Alleviating physiological dormancy

Since physiological dormant (PD) seeds germinate in the field (in situ), then it must also be possible to germinate PD seeds in a laboratory or at home (ex situ).

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

Dormancy Factors

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

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 ex situ.

Seeds in dry storage can lose dormancy

Storing dry seeds in a warm environment, usually in darkness, before sowing them in germination conditions is an ex situ dormancy alleviating treatment more commonly known as ‘dry after-ripening’2.

Mimicking nature

Remember, when trying to alleviate dormancy, treatments should be closely associated with what seeds experience in the field so that they are not only attempts at mimicking the seed’s natural environment, but yield normal, healthy seedlings.

Although dry storage in ambient conditions may gradually alleviate dormancy, conditions are unlikely to be reminiscent of those seeds experience in natural field conditions and will therefore be less effective and provide no ecologically significant insights.

Australian examples of dry after-ripening success

Many PD seeds have been found to germinate following several months of dry after-ripening, including several families native to Australia. Evidence suggests that the longer and warmer the storage environment, the greater the dormancy alleviation.

Actinobole uliginosum seeds

Figure 2. Dry after-ripening alleviates dormancy of tiny Actinobole uliginosum (Asteraceae) seeds. Image courtesy of Gemma Hoyle.

For example, dormancy of Actinotus leucocephalus (Apiaceae) seeds was alleviated by dry after-ripening at alternating 20/50°C or constant 37°C. These temperatures mimicked summer soil conditions for the Mediterranean-type climate region of south-west Australia where the seeds were collected3.

Dry after-ripening also alleviated dormancy in several Australian Asteraceae, although differences were found to exist between species. For example, high temperature storage (25 and 38°C) for 8 months increased germination of Schoenia filifolia subsp. subulifolia, but germination of Craspedia sp. remained low after storage for >16 months4.

Long durations of a DAR treatment designed to mimic in situ temperatures and humidity (34/20oC , 40% relative humidity), alleviated PD of the Australian daisy Actinobole uliginosum (Asteraceae; fig. 2)5.

The warmer the storage temperature, the drier the seeds need to be

An inverse relationship between temperature and seed moisture content has been observed during dry after-ripening of Avena fatua (Poaceae); as the temperature increased, the seed moisture content had to decrease for maximum after-ripening affect, and visa versa6.

Air relative humidity (RH) was equally as important for Draba verna (Brassicaceae) with the most efficient dry after-ripening occurring at 50 and 60% RH. Seeds stored at 25°C and 0, 10 and 20% RH for almost 6 months did not after-ripen, and seeds stored at 70 to 100% RH rotted7.

Dry after-ripening; how does it work?

Dry after-ripening may bring about changes in levels of growth inhibitors or growth promoters within the seed embryo, encouraging the seed to germinate8.

More recently, it has been suggested that gene expression (ß-1,3-glucanase transcription and translation) in Nicotiana tabacum (Solanaceae) seeds may be altered during dry after-ripening treatments9.

References:

  1. Benech-Arnold RL, Sanchez RA, Forcella F, Kruk BC, Ghersa CM. 2000. Environmental control of dormancy in weed seed banks in soil. Field Crops Research 67: 105-122.
  2. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.
  3. Baker KS, Steadman KJ, Plummer JA, Merritt DJ and Dixon KW. 2005. The changing window of conditions that promote germination of two fire ephemerals, Actinotus leucocephalus (Apiaceae) and Tersonia cyathiflora (Gyrostemonaceae). Annals of Botany 96: 1225-1236.
  4. Peishi Z, Plummer J, Turner D, Choengsaat D and Bell D. 1999. Low- and high-temperature storage effects on viability and germinability of seeds of three Australian Asteraceae. Australian Journal of Botany 47: 265-275.
  5. 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.
  6. Foley ME. 1994. Temperature and water status of seed affect after-ripening in wild oat (Avena fatua). Weed Science 42: 200-204.
  7. Baskin C and Baskin J. 1979. Effect of relative humidity on after-ripening and viability in seeds of the winter annual Draba verna. Botanical Gazzette 140: 284-287.
  8. Bell DT. 1999. The process of germination in Australian species. Australian Journal of Botany 47: 475-517.
  9. Leubner-Metzger G. 2005. Beta-1,3-Glucanase gene expression in low-hydrated seeds as a mechanism for dormancy release during tobacco after-ripening. The Plant Journal 41: 133-145.