Every seed contains an embryo which is the new plant in a small size. The seed contains enough nutrients to sustain the embryo until it becomes self-sustaining.
Germination occurs in three phases. Phase one is with a rapid initial uptake followed by a plateau phase (phase two). The third phase will only take place with further uptake of water only after germination is completed, as the embryonic axes elongate. Because dormant seeds do not complete germination, they cannot enter phase three.
Conditions for Germination
Several factors influence seed germination. They include:
1) External factors such as water, oxygen, and suitable temperature.
2) Internal factors such as seed dormancy due to internal conditions.
Water is crucial to seed germination. A dormant seed is generally dehydrated and contains hardly 6-15% water in its living cells. The active cells, however, require about 75-95% of water for carrying out their metabolism. Therefore, the dormant seeds must absorb external water to become active and show germination.
Besides providing the necessary hydration for the vital activities of protoplasm, water softens the seed coats, causes their rupturing, increases permeability of seeds, and converts the insoluble food into soluble form for its translocation to the embryo. Water also brings in the dissolved oxygen for use by the growing embryo. The seed must go through imbibition to activate root growth. However, too much water can be harmful to a germinating seed.
When a plant is still growing underground, during root formation, it relies on the stored food inside the seed, and oxygen from the environment to make energy. If the soil is too saturated with water, there will not be enough oxygen for the plant to survive.
Temperature is also an important factor. Moderate warmth is necessary for the vital activities of protoplasm. Some seeds germinate when it is cold, while others only germinate when the weather reaches warm temperatures. Other seeds only germinate after extreme temperatures,such as after a fire in the grasslands. Though germination can take place over a wide range of temperatures (5-40°C), the optimum for most of the crop plants is around 25-30°C. The germination in most cases stops at 0°C and 45°C.
Oxygen is necessary for respiration which releases the energy needed for growth. Germinating seeds respire very actively and need sufficient oxygen. The germinating seeds obtain this oxygen from the air contained in the soil. It is for this reason that most seeds sown deeper in the soil or in water-logged soils (i.e. oxygen deficient) often fail to germinate due to insufficient oxygen.
Ploughing and hoeing are cultural methods that aerate the soil and facilitate good germination.
Seed dormancy due to internal conditions
In some plants, the embryo is not fully mature at the time of seed shedding. Such seeds do not germinate till the embryo attains maturity. The freshly shed seed in certain plants may not have sufficient amounts of growth hormones required for the growth of the embryo. These seeds require some interval of time during which the hormones get synthesized.
The seeds of almost all the plants remain viable or living for a specific period of time. This viability period ranges from a few weeks to many years. Seeds of Lotus have the maximum viability period of 1000 years.
Seeds germinate before the ending of their viability periods. In many plants, the freshly shed seeds become dormant due to various reasons like the presence of hard, tough and impermeable seed coats, presence of growth inhibitors, and the deficiency of sufficient amounts of food, minerals, and enzymes, etc.
Importance of Seed Dormancy
There are advantages of dormancy in agriculture. These include:
1. Perennation: Seed dormancy allows seeds to pass through drought, cold and other unfavourable conditions.
2. Dispersal: It is essential for dispersal of seeds. The period of dormancy of seed is essential for seeds to be found in other locations, thus help in the natural distribution process.
3. Germination under favourable conditions: Seeds germinate only when sufficient water is available to leach out inhibitors and soften the seed coats. Where the conditions for germination are not favourable, the seed remains intact, thus prolonging their self-life. Dormancy under this condition has helped in the study of the conditions necessary for germination.
4. Storage: It is because of dormancy that we are able to store grains, pulses and other edibles for making them available throughout the year and transport to the areas of shortfall or where there are better prices for these crops.
Breaking of Seed Dormancy
There are two ways of breaking seed dormancy, natural and artificial.
In nature seed dormancy is broken automatically due to:
(i) Development of growth hormones to counter growth inhibitors.
(ii) Leaching of germination inhibitors.
(iii) Maturation and after-ripening of embryo.
(iv) Weakening of impermeable and tough seed coats by microbial action, abrasion, passage through digestive tract of animals, etc.
Artificial Breaking of Seed Dormancy involves:
1. Scarification: Hard, impermeable seed coat is weakened or ruptured by filing, chipping, hot water and chemical treatments.
2. Stratification: Seeds are moistened and exposed to oxygen for variable period at low or high temperature. There is also mechanical stratification e.g. the use of sandpaper, hammer, knife.
3. Counteracting Inhibitors: Inhibitors are destroyed by dipping seeds in KNO3, thiourea, ethylene chlorohydrin and gibberellin.
4. Shaking and Pressure: Vigorous shaking and hydraulic pressure are used to weaken seed coats.
Having overcome the problem of seed dormancy go on to have a healthy garden.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.