern Appalachians and western North America. In P. C. Holt [ed.], The distributional history of the biota of the southern Appalachians, Part II: Flora, 331–404. Virginia Polytechnic Institute and State University Research Division Monograph 2. Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA.
Young, J. A., and C. G. Young. 1992 Seeds of woody plants in North America. Dioscorides Press, Portland, Oregon, USA.
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