seed Flashcards
Reproduction in Plants
Plants reproduce sexually by seeds and asexually by vegetative parts. Seeds are used for multiplication, while grains are for human or animal consumption.
Importance of Seeds
Good planting materials are crucial to profitable crop production, determining the quality and quantity of the produce.
Characteristics of Good Seed
A good seed must be genetically satisfactory, fully developed, free from contamination, deformities, diseases, and pests.
Seed Structure
A seed is a fertilized ripened ovule with three main parts: seed coat, endosperm (food storage organ), and embryo.
Seed Types
Seeds can be exalbuminous (no endosperm at maturity, e.g., beans) or albuminous (endospermic, e.g., maize).
Role of Seeds
Seeds are carriers of new technology, tools for food security, aids in crop yields, and essential for agricultural rehabilitation after disasters.
Characteristics of Quality Seed
Quality seeds should be pure, viable, free from disease, uniform in size, and have high germination rates and moisture levels between 8-12%.
Advantages of Good Seeds
Good seeds offer uniform germination, reduce replanting, ensure strong seedling growth, and maintain quality under storage.
Seed Treatment
Seed treatment involves applying chemicals, nutrients, hormones, or protectants to control diseases and pests. Methods include dry, wet, slurry, and pelleting treatments.
Seed Selection
Seed selection is critical to ensure high-quality, disease-resistant seeds that are free from weed contamination and provided by certified agencies.
Genetic Purity
Genetic purity is the percentage of contamination by other varieties, beginning with the seed planted and affected by prior crops, pollen transfer, and handling.
Isolation in Seed Production
Fields must be isolated by distance or flowering time to ensure pollination occurs only among plants of the desired variety and avoid contamination.
Classes of Seed
The four recognized classes of seeds are Breeder’s seed, Foundation seed, Registered seed, and Certified seed.
Breeder seed
Breeder seed is the seed directly controlled by the originating or the sponsoring breeder or institution which is the basic seed for recurring increase of foundation seed
Foundation seed
is the progeny of breeder seed
The seed stock is handled to maintain specific identity and genetic purity, which may be distributed and produced under careful supervision of an agricultural experiment station
This seed is the source of all other certified seed classes.
Registered Seed
Registered seed is the progeny of foundation seed, maintained for genetic purity and certified by a certifying agency with a minimum purity of 99%.
Certified Seed
Certified seed is the progeny of registered or foundation seed, grown under prescribed conditions, inspected for purity, and used for commercial planting.
Formal Seed System and informal seed system
A centrally planned, mechanized system with quality control, certified seeds, and large-scale marketing by national agencies and private companies.
informal seed system - A locally planned, unmechanized system using local resources, traditional processing, farmer exchanges, and community-level marketing.
SEED DORMANCY
**Seed dormancy is defined as a seed failing to germinate under environmental conditions optimal for germination, normally when the environment is at a suitable temperature with proper soil moisture
This true dormancy or innate dormancy is therefore caused by conditions within the seed that prevent germination
Thus dormancy is a state of the seed, not of the environment
Induced dormancy, enforced dormancy or seed quiescence occurs when a seed fails to germinate because the external environmental conditions are inappropriate for germination, mostly in response to conditions too cold or hot, or too dry
Seed dormancy has two main functions
the first is synchronizing germination with the optimal conditions for survival of the resulting seedling; the second is spreading germination of a batch of seeds over time so that a catastrophe after germination (e.g. late frosts, drought,hervivory) does not result in the death of all offspring of a plant
Often seed dormancy is divided into four major categories
exogenous; endogenous; combinational; and secondary
A more recent system distinguishes five classes of dormancy: morphological, physiological, morphophysiological, physical and combinational dormancy
Exogenous
Physical dormancyorhard seed coatsoccurs when seeds areimpermeable to water
At dormancy break, usually in response to environmental cues, especially temperature, water can enter the seed and germination can occur
Plant families where physical dormancy occurs includeCannaceae,FabaceaeandMalvaceae
Chemical dormancy: species that lack physical dormancy, but where a chemical prevents germination
This chemical can be leached out of the seed by rainwater or snow melt or be deactivated
Leaching of chemical inhibitors from the seed by rain water is often cited as an important cause of dormancy release in seeds of desert plants
Endogenous dormancy
Endogenous dormancy is caused by conditions within the embryo itself, including:
Morphological dormancywhere germination is prevented due to morphological characteristics of the embryo
In some species the embryo is just a mass of cells when seeds are dispersed, it is not differentiated. Before germination can take place both differentiation and growth of the embryo have to occur
In other species the embryo is differentiated but not fully grown (underdeveloped) at dispersal and embryo growth is required before germination can occur
Physiological dormancy
Physiological dormancymeans that the embryo, due to physiological causes, cannot generate enough power to break through the seed coat, endosperm or other covering structures
Abscisic acid is usually the growth inhibitor in seeds and its production can be affected by light
Drying : some plants including a number of grasses need a period of drying before they will germinate
Many herbaceous plants from temperate climate zones have physiological dormancy that disappears with drying of the seeds
Combinational dormancy
In seeds with combinational dormancy the seed or fruit coat is impermeable to water (physical) and the embryo has physiological dormancy
Depending on the species physical dormancy can be broken before or after physiological dormancy is broken
Breaking seed dormancy
Scarification
Hot water treatment
Acid treatment
Other chemicals
Scarification
Scarification
Mechanical scarification is a technique for overcoming the effect of an impermeable seedcoat
Mechanical scarification can be done by rubbing seeds between two pieces of sandpaper or using a file to rupture the seed coat
Seed may also be mixed with coarse sand and rubbed or shaken vigorously
Large seeds like those of lupin can be easily scarified
Hot water treatment
For small to medium-sized seeds, hot water treatment is more practical than scarification
For this treatment seeds should be dipped in 82-95oC pre-heated water (time depends on species)
Seeds should be left to cool, then soak in normal water for 12 to 24 hours, after which they are ready for sowing
With hot water treatments, the seeds should be sown promptly and not stored again
Acid treatment
Acid treatments are often used to break down especially thick impermeable seed coats, e.g. Sulfuric acid
The acid should be used at room temperature for a period of a few minutes to several hours depending on the species
The seeds should be immersed in acid in a glass, or earthenware container, and should be stirred occasionally
Seeds should be removed promptly and washed thoroughly in several changes of water to neutralize
Seeds can be sown immediately or stored
Other chemicals
Three chemicals that have proven very helpful in breaking certain types of dormancy are gibberellic acid (GA3), potassium nitrate, and thiourea
The aqueous solutions of these chemicals should be used at room temperature
The concentration and length of treatment depends on the species to be treated
