Botanically, seed is the ripened ovule which contains embryonic shoots and roots. The outer layer, seed coat, covers the embryo and endosperm protecting it.
Seed formation, development and physiology of seed
Seeds are produced by flowering plants:- angiosperms (enclosed seeds, seeds enclosed by fruits) and gymnosperms (naked seeds, no special structure to cover the seeds).
Seeds in angiosperms, as we discuss here, consist of -embryo, -endosperm and the -seed coat. The two male gametes fuses with the egg cell and the polar nuclei respectively. The haploid sperm and the haploid egg forms the zygote (2n) through syngamy, also called fertilization. The other sperm and the two polar nuclei fuse to form endosperm nucleus (3n) through triple fusion or vegetative fertilization. Thus, with the independent mitotic divisions, diploid embryo and triploid endosperm are formed.
An embryo consist of cotyledons, epicotyl, plumule (tip of the epicotyle and will become the shoot), hypocotyl (connects epicotyle and radical) and the radical (forms into primary root).
[caption id="" align="aligncenter" width="512"]
Source: Commons Seed development in plants consists of six stages: Stage I - Zygote Stage After double fertilization, the endosperm and zygote are visible. Stage II - Proembryo Stage The zygote divides to create a proembryo and a suspensor forms to transfer nutrients from the endosperm to the embryo allowing the embryo to grow.
Stage III - Globular Stage The proembryo contains numerous cells. The root-shoot axis is visible. The embryonic cells near the suspensor will form the root while the embryonic cells at the other end near the top of the embryo will become the shoot. There are cells on the outskirt of the embryo that will become the dermal tissue.
Stage IV - Heart Stage Cell division in the embryo creates a heart shaped embryo revealing the cotyledons.
Stage V - Torpedo Stage The root takes on a torpedo shape. The root and shoot apical meristems appear. Also present is the ground meristem. The cotyledons are clearly visible and start to bend.
Stage VI - Mature Embryo Stage The epicotyl, which contributes to the shoot, and hypocotyl, which contributes to the root, become visible along with the radicle. The cotyledons are extremely visible. Procambium can be seen at the core of the embryo. The integuments of the ovule become the seed coat. 1) Endosperm; 2) Zygote; 3) Embryo; 4) Suspensor; 5) Cotyledons; 6) Shoot Apical Meristem; 7) Root Apical Meristem; 8) Radicle; 9) Hypocotyl; 10) Epicotyl;
11) Seed Coat[/caption]
Endosperm supplies nutrition to the developing embryo. And may contain hormones like zeatin in maize and coconut milk. The seeds can be devoid of endosperm (non-endospermic like peas, legumes) or may be present in mature seeds (endospermic like maize, wheat). The cotyledons are present in non-endospermic dicotyledons and are the source of nutrients.
Seed coat has two layers: testa (produced from outer integument) and tegmen (produced from inner integuments).
[caption id="" align="aligncenter" width="319"]
Labelled Corn Seed Illustration[/caption]Seed is the post fertilized, matured and the ripened ovules. It has seed coat and contains endosperm and embryo. Later, via the process of germination new plants are developed. In germination, seed coats are ruptured and seedlings are emerged from seedling axis.
[caption id="" align="aligncenter" width="386"]
The diagram above compares and contrasts a generalized dicot seed (1) to a generalized monocot seed (2). There are five major parts of a dicot seed. A. Seed coat: the seed coat protects the embryo. B. Cotyledon: the cotyledon stores food; there are two cotyledons in dicot seeds. C. Hilum: the hilum is the point of attachment to its seed vessel. D. Plumule: the plumule is the shoot of the seed where the leaves will first appear. E. Radicle: the radicle is the root of the seed. In the monocot seed there are also five major parts. A. Seed coat: the seed coat protects the seed. B. Cotyledon: the cotyledon is the leaf of the seed; there is only one cotyledon in monocot seeds. D. Plumule: the plumule of the monocot seed is the shoot of the seed. E. Radicle: the radicle of the seed is the root of the seed. F. Endosperm: the endosperm is the food supply for the seed; the dicot seeds contain endosperms in some of the mature seeds.[/caption]Seed quality and seed classes
A good quality seed
- genetically pure,
- pure, viable and can grow vigorously with high yield
- free from pests and seed borne diseases
- whole, not broken and are fresh
- has uniform size, weight
- contains optimum moisture
- high generation percentage.
