Seed Storage and Deterioration
Life Span of Seeds
Long-Lived Seeds
Short-Lived Seeds
Recalcitrant seeds
Orthodox seeds
can be successfully dried to moisture contents as low as 5% without injury
able to tolerate freezing temperatures
cannot be dried to moisture content below 30% without injury
unable to tolerate freezing temperatures
difficult to store because their high moisture content encourages microbial contamination and rapid deterioration
in subzero temperatures causes the formation of ice crystal, this disrupts cell membranes and causes freezing injury.
primarily from perennial trees in moist tropics
come from annual temperate species
mature and exist in their fruits and are covered with fleshy arriloid layers with an impermeable testa.
Physiological maturity: MC- 50-70%
Physiological maturity: MC- 30-50%
tend to be larger than orthodox seeds.
Intermediate seeds
ex. citrus, coffee, cacao
concepts of seed deterioration
Characterization of Seed Deterioration
Inexorable process
all living things must eventually deteriorate and die
possible to stall the rate of deterioration through optimum storage practices.
Irreversible process
once deterioration has begun, the anabolic process cannot be reversed.
low-quality seeds cannot be made into high-quality
Variation in populations
certain varieties will exhibit less deterioration than others
individual seeds also have differing storage potentials
Internal Factors
physical condition + physiological state of seeds influence their life span
broken or cracked seeds deteriorate faster.
can be physiologically impaired, this makes them susceptible to deterioration.
factors that can reduce longevity
deficiency of minerals (N,K, Ca)
Water
temperature extremes
immature seeds do not store well
hard-seeddedness extends seed longevity.
Relative Humidity and Temperature
Two of the most IMPORTANT factors
"Rules of Thumb"
each 1% reduction in seed moisture doubles the life of the seed
each 5C reduction in seed temperature doubles the life of the seed.
Seed Moisture
seed moisture does not apply above 14 or below 5% seed moisture
if stored above 14% begin to exhibit increased respiration.
leads to fungal invasion, heating, which can destroy the seed viability more rapidly than indicated by the first rule of thumb.
if stored below 5%, breakdown of membrane structure hastens seed deterioration.
consequence of reorientation of hydrophilic cell membranes due to the loss of water necessary to retain their configuration.
storage of most seeds between 5 and 6% seed moisture seems to be ideal for maximum longevity.
Moisture Equilibrium
attained when the seed has no further tendency to absorb or lose moisture.
PHASE ONE
very tighly held water that may actually be part of the chemical structure of the seed
this water cannot be removed without destruction of the seed tissue
includes some water held as discrete molecules in bonding interactions with the seed tissue molecules.
PHASE TWO
water that is loosely held compared to phase one.
water is easily removed by drying
lower portion representing strong bonding is difficult to remove
in the upper portion the water contributes significantly to seed deterioration during storage.
PHASE THREE
Easily elimnated water during drying
water is loosely held by weak bonding
if it is not eliminated it can contribute to rapid seed deterioration
Effect of Temperature
moisture equilibrium are slightly influenced by temperatures
increases in temperature cause slight reduction in moisture content at a fixed RH
The Hysteresis Phenomenon
desorption equilibrium curve is slightly higher than the adsorption curve.
desorption- the disappearance of these polar sites was delayed by their tendency to hold and keep the bound water that intervenes to block the collapse of seed tissue.
can only occur in absorptive substances with a high degree of structural rigidity.
seeds stored at low temperatures must be in conditions in which the relative humidity is controlled or place in moisture proof- containers to avoid increases in MC and increased deterioration.
Seed MC + Temperature
High RH: increase seed moisture content, results in biochemical events which increasse hydrolytic enzyme activity, enhance respiration and increases free fatty acids.
High Temp: enhance the rate at which many enzymatic and metabolic reactions occur.
Seed Moisture Content: the most critical factor in maintaining seed longevity
High MC + Hight Temp: quick deterioration
Low MC + High Temp: minimal deteriorative effect.
Low MC seeds store well at temperatures up to 25C
Highs MC seeds store well in temperatures reduced 10C or less than 25C
Genetic Factors
some seeds are better equip genetically and chemically for longer stability in storage than others.
long lived seeds tend to have thicker/impermeable seed coats
seeds possessing high oil content do not store well as those with low oil content
variation can happen as low as variety level in plants.
inheritance is a marked effect on seed longvity, can be a focal point in plant breeding
the environment strongly alters the genetic potential for seed longevity.
