Seeds

and seed raising

News

Macadamia Frozen in Time

All four native species of Macadamia are now threatened in what remains of their Australia rainforest habitats. Conserving them isn't just important for environmental reasons - plants from the wild could hold useful genes for the improvement of commercial nut crops in the future. Unfortunately, its difficult to preserve this genetic resource using conventional long-term storage methods, due to the size and high oil content of the seeds. The Australian Institute of Botanical Science and the University of Queensland are now working together to develop new methods to preserve these important species. One possibility is removal of the tiny embryo from the seed and storing it in liquid nitrogen. They will also try cryopreserving shoot tips grown in tissue culture. Source: Macadamia conservation a tough nut to crack (July 2022)

Seed Dormancy

Raising plants from seed is a fascinating and satisfying aspect of gardening. On a more practical level, it's a necessary skill if you want to fill a vegetable garden cheaply and it provides options for obtaining a variety of ornamental plants that might be otherwise unprocurable. Professional growers, of course, produce many plants (or rootstocks) from seed as a cost-efficient propagation method.

Seed raising isn't always straightforward, however. If your efforts aren't successful, don't necessarily assume the seeds are dead or that your skills as a propagator are to blame. The seeds could simply be dormant.

Seed dormancy factors affect the speed and uniformity of germination, and have implications for seed storage. Therefore, an understanding of dormancy can be useful for anyone involved in raising plants from seed.

Not all seeds have dormancy factors. Some will sprout provided environmental conditions (moisture, oxygen, temperature etc) are suitable for growth.

Many plants, however, employ dormancy mechanisms. In some cases, the seed may require quite particular conditions that make germination difficult to achieve in an artificial environment, but this is not in itself dormancy (although it may be just as frustrating for the gardener). Botanically, dormancy is a "block" or "lock" on germination.

Depending on the species, dormancy may reside in different parts of the seed such as the embryo, the endosperm or the seed coat and more than one type of dormancy can operate on the seed at the same time.

Some forms of dormancy:

Impermeable seed coat - The propagator can mimic the natural action of abrasion, fire or an animals' digestive systems with seed treatments such as cutting, sandpaper, heat or acid, a process known as scarification.

Chilling or warming requirement - A temperature treatment (usually applied with moisture) to overcome dormancy is called stratification.

Presence of chemical inhibitors - These need to be leached away or degraded. (Inhibitors sometimes occur in the fruit tissue surrounding seeds, also).

Requirement for chemical triggers - For example, many fire-adapted species respond to substances in smoke.

Light requirement - Amount and/or wavelength of light can either inhibit germination or stimulate it. Whether a seed is buried, exposed or shaded by a competing plant has obvious implications for its chances of survival in various habitats.

Embryo immaturity - after being shed from the mother plant, the embryo may require further development inside the seed before it can germinate.

A wide range of dormancy mechanisms exist in the plant kingdom. In a given species, these may apply individually or in combination. In the latter case, dormancy-breaking treatments may have to be applied in a particular order for germination to proceed.

Just to complicate matters further, dormancy may vary from one batch of seed to another, and unfavourable environmental factors can induce dormancy in seeds that weren't dormant before (secondary dormancy).

Legumes and scarification

The imposition of dormancy by a hard impermeable seed coat is not widespread in the plant kingdom, but is very familiar to gardeners because it's common in the legume family, including ornamental species like Acacia, Bauhinia and Delonix (Poinciana). This form of dormancy may also be encountered in the hibiscus, geranium, lily and canna families.

Seeds possessing such dormancy generally have a specialised pore or plug that regulates water transfer. In nature, exposure to traumas such as fire or passage through an animal's digestive system cause sufficient disruption to permit entry of water. In a commercial or laboratory setting, abrasive, temperature or chemical treatments can be applied (scarification) to make the seed coat permeable enough to allow water entry and initiate germination.

For the home gardener requiring just a few plants, nicking or sandpapering the seed coat is the most straightforward method, taking care not to damage the seed excessively. A simple hot water treatment can also be used and is popular for Acacia (see bibliography below).

Smoke and Australian natives

Smoke has been found to enable or enhance germination in a diverse array of Australian species, although the phenomenon is not restricted to plants from this continent. The discovery means that a wide variety of Australian native species can now be considered for horticultural production, and could even have implications for gardening and farming more generally.

The smoke effect is distinct from any heat effect of fire. In the nursery, seeds can be exposed to smoke in a special tent or soaked with water through which smoke has been passed. The latter is very convenient for the home gardener because smoke water is now available commercially. Note that additional dormancy factors may also be in play when attempting germination of a given species.

The biochemical basis of smoke stimulation is the subject of ongoing research. One group of plant-active molecules from smoke called karrikins has been recently discovered. The finding that many species not normally associated with fire-prone habitats, even some garden vegetables, are responsive indicates that there's still much to be learned and may even lead to development of new agricultural chemicals.

