Seed Storage and Dormancy
Seed Types Based on Length of Storage
Classification of seeds based on their storage lifespan.
Factors Affecting Plant Seed Viability
The following factors significantly influence seed germination:
Seed Morphology: Structure of seed, including seed coat hardness.
Chemical Composition: Nutrient content of the seed, such as oils and starches.
Seed Maturity: State of development at the time of harvest.
Seed Handling Before Storage: Treatment and care prior to storage.
Moisture: Water content affecting seed metabolic activity.
Temperature: Environmental temperature during storage.
Atmosphere: Composition of air surrounding the seeds.
Seed Morphology
Hard Seed Coat:
Absorbs less water and oxygen.
Reduces metabolic activity.
Improves storage viability.
Protects embryo from mechanical injuries during processes like collection and extraction.
Chemical Composition
The longevity of seeds is affected by their chemical makeup:
Oily Seeds: Tend to have a shorter storage life compared to starchy seeds, though there are exceptions.
Studies suggest that some carbohydrates can improve desiccation tolerance in various seeds.
Seed Maturity
Impact of Maturity on Quality:
Seeds harvested before maturity have lower keeping quality.
Immature seeds lack sufficient food reserves, enzymes, growth regulators, and complete morphological features, hampering germination.
Seed Handling Before Storage
Risks Involved:
Cracks or breaks in seed coats can lead to pest infestation.
Bruised seeds exhibit lower quality and viability compared to healthy seeds.
Poor and delayed transportation to storage facilities post-extraction negatively impacts shelf life.
Moisture Content
Importance of Moisture:
One of the most crucial factors for seed longevity.
Influences the metabolic rate of seeds:
Water Activation: Activates enzymes and solubilizes stored food in seeds.
Hygroscopic Properties: Different components absorb moisture at varying rates:
Proteinaceous components absorb more moisture than carbohydrates, which in turn absorb more than lipids ( ext{Protein} > ext{Carbohydrate} > ext{Lipid}).
Consequently, proteinaceous seeds tend to absorb more moisture than oily seeds, making seed composition critical for storage strategies.
Temperature
Effect of Temperature on Metabolism:
Metabolic activity is temperature-dependent:
Low temperatures lead to reduced metabolic activity and thus extend storage life.
The choice of storage temperature should also consider the seed's moisture content.
Atmosphere Conditions
Oxygen Levels:
Low oxygen concentrations slow metabolic processes and enhance seed storage longevity.
However, controlling oxygen levels is challenging in commercial storage settings.
Utilizing vacuum desiccators for low-temperature seed storage can be effective.
Storage Recommendations for Seeds
Orthodox Seeds:
Recommended conditions:
Temperature: 0 to 5^ ext{°C} with 5 ext{%} to 10 ext{%} moisture for several years.
Temperature: -15 to -20^ ext{°C} with 5 ext{%} to 10 ext{%} moisture for longer-term storage.
Recalcitrant Seeds:
Conditions:
Temperature: 0 to 5^ ext{°C} at moisture content present at harvesting for a few years.
Some species may require temperatures up to 12 to 20^ ext{°C} with an estimated storage life of around 1 year.
Cryogenic Storage:
Ideal for long-term storage to preserve germplasm of orthodox seeds in liquid nitrogen.
Seed Dormancy
Definition of Seed Dormancy:
Seeds may fail to germinate under favorable conditions, representing a survival mechanism for certain species under adverse situations.
Certain plants can germinate right after extraction from fruits (e.g., mango), while others (e.g., apple, cherry) undergo dormancy.
Seeds may either germinate after a set time or require specific treatments to break dormancy.
Types of Seed Dormancy
Seed dormancy can be categorized into two main types:
Exogenous Dormancy: Caused by factors external to the seed embryo.
Endogenous Dormancy: Related to the embryo's development and internal processes.
Exogenous Dormancy
Definition: External factors prevent seed germination, such as the hard seed coat hindering water absorption or oxygen diffusion, impeding metabolic activity.
Types of Exogenous Seed Dormancy
Physical Dormancy:
Most common type characterized by a hard, fibrous, or mucilaginous seed coat restricting water and air diffusion, inhibits metabolic activities, delaying germination until natural erosion occurs.
Examples: Pecan, walnut, cherry, plum, apricot, peach which have hard endocarps.
Mechanical Dormancy:
Seed coats that are excessively strong hinder embryo expansion and radicle emergence. Softening the seed coat is required to break this dormancy.
Example: Olive, walnut.
Chemical Dormancy:
Seeds with inhibitory chemicals (primarily phenols and abscisic acid) naturally present in fruits or the seed coat that impede germination; leaching these through rain or snow can alleviate dormancy.
Example: Common in fleshy fruits like cucurbits and citrus.
Endogenous Dormancy
Definition: Linked to developmental aspects of the embryo itself.
Types of Endogenous Dormancy
Morphological Dormancy:
Occurs when the embryo is immature at seed dispersal, necessitating further growth before germination.
Example: Pawpaw.
Physiological Dormancy (PD):
Caused by physiological mechanisms that inhibit growth, further classified into:
a) Non-deep PD: Weak inhibition; dormancy can be reduced through brief chilling or dry storage.
Example: Grape, tomato.
b) Deep PD: Strong inhibition requiring longer chilling treatment; excised embryonic growth is not viable.
Example: Peach, certain cherry varieties.
c) Intermediate PD: Moderate chilling required to overcome dormancy; hormonal imbalances may exist. Embryos remain viable if excised and placed in growth media.
Example: Most temperate fruit seeds such as apple, pear, and plum.
Photo-Dormancy:
Seed germination dependent on light exposure, mediated by phytochrome; red light promotes and far-red inhibits germination. Mostly found in small-seeded plants like strawberry.
Thermo-Dormancy:
Some seeds require specific temperatures for germination, e.g., lettuce and celery do not germinate below 25^ ext{°C}.
Double Dormancy:
Presence of both physical and physiological dormancies restricts germination.
Example: Seeds with underdeveloped embryos and hard seed coats.
Secondary Dormancy:
Prevents germination of imbibed seeds until environmental conditions are suitable for growth.
Breaking Seed Dormancy
Identification of the type of dormancy is crucial for selecting appropriate methods to break it. The following methods can be employed:
a. Scarification: Breaking or chipping hard seed coats to facilitate water uptake. Can be performed mechanically or chemically, though caution is necessary as it may harm seeds.
b. Stratification:
Cold Stratification: Mimics winter, exposing imbibed seeds to cold temperatures (3^ ext{°C} to 10^ ext{°C}) to alleviate physiological dormancy. - Example: Plum.
Warm Stratification: Mimics summer conditions; multiple cold and warm cycles may be necessary for some species.
c. Dry After Ripening: Mimics dry seasons by placing seeds in intermediate humidity (approximately 65 ext{% RH}) and specific lithium chloride concentrations at warm temperatures (20^ ext{°C} to 30^ ext{°C}).
d. Chemical Treatment: Specific chemicals (e.g., GA, potassium nitrate, ethrel, hydrogen peroxide) can help alleviate physiological dormancy.
e. Light Treatment: Some small seeds, like lettuce and strawberry, require light to break dormancy; red light induces germination while far-red inhibits.
f. Leaching: Inhibitory compounds can be leached away to facilitate germination using water. - Example: Beetroot seeds.
g. Priming: Reducing light and temperature requirements to facilitate breaking dormancy through controlled hydration.