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Keywords:

  • after-ripening;
  • cypsela maturation;
  • dormancy;
  • germination;
  • Onopordum acanthium (Scotch thistle);
  • structural characteristics;
  • temperature

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • • 
    Although Scotch thistle (Onopordum acanthium) is known to have cypselas whose dormancy is affected by maturation temperature, a detailed study of plant development, cypsela structure and germination responses after maturing under contrasting temperatures has not been done.
  • • 
    Plants were grown under high and low temperature regimes in glasshouses and under field conditions in 2000 and 2001. Each year, phenological and developmental characteristics of plants were monitored and cypselas were collected twice. Cypselas were germinated fresh and after 4 months after-ripening, and also examined for surface and internal characteristics by scanning electron microscopy.
  • • 
    Plants from the lower temperature regime were taller, had thicker shoots, larger leaves, larger capitula that appeared sooner, and more, slower-to-mature and larger cypselas with smoother surfaces, thicker coats and higher moisture contents. However, their germination percentages were lower. After after-ripening most cypselas germinated.
  • • 
    In both years, consistent and significant differences in germination patterns and structural characteristics between cypselas from the two temperature regimes indicated that both pre- and postdispersal factors were involved in regulating the germinability of O. acanthium cypselas.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Environmental factors during seed development on the mother plant, or after seed dispersal in the soil, can affect subsequent seed germinability (Gutterman, 2000a). These factors can include moisture, nutrients, light and temperature, and they have different effects on different species (Fenner, 1991; Aamlid, 1992). The effects of temperature on plant growth (Fitter & Hay, 2002; Blackshaw & Entz, 1995) and seed maturation (Fenner, 1992) have been studied. Plants respond to the stress caused by high and low temperatures by changing their metabolic activity (Fitter & Hay, 2002). High temperatures can accelerate metabolic activity, shorten developmental periods (Bewley & Black, 1994) and cause plants to produce small leaves and extensive root systems to offset water loss from the leaves or to maintain water intake relative to leaf area (Gliessman, 1998).

Higher temperatures during development decrease individual seed mass in many species (Shimzu et al., 1979; Ong, 1983; Wulff, 1986; Roach & Wulff, 1987), whereas lower temperatures increase seed mass in others (Lacey et al., 1997). Even small differences in temperature during seed development may affect both seed mass, especially pericarp tissue (Gray et al., 1988), and the germination response of the progeny to temperature (Wulff, 1995).

Higher temperatures during seed development increase seed germinability in many species (Grant-Lipp & Ballard, 1963; Peters, 1982; Alexander & Wulff, 1985; Drew & Brocklehurst, 1990). However, in some cases an inverse relationship has been found, where higher temperatures caused increased seed dormancy (Karssen, 1970; Argel & Humphreys, 1983; Keigley & Mullen, 1986; Govinthasamy, 1994; Hume, 1994).

After dispersal, seeds of many species do not germinate until they experience a period of after-ripening (Baskin & Baskin, 1998). Environmental factors such as moisture, temperature and oxygen can affect after-ripening (Bewley & Black, 1994). During after-ripening, the narrow range of conditions under which seeds can germinate gradually becomes wider. Also, during after-ripening, physical and chemical changes that alter the tensile strength of seed coats often occur in seeds and increase their permeability to water and gases. In some cases, changes can occur in the embryo or the embryo coverings (Kozlowski & Pallardy, 1997).

An inverse relationship between seed moisture content and maximum after-ripening (seen as germination) has been found in several species (Foley, 1994; Mohamed et al., 1998). Seeds of some species lose their impermeability to water after a certain time, even under benign conditions of storage. On the surfaces of many seeds there are cracks that become deeper over time and lead to increased germinability (Werker, 1997).

Whether on the mother plant or in the soil, seeds from one population or one plant or even one flower head do not experience the same environmental conditions during ripening (Gutterman, 1985). For this reason, even seeds from the same genotype can have different degrees of dormancy. Even though the effects of temperature and after-ripening have been reported for many species, little is known about the mechanisms of these effects on germination patterns.

To explore the underlying causes of after-ripening and temperature during seed development, Scotch thistle (Onopordum acanthium) was chosen, because this species responds to changes in temperatures during development or after dispersal and exhibits intermittent germination. This species has both physical and physiological cypsela (seed) dormancy (Qaderi, 2002) and becomes more germinable after dry storage, regardless of conditions and duration (M. Qaderi, A. Presti & P. Cavers, unpublished data). Because every cypsela has its own microenvironment, either on the mother plant or in the soil, its germinability may be different from that of others. Variability in germinability among cypselas leads to intermittency of seedling emergence, which has not been documented extensively and thus needs further study.

Recently, Qaderi & Cavers (2000) have shown that plants grown in a glasshouse produced more readily germinable cypselas than those from the same population grown under field conditions during the same period. From this, we hypothesized that contrasting temperatures during plant growth and cypsela development of O. acanthium, which affect the developmental processes and morphological characteristics of plants and cypselas, result in differences in the germinability of fresh and after-ripened cypselas. Experiments were designed to determine the effects of high and low temperatures on plant developmental processes, including flowering time and period, cypsela developmental stage (e.g. embryogenesis, maturation and desiccation), plant vegetative characteristics (including stem height and diameter, leaf size and above- and below-ground biomass) and reproductive yield (including capitulum size, cypsela number and weight) of O. acanthium. The relationship between germination responses and physical and structural characteristics of cypselas was investigated for cypselas matured under different conditions. This is one of the first studies in which the effects of after-ripening and temperature during maturation on germination patterns and the structural characteristics of seeds have been investigated in the same batch of seeds (cypselas).

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Phenological and morphological characteristics of plants

Scotch thistle (Onopordum acanthium) plants were grown either in the field or in the glasshouse in 2000 and 2001. For details see Table 1. Each year, 60 plants of the same population started at the same time were grown in the field, and 40 or 50 more plants were grown in each of two rooms in the glasshouse, with mean daily temperature regimes of either 29.1°C and 20.8°C (2000) or 32.0°C and 22.4°C (2001) (Fig. 1, Table 1). Throughout the remainder of this paper the terms ‘high(er) temperature’ and ‘low(er) temperature’ are used to describe the two contrasting growth conditions used in each year and are not meant to indicate growth conditions relative to an optimum temperature. In each glasshouse, the plants were maintained at field capacity to prevent drought effects. The relative humidity, measured with a thermohygrometer (Oakton, Taiwan), was 65.6 and 76.3% (2000) or 59.8 and 63.5% (2001) in the higher and lower temperature regimes, respectively. The mean photosynthetically active radiation (PAR), measured in early summer on a sunny day between 1 and 2 pm with an LI-189 Meter (LI-COR, Inc., Lincoln, NB, USA), was 381.7 and 375.4 (2000) or 392.3 and 380.7 (2001) µmol photons m−2 s−1 at the tops of plants (n = 50) for the glasshouses with high and low temperatures, respectively. In 2000 and 2001, dates of anthesis (first flowers) were recorded for both the field- and glasshouse-grown plants and morphological characteristics for the glasshouse-grown plants only. Throughout the growing period, stem height and diameter were measured by means of a measuring tape and leaf area by means of an area meter (LI-3100 Area Meter, LI-COR, Inc.). For each plant, numbers of leaves and capitula were also recorded. However, only data from the last stage of growth, which show the extreme differences between plants that were grown under high and low temperatures, are reported here. At the end of the growing season, roots and shoots were harvested from each plant individually, dried to constant weight (72 h at 60°C in a forced-air oven), weighed and the root to shoot weight ratio was calculated.

