Seed Germination - Types & Factors

 

Seed germination

When a seed is provided with moisture proper temperature and air (oxygen), the dormant embryo (young plant) starts growing. This process is called seed germination.

Once the seed has moved to a new location and is covered with dirt, it can begin germination. Germination is the process of growing seeds in new plants. First, environmental conditions must trigger the seed to grow. Generally, this is determined by how deep the seed is, the availability of water and the temperature. When there is too much water, the seed fills with water in a process called imbibition. Water activates special proteins, called enzymes, that start the process of seed growth. First, the seed grows a root to access water underground. Next, shoots, or above-ground growth, begin to appear. The seed sends a pellet to the surface, where it will grow leaves to draw energy from the sun. In this process, the leaves continue to move towards the light source which is called photomorphogenesis.

Factors Affecting Seed Germination

1. Internal Factors

    (i) Seed maturity

• Seed maturity can affect seed germination and the emergence of seedlings in many cases
• In some species, such as Pinus and Ranunculus, the fruits are shed before the embryo fully matures.
• Such physiologically immature seeds must undergo certain enzymatic and biochemical changes to attain maturity
•  These changes are collectively called after-ripening
• Immature embryos cannot germinate

    (ii) Seed dormancy

• 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 a sufficient amount of growth hormones required for the growth of an embryo
• These seeds require some interval of time during which the hormones get synthesized

    (iii) Seed vigor

• Seeds in low vigor produce weak seedlings that are susceptible to environmental stresses
• A high level of vigor in seeds provide for early and uniform stands which give the growing seedlings good resistance against various environmental stresses

    (iv) Seed size

• Plant species with large seeds have an advantage under low light conditions, when their greater protein and lipid reserves, or their more advanced development, can facilitate growth.
• Large seeds cannot be physically borne on small plants because of the weight of the seed, which may partly explain the association between plant size and seed size.

2. External factors

    (i) Water

• Water is required for germination. Mature seeds are often extremely dry and need to take in significant amount of water, relative to the dry weight of seed, before cellular metabolism and growth can resume.
• Most seeds need enough water to moisten the seeds but not enough to soak them.
• The uptake of water by seeds is called imbibition, which leads to the swelling and breaking of the seedcoat
• When seeds are formed, most plants store a food reserve within the seed, such as starch, proteins, and oils.

    (ii) Temperature

• The temperature influences strongly the percentage and rate of seed germination, the optimum temperature varying with the kind of seeds.
• In general, seeds of cool-season crops, such as lettuce, celery, etc. germinate best at relatively low temperature of 0 to 10°C, whereas seeds of warm-season crops like pumpkin, bottle gourd, etc. require relatively high temperature ranging from 20 to 30°C
• Seeds of several other species germinate over a wide range of temperatures. Many other kinds of seeds germinate better when the temperature fluctuates daily about 10'C between maximum and minimum.

    (iii) Light

• Light or darkness can be an environmental trigger for germination and is a type of physiological dormancy
• Most seeds are not affected by light or darkness, but many seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for the growth of the seedling.

    (iv) Aeration

• Germinating seeds respire at a very high rate.
• Thus the adequate supply of oxygen is necessary for germination to take place and the seedling to grow.
• Germination rate and germination percentage of most kinds of seeds are adversely affected if the germinating medium is poorly aerated.
• In seed-beds that are over-watered or poorly drained with clayey soil, the soil pore spaces may be so filled with water that oxygen supply to the seeds is retarded or prevented.

    (v) Atmospheric gasses

• Most crop seeds germinate well in the ambient composition of air with 20% oxygen, 0.03% carbon dioxide, 78.2% nitrogen. 

Types of germination

There are two types of germination:

(1) Epigeal germination

(2) Hypogeal germination

   (1) Hypogeal germination

Portion of the axis above the cotyledons (epicotyl) grows rapidly and carries the

plumule above the surface of the soil.

Examples: Litchi, groundnut, peas, gram and jackfruit

    (2) Epigeal germination

Portion of the axis below the

cotyledons (hypocotyl) grows

relatively fast and pushes the

cotyledons above the surface of the soil.

Examples: Muskmelons, gourds,

luffa, beans and cucumbers