Presence of Microflora
field fungi
storage fungi
infects seed that are developing on the mother plant
typically requires high RH (90-95%) or high MC (30-35%)
rarely contributes to seed deterioration during storage
capacity to grow without free water
grow at seed MC in equilibrium and RH of 65-90%
Aspergilius and Penicillium- two main fungal genera
saprophytes and survive on dead tissues
invade the embryo in the seed
Mechanical Damage
delayed effects on longevity are much more troublesome
small mechanically damaged areas initially have little effect, but eventually leads to deterioration of vital embryonic tissues.
Direct injuries to embryonic tissues
much more detrimental to seed longevity
promotes invasion by storage fungi
can enter the seed through cracks in the testa
Seed Maturity
temp, moisture, variety, nutrient status all affect seed maturity, which affects seed storability
greatest storage potential happens in physiological maturity/max dry weight of the seed.
seeds with lack of N,K, Ca did not store well
seeds lacking in P did not exhibit less storage potential.
Predicting Seed Deterioration
low moisture, high quality seeds stored under cool conditions maintain seed quality better than high moisture.
Factors with seeds being stored and storage conditions.
cultivar and harvest viability
pre- post- harvest conditions
oxygen pressure effects during storage
fluctuating environmental conditions
Maintaining Seeds in Storage
Principle approaches for storing seeds
conditioned
cryogenic
hermetic
containerized storage
Conditioned Storage
careful control of temperature and RH
Type of Seed to be Stored
the differential between seed and grain prices
prevailing price of seed to be stored and its storage potential
weight per volume of seed (cost of construction)
extent of loss resulting from disposing the seed in alternative markets (selling grains)
projected seed inventory (volume)
Loss of Seed Weight During Conditioned Storage
high moisture seeds are placed in a conditioned storeroom at 40% RH, this will decrease 10% as moisture equilibrium is attained
seed will weigh less following conditioned storage as a result of water loss
conditioned storage will eventually result in higher density seed lots with more seed per unit volume
important economic consideration for seed producers
Requirement of Conditioned Storage
involves placing seeds in a dry and cool environment for extended periods
for longer storage, seed MC should be less than 11% with temperatures not increasing higher than 20C
temperate zones- seed MC 10-11% under normal warehouse conditions
Cryogenic Storage
Places seeds into liquid nitrogen at temps of -196C
advantage: seeds are place at a temperature where little detrimental physiological activity occurs. This prolongs storage life.
objective is to replace the water in seeds by infiltrating the protective compounds into the seeds without inducing subsequent seed injury
Hermetic Storage
packaging seeds in moisture-resistant or hermetically sealed containers for storage
to maintain seeds at safe storage moisture levels
completely moisture-proof packages hermetically seal the seed and are effect or long-term seed storage up to 10 years.
moisture content of seeds place in storage must be lower than 2-3% or deterioration will occur in sealed storage
starchy seeds above 12%
oily seeds above 9%
pro- removing ambient air from the seeds and replace it with specific gases known to prolong seed life.
Containerized Seed Storage
absolute control in seed storage
humidity can be closely regulated in a closed container by use of salts or acidic solutions.
Sulfuric acid (H2SO4) is a common acid desiccant, when diluted with water.
common seed desiccant- silica gel
Advantages
construction of conditioned storage facilities is not necessary
maintenance and operation costs are minimal
metal storage boxes are insect, rodent, and moisture proof
seeds are not damaged by storage fungi since they are maintained at 45% RH
Symptoms of Seed Deterioration
difficulties in evaluations
focusing on whole seed germination- not all seeds deteriorate uniformly.
