The process by which a seedling sprouts from a dormant seed is called germination. This article provides an overview of what happens when seeds begin to grow.
Before Germination – Seed Dormancy and Hibernation
Most seeds undergo a period of dormancy and/or hibernation before they begin to grow. Seed hibernation occurs when the environmental conditions are not suitable for growth – for example it may be too cold/hot, or the atmosphere may contain toxins such as smoke. Some seeds have been known to grow after thousands of years in hibernation.
Seed dormancy differs from seed hibernation in that dormant seed is one that is not growing even when the environmental conditions are ideal for growth. Dormancy can be induced by a large number of factors, and it helps the plants to reproduce successfully, by preventing all of the seeds from germinating at once. Instead, by staggering germination over a period of time, some of the seeds are protected from the influence of harmful but transient environmental factors. Seed dormancy also helps to reduce the competition for nutrients and light from other plants.
Seed dormancy and hibernation are complex phenomena that are still not fully understood.
Germination Triggers
So, it’s apparent that seeds don’t always grow at the first opportunity. In order for germination to occur, the seed must be viable, the conditions (including the levels of water, oxygen, light and heat) must be right for that species, and the seed must not be in a dormant state.
Growth
Once these conditions are met, the internal metabolic changes which trigger growth begin. Firstly, the embryo plant must break out of the seed pod by splitting it. To do this it soaks up water, a process known as imbibition. The rate at which this occurs varies between species, and with the amount of water present in the environment. Large amounts of water are often needed, because mature seeds can be very dry.
The water that the seed imbibes is used to break down the food (fats, proteins and starches) that is stored within the seed. The embryo plant can now use these to fuel cell division, and its respiratory rate increases dramatically. Assuming that conditions are good, growth is now rapid, and once the embryo becomes too large for the seed case, it bursts through. When these food reserves are all used up, the seedling must obtain further nutrients from its environment. The growing seed also makes use of oxygen to fuel its metabolic processes.
Seed growth is dependent on adequate nutrients and oxygen, but the temperature and light levels must also be right. Optimal levels vary between species, particularly the influence of temperature, with some plants preferring cold environments, and others warmth.
Establishment
The root tip, or radicle, is the first part of the seedling to emerge from the seed case, which is important because it helps to ensure that the seedling can anchor itself in the soil quickly. According to some authorities, the emergence of the radicle marks the end of germination, and the beginning of the ‘establishment’ phase, during which the seedling becomes an independent plant.
After the radicle becomes established, the next parts of the seedling to emerge are the cotyledons, or seed-leaves (some seeds only have one of these). The earth’s gravitational pull plays a role here; the root grows towards the earth, whereas the cotyledons orient themselves away from gravity, towards the surface. When the shoot reaches the surface, the cotyledons open up and develop a green colour, caused by the photosynthetic pigment chlorophyll. The young seedling can now begin to produce energy and is no longer dependent on the food reserves of the seed.
Establishment ends when the seed’s food reserves are exhausted, and the seedling obtains its nutrients from the environment. After this, the seedling develops its first ‘true leaves’, which, unlike the rounded cotyledons, usually conform to the normal shape for that species. The continued development of the shoot influences the root, which also continues to grow and branch off.
Germination and establishment are vulnerable times in the life cycle of the plant, and many seedlings do not survive. But once past this stage, the established seedling’s continued growth to maturity (which is hormonally regulated), is now heavily influenced by the light levels of the environment, as well as by factors such as the soil quality, water levels, temperature and the presence of predators. Fortunately these factors can often be controlled to some extent, so careful gardeners can play a significant role in helping their seedlings to continue to grow and thrive.
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