Physiological dormancy alleviation

The red arrows in fig. 1 represent the effect that temperature and particularly seed moisture content can have on increasing and decreasing the amount of physiological dormancy a seed possesses, or the seed’s dormancy status.

Since dormancy status can increase and decrease cyclically in response to temperature and seed moisture content, treatments that involve controlling temperature and moisture can be used to alleviate physiological dormancy in seeds.

Dormancy Factors

Figure 1. Temperature and seed moisture content are the most important factors regulating seed physiological dormancy (red arrows)1.

Just as some dispersed seeds may lose dormancy during a period of dry after-ripening in their natural environment, others may require predominantly moist (imbibed) conditions. Indeed for some species, maintaining seeds in an imbibed state has been found to alleviate dormancy. This treatment is known as ‘stratification’.

Why ‘stratification’?

The term stratification in relation to seeds derives from the horticultural practise of germinating ornamental tree and shrub seeds. Traditionally, seeds were stratified in deep seed trays between layers of moist sand and left in a shaded garden frame for several months. Seeds would later be recovered and sown into pots of soil prior to the season when germination was expected. Since the practice was developed in the northern hemisphere, the term ‘stratification’ originally related to the exposure of seeds to low temperatures. However as knowledge of seed germination has increased, the terms ‘warm-‘ and ‘cold stratification’ are now commonly used to differentiate between treatment temperatures.

Common practice is still to sow seeds in pots of moist soil outdoors (warm or cold stratification), or to mix seeds with wet vermiculite in a sealed plastic bag in the fridge (cold stratification). In the laboratory, seeds are commonly held over water in a sealed container, or placed directly on water agar, and stored at the relevant temperature(s).

Warm or cold stratification?

The local environment and lifecycle of a species are likely to reveal clues about possible stratification temperatures. As you would expect, in temperate environments cold stratification is effective at governing changes in dormancy status of imbibed seeds1. Within Australia, cold stratification pre-treatments have increased seed germination of some species from sub-alpine, Mediterranean-type and temperate climates2,3,4).

Several weeks of warm stratification at 33/18°C or 26/13°C have been shown to successful alleviate dormancy of Acanthocarpus preissii (Dasypogonaceae) seeds, enabling subsequent germination at 18/7°C. It was concluded that A. preissii seeds require treatments mimicking temperature and soil moisture conditions representative of early to mid-autumn in south-west Western Australia, prior to germination temperatures reminiscent of winter5.

Germination of Juncus antarcticus

Figure 2. Juncus antarcticus (Juncaceae) seeds have responded to cold stratification at the TSCC.

At the TSCC, physiological dormancy of the alpine rush species Juncus antarcticus (Juncaceae; fig. 2), collected at 1310 m near the summit of Mt Rufus, has been alleviated by a cold stratification treatment (more details here). Similarly, the dwarf sedge Isolepis montivaga (Cyperaceae – collected at around 500 m from the shores of Lake Leake, Tasmania) shows a marked improvement in germination following 8 to 12 weeks of cold stratification at 5°C.

Goodenia fascicularis

Figure 3. Goodenia fascicularis (Goodeniaceae) in flower. Photo courtesy of Gemma Hoyle.

All this germination data is available on the TSCC Germination Database.

In contrast, seeds of species native to Australia’s semi-arid tropical regions, lying dormant over summer, are more likely to require warm stratification treatment to alleviate dormancy.For example recent studies have shown that warm (34/20°C), wet storage of Goodenia fascicularis (Goodeniaceae) seeds alleviated physiological dormancy and enabled seeds to germinate at cooler temperatures reminiscent of autumn months (fig. 4)6.

Percentage germination of Goodenia fascicularis

Figure 4. Final percentage germination (mean ± s.e.) of Goodenia fascicularis seeds at 15/25°C 12/12 hour light/dark after various durations of warm stratification at 20/34°C in darkness. Final germination of control seeds (in storage at 15°C/15% RH) is also shown.

