SEED GERMINATION

What Is Germination?

Figure 1. Vittadinia muelleri seeds germinating on 1% agar following 5 days at 20°C.jpg

Figure 1. Vittadinia muelleri seeds germinating on 1% agar following 5 days at 20°C.jpg

Every viable (living) seed has the potential to become a plant. For this to happen, the seed must germinate, and for germination to occur, a seed essentially needs:

  1. Water (during absorption and subsequent stages of growth),
  2. Oxygen (for respiration) and;
  3. Temperatures adequate for metabolism and growth.

Some seeds also require light and therefore must be on the soil surface in order to germinate, and not buried beneath the soil surface.

Figure 2 Lepidium desvauxii (Brassicaceae) germinating at 15°C at the TSCC Note the mucilage produced by seed This helps bind the seed to surrounding soil particl

Figure 2 Lepidium desvauxii (Brassicaceae) germinating at 15°C at the TSCC Note the mucilage produced by seed This helps bind the seed to surrounding soil particles

The seed coat ruptures and the radicle pushes through

When the temperature is suitable and there is an adequate supply of water and oxygen, the seed absorbs water and swells. For most plant species, the first visible sign of germination is when the seed coat ruptures to make way for the emerging root tip or radicle (fig. 2). It is generally agreed that when the radicle pushes out through the seed coat, germination has begun.

The shoot tip grows

Figure 3. Close up of Vittadinia muelleri seeds germinating on 1% agar at 20°C displaying classic epigeal germination.jpg

Figure 3. Close up of Vittadinia muelleri seeds germinating on 1% agar at 20°C displaying classic epigeal germination.jpg

Once the radicle has appeared the seed can develop in one of two ways depending on the species. In seeds with epigeal germination, the cotyledons emerge form the seed coat green, enabling the plant to manufacture its own food from sunlight via photosynthesis (fig. 3). This type of germination is very common, typically displayed by small, light requiring seeds, and will be familiar to many who grow plants from seed.

Alternatively some seeds display hypogeal germination whereby the cotyledons remain in the seed but the epicotyl is released from the seed through extension of the cotyledonary petioles. This germination behaviour is more typical of larger seeded species.

Whilst the radicle is the beginnings of the plant’s root system, the epicotyl/plumule will develop into the future stem and leaves. The plumule will turn green and photosynthesis begins.

Variations

As elsewhere in biology there is huge variation to seed germination behaviour. For example Arum maculatum (Cuckoo Pint, Araceae) develops from seed to tuber in the first year, without leaves being produced or photosynthesis taking place1. Germination of Hyphaene thebaica (Gingerbread palm, Palmae) seeds is through extension of the cotyledonary petiole. Growth of up to 25 cm can take place before radicle development is initiated. A number of wetland sedge species also seem to initiate petiole extension followed rapidly by epicotyl development, prior to growth of the radicle. In seeds with ‘double dormancy’ (i.e. differing levels of dormancy in various tissues – typically radicle and epicotyl), there can be a one year delay between growth of the radicle and subsequent growth of the epicotyl. This behaviour has been documented for Lilium martagon (Liliaceae) and Paeonia (Paeoniaceae) species.

A plant’s own food

Figure 4. Cross section of a stylised dicotyledonous seed. The relative size of the endosperm and cotyledons is highly variable across seed bearing plant species

Figure 4. Cross section of a stylised dicotyledonous seed. The relative size of the endosperm and cotyledons is highly variable across seed bearing plant species

Most seeds store enough sustenance in their endosperm and/or cotyledons to give the growing seedling energy until it is big enough to photosynthesise (fig. 4). This level of provision is often tied to the germination strategy of the seed. Light requirement for germination is common for tiny seeds which have little reserve to grow through soil or heavy leaf litter.

However some tiny seedlings can suspend the need to get to sunlight for quite some time. For example orchid seeds (Orchidaceae) are minute and contain very little sustenance. In the wild, orchids develop a parasitic relationship with a fungus (termed myco-heterotrophy) which provides the emerging seedling with food until it is large enough to photosynthesise. For some orchid species this can be a long time, for example the Military Orchid (Orchis militaris) can enjoy up to ten years feeding off its fungal relationship before developing its first leaf! Other species of orchid remain dependent on this relationship all their lives.

A non-dormant seed can germinate over the widest range of environmental conditions possible for that species2, and will usually germinate within a few days of receiving water, oxygen and adequate temperatures.

Further reading on germination

References:

  1. Pritchard HW, Wood JA and Manger KR. 1993. Influence of temperature on seed germination and the nutritional requirements for embryo growth in Arum maculatum L. New Phytologist 123: 801-809.
  2. Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.

