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 (see p185). 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 Chapter 12.
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.
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