Seeds classes include:
Nucleus seed: produced by breeder, genetically pure
Breeder seed: source for foundation seed, controlled by research institute or organization, supervision under qualified breeder
Foundation Seed: produced from breeder seed, white coloured tags
Certified Seed: produced from foundation seed, determined by seed certification agency, blue coloured tag; green colour for certified ii seed class
Improved Seed: produced from certified seed, yellowish tags, producers responsible for it
Source Seed: seeds used to produced seeds of the same variety.
Principles and practices of seed production
Types of Seeds:
~ food crop seeds
~ Vegetable Seeds
~ Forage and Pasture Seeds
~ Cultural Crop Seeds
~ Other Crop Seeds
~ Other planting materials
Types of Seed Protection:
× Varietal Seed production: It is single parent multiplication where production is by open pollination. Here, production techniques are uniform, and can be continued for multiple generations.
× Hybrid Seed Production: It needs two to many parents where pollination is managed. The techniques differ from crop to crop. And it requires more production care.
Seed processing, handling and storage
Seed processing is important to remove undesired impurities from the seed, to maintain the moisture levels in seeds and to maintain seed quality. The seed processing has to make sure the seed loss to be minimum with complete removed of unwanted materials from the seed. The process should be efficient.
Seed processing grades the seeds according to size, length, weight, shape, surface texture, colour, electrical conductivity, liquid affinity accordingly to the type of seed, thus separating the seeds with its physical quality.
Different equipments in seed processing have been listed:
- Air Screen Cleaner
- Cleaner cum grader
- Disc Separator
- Indented Cylinder Separator
- Specific Gravity Separator
- Roll mill
- Magnetic Separator
- Colour separator
- Spiral Separator
Seed Storage are the duration when seed are stored in its fine condition between harvest period to plantation. Or, the seed can be stored for other extended period of time for different purposes such as research.
The types of seed storage include:
Storage for commercial truthfully, *labeled and certified seed, for *carry over seed, *foundation seed stocks and enforcement seed samples,and stotage for *germplasm seeds.
The seed longevity during can vary from seed to seed, with its previous quality before storage.
Seed testing principles
Seed testing includes test of seeds for research purposes and also to check if the storage techniques are functioning:
They include:
- Germination test
- Viability test
- Purity test
- Weed test
Seed certification procedures and seed standards of major crops in Nepal
Sees certification procedures and seed standards of major crops in Nepal has been explained in Seed Certification Directive 2074.
Importance of Varietal Replacement and Seed Replacement Rate
Seed replacement rate (SRR) is the ratio of area sown to the area of total crop planted in the season by using certified seeds expressed in percentage.
SRR: ( I * 100 ) / (A * K)
where,
I = improved seeds in kg
A = area under the crop a
K = seed rate per unit of area in kg/ha
Seed replacement rate gives the idea of quality seeds used by the farmers.
Seed self-sufficiency and seed production programs in Nepal
Different seed production programs in Nepal has been listed in Crop Development Directorate (cddnepal).
Important Resources:
* National Seed Vision 2013 - 2025, Government of Nepal, Ministry of Agricultural Development, National Seed Board, Seed Quality Control Centre, Hariharbhawan, Lalitpur, Nepal
* PDFs from Seed Quality Control Centre's [website]
* Principles of Seed Technology by agrimoon.com
* An overview of different seed production initiatives in Nepal, MN Paudel et al., Agronomy Journal of Nepal, Vol 3, 2013.
* wikipedia article: Seed
* Seed Production Technology Manual, Government of Nepal, MOAD (KUBK-ISFP), Program Management Office, Jankinagar-3, Rupandehi
No comments:
Post a Comment