Seed Germination Requirements

At a basic level, seeds have three requirements to permit germination. These are:

  1. Water (during absorption and subsequent stages of growth).
  2. Oxygen (for respiration).
  3. Temperatures adequate for metabolism and growth.

In addition, light may or may not be necessary for germination.

Let’s examine each basic requirement separately, bearing in mind that, as is common among all living organisms, there are always
exceptions to the rules:

Velleia paradoxa germination

Figure 1. Velleia paradoxa (Goodeniaceae) seeds germinating at 20°C. Seed produce a mucilage coat soon after taking up water.

 

Water and seed imbibition

Imbibition simply means the absorption of a liquid. Before attempting to germinate a seed, it is important to know whether the seed (or fruit) will imbibe water. In the laboratory this is determined by placing the seeds on moist filter paper at room temperature and then at hourly intervals for 8-10 hours, blotting the seeds dry and weighing them1. A gradual increase in seed weight indicates that the seed is absorbing water and is therefore ‘water permeable’.

Water permeable seeds will absorb or lose water from the atmosphere until they come into equilibrium with their surroundings. When the temperature is suitable and there is an adequate supply of water and oxygen, most seeds absorb water and swell. The seeds are now well on their way to germinating.

However, at least 15 families of angiosperms produce seeds with impermeable seed (or fruit) coats1. Such seeds cannot readily absorb water and therefore possess what is known as physical dormancy. Such seeds require very specific conditions in the field and specific treatments in the laboratory or at home, before they will germinate. Essentially this involves perforating the seed coat or removing the tissues surrounding the seed, to allow imbibition of water to help overcome physical dormancy.

Bizarrely, germination of many Mistletoe species may actually be an exception to the rule that seeds must absorb water before they can germinate. Amyema preissii seeds (Wire-leaf Mistletoe, Loranthaceae), seem to be inhibited when the seeds are fully hydrated. Removal of the seeds from inside the berries allows seeds to lose moisture (known as seed desiccation), and germination then commences quickly2.

Oxygen and respiration

The vast majority of seeds of plant species require oxygen to support germination, such as that found in air pockets between particles of lightly packed soil. In contrast, a number of aquatic and marshland species have been shown to require anaerobic conditions (conditions lacking oxygen), to initiate germination. For example Najas marina (Najadaceae), a pondweed of brackish water with a global distribution, has been shown to require total exclusion of oxygen to initiate germination. Subsequent seedling growth however did require oxygen (MSBP, unpublished data).

Germination temperatures

Germination temperatures vary greatly among plants. A plant’s natural environment, and more specifically the environmental conditions at the time of natural seed dispersal, will reveal information regarding the optimal temperature range for germination. More specifically, the conditions between the time of seed dispersal and the time of germination will reveal clues regarding not only germination temperatures, but also potential dormancy alleviating temperatures.

Germination of Juncus kraussii

Figure 2. Germination of Juncus kraussii (Juncaceae) at the TSCC. This species show a clear requirement for alternating temperatures to stimulate germination.

Alternating temperature regimes are often more favourable for germination than constant temperatures and some species will only germinate at alternating temperatures (for example, Juncus kraussii (Juncaceae), fig. 2). Such regimes are of course more reminiscent of what seeds would experience in their natural habitat, depending on environmental conditions and seed exposure.

It has been reported that for a high percentage of a seed lot to germinate, the difference between high and low temperatures must be 10°C or more. However in some species, changes of only 1°C can be the difference between seeds and seedlings (studies cited in 1).

Beneath well developed vegetation, soil is often insulated from the sun during the day and retains warmth during the night. Alternatively, when vegetation is cleared the soil surface receives a lot of warming during the day and loses heat rapidly at night, resulting in temperature fluctuations. These fluctuations diminish as a seed becomes further buried into the soil. Alternating temperatures are therefore significant to a seed as they indicate shallow burial and low levels of local plant competition; a potentially prime spot for germination.

A preference for alternating temperatures has also been documented in aquatic plant species. In an aquatic environment, alternating temperatures indicate shallow water and a lack of neighbouring plants.

Light requirements

Seeds of many species germinate equally well in light and darkness. Others have been found to germinate to higher percentages in light while a smaller proportion germinate better in darkness1.

Studies have shown that small seeds often require light for germination while large seeds are usually indifferent to their exposure to light. These results support the general gardening rule that says that the larger the seed, the deeper it should be planted in the soil.

Germination of Goodenia fascicularis (Goodeniaceae) seeds collected in south-west Queensland, Australia required a warm stratification pre-treatment to alleviate dormancy, and light to terminate dormancy and trigger germination (fig. 3).
Despite dormancy alleviation, seeds in darkness did not germinate. Obviously these seeds need to be on the soil surface in order to germinate after a warm, wet summer typical of south-west Queensland3.

