A seed contains the embryonic new plant and the beginnings of a new generation. For annuals in particular, the parent generation dies leaving their entire species as progeny in their seeds. To this end, it is important that the seeds are protected and germinate when conditions are most favorable for sucessful growth.
Seeds therefore have many adaptation to enable them to be triggered to germinate in favorable times. Seeds may look dead but in fact they are just a dormant phase in a plant's life cycle and when times are right, they germinate rapidly. Some seeds wait days, some weeks and some many years until the conditions are right.
Factors which affect seed germination include moisture, temperature and light.
For seeds to grow, even if the mature plant has adaptation for dry conditions, the seedlings need ample moisture. Seeds are triggered into germinating by the presence of water. Water is imbibed into the seed coat (testa) and either triggers germination directly by reacting with chemicals in the endosperm or gradually washes away chemicals which inhibit germination which block the passage of water to the endosperm. These inhibitors are present in seeds which need a sustained period of water for germination such as tree seeds and ensure that one quick shower in a dry season does not trigger germination.
Temperature affects seed germination too. The chemical processes which enable the cotyledons to form, the radicle to grow down and the hypocotyl (embryonic leaf) to grow up are faster in warmer condtions so germination is triggered when temperatures rise. Along with the presence of available (ie not frozen) water, these factors are important together for plants in temperate areas because otherwise, if germination was triggered by water alone, the seeds might germinate going into the coldest part of the year (winter). Because the two factors work together, germination is triggered only when temperatures rise and there is adequate water (spring).
LIght levels also affect germination. Many seeds are tiny and in nature land on the surface of the soil. Others are heavy or pass through the gut of a bird or animal and are deposited in soil or droppings. For both kinds of seed, light is important in germination. High light levels trigger it as the plant knows longer days mean better conditions whilst low levels such as winter days slow the process down.
We can mimic factors affecting germination and in horticulture two methods are used to trick the seeds into germination. The first is scarification where the seed coat is scraped or cut using sand paper or small knives. This imitates the seed passing through the gut of an animal or getting trodden on and moved in soil over winter and reduces the thickness of the testa, allowing water into the endosperm. Along with warm conditions, this will trigger earlier germination.
Stratification is another way in which horticulturalists use factors which affect seed germination. By placing the seed in a cold environment like a fridge for several weeks and then placing them in a warm place, the process of overwintering is mimicked and the seed will germinate within a few weeks so the grower does not have to wait over winter for spring as in nature.
Knowing factors which affect germination helps us to understand our plants and helps us to grow them in the most favourable conditions It also teaches us that sometimes, patience is a must!