Lettuce

One of the most common backyard crops with dormancy issues is lettuce. It's response to light has been well-studied. Red wavelengths (strong in direct sunlight) stimulate germination and far-red light (predominant in light filtered through leaves) inhibit it. This has an obvious evolutionary advantage, as seeds germinating under established plants are unlikely to be successful.

Lack of light can also induce dormancy in weed seeds. If buried, they can remain viable for years, waiting for exposure to spring into action. The gardener can exploit this knowledge to reduce weed problems with appropriate cultural techniques such as no-dig/low-till methods and mulching.

An additional problem with lettuce is thermoinhibition. High soil temperatures will inhibit germination, and if they last long enough, may cause the seed to enter secondary dormancy.

In the garden, the actual response will depend on the cultivar of lettuce as these responses have a genetic basis. Like other forms of dormancy, conditions under which the seeds were produced and stored can also potentially affect subsequent dormancy behaviour.

In Conclusion

Dormancy problems can be eliminated by breeding, or minimised via production methods in the case of commercially produced seed. They can nevertheless provide hurdles for the home gardener, who is likely to be propagating plants that have received relatively little scientific study, (compared to the most important crops), or home-produced seed.

The precise mechanism(s) employed by a particular species will be greatly influenced by the habitat in which it has evolved. Undoubtedly there is much still to be discovered on the subject of seed dormancy in the plant world.

Besides the economic advantages to the farmer or production nursery of faster or more uniform germination, unlocking the secrets of dormancy could make a difference to the survival of endangered species. If the dormancy-release mechanism (e.g. fire, animal) is removed from a habitat, the plant too may eventually disappear. While vegetative propagation may be one way of rescuing plants from the brink, greater numbers can be propagated and distributed, and greater genetic diversity preserved, more cheaply and easily with seeds.

Older News

Better germination may be a matter of "the vibe"

Research out of the University of Western Australia suggests communication between germinating seeds of the same or different species can affect growth. Chilli seed germination was helped by the presence of other chillis or basil plants, but hindered by fennel. Plastic barriers blocking chemical transfer suggests "nanomechanical vibrations" may be the mode of comminication. Source: Plants 'talk' to plants to help them grow (May 2013)

Heat-inhibition of lettuce seed explored

Researchers have discovered a gene responsible for stopping lettuce germination in hot weather. When the seeds are exposed to moisture at warm temperatures, the gene switches on and increases production of absicic acid. This plant hormone inhibits germination. The mechanism may have helped the lettuce's Mediterranean ancestors avoid germinating in the summer, when conditions are usually too hot and dry to allow completion of the life cycle. Identification of the gene may lead to new varieties that don't require seed priming and other expensive and unreliable techiques to achieve year-round crop production. Source: Gene discovery may yield lettuce that will sprout in hot weather (March 2013)

e-Clean seeds

A new chemical-free method to kill pathogens on the surface of seeds has been developed in Germany. Low-energy accelerated electrons are used to destroy the DNA of organisms on the seed surface without harming the plant embryo inside. Besides the obvious benefits to humans and environment the avoidance of chemicals presents, this method means that pathogens can't build up resistance. Source: Healthy seeds - treated environmentally friendly (February 2013)

Biochemical memory helps plants respond to future drought

Scientists have found that "training" plants to tolerate periods of water stress can help them cope better next time. While this may have been common knowledge to horticulturalists, the study provides insights into a biological mechanism. Via changes in chromatin, the plants appear to "remember" prior stress and respond to subsequent dehydration by increasing transcription of certain genes. The findings may be used in the engineering of drought-tolerant crops in the future, but can be applied to current plant management techniques. For example, by "hardening" off seedlings prior to transplanting by deliberate withholding of water. Source: UNL Scientists Find Plants 'Remember' Drought, Change Responses to Survive (March 2012)

Australians return from space mission

The Australian native plant seeds that were launched into space in 2008 have returned to Earth. While Wollemi Pine germination tests are still underway, the Golden Wattle, Flannel Flower and Waratah seeds have germinated normally. They appear to have survived six months of microgravity and low-level ionising radiation in the International Space Station without damage, but will continue to be monitored and tested. The ability to withstand such conditions will be important if plants are to be cultivated in space greenhouses in the future, or perhaps on other planets. Source: Astronaut announces seedlings-from-space (June 2009)

A date with history

A Judean date palm has been grown from a seed found in the ruins of Masada, the Jewish fortress that fell to the Romans abround 2000 years ago. Believed to be the oldest seed ever to germinate, the resulting plant has been nicknamed Methuselah. Furthermore, it represents a previously extinct form of date palm, the fruit of which could have properties not present in modern dates. More from the American Association for the Advancement of Science: Researchers Resurrect Extinct Judean Date Palm Tree from 2,000-Year-Old Seed (June, 2008)