Percentage germination of Goodenia fascicularis

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.

Studies suggest that seeds may have evolved to use light as an indicator of whether they are under the soil surface or beneath a canopy or relatively close to other plant species that may complete not only for light but other resources.

Tests carried out at the TSCC on the rare annual eyebright species Euphrasia scabra (Orobanchaceae) suggest that light is necessary for germination. On the contrary, studies at the Millennium Seed Bank Project have found that seeds of Trachyandra divaricata (Asphodelaceae), a tuberous rooted lily from South Africa, prefer to germinate in the dark. Trachyandra divaricata achieved 100% germination within 21 days at 10°C in darkness, but in the light it took over 140 days to achieve 50% germination (MSB, unpublished data).

Ethylene, apples and flooding; a specific germination requirement!

Some non-dormant seeds are particularly fussy and insist on something additional for germination. Schoenoplectus hallii (Cyperaceae) disperses dormant seeds in autumn and dormancy is alleviated over winter if seeds are buried in moist soil. Once dormancy is lost, non-dormant S. hallii seeds germinate throughout summer if exposed to flooding, light and ethylene.

Studies found that seeds of S. hallii produce a small amount of ethylene when they are flooded and that, when large numbers of them are flooded in a relatively small volume of water, enough ethylene accumulates to promote germination. The ethylene germination cue may serve as a ‘flood-detecting’ mechanism signaling that water is available and competing species are absent4.

In the laboratory 97% of S. hallii seeds germinated following 3 days of exposure to slices of apple, which are known to produce ethylene4.

Host root exudates (xenognosins)

Some seed have been shown to respond to the presence of roots belonging to other plants. This response is particularly well recorded for certain members of the Orobanchaceae, a family of parasitic flowering plants. A number of Orobanchaceae general produce large quantities of very small seeds (not unlike orchids). Studies have found that the tiny seeds of Orobanche (Broomrape) and Striga (Witchweed) species are stimulated to germinate by complex organic compounds (termed xenognosins) excreted by the fine roots of certain plants. These chemicals decay rapidly in the soil therefore the seeds must to be within a few millimetres of the root. When the seed ‘smells’ the close presence of a suitable root it initates germination and grows towards the root (where the chemical concentration is highest) before pushing a specialised root (haustorium) into the unlucky host and stealing nutrients!

Studies have shown that some Orobanche species exhibit a physiological dormancy and that their sensitivity to xenognosins fluctuate through the year due to changes in dormancy status.

References:

  1. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.
  2. Lamont and Perry. 1977. The effects of light, osmotic potential and atmospheric gases on germination of the Mistletoe Amyema preissii. Annals of Botany. 41: 203-209
  3. 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.
  4. Baskin CC, Baskin JM, Chester EW and Smith M. 2003. Ethylene as a possible cue for seed germination of Schoenoplectus hallii (Cyperaceae), a rare summer annual of occasionally flooded sites. American Journal of Botany 90: 620-627.

Germinating Seeds For The First Time

The Challenge

Allocasuarina duncanii twins

Figure 1. Allocasuarina duncanii (Casuarinaceae) seedlings. Click image for more details.

Over time seed banks develop a wealth of knowledge and experience in regarding to germination. Even so it should be appreciated that much of the germination testing carried out, follows a process of trial and error. Currently the germination of only a tiny fraction of seed bearing plants has fallen under scientific scrutiny. To compound matters, some of the very early published work failed to record factors that we now know are crucial to interpreting results; where seeds came from, when they were collected, how they were stored and for how long.

Chorizandra australis (ii)

Figure 2. Woody achenes of the Australian sedge Chorizandra australis (Cyperaceae). Achenes are small, single seeded fruits and are commonly dispersed by many plant species.

It is well documented that the germination behaviour exhibited by many plants is quite plastic. Optimal germination conditions vary not only from species to species, but also within a species from location to location. Even collections made from the same species in the same location can have differing germination requirements from one year to the next.

Consequently, seed workers can not assume that conditions that evoke germination for one collection will evoke germination from another of the same species.

We do know however that some seed traits are strongly linked to taxonomy, morphology or location, therefore we can at least apply some framework to our investigations.

So how do we do that?

Investigating seed germination requirements

Persoonia gunnii seeds

Figure 3. Cross section of Persoonia gunnii (Proteaceae) stones revealing a pair of seeds with well developed embryos. Thick woody coats like this seldom pose a barrier to water or seedling growth.

