To recap…

Dormancy is a seed characteristic, manifesting as a block or series of blocks that prevent germination under otherwise favourable moisture, temperature and gaseous conditions1.

Dormancy is a ‘plastic’ seed characteristic, the status of which can range from any value between all (maximum dormancy) and nothing (non-dormant).

Physiological dormancy (PD) is an endogenous dormancy (on the inside of the seed) and the most common expression of seed dormancy in the world2. Physiological dormancy is thought to be caused by a physiological inhibiting mechanism (PIM) of the seed embryo.

Overcoming physiological dormancy

If PD seeds germinate in the field (in situ), as indeed they must do, then it must also be possible to germinate PD seeds in a laboratory or at home (ex situ). You may have heard the general term ‘breaking dormancy’ in reference to achieving germination of dormant seeds. However there are many different approaches to dealing with the physiological blocks to germination including alleviating, terminating and bypassing them. To learn more, read on…

Fig. 1 shows the two kinds of factors that affect seed dormancy;

  1. Those that cause increases and decreases in the dormancy status of a seed (red arrows), and
  2. Those that terminate dormancy altogether, once the level of dormancy is sufficiently low (orange arrows)3.
Dormancy Factors
Figure 1. Factors that can enforce or alleviate (red) and terminate (orange) physiological dormancy to achieve germination (green). Adapted from Benech-Arnold et al., 2000

Alleviating dormancy

Dormancy status can increase and decrease cyclically in response to temperature and seed moisture content (red arrows, fig. 1). ‘Dormancy alleviating factors’ cause dormancy to decreases so that seeds are able to germinate over a wider range of conditions. In contrast, ‘dormancy inducing factors’ cause dormancy to increase until seeds are unable to germinate under any conditions usually conducive to germination1.

Temperature and seed moisture content have been found to increase and decrease dormancy status3,4. Good evidence of this has come from the study of weed soil seed banks5,6.

Therefore treatments that involve controlling temperature and moisture can be used to alleviate physiological dormancy in seeds. Such treatments include;

  1. Dry after-ripening
  2. Stratification
  3. Dry/wet cycling

Treatments that alleviate dormancy are closely associated with conditions seeds experience in the field and are therefore not only attempts at mimicking the seed’s natural environment, but are also likely to result in germination of normal, healthy seedlings.

Optimising germination in time…

The alleviation of dormancy in the field, as a result of seasonal environmental changes in temperature and seed moisture content, can be thought of as optimising germination in time. In other words seeds can postpone germination until a time when subsequent seedling development is optimised.

Terminating dormancy

Alleviating dormancy may or may not be enough to result in seed germination (dashed red arrows, fig. 1). For many species, after dormancy has decreased, constraints to germination still persist and dormancy needs to be ‘terminated’ before seeds can germinate (orange arrows, fig. 1).

For example, seasonal temperature changes alleviated dormancy of Leucopogon species (Ericaceae), and then smoke terminated dormancy7.

Factors that terminate dormancy may also be referred to as ‘germination inducing factors’1 or ‘germination stimulants’8.

Changes in dormancy status may also change a seed’s sensitivity to the effect of germination stimulants such as light, nitrate9, and fluctuating temperatures10 (green arrows, fig. 1).

Smoke is a very effective chemical agent for terminating dormancy / stimulating germination in many Australian plant species11. Dormancy of Schoenia filifolia subsp. subulifolia (Asteraceae) was overcome by smoke-water, which also eliminated the requirement of light for germination12. This could be interpreted as one germination stimulant substituting for another.

Karrikinolide chemical structure

Figure 2. Chemical structure of 3-methyl-2H-furo[2,3-c]pyran-2-one (karrikinolide)

Figure 2. Chemical structure of 3-methyl-2H-furo[2,3-c]pyran-2-one (karrikinolide)

An active component of plant- and cellulose- derived smoke has been identified as ‘karrikinolide’;

3-methyl-2H-furo[2,3-c]pyran-2-one (fig. 2)13.

Whilst we don’t know how this compound works yet, butenolide (karrikinolide) and gibberellic acid (GA3) were found to have a similar effect in promoting seed germination of three Australian Asteraceae species14 and a range of arable weed species15.

Light stimulated the germination of native Asteraceae seeds belonging to the genera Brachyscome, Chrysocephalum, Hyalosperma, Leucochrysum, Rhodanthe, Erymophyllum, Craspedia and Waitzia genera16, 12. A light requirement for germination ensures that non-dormant seeds will only germinate on or close to the soil surface or beneath a gap in the canopy.