Table 1.  Time-course of events for Scotch thistle (Onopordum acanthium) cypselas from two populations matured under high and low temperatures in two glasshouses of the Department of Plant Sciences, University of Western Ontario, London, Canada, in the summers of 2000 and 2001
Population1Cypselas set to germinateSeedlings transferred to flatsSeedlings started hardening in cold frameSeedlings planted outside at ESWRosettes excavated and pottedRosettes recovery from shock2Rosettes placed under designated temperatureCypselas collectedPlants harvested
  • 1

    The source of these populations was cypselas collected on September 4, 1996.

  • 2

    Until placed under designated temperature, all rosettes were kept in a glasshouse with mean daily temperature of c. 20°C, 65% RH and 380 µmol photons m−2 s−1 PAR.

  • 3

    In 1996, plants from the Fanshawe Conservation Area (FCA) population had grown in situ, while those of the Gibbons Park (GP) population were transplanted to the ESW Research Station (on May 17), because they would have been destroyed in their original site.

  • 4

    4 E, early; L, late.

2000
FCA3May 04, 99May 17, 99June 21, 99June 25, 99April 06, 00April 07, 00May 18, 00High temp:August 1, 00
E4– June 23, 00
L – July 07, 00
Low temp:
E – July 05, 00
L – July 18, 00
2001
GPMarch 31, 00April 09, 00June 29, 00July 04, 00April 16, 01April 17, 01May 10, 01High temp:August 16, 01
E – June 28, 01
L – July 09, 01
Low temp:
E – July 19, 01
L – August 01, 01
image

Figure 1. Mean daily high (open circles) and low (solid circles) temperatures in the Plant Sciences glasshouses of the University of Western Ontario, London, Canada, in (a) 2000 and (b) 2001.

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Cypsela collection: fully matured stage

Two bulk collections of cypselas of O. acanthium were made from the glasshouse-grown plants in each of the high and low temperature rooms (Table 1). After harvest, the cypselas were cleaned manually. Aborted cypselas were discarded. Cypselas were either tested fresh or after dry storage for 4 months at 25°C.

Cypsela collection: developmental stages

Cypselas were collected six times during their development (two collections from each of the embryogenesis, maturation and desiccation stages) from field-grown plants at the ESW Research Station and from glasshouse-grown plants (under high and low temperatures) in 2000 and 2001. The cypselas were either used fresh or stored dry inside the capitula at −20°C until used for further structural examination.

Germination tests

Cypselas were tested for germination essentially as described by Qaderi & Cavers (2000). From each collection from each of the growth conditions, five replicates of 100 fresh or stored cypselas were tested. Each replicate was placed in a 9-cm diameter glass Petri dish on one layer of blue germination filter paper (Anchor Paper Co.; St Paul, MN, USA) initially moistened with 10 ml of distilled water. More water was added each day as needed. The cypselas were set to germinate in an incubator at 25°C for 14 h in light, and at 10°C for 10 h in dark. Light was provided by two cool white fluorescent tubes (mean PAR of 42.2 µmol photons m−2 s−1 at the level of the Petri dishes, n = 60) situated c. 25 cm above the surfaces of the Petri dishes. Germinated cypselas (radicle 2 mm or longer) were counted and removed daily. The dishes were arranged randomly in the incubator at the beginning of the experiment and replaced in different random patterns after each daily germination count. The experiment was terminated after a 5-d period with no germination (typically after 45 d). At the end of the experiment, firm nongerminated cypselas were tested for viability by cutting them 1 mm from the cotyledonary end and setting them to germinate in the incubator as above.

Cypsela imbibition

Four replicates of 50 fresh cypselas from the glasshouse-grown plants from each of two rooms (2001) were weighed by means of an electrobalance (Precisa 80 A, PAG Oerlikon AG, Zurich, Switzerland). Each replicate was placed in a 9-cm diameter glass Petri dish on one layer of blue germination filter paper initially moistened with 10 ml of distilled water. Then, the plates were covered with lids to prevent water loss and placed in an incubator set at 25°C for 14 h in the light and 10°C for 10 h in the dark. Every two hours for 12 h and again after 24, 36 and 48 h, the cypselas were removed from the dishes, blotted dry with a paper towel and weighed (method of Bansal et al., 1980). In each case, except for the last measurement of the time course, cypselas were replaced in the dishes. More water was added as needed. The increased cypsela weight after imbibition was calculated according to eqn 1, where CWI (%) is the cypsela weight increase in percent, CWAI the cypsela weight after imbibition and CWBI the cypsela weight before imbibition.

  • CWI (%) = [(CWAI−CWBI)/CWBI] × 100(Eqn 1)

Determination of the physical characteristics of capitula and cypselas

Shortly after each collection, 10 mature capitula were selected randomly from plants grown under high and low temperatures (in the glasshouse) in 2000 and 2001. From each capitulum, cypselas were removed by means of forceps, separated into sound (fully developed) and aborted (not developed) categories, counted and weighed. From each growth condition, 50 cypselas were weighed individually by means of an automatic Cahn 25 electrobalance. Capitulum width was measured by means of a digimatic caliper (Mitutoyo Corp., Kanagawa, Japan). Moisture content of cypselas was determined by weighing four replicates of 1 g cypselas, drying them for 1 h at 130°C in a forced-air oven and reweighing, then calculated according to eqn 2, where CMC (%) is the cypsela moisture content in percent, CIW the cypsela initial weight and CWAD the cypsela weight after drying.

