Seeds

Seed germination
Details of the structure of seeds can be found in Plant reproduction. The main requirements for the successful germination of most seed are as follows:

• Water supply to the seed is the first environmental requirement for germination. The water content of the seed may fall to 10 per cent during storage, but must be restored to about 70 per cent to enable full chemical activity. Water initially is absorbed into the structure of the testa in a way similar to a sponge taking up water into its air space, i.e. by imbibition. This softens the testa and moistens the cell walls of the seed so that the next stages can proceed. The cells of the seed take up water by osmosis, often assuming twice the size of the dry seed. The water provides a suitable medium for the activity of enzymes in the process of respiration. A continuous water supply is now required if germination is to proceed at a consistent rate, but the growing medium, whether it is outdoor soil or compost in a seed tray, must not be waterlogged, because oxygen essential for aerobic respiration would be withheld from the growing embryo. In the absence of oxygen, anaerobic respiration occurs and eventually causes death of the germinating seed, or suspended germination, i.e. induced dormancy.
• Temperature is a very important germination requirement, and is usually specific to a given species or even cultivar. It acts by fundamentally influencing the activity of the enzymes involved in the biochemical processes of respiration, which occur between 0°C and 40°C. However, species adapted to specialized environments respond to a narrow range of germination temperatures. For example, cucumbers require a minimum temperature of 15°C and tomatoes 10°C. On the other hand, lettuce germination may be inhibited by temperatures higher than 30°C and in some cultivars, at 25°C, a period of induced dormancy occurs. Some species, such as mustard, will germinate in temperatures just above freezing and up to 40°C, provided they are not allowed to dry out.
• Light is a factor that may influence germination in some species, but most species are indifferent. Seed of Rhododendron, Veronica and Phlox is inhibited in its germination by exposure to light, while that of celery, lettuce, most grasses, conifers and many herbaceous flowering plants is slowed down when light is excluded. This should be taken into account when the covering material for a seedbed is considered (see tilth). The colour (wavelength) of light involved may be critical in the particular response created. Far red light (720 nm), occurring between red light and infra-red light and invisible to the human eye, is found to inhibit germination in some seeds, e.g. birch, while red light (660 nm) promotes it. A canopy of tall deciduous plants filters out red light for photosynthesis. Seeds of species growing under this canopy receive mainly far red light, and are prevented from germinating. When the leaves fall in autumn, these seeds will germinate in response to the now available red light and to the low winter temperatures.

Typical germination process
Seed dormancy
As soon as the embryo begins to grow out of the seed, i.e. germinates, the plant is vulnerable to damage from cold or drought. Therefore, the seed must have a mechanism to prevent germination when poor growing conditions prevail. Dormancy is a period during which very little activity occurs in the seed, other than a very slow rate of respiration. Seeds will not germinate until dormancy is broken.

A thick testa prevents water and oxygen, essential in germination, from entering the seed. Gradual breakdown of the testa, occurring through bacterial action or freezing and thawing, eventually permits germination following unsuitable conditions. The passing of fruit through the digestive system of an animal, such as a bird, may promote germination, e.g. in tomato, cotoneaster and holly. Many species, e.g. fat hen, produce seed with variable dormancy periods, to spread germination time over a number of growing seasons. Spring soil cultivations can break the seed coat and induce germination of weed seeds. This structural dormancy, in horticultural crops, may present germination problems in plants such as rose rootstock species and Acacia . Physical methods using sandpaper or chemical treatment with sulphuric acid (collectively known as scarification) can break down the seed coat and therefore the dormancy mechanism.
Chemical inhibitors may occur in the seed to prevent the germination process. Abscisic acid at high concentrations helps maintain dormancy while, as dormancy breaks, progressively lower levels occur, with a simultaneous increase in concentrations of growth promotors such as gibberellic acid and cytokinins. Inhibitory chemicals located just below the testa may be washed out by soaking in water.

Cold temperatures cause similar breaks of dormancy in other species (stratification), the exact temperature requirement varying with the period of exposure and the plant species. Many alpine plants require a 4°C stratification temperature while other species, e.g. Ailanthus, Thuja, ash and many other trees and shrubs, require both moisture and the chilling treatment. The chemical balance inside the seed may be changed in favour of germination by treatment with chemicals such as gibberellic acid and potassium nitrate.

An undeveloped embryo in a seed is incapable of germinating until time has elapsed after the seed is removed from the parent plant, i.e. the after ripening period has occurred, as in the tomato and many tropical species, such as palms. Some seeds such as Acacia are recorded to have a dormancy of more than a hundred years.

The practical implications of the above are considered in detail in Plant propagation.

Seed viability
There are a number of essential germination requirements for a successful seedling emergence to occur. A viable seed has the potential for germination, given the required external conditions. Its viability, therefore, indicates the activity of the seed’s internal organs, i.e. whether the seed is ‘alive’ or not. Most seeds remain viable until the next growing season, a period of about eight months, but many can remain dormant for a number of years until conditions are favourable for germination. In general, viability of a batch of seed diminishes with time, its maximum viability period depending largely on the species. For example, celery seed quickly loses viability after the first season, but wheat has been reported to germinate after scores of years. The germination potential of any seed batch will depend on the storage conditions of the seed, which should be cool and dry, slowing down respiration and maintaining the internal status of the seed. These conditions are achieved in commercial seed stores by means of sensitive control equipment. Packaging of seed for sale takes account of these requirements and often includes a waterproof lining of the packet, which maintains constant water content in the seeds.

The Seeds Acts
In the UK the Seeds Acts control the quality of seed to be used by growers. A seed producer must satisfy the minimum requirements for species of vegetables and forest tree seed by subjecting a seed batch to a government testing procedure. A sample of the seed is subjected to standardized ideal germination conditions, to find the proportion that is viable (germination percentage). The germination and emergence under less ideal field conditions (field emergence), where tilth and disease factors are variable, may be much lower than germination percentage.

The sample is also tested for quality which provides information, available to the purchaser of the seed, covering trueness to type; that is, whether the characteristics of the plants are consistent with those of the named cultivar; the percentage of non-seed material, such as dust; the percentage of weed seeds, particularly those of a poisonous nature (see Weeds Act, Weeds). The precise regulations for sampling and testing, and requirements for specific species, have changed slightly since the 1920 Act, the 1964 Act (which also included the details of plant varieties), and the entry of Britain into the European Community. Some control under EC regulations is made of the provenance of forestry seed, as the geographical location of its source is important in relation to a number of factors, including response to drought, cold, dormancy, habit, and pest and disease susceptibility.