Seed lots are composed of individual seeds, possessing their own unique capabilities to perform in the field.
observations made regarding seed deterioration
proportion of high-quality seeds within a population decreases with increasing storage time
curves shift from high to low seed quality and range becomes wider with increased storage time.
total population studies of seed deterioration do not represent what is occurring at the individual seed level
Seed Symptoms
testa color (morphological)
darkening of the seed coat in deteriorating clover, peanuts and soybeans is an early indicator.
oxiddative reaction which can be accelerated under conditions of hgh temperatures and RH
some seeds can develop necrotic lesions in the cotyledons
Ultrastructural Changes
using an electron microscopy
Cell Membranes
inability to retain cellular constituents which leak out during imbibition
important seed quality implications
many cellular constituents arer essential for normal, vigorous germination
exude compounds that are necessary for maintenance of internal osmotic potential which is responsible for normal water uptake. This provides turgor pressure requires for radicle protrusion
external leakage encourages the growth of pathogenic microflora
membrane leakage have been attributed to the loss of phospholipids
decline in phospholipids under accelerated aging conditions could increase the rate of seed deterioration.
Loss of Enzyme Activity
measuring incipient seed deterioration include activity of enzymes associated with breakdown of food reserves or biosynthesis of new tissues during germination.
Reduced Respirations
composite expression of activity of a large group of enzymes that react together in breaking down food reserves.
respiration becomes progressively weaker as seeds deteriorate.
reduction in the rate of respiration are closely associated with seed deterioration.
Increased Seed Leachates
increased seed leachate content when soaked in water is a frequently observed symptom of deteriorating seeds.
reflection of membrane degradation.
Increase in Fatty Acid Content
hydrolysis of phospholipids leads to the release of glycerol and fatty acids. This reaction accelerates with increasing seed moisture content
Performance Systems
delayed seedling emergence is among the first noticeable symptoms.
slower rate of growing
decreased germination
decreased resistance to environmental stresses during seed germination.
ultimate performance symptom is the complete loss of germinability and death of the seed.
Causes of Seed Deterioration
Lipid Peroxidation
damages the cell membranes
generates free radicals
Minimize Lipid Peroxidation
FR attack compounds like fatty acids
changes protein structure
damage the cytochrome
assault chromosomal DNA
Lipid modification- breeders have focused on changing the proportion of saturated/unsaturated fatty acids. Changing to saturated fatty acids bc they are less prone to lipid peroxidation.
Regulation of Oxygen Pressure- reducing the quantity of oxygen surrounding the seed can decrease the initiation of free radicals
Antioxidant Treatments- antioxidants are free radical scavengers, one antioxidant can protect a bunch of fatty acid molecules.
Hydration/dehydration treatments- also known as priming, doesn't extend seed storage but leads to improving seedling performance during germination.
Degradation of Functional Structures
loss of selective permeability (in cellular membranes)
cytoplasmic metabolites leach into the intercellular spaces.
membrane degradation occurs from both hydrolysis and phospholipids by phospholipase and autoxidation
Mitochondrial degradation and functional changes play a major role in seed deterioration
functional changes in mitochondria can be repaired by unsaturated fatty acids.
certain growth regulators contribute to the protection of mitochondria and other cytoplasmic membranes
Inability of Ribosomes to Dissociate
degradation of function in deteriorating seeds is the dissociation of ribosomes
dissociation of polyribosomes must occur before attachment of preformed mRNA, leading to the protein synthesis in germinating seedlings
Enzyme Degradation and Inactivation
general decrease in enzyme activity in seed lowers its respiratory potential
lowers the energy (ATP)
lowers the food supply
configurational changes
partial folding or unfolding of ultrastructure
condensation to form polymers
degradation to subunits
Breakdown in Mechanisms for Triggering germination
breaking down GA can cause seed deterioration
CK included as well
Genetic Degradation
seed deterioration is associated with random somatic mutations that impair cellular functions of vital seed tissues.
- extracts from aged seeds retard the germination of fresh seeds
- mutations in seeds are highly correlated with seed age and decline in viability
- spontaneous mutations arise in aging seeds and become evident by chromosome breakage in the presplit phase
- differences in the shoot and root tips in aged seed closely parallel those in irradiated seeds
Starvation of Meristematic Cells
noted that in respiration may deplete the tissues involved in the transfer of nutrition from reserve storage areas. This prevents them from reaching the embryo
meristematic cells exhaust their energy supply with no way of converting ADP to ATP
Accumulation of Toxic Compounds
low-moisture storage, the reduced respiration and enzyme activity may be responsible for the accumulation of toxic substances that reduce seed viability.