  • CMC (%) = [(CIW−CWAD)/CIW] × 100(Eqn 2)

The f. wt, d. wt and water content of cypselas from different developmental stages were determined by weighing four replicates of 0.5 g cypselas, drying them in an oven at 130°C and reweighing. Also, the length and width of each of the 20 cypselas were measured with a caliper. The colours of fresh cypselas at different developmental stages were determined according to the Munsell book of color (Munsell Color Company, 1960).

Ratio between cypsela coat and embryo

Fifty after-ripened cypselas from each of the early and late collections of glasshouse-grown plants (under high and low temperatures) of the GP population (2001) were weighed individually by means of an automatic Cahn 25 electrobalance and then dissected under an illuminating stand magnifier. Embryos were separated from the cypsela coats, and the coat from each cypsela was examined under a dissecting microscope (Wild M3, Wild Leitz Canada) to ensure complete removal of the embryo. Then, each cypsela coat was weighed individually and the ratio between the coat and embryo was calculated.

Scanning electron microscopy

The surface and a transverse cut of cypselas matured under high and low temperatures were examined by means of a scanning electron microscope (S-4500 SEM, Hitachi) using Quartz PCI, Version 5.1 software (1993–2001). To view the fine structures with the SEM, cypselas were coated with gold using the sputter coating method (Flegler et al., 1993). Each treatment consisted of 12 replicates.

Statistical analyses

Final germination percentages, moisture percentages and components of reproductive yield from fresh and stored cypselas matured under high and low temperatures and collected twice in the glasshouses were each analysed by means of a balanced anova. Structural characteristics of plants, data for the imbibition curve and physical characteristics of cypselas from different developmental stages were analysed by means of a one-way anova (Sokal & Rohlf, 1995; SAS, 1999). Before the analyses, percentage data were subjected to the test for homogeneity of variance and then transformed to arc-sine square root of percentage values to normalize the variance (Zar, 1999). A Tukey's multiple comparison test was used to determine differences between treatments (SAS, 1999). Also, Pearson's correlation (r) was used to show both the relationships among components of reproductive yield and the relationship between cypsela d. wt and mean daily temperature during cypsela maturation (Zar, 1999).

The coefficient of germination rate (CGR) was calculated for each replicate according to eqn 3, where N is the final germination percentage, ni the number of germinated cypselas on the particular day on which a count was made, and di the number of days from the start of the experiment (Alm et al., 1993).

  • CGR = N/Σnidi(Eqn 3)

All values of CGR are between 0 (no germination) and 1 (fastest germination rate). A balanced anova and a Tukey's multiple comparison test were then applied (Sokal & Rohlf, 1995; SAS, 1999).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Phenology and morphology of plants

Under higher temperatures, the dates of first flowering were earlier and more synchronized (7 and 10 d between the first flower of the earliest and latest plants; 2000 and 2001, respectively) than those for plants grown under lower temperatures (16 and 22 d; 2000 and 2001, respectively) (Table 2). Also, plants grown under higher temperatures produced more capitula and relatively larger root systems, based on measurements of root : shoot ratios. However, the capitula produced under lower temperatures were larger, with more sound cypselas and fewer aborted ones (Table 5). Also, plants under lower temperatures grew taller, produced thicker stems and had larger leaves (Table 2). The number of leaves per plant was similar between higher and lower temperatures in 2000, but higher for plants grown under higher temperatures in 2001.

Table 2.  Descriptive characteristics for Scotch thistle (Onopordum acanthium) populations grown under two temperature regimes in glasshouses in the summers of 2000 (Fanshawe Conservation Area population) and 2001 (Gibbons Park population)
YearDescriptive characteristicTemperature regime
HighLow
  • 1

    Means followed by different letters within rows are significantly different (P < 0.05) according to Tukey's multiple comparison test.

2000Date of anthesisMay 29–June 04June 06–June 21
Stem height (cm)96.47 ± 2.31 B1119.14 ± 3.11 A
Stem diameter (mm) 9.72 ± 0.23 B 12.01 ± 0.31 A
Leaf number per plant21.33 ± 0.88 A 21.17 ± 0.63 A
Average area per leaf (cm2)89.77 ± 3.08 B141.52 ± 4.26 A
Capitulum number per plant10.27 ± 0.42 A  7.69 ± 0.35 B
Root : shoot weight ratio 0.42 ± 0.02 A  0.23 ± 0.01 B
2001Date of anthesisJune 06–June 15June 21–July 12
Stem height (cm)49.84 ± 1.67 B 85.24 ± 2.35 A
Stem diameter (mm) 6.25 ± 0.18 B  8.59 ± 0.19 A
Leaf number per plant26.11 ± 0.61 A 20.91 ± 0.51 B
Average area per leaf (cm2)51.90 ± 2.74 B108.29 ± 3.69 A
Capitulum number per plant12.61 ± 0.77 A  9.02 ± 0.39 B
Root : shoot weight ratio 0.74 ± 0.04 A  0.41 ± 0.02 B
Table 5.  Components of reproductive yield for Scotch thistle (Onopordum acanthium) populations grown under two temperature regimes in the Plant Sciences glasshouses in the summers of 2000 (Fanshawe Conservation Area (FCA) population) and 2001 (Gibbons Park (GP) population)
YearDescriptive characteristicCollectionTemperature regime
HighLow
  • 1

    Means followed by different upper-case letters within rows or by different lower-case letters within columns for each attribute (early vs late) are significantly different (P < 0.05) according to Tukey's multiple comparison test.

