Seed germination and seedling growth

Seed germination  

The seed-

Seed is the result of fertilization, which is formed in all spermatophytes (gymnosperms and angiosperms). In gymnosperms, the ovules are exposed and so, also the seeds. In angiosperms, the seeds are enclosed within the fruit.

Parts of the seed -

A mature seed consists of two essential parts- the seed coat and the embryo.

The seed coat -

The outer covering of the seed is known as seed coat. It develops from the integument of the ovule. Mostly; it is made of the two layers- The outer thick and leathery layer testa and the inner thin and papery layer tegmen.

The seed is attached to the pericarp (fruit wall) by a short stalk called seed stalk or funiculus. In a mature seed, the position of seed stalk is marked by a small oval    depression called hilum. Just below the hilum is a small pore, the micropyle.

Seed coat has functions like -

1) It protects the embryo from desiccation, unfavorable temperatures, mechanical injury and attacks by bacteria fungi and insects.

2) Seed coat helps in seed dispersal by developing special structures in some plants such as wings, hairs, fleshy colored tissues and air filled cavities.

3)  Some seeds such as Pinus, contain starch and protein as a reserve food material which are metabolized during seed germination

The embryo -

The embryo is a young or miniature plant enclosed within the seed coat. It develops from the fertilized egg. The embryo of a mature seed consists of 4 distinct parts.

Cotyledons, plumule, hypocotyl and radicle

           

The radicle, plumule and hypocotyl together form the embryonic axis or tigellum.

                       

The cotyledons are attached to the embryonic axis. Dicotyledons typically have two cotyledons, situated opposite to each other. Monocotyledons have only one cotyledon which is often highly modified. In most of the plants, the cotyledons store the reserve food material ( eg. Pea, gram, bean etc), and in others they also serve as photosynthetic organs.

           

The part of the embryonic axis just above the point of attachment of the cotyledon is known as epicotyl and at the tip of it is the plumule. It has1or more leaf primordia at its apex.

           

The part, lying below the point of attachment is hypocotyl. It represents the root stem transition region.ie. The part where stem changes into root.

           

The radicle is the basal tip of the hypocotyl. When seed germinates, the radicle becomes the primary root of seedling.

In seeds, food is stored in the cotyledons or in a special food storage tissue, the endosperm. When endosperm is present the seed is known as endospermic or albuminous seed. When the endosperm is fully utilized during the development of seed the seed is known as non-endospermic or ex-albuminous seed.

           

Food materials stored in seeds are carbohydrates, proteins and lipids. On dry weight basis seeds of cereals contain 70-80% starch, those of peas and beans about 50% starch. In maize, principal food is starch but embryo contains 50% oil. The seeds of rape and mustard contain 40% oil and 30% protein. 

Nucleic acids -

                       

Nucleic acids are found in very low amounts in storage tissue. These nucleic acids are converted into nucleotides or nucleosides by the activation of the RNA ase, DNA ase and polynucleotide phosphorylases .These nucleotides or nucleosides are later transported to the embryo and converted into its nucleic acids.

 

Carbohydrates -

           

Starch grains are found in amyloplasts. Large starch grains appear first and smaller grains appear at later stages of development. Few outermost layers of endosperm become highly specialized to form aleuron layers. Aleuron layers have thick walls and non - vacuolated cytoplasm. They are interconnected by plasmodesmata.

Proteins -

           

In fabaceae, storage proteins are globulins. They occur as discrete protein bodies. Protein bodies may lack inclusions or may contain globoids consisting of insoluble salts of phytic acid, protein crystalloids, protein - carbohydrate bodies and calcium oxalate crystals. Protein bodies accumulate in vacuoles.

Lipids -

Lipids are stored as oil bodies and triglycerides. Monomolecular layer of phospholipids forms an interface with the surrounding cytoplasm.

Germination of seeds -

           

The process by which the dormant embryo of the seed resumes active growth and forms a seedling is known as germination.

Conditions necessary for germination

Seeds of all plants require supply of oxygen, water and favorable temperature for the germination.

1. Oxygen - During germination embryo resumes growth and for this energy is required. The energy comes from oxidation of food material.

2. Water - In dormant seeds, the food material is stored in concentrated form and they have low physiological activity. Water is essential as the concentrated food is converted in the form it can be utilized by the seedling. Water also serves as a medium where enzymatic reactions occur. It also softens the seed coat and allows the embryo to come out.

3. Temperature - A number of physiological processes occur within the seed during germination. The favorable temp ranges from 20-350c.

The germination process - 

          

Under favorable conditions, the seeds absorb water from the soil through micropyle. The first visible indication is the swelling of seed, followed by the softening of the seed coat.         

Absorption of water causes a series of physiological changes. It first dilutes the cell contents and there by activates enzymes. These enzymes convert the stored food material into the soluble form that can be used by the growing embryo. Carbohydrates are hydrolyzed by the activities of phosphorylase, amylase and maltase. Hydrolysis of starch releases glucose, which is taken up by scutellum to polysaccharides that are transported to growing seedling. Hemicellulose is hydrolyzed to mannose and other monosaccharides.

           

During seed germination, protein is converted to amides and amino acids by proteinases and peptidases. These smaller, more soluble compounds are translocated to the embryo for their utilization.

           

While, lipases hydrolyze triglycerides, to glycerol and fatty acids; some of the fatty acids are utilized to synthesize phospholipids and glycolipids which are required as constituent of organelles, but most of the fatty acids are converted to sugars which are utilized for growth of seedling.

            

Cell-division starts in the growing parts of the embryo (i.e., radicle and plumule) when they get food material.   The radicle is the first part of the embryo to come out of the seed coat. It is positively geotropic and soon grows towards the soil regardless of its initial orientation. With the expansion of embryo, the seed coat ruptures and the plumule lying between cotyledons comes out. It forms the shoot.

           

On the basis of   the behavior of   cotyledons, the germination may be following two types:  

           

Epigeal germination-

In seeds with epigeal   germination   the   cotyledons are brought above the ground due to the elongation of the hypocotyls. In cotton, papaya, onion and castor, flat green leaf like cotyledons can be seen in the young seedlings. Here the cotyledons, besides food storage, also perform photosynthesis till the seedling becomes independent. In some other plants like tamarind and bean, the cotyledons being thick, do not become leaf like; they shrivel and fall off.

Hypogeal germination-

In hypogeal germination the cotyledons do not come out of the soil surface. In such seeds the epicotyl elongates pushing the plumule out of the soil. All monocotyledons show this type of germination. The radicle and plumule come out by piercing the coleoptile and coleorrhiza respectively. The plumule grows upward and the first leaf comes out of the coleoptile. The radicle forms the primary root which is soon replaced by many fibrous roots. Among dicotyledons, gram, pea, groundnut, mango etc. are common examples of hypogeal germination.