Light also stimulated germination of Goodenia fascicularis (Goodeniaceae) to allow seeds to germinate after physiological dormancy had been alleviated by warm stratification17 (fig. 3).

Percentage germination of Goodenia fascicularis

Figure 3. Percentage germination (mean ± s.e.) of Goodenia fascicularis seeds at 20°C,

Figure 3. Percentage germination (mean ± s.e.) of Goodenia fascicularis seeds at 20°C, 12/12 hour photoperiod and constant darkness, after receiving increasing durations of warm stratification to alleviate physiological dormancy.

Other known chemical germination stimulants include potassium nitrate (KNO3) which triggered germination of several arid-zone, winter ephemerals including Leucochrysum fitzgibbonii and Craspedia sp. (Asteraceae), in red light18. In the dark, application of GA3, (1 to 100 mg L-1) substituted light to stimulate germination of several genera of Asteraceae to similar levels observed in light-treated seeds without GA3 19, and the application of nitrate or GA3 meant that the arid-zone Asteraceae Shoenia filifolia subsp. subulifolia required less dry after-ripening to alleviate dormancy12.

Optimising germination in space

Factors known to terminate dormancy can be thought of as optimising germination in space. In other words seeds can postpone germination until a place when subsequent seedling development is optimised. For example, light and alternating temperatures may indicate to the seed that it is near the soil surface, in an open site or beneath a canopy gap, while KNO3 and smoke will indicate fire20, 21.

If the appropriate germination trigger is not received then seeds may cycle back into dormancy, otherwise known as secondary dormancy.

Treatments that stimulate germination are also important for non-dormant seeds that require no dormancy alleviation.

The alleviation and termination of dormancy is confirmed by germination of healthy seedlings.

Bypassing dormancy

Ex situ, germination triggers working alone in the presence of dormancy, may simply bypass dormancy to stimulate a germination response. Bypassing the blocks to germination does not attempt to alleviate dormancy and does not mimic what happens to seeds in situ. Therefore bypassing dormancy does not reveal much about the germination ecology of a species or the seed dormancy mechanisms.

Bypassing dormancy can be considered a last resort in the quest to germinate dormant seeds, and is currently the only way in which some dormant seeds are germinated for use in land revegetation.

Gibberellic acid and cytokinins are known to bypass dormancy in at least 36 Australian plant genera11. Application of GA3 promoted germination of Lomandra sonderi seeds but not diaspores (seed with inner pericarp intact), and excised embryos did not grow unless treated with GA3 or zeatin, indicating embryo dormancy as well as a pericarp inhibitor-induced dormancy19. Application of GA3 bypassed the endogenous dormancy of Brachyscome iberidifolia, Chrysocephalum apiculatum, Leucochrsyum fitzgibbonii, L. molle, Myriocephalus stuartii, Rhodanthe polygalifolia and R. moschata16.

However, application of GA3 is known to have negative effects upon seedling morphology, typically resulting in elongated seedlings15.

Treatments that bypass dormancy can be considered as ecologically irrelevant or ‘artificial’ ways in which to achieve germination of dormant seeds. However, application of GA3 and surgical treatments may be effective in a) determining seed viability and b) classifying dormancy level.

Avoiding dormancy

For some species dormancy can be avoided altogether by collecting very young (immature) seeds. For example, Gahnia grandis (Cyperaceae) achenes will rapidly germinate when collected immature and sown immediately, but when fully mature these achenes are deeply dormant. Unfortunately immature achenes have poor longevity potential making them unsuitable for conservation seed banking. Immature material will rapidly degrade in storage to become nonviable. Hence the need to be sown soon after collection.

 

Figure 1. Final percentage germination (mean ± s.e.) of Actinobole uliginosum seeds

Figure 4. Final percentage germination (mean ± s.e.) of Actinobole uliginosum seeds

Figure 4. Actinobole uliginosum (Asteraceae, foreground) and Goodenia fascicularis (Goodeniaceae, background) growing in a warm, dry experimental glasshouse prior to seed production. Work conducted at University of Queensland. Image courtesy of Gemma Hoyle.

Mature Gahnia achenes are notoriously difficult to germinate. Presently the TSCC has three species of Gahnia in its collections and is yet to determine how to alleviate their dormancy.

Glasshouse experiments have found that some Australian forb seeds collected from plants grown in warmer, drier environments (fig. 4), respond better to dormancy alleviating treatments than seeds collected from plants grown in cooler, wetter environments22,17.