The seed itself and the environment from which it came can reveal many clues regarding germination requirements. Here we list a few of the important questions you may want to ask yourself before attempting to germinate viable seeds that you know very little about.

Is it a seed?

The very first thing to check is that you are actually dealing with seeds. This may sound obvious but many plant species will shed seeds that appear to be perfectly healthy, but in fact have little or no content (see an example here). Seed bank workers and seed scientists always conduct seed viability tests to assess seed quality prior to germination testing.

Seed morphology

Seed morphology plays an important role in germination requirements. You may need to ask yourself:

  • Are seeds dispersed within a fleshy fruit or other covering structure that needs to be removed prior to sowing seeds?
  • Are seeds able to absorb water? If not, they probably have physical dormancy.
  • How small are the seeds? Germination of tiny seed is more likely to be light dependent.
  • How small is the embryo? Diminutive and/or undifferentiated embryos may require time to develop post dispersal i.e. seeds have morphological dormancy.
Seed Embryo Variation

Figure 4. Sample of internal seed morphology.

Figure 4 is an illustration showing a small sample of the variation observed in internal seed morphology (Seed embryos are yellow. Endosperm is shaded white):

  1. Tiny Undifferentiated embryo (e.g. Ranunculaceae)
  2. Small differentiated embryo (e.g. Apiaceae)
  3. Linear embryo (e.g. Oleaceae)
  4. Bent embryo (e.g. Brassicaceae)
  5. Large investing embryo (e.g. Mimosaceae)

Consider the plant life cycle and natural environment

Equally as important to consider is the plant life cycle and the environment it has evolved in:

  • What is the life cycle of the plant?
  • What time of year are mature seeds dispersed in the wild?
  • When do they germinate in the wild? Level of documentation for this varies – it’s very good in the UK, but elsewhere information can be very scarce. If this can’t be determined then…
  • When and/or what is the most likely window for seedling recruitment? i.e. Does the habitat offer clear disturbance events (such as fire), that offer the opportunity for seedlings to establish with little competition?
  • What are the environmental conditions in the plant’s natural habitat before, during and after seed germination?

Whenever possible, germination testing should begin within 7 to 10 days of harvesting seeds1. This is because post-harvest handling may cause seeds to undergo changes, and then there is no way of knowing what the germination response of fresh seeds is, or how germination requirements may have changed over time. This is particularly important if seeds exhibit physiological dormancy.

N.B. Almost all tests conducted by the TSCC are performed on stored seeds, not fresh seeds, and usually commence about 6 months after collection. This is standard operation for seed banking as germination of stored seed is the focus of the program.

For some species dormancy can be avoided by collecting very young / immature seeds. For example, Gahnia grandis (Cyperaceae) seeds will rapidly germinate when collected immature and sown immediately, but when fully mature (and possessing greater storage longevity), these seeds are deeply dormant. Since immature seeds have lower longevity potential, they are useless for conservation seed banking as they will rapidly degrade in storage to become nonviable.

Time between seed dispersal and germination

A useful strategy is to think about the journey the seed takes between dispersal and germination. It is particularly important when dealing with physiologically dormant seeds, to have some information about conditions during the time between natural seed dispersal and when they might germinate:

  • Are seeds exposed to wet or dry conditions?
  • Are seeds exposed to warm or cool conditions?
  • If so, for how long?
  • Are these conditions necessary for germination or not?
  • Are seeds buried or on the soil surface when they germinate?

Much information can be gained from collecting seeds at the time of natural dispersal, and sowing them on soil where they receive natural temperature and soil moisture conditions, either outside or in a non-heated greenhouse. How long is it before seeds germinate? What conditions do they experience in the meantime?

Air and/or soil temperature records can reveal the temperatures seeds are exposed to before and during germination. You might consider purchasing a temperature data logger or obtaining temperature, humidity and rain event data for a general area from www.bom.gov.au.

If seeds do not germinate whilst under the soil, they may require light for germination. It may be worth sowing seeds both on and under the soil surface and noting germination patterns. Seeds less than 3 or 4 mm in size are more likely to require light for germination and should be sown on or close to the soil surface.

The TSCC Germination Database

The TSCC Germination Database provides you with a listing of the relative success of various seed germination protocols. This information is generated through germination testing of seeds currently held in the Tasmanian Seed Conservation Centre.

Please note, the TSCC seed bank focuses on the preservation of Tasmania’s wild flora, therefore the database holds little information on the germination behaviour of cultivated garden plants. However we do have some data on a few introduced weed species.

You can search for germination data via plant family or genus. Simply select the family or genus that you are interested in and press the report button next to it. This will generate a PDF file for downloading.

References:

  1. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.