Organic Farming: Seed Structure And Its Germination

Introduction

We know, the seed is the most important input in crop cultivation. Good quality seeds/planting materials have high germination and vigor that form the basis for obtaining high yields. Good quality seeds/planting materials should be reasonably pure and free from insect damage and diseases. Only healthy seeds of suitable variety should be selected. There should not be any mixture of seeds or other varieties of weeds. The seed should be purchased from a reliable source and it should be organically certified. Before sowing, the seed lot must be tested for its germination percentage, while sowing the quantity of the seed should be adjusted accordingly. Time of sowing is one of the important aspects in obtaining good germination of the seeds. It has also a marked influence on the yield.

Farmers have been producing seeds and planting materials over the years to suit their ecological, nutritional, tastiness, medicinal, fodder, fuel, and other demands. These are called Farmer’s varieties or Land Races. Farmer’s varieties are also referred to as indigenous seeds, native seeds, organic seeds [if the farm is certified] heritage seeds, jwaari, nate, desi, etc. Farmers’ varieties are sustainable.

Crops differ in their seed structure, germination pattern, and planting geometry. According to the crop plants, we should adopt the geometry.

 

Seed Structure And Its Germination

• What Is Seed?

A seed is a small embryonic plant enclosed in a covering called a seed coat with some stored food to give the seedling a faster start. It is the product of the ripened ovule which occurs after fertilization and also has some growth within the mother plant. Seeds are used as propagating materials for crops such as cereals, legumes, and trees. An organic seed is the seed from plants grown organically and certified or could be seeded from farmers’ varieties or desi varieties [subject to certification by an accredited certifying agency].

• Seed Structure

A seed contains an embryo from which a new plant emerges under ideal conditions. Seeds also contain storage of energy which is wrapped in the seed coat or testa. Seed protects and nourishes the embryo. The embryo has one cotyledon or seed leaf in monocotyledons, two cotyledons in almost all dicotyledons. When it germinates, the radicle and plumule emerge out.

The radicle is the embryonic root. The plumule is the embryonic shoot. The embryonic stem above the point of attachment of the cotyledon(s) is called the epicotyl. The embryonic stem below the point of attachment is called the hypocotyl.

• Seed Germination

It is a process of emerging out the radical and plumule from the seed coat of seed under favorable conditions. First the seed absorbs water which causes it to swell and activate different enzymes in the endosperm. This process is termed imbibition. However, the nature of the seed coat determines how rapidly water can penetrate and subsequently initiate the metabolic process of germination. The nature of germination differs with the types of seed. For example, monocot seed has different germination pattern in comparison to that of dicot seeds. Let us examine both germination patterns separately.

Monocot Seed Germination

In monocot seeds, the radicle and cotyledon are covered by a coleorhiza and coleoptile, respectively. The coleorhiza is the first part to come out of the seed, followed by the radicle. The coleoptile is then pushed up through the ground until it reaches the surface. Then it stops elongation and then the first leaf emerges out from the seed and begins growing in the upward direction.

Example Corn
Coleorhiza
Radicle
Coleoptile
Coleoptile
Foliage Leaves

Dicot Seed Germination

In this case of seed germination, the first plant part that is emerging out of the embryonic root is termed as radicle or primary root. This allows the seedling to become anchored in the ground and start absorbing water. At the same time, the hypocotyls begin to elongate. While elongating, it pulls up the cotyledons and enclosed shoots above the soil surface. Once the cotyledons are above the soil surface, the hypocotyl moves upward up and the cotyledons begin to unfold. After unfolding, the shoot growth continues and subsequently new leaves are differentiated. Two types of seed germination have been found in the case of dicot seeds.

[A] Epigeous: In epigeous germination, the hypocotyl elongates and forms a hook, pulling the cotyledons and shoot above the soil. Once it reaches the surface, it pulls up the cotyledons and shoots the tip of the growing seedlings into the air [ Examples: beans and tamarind ].

Example Beans
Radicle
Hypocotyl
Foliage Leaves
Cotyledon
Epicotyl
Hypocotyl

[B] Hypogeous: In hypogeous germination, the epicotyl elongates and forms the hook. The cotyledons stay underground and get decomposed later on [Example: Peas].

Example Peas
Radicle
Hypocotyl
Hypocotyl
Epicotyl
Cotyledon
Foliage Leaves

• Requirements For Seed Germination

The seed germination depends on many factors. The external factors include: water, oxygen, temperature, light, and the right soil conditions. Different sets of variables operate for the successful germination of seeds of different varieties. Now, let us study the impact of different factors on seed germination.

