SEED DEVELOPMENT AND GERMINATION

• The life of an individual plant begins when an egg nucleus in maternal organ of a flower is fertilized by a sperm nucleus to form a zygote.
• Growth and differentiation of the zygote produces an Embryo, contained in a protective structure called Seed.
• When the seed gets appropriate conditions, the embryo within the seed will renew its growth and  will continue to develop into a mature plant.

SEED DEVELOPMENT

• Seed is a mature embryo surrounded by nutritive tissues and encased in protective seed coat.
• The development of seed begins with the fertilized ovule or zygote. The early stage of seed development is characterized by excessive cell divisions that form the embryo and ,in endospermic seeds the tissue that store nutrients that will support the eventual germination of the seed and seedling development.
• The first division of the zygote is usually transverse and immediately establishes polarity of the embryo. The upper cell  becomes the embryo while the lower produces a stalk like suspensor that anchors  the embryo at the base of the embryo sac.
• During the earlier stages of embryo development, cell divisions occurs throughout the entire cell mass but at the heart shaped embryo stage both root and shoot apical meristem begins to organize as centres of cell divisions.
• Through out the development of embryo, there is a continuous flow of nutrients from the parent plant into the endosperm or Cotyledons.
• In some  cereal grains and many monocots, the endosperm is retained until maturity and may comprise the bulk of the seed. These are called Endospermic seeds.
• The endosperm of mature endospermic seeds consists of cells filled with starch along with proteins and some amounts of lipids.
• In some monocot seeds ,most notably the cereal grains such as Tritium (wheat),Hordeum(barley) and Avena (oats),the endosperm is surrounded by one or more distinctive layers called Aleurone. Aleurone cells are observed to have , presence of numerous protein bodies and are the source of enzymes needed to mobilize nutrients during germination.
• In endospermic dicot seeds, such as Castor bean (Ricinus  communis) have retained a significant amount of endosperm and at maturity the cotyledons are thin leaf like structures.
• In non endospermic dicot seeds ,such as Pisum (pea) &Phaseolus (Bean), the cotyledons enlarge at the expense of the endosperm which occupy 90% of seed volume at maturity .
• Both endosperm and cotyledons contain a large quantity of stored carbon (in form of carbohydrates,  proteins and lipids ),mineral elements and hormones that support the growth and development of the seedlings until it can establish itself as a photosynthetically competent plant.
•  The maturation of seed development is characterized by cessation of embryo growth and development of desiccation resistance. Maturation is terminated by a dramatic desiccation in which the water content of the seed is reduced from  80 or 90 percent to approximately 5 percent.
• Surrounding the mature seed is a hard coat derived from maternal tissue (the Integuments) which surround the seed during its development in ovary .
• Comprised of heavy walled cells and covered with a thick ,waxy cuticle ,the Seed coat often presents a significant barrier to the uptake of both water and oxygen by seed.

SEED GERMINATION

• Seeds are quiescent or resting organs because seeds are severely dehydrated, any metabolic activity takes place so slowly that it is scarcely detected.
• Resumption of embryo growth is called Germination.
• Germination is based on a number of factors but three are specifically important-1)adequate water to rehydrate the tissue 2)the presence of oxygen to support aerobic respiration and 3)a physiological temperature.
• Although many seeds will germinate over a wide range of temperatures, the optimum range for most of the seeds is 25°C -45°C.
• The initial step in germination is the uptake of water and regulation of seed tissues by the process Imbibition. Imbibition is the movement of water down a water potential gradient. Unlike osmosis it doesn't require differentially permeable membrane and is carried out  primarily by surface acting or Matric forces.
• Hydration causes swelling of the imbibing material ,which may generate substantial force (called Imbibition pressure). Imbibition pressure developed by a germinating seed will cause the seed coat to rupture, thus permitting the embryo to emerge.
•  After Imbibition of water the next step is the general activation of seed metabolism within minutes of water entering the cells, initially utilizing a few mitochondria and respiratory enzymes that had been considered in the dehydrated state.
• Renewed protein synthesis is also an early event, utilizing previously existing RNA transcripts and ribosomes, as existing organelles are repaired and new organelles are formed.
• This is events are followed by -1)the release of hydrolytic enzymes that digest and mobilize the stored reserves 2)renewed cell divisions and cell enlargement in the embryonic axis.
• In non endospermic dicot seeds such as the legumes (peas,beans) the initial stages of radicle elongation appear to depend on reserved stored in the tissue of radicle itself. Later carbon reserves are mobilized from the cotyledons and transported to elongating axis.
• Generally germination is considered to be complete when the radicle emerges from the seed coat. Radicle emergence occurs through a combination of cell enlargement within radicle itself and imbibition pressures developed within the seed. The  seed coat is ruptured and  the radicle is protruded which allows it to make direct contact with water and nutrient salts required to support further growth of the young seedling.

Cross section of a mature Maize kernel showing the principal structure of an Embryo