In this article we will discuss about Seed. After reading this article you will learn about: 1. Structure of Seed 2. Imbibition of Seed.
Structure of Seed:
The seed is derived from the fertilized ovule. The structures that can be recognized during the development of seed are:
(i) The Testa:
Derived from one or both integuments of the ovule. The presence of fatty or waxy cuticle makes the testa somewhat impermeable to water and gases. In some cases, the testa may be mucilaginous and thereby play an important role in water retention and seed dispersal.
A common feature of the testa is the hilum which is the point of attachment of the seed to the funiculus. A small hole, the micro Pyle, is at one end of the hilum. Outgrowths like strophiole and arils may be present in certain seeds. In some seeds, the outer coat is not the testa but the pericarp. In some other seeds, the testa remains fused with the pericarp.
(ii) Perisperm and Endosperm:
Seeds are described as endospermic and non-endospermic depending on the presence or absence of well-formed endosperm. The true endosperm is derived from two polar nuclei of the ovule and one male nucleus and it is generally a triploid tissue.
The endosperm of the members of the Graminae is characterized by the presence of an outer aleurone layer. In the majority of seeds, the perisperm fails to develop as the seed attains maturity, but in a few cases (e.g. Coffee, Yucca), this tissue becomes the main storage site,
(iii) The Embryo:
This consists of the embryonic axis bearing one or two cotyledons. The embryonic axis is composed of the hypocotyls to which the cotyledons are attached, the radicle and the plumule. In some cases, an internode between the cotyledons called mesocotyl is present. In monocots, especially in Graminae, the single cotyledon is the scutellum.
Seeds contain large amounts of food reserves which support the growth and development of the seedling until it becomes an autotrophic plant capable of photosynthesis. These reserves include carbohydrate, protein, lipid, organic phosphate and various inorganic compounds. In cereals, carbohydrates, mainly starch, predominates.
Some seeds store lipids as the main reserve. High levels of protein and starch but only little lipids are present in many legumes.
Imbibition of Seed:
Water is essential for rehydration of seeds as the initial step towards germination. A number of factors like size, structure and hydrophilic nature of content determine the amount of water taken up by an imbibing seed. Water uptake by a seed is influenced by two main factors.
These are:
(i) The water relations of the seed
(ii) The relationship between the seed and the substrate, viz., the soil
The concept of water potential (P) is an important consideration while understanding water relations of a seed.
The water potential (P) which indicates the energy status of water can be written as:
Ψ of cell = Ψπ + Ψm + Ψp
Net diffusion of water takes place down an energy gradient from high to low potential.
Thus, Ψ of cell is affected by three factors:
(i) Ψπ – this is the osmotic or solute concentration effect,
(ii) Ψm – this is the matric or hydrational potential displayed by the ability of the matrices (e.g., cell wall, protein bodies and other contents) to become hydrated and bind water, and
(iii) ΨP – this is the turgor or hydrostatic pressure on the wall created by swelling of the contents following entry of water.
The potential of pure water at atmospheric pressure reaches the highest value and by convention, is expressed as zero. Values for Ψπ and Ψm are negative, while Ψp value is positive. The water potential, i.e., the sum of the three terms is a negative quantity, except in full turgid condition.
Thus, it is obvious that increasing osmotic and matric effects with a cell wall reduce the water potential to lower (more negative) values while increasing turgor pressure will increase the water potential to higher (less negative) values.
Water potential is expressed in terms of pressure or energy. The common practice is to use the bar (1 bar = 103 dynes/’cm2 or 102 1/kg or 0.987 atoms). Again, soil environment has its own potential expressed as Ψe.
The flow of water through the soil into the seed can be formulated as:
F = Ψe – Ψs
where F = rate of flow from environment (soil) to seed, Ψe = potential of the environment. Ψs = potential of the seed. Thus, the difference in water potential between seed and soil (Ψe – Ψs) is one of the factors determining the flow rate of water from the environment to the seed.
During the initial imbibitional phase, this difference is quite large, but as water uptake by seed increases, the water potential of the seed also increases whereby it becomes less negative.
In a non-dormant seed, water uptake, increased respiration and biochemical events lead to the growth of the embryo and the development of the seedling. The first visible sign of germination is the increase in length and fresh weight of radicle. In some seeds, however, growth of the hypocotyl is the first visible sign of germination.
Initial water uptake by seeds is accompanied by a rapid leakage of substances, e.g. sugars, organic acids and amino acids. Seeds with cracked or scarified seed coats and seeds with seed coats completely removed leak more solutes than those with intact seed coats.
The seedlings formed after germination can be divided into two types:
(i) Epigeal (or epigeous) in which the cotyledons are raised above the soil and generally become green and carry on photosynthesis
(ii) Hypogeal (or hypogeous) in which the cotyledons remain buried under the soil.