2000Capitulum width (mm)Early 22.35 ± 0.42 Ba1 24.94 ± 0.36 Aa
Late 14.61 ± 0.46 Bb 20.30 ± 0.48 Ab
Sound cypsela no/cap.Early171.20 ± 4.86 Aa179.30 ± 4.28 Aa
Late 51.80 ± 4.79 Bb110.70 ± 7.77 Ab
Aborted cypsela no/cap.Early 13.20 ± 2.02 Ab 10.80 ± 2.14 Aa
Late 58.20 ± 9.48 Aa 12.30 ± 3.30 Ba
Sound/aborted no. ratioEarly 15.78 ± 2.42 Aa 24.27 ± 5.13 Aa
Late  1.37 ± 0.38 Bb 23.31 ± 8.63 Aa
Sound cypsela weight (g)Early  2.26 ± 0.07 Ba  2.80 ± 0.09 Aa
Late  0.59 ± 0.06 Bb  1.60 ± 0.10 Ab
Aborted cypsela weight (g)Early  0.03 ± 0.01 Ab  0.04 ± 0.01 Aa
Late  0.10 ± 0.03 Aa  0.04 ± 0.01 Ba
Sound/aborted weight ratioEarly 93.41 ± 14.37 Aa102.54 ± 19.06 Aa
Late 10.76 ± 2.79 Bb121.86 ± 46.77 Aa
Individual cypsela wt. (mg)Early 13.17 ± 0.15 Ba 15.62 ± 0.23 Aa
Late 11.31 ± 0.16 Bb 14.53 ± 0.42 Ab
Indiv. cypsela dry wt. (mg)Early 11.88 ± 0.14 Ba 13.29 ± 0.20 Aa
Late 10.54 ± 0.15 Bb 12.98 ± 0.38 Aa
2001Capitulum width (mm)Early 20.54 ± 0.34 Ba 24.75 ± 0.47 Aa
Late 15.66 ± 0.59 Bb 20.65 ± 0.59 Ab
Sound cypsela no/cap.Early133.00 ± 4.13 Ba173.10 ± 12.48 Aa
Late 46.00 ± 9.17 Bb101.90 ± 7.46 Ab
Aborted cypsela no/cap.Early 20.10 ± 3.16 Ab 31.90 ± 8.16 Aa
Late 83.60 ± 13.57 Aa 40.90 ± 7.40 Ba
Sound/aborted no. ratioEarly  8.96 ± 1.45 Aa 12.95 ± 3.13 Aa
Late  1.59 ± 0.68 Ab  9.98 ± 5.88 Aa
Sound cypsela weight (g)Early  1.86 ± 0.06 Ba  2.72 ± 0.21 Aa
Late  0.55 ± 0.10 Bb  1.53 ± 0.13 Ab
Aborted cypsela weight (g)Early  0.06 ± 0.01 Ab  0.14 ± 0.04 Aa
Late  0.28 ± 0.07 Aa  0.15 ± 0.03 Aa
Sound/aborted weight ratioEarly 42.87 ± 6.92 Aa 58.36 ± 14.49 Aa
Late 11.04 ± 4.42 Ab 55.02 ± 35.73 Aa
Individual cypsela wt. (mg)Early 13.96 ± 0.09 Ba 15.75 ± 0.41 Aa
Late 12.63 ± 0.50 Ba 14.96 ± 0.53 Aa
Indiv. cypsela dry wt. (mg)Early 12.67 ± 0.09 Aa 13.39 ± 0.38 Aa
Late 11.69 ± 0.51 Aa 12.96 ± 0.50 Aa

Developmental stages of cypselas

Ripening of O. acanthium capitula was divided into three stages from flowering to desiccation. Capitula were collected during embryogenesis (days 1 and 6), maturation (days 14 and 26) and desiccation (days 34 and 40) (Fig. 2, Table 3). In the early embryogenesis stage, cypselas had a pale yellow colour, which gradually changed during development and turned to dark brown by the late desiccation stage. The length and width of cypselas increased until the later stages of maturation, and there was no significant change in these characteristics afterwards (Table 3).

image

Figure 2. Developmental stages of Scotch thistle (Onopordum acanthium) capitula and cypselas. Collections were made from the Environmnetal Sciences Western (ESW) population grown in the field at the ESW Research Station in 2001. Developmental stages: (a–b) embryogenesis, (c–d) maturation and (e–f) desiccation. Capitula and cypselas are shown at 55% of actual size.

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Table 3.  Descriptive characteristics of developing Scotch thistle (Onopordum acanthium) cypselas collected from the ESW population grown at the ESW Research Station, London, Canada in 2001
Developmental stage (days after flowering)Collection date (days after flowering)Cypsela length (mm)Cypsela width (mm)Cypsela f. wt (mg)Cypsela d. wt (mg)Cypsela moisture content (%)Cypsela colour
  • 1

    Means followed by different letters within columns are significantly different (P < 0.05) according to Tukey's multiple comparison test.

Embryogenesis
(1–5) 12.96 ± 0.09d11.12 ± 0.02c 3.44 ± 0.26e 0.75 ± 0.05f78.00 ± 0.44aPale yellow
(6–12) 64.63 ± 0.09c1.69 ± 0.09b10.34 ± 0.49d 2.52 ± 0.11e75.56 ± 0.29bLight green
Maturation
(13–20)144.85 ± 0.07bc1.77 ± 0.07b14.39 ± 0.35c 3.97 ± 0.08d72.39 ± 0.17cDark green
(21–32)265.13 ± 0.06ab2.68 ± 0.05a16.63 ± 0.26ab10.43 ± 0.08c37.24 ± 0.56dLight brown
Desiccation
(33–37)345.33 ± 0.06a2.93 ± 0.07a18.02 ± 0.36a14.77 ± 0.24a18.01 ± 0.51eMid-brown
(38–40)405.39 ± 0.06a2.88 ± 0.01a15.52 ± 0.14bc14.01 ± 0.13b 9.73 ± 0.18fDark brown

During embryogenesis and the early stages of maturation, the water content of cypselas was high, but it gradually decreased during the maturation and desiccation stages. After flowering, weights of both fresh and dry cypselas increased until the early stage of desiccation; thereafter they decreased (Table 3).

In both years, all three developmental stages were shorter for cypselas that matured under higher temperatures. The period of maturation was almost twice as long as those of embryogenesis and desiccation, the latter of which was the shortest (Fig. 3).

image

Figure 3. Durations of developmental stages of Scotch thistle (Onopordum acanthium) cypselas. Collections were made from plants grown under high and low temperatures in the Plant Sciences glasshouses, University of Western Ontario, London, Canada, in 2000 (Fanshawe Conservation Area (FCA) population) and 2001 (Gibbons Park (GP) population). Developmental stages: embryogenesis (open bars), maturation (slanted-line bars) and desiccation (hatched bars).

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Reproductive yield

Differences between growth temperatures, collections and in some cases their two-way interactions were significant for the width of capitula, the number and weight of sound cypselas and the number of aborted cypselas (Table 4).

Table 4.  Summary of analyses of variance for components of reproductive yield of Scotch thistle (Onopodum acanthium) populations grown in glasshouses under two temperature regimes in the Plant Sciences glasshouses in the summers of 2000 (Fanshawe Conservation Area (FCA) population) and 2001 (Gibbons Park (GP) population)
Descriptive characteristicSource20002001
dfMSFdfMSF
  1. Significance values: *P < 0.05; **P < 0.01; ***P < 0.001. Abbreviations: d.f., degrees of freedom; MS, mean square (variance); F, variance ratio.