References:

  1. Vleeshouwers LM, Bouwmeester HJ and Karssen CM. 1995. Redefining seed dormancy: An attempt to integrate physiology and ecology. The Journal of Ecology 83: 1031-1037.
  2. Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
  3. Benech-Arnold RL, Sanchez RA, Forcella F, Kruk BC and Ghersa CM. 2000. Environmental control of dormancy in weed seed banks in soil. Field Crops Research 67: 105-122.
  4. Vleeshouwers LM and Bouwmeester HJ. 2001. A simulation model for seasonal changes in dormancy and germination of weed seeds. Seed Science Research 11: 77-92.
  5. Bouwmeester HJ and Karssen CM.1992. The dual role of temperature in the regulation of the seasonal changes in dormancy and germination of seeds of Polygonum persicaria L. Oecologia 90: 88-94.
  6. Batlla D and Benech-Arnold RL. 2003. A quantitative analysis of dormancy loss dynamics in Polygonum aviculare L. seeds: Development of a thermal time model based on changes in seed population thermal parameters. Seed Science Research 13: 55-68.
  7. Ooi MKJ, Auld T D and Whelan RJ. 2006. Dormancy and fire-centric focus: Germination of three Leucopogon species (Ericaceae) from South-eastern Australia. Annals of Botany 98: 421-430.
  8. Merritt DJ, Turner SR, Clarke S and Dixon KW. 2007. Seed dormancy and germination stimulation syndromes for Australian temperate species. Australian Journal of Botany 55: 336-344.
  9. Derkx MPM and Karssen CM. 1993. Changing sensitivity to light and nitrate but not to gibberellins regulates seasonal dormancy patterns in Sysymbrium officinale seeds. Plant, Cell and Environment 16: 469-479.
  10. Benech-Arnold RL, Ghersa CM, Sanchez RA and Insausti P. 1990. Temperature effects on dormancy release and germination rate in Sorghum halepense (L.) Pers. Seeds: A quantitative analysis. Weed Research 30: 81-89.
  11. Merritt DJ and Dixon KW. 2003. Seed storage characteristics and dormancy of Australian Indigenous plant species: From the seed store to the field. In: Seed conservation: Turning Science into Practice. RD Smith, JB Dickie, SH Linington, HW Pritchard and RJ Probert, eds. Royal Botanic Gardens Kew, 807-823.
  12. Plummer JA, Rogers AD, Turner DW and Bell DT. 2001. Light, nitrogenous compounds, smoke and GA3 break dormancy and enhance germination in the Australian everlasting daisy, Shoenia filifolia subsp. subulifolia. Seed Science and Technology 29: 321-330.
  13. Flematti GR, Ghisalberti EL, Dixon KW and Trengove RD. 2004. A compound from smoke that promotes seed germination. Science 305: 977.
  14. Merritt DJ, Kristiansen M, Flematti GR, Turner SR, Ghisalberti EL, Tengrove RD and Dixon KW. 2006. Effects of a butenolide present in smoke on light-mediated germination of Australian Asteraceae. Seed Science Research 16: 29-35.
  15. Daws MI, Davies J, Pritchard HW, Brown NAC and Van Staden J. 2007. Butenolide from plant-derived smoke enhances germination and seedling growth of arable weed species. Plant Growth Regulation 51: 73-82.
  16. Bunker KV. 1994. Overcoming poor germination in Australian daisies (Asteraceae) by combinations of gibberellin, scarification, light and dark. Scientia Horticulturae 59: 243-252.
  17. Hoyle GL, Steadman KJ, Daws MI and Adkins SW. 2008. Pre- and post-harvest influences on seed dormancy status of an Australian Goodeniaceae species: Goodenia fascicularis F. Muell. & Tate. Annals of Botany. 102: 93-101.
  18. Bell DT. 1999. The process of germination in Australian species. Australian Journal of Botany 47: 475-517.
  19. Plummer JA and Bell DT. 1995. The effects of temperature, light and gibberellic acid (GA3) on the germination of Australian everlasting daisies (Asteraceae, Tribe Inuleae). Australian Journal of Botany 43: 93-102.
  20. Daws MI, Burslem DFRP, Crabtree LM, Kirkman P, Mullins CE and Dalling JW. 2002. Differences in seed germination responses may promote coexistance of four sympatric Piper species. Functional Ecology 16: 258-267.
  21. Brown NAC, van Staden J, Daws MI and Johnson T. 2003. Patterns in the seed germination response to smoke in plants from the Cape Floristic Region, South Africa. South African Journal of Botany 69: 514-525.
  22. Hoyle GL, Daws MI, Steadman KJ and Adkins SW. 2008. Pre- and post-harvest influences on seed dormancy status of an Australian Asteraceae species: Actinobole uliginosum (A.Gray) H.Eichler. In preparation for submission to Seed Science Research 18: 191-199.