Seeds

Contents

Seed structure

Seeds are the product of sexual reproduction in Spermatophytes, which consists of the Gymnosperms (conifers and allies) and the Angiosperms, which themselves are broadly divided into the monocots and dicots. A seed consists of several distinct tissues:

Fruits

Gymnosperms have naked seeds (and therefore no fruits), but angiosperms have enclosed seeds (fruits: the word angion means 'vessel'). There are three principle types of fruits:

Fruits form from the ovary wall, and contain the seeds in angiosperms.

Fruits may be distributed by dehiscence (splitting), zoochory (animals), anemochory (wind), and various other common mechanisms. The form of the fruit is telling here: small dry fruits are usually anemochorous, large fleshy fruits are zoochorous..

Industrial seed production

Seeds are used by humans for both food stuffs and fibres, and their production is extremely economically important.

Seeds are also sold as commodities in their own right, usually as horticultural seeds (F1 hybrids), or as seeds for growing more seeds with, i.e. farming.

Harvesting seeds requires good agricultural practice such as removing weeds, preventing cross pollination and avoiding edge harvesting (where cross pollination is likely). Wild flower seeds generally disperse over a long period, as do ornamentals with long flowering periods, so these may have to be hand harvested, which is obviously expensive. On the other hand, crops tend to flower simultaneously and retain their seeds (this is basically accidental artificial selection), so they are easier to harvest mechanically, but even here, wheat may sprout on the ear, useless for planting or for food.

Mechanical harvesting with a combine harvester, which both cuts and threshes the seeds, is much used in industrial seed production. Threshing must be carried out at correct moisture content (MC) or the seeds will be bruised (if too wet) or cracked (if too dry). MC depends on both the parent plant (which dries its seeds as they mature) and on the relative humidity (RH) to which the seeds are exposed. Wheat wants about 14% moisture content for optimal harvesting, which can be tested for by measuring the electrical conductivity of ground seeds, or simply by biting them.

Seed physiology

Seeds are resistant to environmental extremes because they are metabolically inactive: they are either quiescent (in seeds that are too dry to germinate) or dormant (in seeds wet enough to germinate, that don't).

Seeds contain hygroscopic proteins: at 60% RH, seeds will have about 12% MC at equilibrium, but the water is tightly bound to the matrix, so the water potential is very low, and solutes cannot diffuse. Most seeds are quite stable in this quiescent state for years, but will slowly deteriorate as no metabolism occurs to repair damage sustained during storage.

The MC affects viability in storage, at MC<5%, seeds become brittle and crack, but at MC>14%, fungi attack. Inviable seeds usually exhibit membrane damage during imbibition, especially in the tonoplast. The vacuoles burst, causing organelle digestion, loss of osmotic ability, and cell collapse. Membranes are destroyed by chain reaction lipid peroxidation: vitamin E is a common protectant (hence seeds are rich in this vitamin), but high MC and high temperature (T) encourage peroxide formation. Absolute viability depends on many things, but generally, a 5°C rise in temperature halves the seeds likely lifespan, as does every 1% rise in MC above the optimum.

The storage of quiescent seeds in seed banks requires very dry (5% MC) and cold (−20°C) conditions, especially in seed banks of germplasm, but unfortunately, some seeds will not tolerate freezing. However, even before loss of viability (which is defined as the time taked for germination rates to drop to 50%, i.e. t1/2) seeds accumulate mutations, take longer to germinate and may be abnormal when grown. It's best to keep them only for 95% viability before regenerating them. After imbibing, lettuce seed will remain dormant above 30°C. If then stored at a range of MCs, then set to germinate at normal temperature, high MC seeds die, as expected, but above 20%, seeds can metabolise, fix themselves (regenerate), and in fact survive better than dry seeds. This depends on oxygen though.

In agricultural seed storage, we are only keeping seeds quiescent for one season, but cool and dry is still best if it's economical. Many small seeds (carrots) are harvested from the Mediterranean, where seeds can dry on the parent plant, then kept at 10°C. Heavy seeds (wheat) are not viable to import, but cereals are largely robust and are not stored for long, so they are just kept dry enough to prevent fungi. Artificial drying by spreading seeds out and allowing them to equilibrate with dry air, is useful, but a lot of space is required. Alternatively, drying can be achieved by piling seeds up and pumping through air. Hot airs can be used if the seeds are just being stored prior to milling, but cool air is required if the seeds are destined for for sowing or malting.

Dormancy

Dormancy may last for days to years after imbibing, and serves to spread seed germination temporally. Cultivated seeds have rather low dormancy, but this has been investigated in detail as crops are more genetically uniform than wild seeds. There are several dormancy mechanisms:

There may be several mechanisms in any given seed, but all are under, and influence, hormonal control. Abscisin generaly encourages dormancy, cytokinin, auxin and gibberellin the breaking thereof. You can artificially add hormones to induce germination: malt barley is often encouraged with gibberellin, and lettuce germination with cytokinin.