Water

Germination requires moist conditions. Mature seeds are extremely dry and need to take up significant amounts of water before their metabolism is resumed. The pressure caused by imbibing water aids in cracking the seed coat. When seeds are formed, most plants store large amounts of food in the form of starch, proteins, or oils, for the activity of the embryo inside the seed. When the seed imbibes water, hydrolytic enzymes [such as Amylases] are activated, breaking down of the stored food resources takes place and gives energy to the seedling to grow.

Oxygen

Most seeds respond best when water levels are enough to moisten the seeds and oxygen is readily available. Once the seed coat is softened and cracked, the germinating seedling requires oxygen for its metabolism. If the soil is waterlogged, then there will be a dearth of oxygen supply and prevent the seed from germination as it prevents aerobic respiration, which is the main source for the seedling’s energy until it starts to photosynthesize.

Temperature And Light

Temperature is an important factor for seed germination. Often, seeds have a set temperature range for germination and will not germinate above or below a certain temperature. There are minimum, maximum and optimum ranges of temperature. Collectively, these are called cardinal temperatures.

 

Seed Dormancy And Methods Of Breaking Dormancy

• Seed Dormancy

Seed dormancy refers to the failure of a viable seed to germinate under favorable environmental conditions. In some dormant seeds, morphological changes take place before the start of germination. Under natural conditions, necessary changes take place gradually under varying combinations of aeration, moisture, temperature, and light.

In general, there are two types of seed dormancy: [1] Seed Coat Dormancy [2] Internal Dormancy. Seeds with seed coat dormancy usually have a hard seed coat. Such seed coats are impermeable to oxygen and/or water. Occasionally the dormancy is caused by an inhibiting chemical in the epidermis or adjacent interior membranes. Under natural conditions, these seeds remain on or in the ground without germinating until they have been weathered sufficiently. The weathering allows penetration of water, exchange of gases, or neutralization of inhibiting chemicals.

Seed coat dormancy is common in legumes. If the seed of such plants are harvested at green [Partial Immature] and sown before they dry out, germination is reduced. However, once the seed is dried out, the dormancy is present which must be counteracted to obtain prompt germination. Methods of breaking seed coat dormancy include scarification, hot water, dry heat, mulch, water, cold and warm stratification.

In another case, there are certain chemicals present in the endosperm or testa of seed which does not allow the hydrolytic enzymes to function. Until they are inactivated or leached out from the seeds, the seed does not germinate.

• Methods Of Breaking the Seed Dormancy

Scarification: Scarification is a mechanical technique used in some legumes. It tampers the seed coat and reduces its impermeability. It is done by rubbing seeds between two pieces of sandpaper or using a file, a pin, or a knife to rupture the seed coat. Seed may also be mixed with coarse sand and shaken vigorously in a jar. Care must be taken to avoid injury to the seed embryo.

Hot Water: For small to medium-sized seeds or when we have a large number of seeds, the hot water treatment is more practical than scarification. For this treatment, seeds should be dropped into about six times their volume preheated water [82 to 93°C]. The seed should be left to cool and soak in the water for 12 to 24 hours, after which they are ready for sowing. The container used for this treatment should not be made of aluminum as it may be toxic to the seeds. With hot water treatments, the seeds should be sown promptly and not stored again.

Mulch: The mulch treatment hastens the microbial breakdown or softening of the seed coats. It is a slow method but often occurs in nature. For this treatment, fill a six to eight-inch deep container half with the seed-bed medium. Then the seeds should be covered with a mulch of wood shavings. Rice straw is used for soil bed nurseries. An one-inch thick layer of old composted shavings is best; but if not available, a three-inch layer of fresh shavings may be used. If fresh shavings are to be used, they should be soaked a few hours in a bucket of water first and mixed with compost [starter of microbial inoculants]. Neither the seeds nor the medium should be treated with a fungicide.

Water: For the occasional species whose seed coats contain a readily water-soluble, germination-inhibiting chemical, this substance can be removed by soaking the seeds in tap water or by leaching the seeds in slowly running tap water for various lengths of time just prior to soaking. The length of time depends on the species.

With the water bath, changing the water every 12 to 24 hours will hasten this leaching process. Softened water should not be used for this treatment.

Cold Stratification Or Pre-Chilling: Cold stratification or pre-chilling is useful for seeds with internal dormancy. The embryo of many seeds fails to germinate because oxygen does not diffuse through the seed coat. At cold temperatures, more oxygen is soluble in water, so the oxygen requirements of the embryo is met readily.

Warm Stratification: The exposure of seeds to moist, warm conditions at room temperature or above is called warm stratification. Sometimes this treatment is necessary for seeds with internal dormancy to facilitate the after-ripening of the embryos.