Capitulum widthTemperature (T) 1  171.6 91.2*** 1  211.568.2***
Collection (C) 1  383.3203.8*** 1  201.364.9***
T × C 1   24.0 12.8** 1    1.5 0.5
Error36    1.9 36    3.1 
Sound cypsela no/cap.Temperature (T) 111223.0 35.8*** 123040.024.6***
Collection (C) 188360.0282.0*** 162568.066.7***
T × C 1 6452.0 20.6*** 1  624.0 0.7
Error36  313.0 36  938.0 
Aborted cypsela no/cap.Temperature (T) 1 5832.2 21.3*** 1 2387.0 2.5
Collection (C) 1 5405.6 19.8*** 113140.613.9**
T × C 1 4730.6 17.3*** 1 7425.6 7.8**
Error36  273.6 36  946.3 
Sound/aborted no. ratioTemperature (T) 1 2316.3  8.7** 1  383.7 2.7
Collection (C) 1  590.6  2.2 1  267.0 1.9
T × C 1  452.1  1.7 1   48.5 0.3
Error36  267.0 36  140.9 
Sound cypsela weightTemperature (T) 1    6.1 87.4*** 1    8.538.3***
Collection (C) 1   20.7297.8*** 1   15.670.2***
T × C 1    0.5  7.7** 1    0.0 0.1
Error36    0.07 36    0.2 
Aborted cypsela weightTemperature (T) 1    0.006  2.2 1    0.007 0.3
Collection (C) 1    0.011  3.7 1    0.148 5.9*
T × C 1    0.016  5.5* 1    0.106 4.2*
Error36    0.003 36    0.025 
Sound/aborted wt. ratioTemperature (T) 136136.0  5.2* 1 8842.0 1.9
Collection (C) 110026.0  1.4 1 3093.0 0.7
T × C 126000.0  3.8 1 2029.0 0.4
Error36 6912.0 36 4663.0 
Individual cypsela weightTemperature (T) 1   80.3112.3*** 1   42.319.9***
Collection (C) 1   21.7 30.4*** 1   11.2 5.2*
T × C 1    1.5  2.1 1    0.7 0.3
Error36    0.7 36    2.1 
Individual cypsela d. wt.Temperature (T) 1   37.1 65.5*** 1    9.9 5.9*
Collection (C) 1    6.8 12.1** 1    5.0 3.0
T × C 1    2.6  4.7* 1    0.7 0.4
Error36    0.6 36    1.7 

In both years, plants grown under lower temperatures produced significantly wider capitula containing heavier sound cypselas than those grown under higher temperatures. In both years, the number of aborted cypselas was lower for the late collection from the lower temperature treatment. In both years, under higher temperatures, the number of aborted cypselas was higher for the late-maturing cypselas, while the number and weight ratios between sound and aborted cypselas were significantly higher for the early-maturing cypselas. These differences were not seen under lower temperatures. In both years, within each temperature regime, sound cypselas from the early collection were heavier than those from the late collection (Table 5). In both years, cypselas that matured under lower temperatures were heavier. After drying, however, there was no difference in weight between cypselas matured in 2001 (Table 5). Pearson's correlation analysis demonstrated that cypsela dry weight was negatively correlated with mean daily temperature during cypsela maturation (r = −0.74, P < 0.001 and r = −0.36, P < 0.05; 2000 and 2001, respectively).

Pearson's correlation analysis showed many significant relationships between different reproductive parameters. For example, sound cypsela number (0.93 and 0.78; 2000 and 2001, respectively) and weight (0.96 and 0.84; 2000 and 2001, respectively) were positively correlated with capitulum width, while aborted cypsela number (−0.64 and −0.40; 2000 and 2001, respectively) and weight (−0.34 and −0.18; 2000 and 2001, respectively) were negatively correlated with it. In both years, there were significant correlations between capitulum width and individual cypsela weight (0.77 and 0.72; 2000 and 2001, respectively) (Table 6).

Table 6.  Pearson's correlation coefficients between capitulum width, cypsela number, cypsela weight, the number and weight ratio between sound and aborted cypselas and one cypsela weight from Scotch thistle (Onopordum acanthium) populations grown in glasshouses in the summers of 2000 (Fanshawe Conservation Area (FCA), bottom-left) and 2001 (Gibbons Park (GP), top-right)
 Capitulum widthSound cypsela numberAborted cypsela numberSound/ aborted cypsela numberSound cypsela weightAborted cypsela weightSound/ aborted cypsela weightOne cypsela weight
  • 1

    Correlation coefficients are significant if |r| is greater than 0.27 (P < 0.1), 0.32 (P < 0.05), 0.38 (P < 0.02), 0.42 (P < 0.01), 0.45 (P < 0.005) or 0.52 (P < 0.001).

Capitulum width1.000.78−0.400.360.84−0.180.270.72
Sound cypsela number 0.931.00−0.790.530.98−0.680.420.39
Aborted cypsela number−0.64−0.731.00−0.56−0.720.89−0.51−0.12
Sound/aborted cypsela number0.480.40−0.551.000.53−0.460.980.24
Sound cypsela weight0.960.98−0.730.491.00−0.580.420.55
Aborted cypsela weight−0.34−0.490.77−0.46−0.461.00−0.430.16
Sound/aborted cypsela weight0.420.37−0.550.960.40−0.491.000.18
Individual cypsela weight0.770.61−0.650.560.75−0.300.451.00

Structural and physical characteristics of cypselas

Cypsela weight ratio Overall, cypselas that matured under lower temperatures and those from early collections had higher weights for intact cypselas, cypsela coats and embryos. For the weights of intact cypselas, cypsela coats and embryos, in 2000, temperature and collection date, and in 2001, only temperature, had significant effects (Table 7). For the ratio between cypsela coats and embryos, in 2000, collection date and the two-way interaction between temperature × collection, and in 2001, temperature, had significant effects (Table 7).

Table 7.  Summary of analyses of variance for weights of various cypsela for two collections of Scotch thistle (Onopordum acanthium) components matured under high and low temperatures in glasshouses in the summers of 2000 (Fanshawe Conservation Area (FCA) population) and 2001 (Gibbons Park (GP) population)
Descriptive characteristicSource20002001
dfMSFdfMSF
  • 1

    CC, Cypsela coat; E, Embryo. Significance values:

  • *

    P < 0.05;

  • **

    P < 0.01;

  • ***

    P < 0.001. Abbreviations: d.f., degrees of freedom; MS, mean square (variance); F, variance ratio.