An experimental approach termed ‘the move-along’ involves exposing imbibed seeds to simulated seasonal temperatures that might be experienced in the field post dispersal and before germination7. This approach successfully alleviated dormancy of the Australian shrub Marianthus bicolor (Pittosporaceae)8.

Overseas, the North American woodland herb Cardamine concatenate (Brassicaceae) was found to require warm plus cold stratification to germinate in spring, after dispersal the previous spring. Although cold stratification alleviated dormancy, a warm stratification pre-treatment decreased the length of cold stratification necessary9.

At the TSCC, germination testing of the endemic rush species Juncus curtisiae (Juncaceae) has revealed a need for dormancy alleviation treatments. No germination of J. curtisiae was observed at 15°C, 20°C or alternating 27/15°C, with or without the application of potassium nitrate (KNO3). However following eight weeks of cold stratification (5°C) 18% germination was achieved at 27/15°C with KNO3. In the final round of testing, seeds were given warm stratification (20°C) for four weeks followed by cold stratification (5°C) for eight weeks before being transferred to 27/15°C with KNO3, and the result; a highly significant 94% germination (fig. 5).

Germination of Juncus curtisiae

Figure 5. Germination of Juncus curtisiae (Juncaceae).

Stratification in light or dark?

Stratification treatments at the TSCC and other institutes are usually applied to seeds in light, however in some species light exposure can inhibit stratification. The effect of warm stratification followed by cold stratification on germination of Solanum melongena (Solanaceae) seeds was more pronounced in the dark than the light10, suggesting some inhibition of stratification in the light. Studies of Lolium rigidum (Poaceae) seeds have concluded that in the field, these seeds need to be buried or under a complete canopy for stratification to effectively alleviate dormancy11. Some dark stratification has been conducted at the TSCC but in the species so far looked at, no effect has been observed.

References:

  1. 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.
  2. Beardsall D and Mullett J. 1984. Seed germination of Eucalyptus pauciflora Sieb. ex Spreng. from low and high altitude populations in Victoria. Australian Journal of Botany 32: 475-480.
  3. Curtis NP. 1996. Germination and seedling survival studies of Xanthorrhoea australis in the Warby Range State Park, North-eastern Victoria, Australia. Australian Journal of Botany 44: 635-647.
  4. Moncur MW, Boland DJ and Harbard JL. 1997. Aspects of the floral biology of Allocasuarina verticillata (Casuarinaceae). Australian Journal of Botany 45: 857-869.
  5. Turner SR, Merritt DJ, Ridley EC, Commander LE, Baskin JM, Baskin CC and Dixon KW. 2006. Ecophysiology of seed dormancy in the Australian endemic species Acanthocarpus preissii (Dasypogonaceae). Annals of Botany 98: 1137-1144.
  6. Hoyle GL, Daws MI, Steadman KJ and Adkins SW. 2008. Mimicking a semi-arid tropical environment achieves dormancy alleviation for seeds of Australian native Goodeniaceae and Asteraceae. Annals of Botany 101: 701-708.
  7. Baskin CC and Baskin JM. 2004. Determining dormancy-breaking and germination requirements from the fewest seeds. In: Guerrant E, Havens K, Maunder M, eds. Ex situ plant conservation: Supporting species survival in the wild. Covelo, CA: Island Press, 162-179.
  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. Baskin C and Baskin J. 1995. Warm plus cold stratification requirement for dormancy break in seeds of the woodland herb Cardamine concatenata (Brassicaceae) and evolutionary implications. Canadian Journal of Botany 73: 608-612.
  10. Watanaba H, Kusagaya Y and Sigusa M. 2002. Environmental factors affecting germination of apple of Peru. Weed Science 50: 152-156.
  11. Steadman K. 2004. Dormancy release during hydrated storage in Lolium rigidum seeds is dependent on temperature, light quality, and hydration status. Journal of Experimental Botany 55: 929-937.