Seed testing

Seeds may be tested for various quality-determining factors, such as purity and vigour. Purity checks that the seed lot is not riddled with weed seeds or unwanted hybrids. Percentage germination tests determine the basic viability of a seed lot: seeds are germinated on warm, moist filter paper, maybe with hormones. Chitting (formation of first root) is the layperson's %G, but officially, seed merchants have to grow seed until they are sure it is not abnormal.

However, the field is much more stressful than the conditions of a germination test: about 30% of viables will still not survive, especially if cold, so farmers increase seed rate to account for this. Some seedlots have very low vigour (the ability to germinate under stress): these seed lots can be weeded out by using vigour tests: germinate under 10°C cold stress in a box of vermiculite or similar. Low vigour has many causes:

Uniformity is required in germination and therefore time of eventual cropping, hence various post harvest treatments have been developed to encourage uniform and vigorous germination.

Seed banks

Storage of seeds (and other 'germplasm') is a plant version of culture collections for micro-organisms. Advantages of seeds, as with bacteria & fungi, is that they require little space, are long lived (like cryopreserved bacteria and spores), and the running costs are lower than those of a huge greenhouse. There are many reasons to bother storing biodiversity like this. Firstly, to preserve genetic diversity: the storage of genes that might be lost or become uncommon or important, especially pest/disease resistance in crops, whose frequencies change frequently; and the storage of species that may become extinct, for reintroduction (frozen zoo). Secondly, we get a time series record of gene frequencies and genotypes in a habitat by storing seeds over a period of time. This gives up a way to study evolutionary pressures.

There are three principle types (or rationales) of seed bank.

The storage conditions in seed banks vary, but dry and cold is the rule. Germination and vigour should be tested before and after storage to ensure the regime is optimal, and that the seed are 'orthodox' and don't respond adversely to cold and dry treatments.

When drying, ambient temperatures at low (15%-30%) RH are used to desiccate seeds. Some seeds are more hygroscopic than others so you need to check the MC by fresh mass/dry mass ratio to optimise it. A 1% fall in MC usually doubles the lifespan of a seed, down to a point (5% often best). Dry seeds will tolerate freezing, as the MC is so low that no free water (unbound to matrix) is available to freeze and cause damage by intracellular ice formation, which tears open membranes. A 5°C fall will usually double the lifespan, down to a point (−18°C is often best).

Seed viability drops with increasing temperature

Seed viability drops with increasing moisture content

Seed banking problems and alternatives

Before even trying to preserve seeds, it is imperative to know the purity, provenance, viability and taxon to which the accessions belong. Accessions are given serial numbers, which link the seed to a database of information about what the seed actually is. The seeds are then cleaned of chaff and fruit before preservation. Much research is now going into looking at markers for seed bank tolerance, i.e. finding gene probes, protein assays, etc., to predict the orthodox or recalcitrant nature of seeds.

Problem one of seed bank storage is getting it to work in the first place: about 20%-30% of seeds are recalcitrant and will not survive dry & freeze regimes: in particular, they are desiccation intolerant, and therefore cannot be frozen.

Seeds that store well when cold and dry are called orthodox. Indications for successful storage under seedbank conditions include:

Seeds that store badly are called recalcitrant. These are largely desiccation intolerant. Contraindications against successful storage include:

Problem two of seedbank storage is that some plants never produce seeds, or have poor fertility, especially many crops. Such species are usually reproduced by cloning: seed potatoes, suckering, grafting, etc. Bananas are triploid and sterile, and potatoes are polyploid and have aberrant seed and low fertility. Problem three is that some plants need extra care: orchid seeds need cosseting to germinate after storage: they require symbiotic fungi or tissue culture for effective germination as they have no food reserves. Problem four is that cold storage is not a typical situation for a seed to find itself in for extended periods of time. There will inevitably be a loss of biodiversity, and increase in damage during storage

Alternatives to seedbanking for sterile, difficult and recalcitrant seeds include the storage of tubers, seeds, corms, bulbs, etc. under cool (not freezing) conditions.

This allows the storage of some sterile plants and some recalcitrant seed, but only for a few years. Tissues culture banks of callus or small plants at temperatures severely restricting growth take up less space, and we can keep sterile crops and plants whose seeds are recalcitrant, but these are much more labour intensive, there is always the risk the tissue cultures will become infected or inviable, and we subject the plants to even more bizarre selection pressures (agar tolerance?!). Cryopreservation of seeds, embryos dissected from seeds, pollen, or tissue cultures in liquid nitrogen with cryopreservants has many advantages: small recalcitrant seeds can be impregnated with cryopreservant and become orthodox, and the system is simpler than growing cultures to maintain. However, there are higher refrigeration costs.

The Millennium seedbank is an £80M project at Wakehurst place (Kew in the country), funded by the Millennium Commission and Wellcome trust. They plan to:

Peer Review.
This page has been peer reviewed by 2 people. Thanks to Mary Barkworth for her correction.