Intact cypsela weightTemperature (T) 161.870.8*** 114.617.2***
Collection (C) 134.139.0*** 1 1.5 1.8
T × C 1 0.9 1.1 1 0.5 0.6
Error44 0.9 44 0.8 
Cypsela coat weightTemperature (T) 124.763.0*** 1 4.410.3**
Collection (C) 120.351.7*** 1 0.9 2.1
T × C 1 0.7 1.8 1 0.2 0.4
Error44 0.4 44 0.4 
Embryo weightTemperature (T) 1 8.473.0*** 1 3.032.9***
Collection (C) 1 1.815.5*** 1 0.1 0.9
T × C 1 0.0 0.2 1 0.1 0.9
Error44 0.1 44 0.1 
CC/E weight ratio1Temperature (T) 1 0.001 0.3 1 0.043 9.2**
Collection (C) 1 0.23754.8*** 1 0.006 1.3
T × C 1 0.026 6.0* 1 0.000 0.1
Error44 0.004 44 0.005 

In 2000, plants that were grown under lower temperatures produced cypselas that had heavier cypsela coats, embryos and total weights than those that were grown under higher temperatures. However, there was no difference in the ratio between cypsela coats and embryos for cypselas matured under either condition. In 2001, plants that were grown under lower temperatures produced heavier cypselas and cypsela coats for the late collection only and heavier embryos for both collections than those that were grown under higher temperatures (Table 8).

Table 8.  Weights (mean ± SE) of intact cypselas, cypsela coats and embryos and the weight ratio between cypsela coats and embryos for two collections of Scotch thistle (Oropordum acanthium) cypselas matured under high and low temperatures in glasshouses in the summers of 2000 (Fanshawe Conservation Area (FCA) population) and 2001 (Gibbons Park (GP) population)
YearDescriptive characteristicCollectionTemperature regime
HighLow
  • 1

    Means followed by different upper-case letters within rows or by different lower-case letters within columns for each attribute (early vs late) are significantly different (P < 0.05) according to Tukey's multiple comparison test.

  • 2

    2 CC, Cypsela coat; E, Embryo.

2000Intact cypsela weight (mg)Early12.72 ± 0.29 Ba114.71 ± 0.21 Aa
Late10.75 ± 0.28 Bb13.30 ± 0.30 Ab
Cypsela coat weight (mg)Early 8.10 ± 0.19 Ba 9.30 ± 0.14 Aa
Late 6.56 ± 0.18 Bb 8.24 ± 0.21 Ab
Embryo weight (mg)Early 4.62 ± 0.10 Ba 5.41 ± 0.08 Aa
Late 4.19 ± 0.11 Bb 5.07 ± 0.10 Aa
CC : E weight ratio2Early 1.75 ± 0.01 Aa 1.72 ± 0.02 Aa
Late 1.57 ± 0.02 Ab 1.62 ± 0.02 Ab
2001Intact cypsela weight (mg)Early13.89 ± 0.21 Aa14.79 ± 0.25 Aa
Late13.33 ± 0.29 Ba14.63 ± 0.31 Aa
Cypsela coat weight (mg)Early 9.03 ± 0.15 Aa 9.52 ± 0.18 Aa
Late 8.64 ± 0.20 Ba 9.36 ± 0.21 Aa
Embryo weight (mg)Early 4.86 ± 0.06 Ba 5.27 ± 0.07 Aa
Late 4.69 ± 0.09 Ba 5.27 ± 0.11 Aa
CC : E weight ratioEarly 1.86 ± 0.02 Aa 1.80 ± 0.02 Aa
Late 1.84 ± 0.02 Aa 1.78 ± 0.02 Aa

In 2000, except for the embryo weights of cypselas matured under lower temperatures, weights of the intact cypselas, cypsela coats and embryos, and the cypsela coat to embryo ratio were higher for the early-collected cypselas that matured under both higher and lower temperatures. However, in 2001, there was no difference in any of these parameters between early and late-collected cypselas matured under either condition (Table 8).

Scanning electron microscopy Scanning electron microscopy of surface and transverse sections revealed differences between fresh cypselas matured under higher and lower temperatures. Cypselas that matured under higher temperatures had rougher surfaces (Fig. 4a) with more grooves (Fig. 4b). The grooves were narrow (Fig. 4c) and deep (Fig. 4e) for the cypselas matured under higher temperatures and wide (Fig. 4d) and shallow (Fig. 4f) for those matured under lower temperatures. Also, cypselas that matured under higher temperatures had thinner cypsela coats (Fig. 4g–h). In both years, similar patterns were found. No obvious changes in surface or internal structure were apparent after dry storage (data not shown).

image

Figure 4. Scanning electron microscopy of Scotch thistle (Onopordum acanthium) cypselas. Cypselas were matured under high (a, c, e, g) and low (b, d, f, h) temperatures in the Plant Sciences glasshouses, University of Western Ontario, London, Canada, in 2000. (a–f) Surface view and (g–h) transverse view. cc, cypsela coat; emb, embryo. Magnification: a–b, X200; c–d, X600; e–f, X6000; g–h, X200.

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Water content of cypselas Fresh cypselas collected early or those matured under lower temperatures had the highest water content. In both years (2000–01), temperature, collection date, cypsela state (fresh or stored), the two–way interactions between temperature × state and collection × state and the three–way interaction among temperature × collection × state were significant (Table 9). In both years, fresh cypselas that matured under lower temperatures had higher water content than those that matured under higher temperatures. However, after storage these differences had disappeared (Table 10).

Table 9.  Summary of analyses of variance for germination percentages, coefficients of germination rate and moisture percentages of Scotch thistle (Onopordum acanthium) cypselas matured under high and low temperatures in glasshouses in the summers of 2000 (Fanshawe Conservation Area (FCA) population) and 2001 (Gibbons Park (GP) population)
CharacteristicSource20002001
dfMSFdfMSF
  1. Significance values: *P < 0.05; **P < 0.01; ***P < 0.001. Abbreviations: d.f., degrees of freedom; MS, mean square (variance); F, variance ratio.

Moisture percentageTemperature (T)1  32.6789.8***156.6863.6***
Collection (C)1  27.7671.7***14.772.1***
State (S)1 256.96231.4***1323.34936.4***
T × C1   0.01.010.00.1
T × S1  37.6912.6***156.5863.0***
C × S1  18.1438.4***15.483.1***
T × C × S1   0.36.9*10.69.4**
Error24   0.0 240.1 
Germination percentageTemperature (T)17191.7628.0***12931.5429.2***
Collection (C)11366.1119.3***1694.8101.7***
State (S)14931.8430.7***13022.7442.5***
T × C1   7.70.710.90.1
T × S1 117.510.3**1269.239.4***
C × S11877.5164.0***1227.533.3***
T × C × S1 333.929.2***1135.119.8***
Error32  11.4 326.8 
Germination rateTemperature (T)1   0.00052.0 10.00192.6
Collection (C)1   0.006426.5***10.00739.9**
State (S)1   0.0325134.1***10.0766103.9***
T × C1   0.00000.010.00334.5*
T × S1   0.00010.3 10.00395.3*
C × S1   0.003413.9**10.00121.6
T × C × S1   0.00072.910.00131.7
Error32   0.0002 320.0007 
Table 10.  Germination percentages, coefficients of germination rate and moisture percentages (mean ± SE) of Scotch thistle (Onopordum acanthium) cypselas matured under high and low temperatures in the Plant Sciences glasshouses in the summers of 2000 (FCA population) and 2001 (GP population)
YearCollectionCypsela stateGermination percentageCoefficients of germination rate2Moisture percentage
High temperaturesLow temperaturesHigh temperaturesLow temperaturesHigh temperaturesLow temperatures
  • 1

    Means followed by different upper-case letters within rows and treatments or by different lower-case letters within columns and years are significantly different (P < 0.05) according to Tukey's multiple comparison test.

  • 2

    2 For coefficients of germination rate, all means have been multiplied by 100 to facilitate data interpretation.

  • 3

    Both fresh and stored (4 months at 25°C) cypselas were incubated under 25°C for 14 h light and 10°C for 10 h dark..

2000EarlyFresh386.80 ± 2.27 Ab139.60 ± 1.63 Bb 6.70 ± 0.60Ac 5.50 ± 0.49Ab9.82 ± 0.13Ba14.91 ± 0.23Aa
Stored93.40 ± 0.68 Aab58.40 ± 0.98 Ba13.68 ± 0.58Aa13.60 ± 1.20Aa5.41 ± 0.01Ac 5.20 ± 0.00Ac
LateFresh36.20 ± 0.97 Ac12.00 ± 0.55 Bc 5.22 ± 0.64Ac 5.58 ± 0.33Ab6.84 ± 0.09Bb10.69 ± 0.15Ab
Stored96.80 ± 1.50 Aa53.40 ± 1.25 Ba10.21 ± 0.18Ab 8.32 ± 0.98Ab5.04 ± 0.03Ac 5.01 ± 0.01Ac
2001EarlyFresh88.60 ± 2.48 Aa49.00 ± 2.24 Bc13.65 ± 1.40Abc10.97 ± 0.71Ab9.28 ± 0.18Ba14.95 ± 0.18Aa
Stored92.80 ± 0.49 Aa83.20 ± 1.62 Ba18.21 ± 1.64Aab21.74 ± 1.23Aa5.01 ± 0.03Ac 5.20 ± 0.02Ac
LateFresh64.80 ± 1.65 Ab33.40 ± 0.51 Bd10.56 ± 0.38Ac 6.49 ± 0.47Ab7.44 ± 0.02Bb13.39 ± 0.33Ab
Stored92.60 ± 0.68 Aa73.40 ± 1.12 Bb19.54 ± 1.53Aa17.19 ± 1.57Aa5.24 ± 0.01Ac 5.06 ± 0.02Ac

Within both temperature treatments, in both years, fresh cypselas that were collected early had higher water content than those that were collected late, but after storage there was no difference between them (Table 10). Fresh cypselas from the early collection of the lower temperature treatment (2001) had the highest water content (14.95%), while after-ripened cypselas from both the late collection of the lower temperature treatment (2000) and the early collection from higher temperatures (2001) had the lowest (5.01%) (Table 10).

Cypsela imbibition The weights of cypselas from both maturation conditions increased after two hours soaking in water and the pattern of increase was similar between them over 48 h, but the cypselas from the lower temperatures were heavier throughout the imbibition period. Initially, the weight of 50 cypselas was 0.07 g higher for those matured at lower temperatures and the difference was even greater (0.1 g) after 24 h of imbibition.

Germination responses of cypselas Germinability was higher for the cypselas that matured under higher temperatures and were collected early in the season or were stored dry. In both years, temperature, collection date, cypsela state, the two–way interactions between temperature × cypsela state and collection × cypsela state and the three–way interaction among temperature × collection × cypsela state were significant (Table 9). In both years, germination percentages were higher for both fresh and after-ripened cypselas that matured under higher temperatures than for those that matured under lower temperatures (Table 10). Both the highest and the lowest germination percentages were from cypselas that were collected late in 2000. The highest value (96.8%) was from the after-ripened cypselas that had matured under higher temperatures, and the lowest value (12%) was from the fresh cypselas that had matured under lower temperatures (Table 10).

In both years, within the higher temperature treatment, there was no difference between fresh and stored cypselas of early collections, but for the late-collected cypselas after-ripening significantly increased germinability. Within the lower temperature treatment, in both years, the after-ripened cypselas of both collections had significantly higher germination percentages than the fresh ones (Table 10). In all treatments, nongerminated cypselas had more than 99% viability (data not shown).

Cypselas that experienced after-ripening and those that were collected early, germinated quickly. In both years, collection date and cypsela state were significant. However, in 2000, only the two-way interaction between collection × cypsela state, and in 2001, the two-way interactions between temperature × collection and temperature × cypsela state, were significant (Table 9). In both years, the rates of germination for cypselas that matured under higher and lower temperatures were similar (Table 10).

Within the higher temperature treatment, there was no difference in the rate of germination between fresh cypselas from the early and late collections. In 2000, after-ripened cypselas from both collections germinated faster than the fresh ones, but in 2001, only after-ripened cypselas from the late collection germinated faster (Table 10). Within the lower temperature treatment, the fresh cypselas of the two collections each year had similar rates of germination. Except for the late collection of 2000, after-ripening increased the rate of germination (Table 10). The fastest rate of germination (19.54) was from the after-ripened cypselas of the late collection in 2001 and the slowest (5.22) from the fresh cypselas matured under high temperatures and collected late in 2000 (Table 10).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Vegetative and reproductive characteristics of plants

The overall morphology of O. acanthium is dependent on temperatures during growth and development. At the higher growth temperatures, plant height, leaf area and stem diameter were lower but the root system was larger than at lower growth temperatures (Table 2). These characteristics may help plants reduce water loss and withstand high temperatures. The smaller size of plants grown under higher temperatures could have resulted from the relatively smaller differences between day and night temperatures (7.2°C and 9.9°C, 2000 and 2001, respectively) in that treatment compared with those in the lower temperature regime (11.5°C and 13.6°C, 2000 and 2001, respectively) (Fig. 1). At higher temperatures, growth inhibition can also be related to excessive transpiration, which can deplete carbohydrate pools used for plant growth (Kozlowski & Pallardy, 1997). There is physiological evidence that plants sacrifice photosynthesis and growth for the sake of reproduction (Abrahamson & Caswell, 1982) and this was evident in our study because plants flowered earlier when grown under higher temperatures.

In most species, photoperiod regulates flowering. However, photoperiod effects are modified by other factors, such as temperature and light quality (Bewley et al., 2000). Plants grown under the warmer regime reproduced more quickly (Tables 2 and 10, Fig. 4). However, a higher proportion of developing cypselas from the warmer regime aborted (Table 5). This could have been related to changes in physiological processes during plant development (Larcher, 1995). Abortion of some cypselas may have beneficial effects on the remaining cypselas because larger amounts of necessary resources can be allocated to them (Bookman, 1983).

Developmental stages of cypselas

The different embryo developmental stages have been described for many species (Norton & Harris, 1975; Crouch & Sussex, 1981; Goldberg et al., 1989; Comai & Harada, 1990; Dasgupta & Mandal, 1993; Ren & Bewley, 1998; Bhattacharya et al., 2002). However, no previous studies have reported embryo development in thistle species. Here, embryo development in O. acanthium was examined and divided into three stages based on water content, colour and morphology: embryogenesis, embryo maturation and desiccation (Table 3). These stages were easily distinguishable from each other (Table 3, Fig. 2). Overall, these stages are similar to the sequence described by Bewley & Black (1994).

Although there are many reports on seed developmental stages for different species, no comparative study has been found to show differences in developmental stages for a single species grown under contrasting temperatures. In this study, all stages of embryo development were of shorter duration under the warmer growth conditions (Fig. 3) and this acceleration of development may be related to higher metabolic activity than would occur under cooler conditions (Fitter & Hay, 2002). Because seed development can be affected by different genetic and physiological as well as environmental factors, it cannot be temporally uniform even if plants are grown under identical environments (Ren & Bewley, 1998).

Structural and physical characteristics of cypselas

Under higher temperatures O. acanthium plants produced smaller and lighter cypselas with thinner coats and rougher surfaces (Tables 5 and 8, Fig. 4). Because the duration between flowering and maturation was shorter for these plants, it is possible that the physiological and biochemical processes that led to cypsela maturation were accelerated by the higher temperatures (Bewley & Black, 1994). High temperatures increase the rate of ripening and decrease the period of time available for assimilation of photosynthates by seeds (Fenner, 1992). There is good evidence that the environment, by changing the maternal phenotype during seed development, may affect seed mass and quality (Gutterman, 1980/81). However, the physiological basis for nongenetic maternal effects in seed production is unclear (Roach & Wulff, 1987). In Daucus carota, Gray et al. (1988) found that higher temperatures caused a significant reduction in pericarp tissue but not in weights of embryo and endosperm.

Onopordum acanthium cypselas that matured under higher temperatures also had lower water content. This could have resulted from a higher rate of transpiration through leaves, even though their relatively larger root system might have compensated for loss of water from aerial parts of the plant (Gliessman, 1998). In the higher temperature regime, the cypselas with lower water contents would need less desiccation during maturation and thus might ripen faster (Table 10, Fig. 3). Also, the lack of water during maturation could affect maturation processes that require a minimum content of water to proceed.

Germination patterns of cypselas

Temperature during development can affect seed germinability (Fenner, 1992) although different species respond differently to it. For example, seeds of Amaranthus retroflexus had a higher percent germination when matured under 27/22°C than under 22/17°C day/night temperature regimes (Gutterman, 2000b). In our study, higher temperatures during maturation also resulted in cypselas that were more readily germinable. Higher and faster germinability of these cypselas was correlated with their lower moisture content (Table 10), thinner cypsela coats (Table 8) and rougher surfaces (Fig. 4a,c). A thinner cypsela coat with a rougher surface could have been broken easily by internal pressure caused by growth and expansion of the embryo. By contrast, a thicker seed coat with a smoother surface (covered with wax) could cause physical dormancy in seeds (Werker, 1997). In several cases, it has been shown that a seed coat is less permeable to oxygen than to water (Bewley & Black, 1994). Similar effects of seed coat structure on seed germinability have been reported for several species including Glycine max (Harris, 1987), Lupinus angustifolius (Valenti et al., 1989) and Arabidopsis thaliana (Léon-Kloosterziel et al., 1994).

Increased seed germinability after a period of after-ripening has been reported for Lonicera spp. (Hidayati et al., 2002), Bromus tectorum (Allen et al., 1995; Bauer et al., 1998), Striga asiatica (Mohamed et al., 1998) and Cenchrus ciliaris (Hacker & Ratcliff, 1989). Increased germinability after dry storage for O. acanthium cypselas matured under both conditions (Table 10) might have been related to decreases in their water content and changes to their structural and chemical composition. Although the mechanisms of after-ripening are not known, after-ripening has an inverse relationship with seed moisture content (Foley, 1994) and germinability may increase because of an increase in the size of cracks on the seed surface (Werker, 1997) and/or physical and chemical changes in the seed coat (Kozlowski & Pallardy, 1997).

Implications

Cypselas with different morphological characteristics can have different responses in the seed bank. In warmer summers in the field, plants may be stressed and thus produce thin-walled, highly germinable cypselas that germinate quickly under conditions that meet their germination requirements. However, in cooler summers, plants usually produce more highly dormant cypselas that enter the seed bank and germinate sporadically over time (Qaderi & Cavers, 2002). Based on these two reproductive strategies, a germination model can be produced for a particular environment to predict emergence of this species in a subsequent year. For example, highly germinable cypselas from a warm summer may germinate shortly after dispersal in the autumn of that year and produce inflorescences in the following spring, but highly dormant cypselas from a cool summer can be incorporated into a persistent seed bank and have an extended, attenuated germination pattern. An irregular pattern of cooler and warmer summers would lead to a more diverse seed bank for O. acanthium than would be produced under a series of summers with very similar temperature regimes. These results could help to explain why populations of O. acanthium arise intermittently, often with several years between the appearance of large seedling cohorts (Qaderi et al., 2002). By considering air temperature and rainfall data, it may be possible to estimate the probability of new cohorts of seedlings appearing in the following year and develop an effective control method for O. acanthium and related species that grow in pastures, agricultural fields and waste places.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank the Natural Sciences and Engineering Council of Canada for financial support through operating grants to P. B. Cavers and M. A. Bernards. The authors also thank Peter Duenk and Caroline Rasenberg for their assistance with planting rosettes at the ESW Research Station, Magdalena van Hal and Vicki Cuthbertson for their help with maintaining plants in the Plant Sciences glasshouses and Zakera Qaderi and Roselyne Labbé for helping with cypsela collection and cleaning. We appreciate helpful comments on previous versions of this manuscript from Drs Jerry Baskin, Norman Hüner, Allan Hamill and two anonymous referees.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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