SEED STORAGE
The seeds are considered to be in storage from the moment they reach at physiological maturity until they germinate, or they are thrown away because they are dead or otherwise worthless.
Purpose of seed storage:
As, seeds are uniquely equipped to survive as a viable regenerative organisms until the time and place are right for the beginning of a new generation, so, the purpose of seed storage include-
to preserve planting stocks from one season to the next
to maintain seed quality for the longest duration possible
to maintain germplasm over time for improved plant breeding program
to supply seed in emergency case of seed or seedling damage caused due to natural or any other hazards.
Factors influencing the life span of seed:
A)Relative Humidity and Temperature:
The two most important factors that influence the life span of seeds are relative humidity and temperature. The effects of relative humidity (and its subsequent effect on seed moisture) and temperature of the storage environment are highly interdependent.
Seed moisture:
Seed moisture contents means that water is held by the seed. Generally, there three types of water in seed.
►Bound water is tightly held to ionic groups such as amino or carboxyl groups and exists as a monolayer around macromolecules of the seed.
►Adsorbed water is considered to exist in multilayers, loosely held by bonding to hydroxyl and amide groups above the monolayer of bound water.
►Free water is considered as capillary or solution water held only by capillary forces to the seed tissues.
Moisture Equilibrium:
Seeds are highly hygroscopic living material and their moisture content depend on temperature and relative humidity of surrounding air. The hygroscopic nature of seeds allows them to maintain equilibrium moisture content with any given relative humidity at constant temperature. Equilibrium is attained when the seed has no further tendency to absorb or loss moisture. Hygroscopic equilibrium curves also called absorption isotherms are graphical expressions of the relationship between the moisture content of seeds and their ambient relative humidity. It is a sigmoid like curve, with three distinct phases representing different stages of water absorption or adsorption. Phase one represents very tightly held water that may actually be a part of the chemical structure of the seed. This kind of water cannot be removed without destruction of the seed tissues. This phase may also include some water held as discrete molecules in bonding interactions with the seed tissue molecules. Phase two represents water that is more loosely held than that of phase one. For most seeds, this portion of the moisture equilibrium is represented by a straight –line relationship between relative humidity and moisture content. Water represented by the upper portion of the phase two is easily removed by drying; however, the lower portion representing strong bonding is difficult to remove. Water in phase three represents water loosely held by very weak bonding and free water in the intercellular and intertissue space. It is easily eliminated during drying; but if not eliminated, contributes rapid seed deterioration. This curve can be used to predict seed moisture content at a given relative humidity.
The Hyteresis Phenomenon:
The desorption equilibrium curve will usually be slightly higher than the adsorption curve. The difference in the equilibrium moisture content during adsorption and desorption is known as hysteresis .The higher desorption isotherm has attributed to the appearance of additional point of attachment (polar sites) for bound water. On desorption, the disappearance of these polar sites was delayed by their tendency to hold and keep the bound water. On the other hand the lower adsorption isotherm has attributed to the absence of sufficient point of attachment (polar sites) for bound water.
Besides from the influences of temperature and relative humidity of surrounding air on seed moisture content, the adsorption or desorption of water by the seed depends on the chemical composition. The hydrophilic nature of starch, protein and lipid is not same. Protein adsorbs more water than both starch and lipid while starch precedes the lipid. So, the the moisture equivalent of seeds of different cultivars is different at same relative humidity (RH) and temperature.
| Crop Species | % Relative Humidity | |||||||||
| 30 | 45 | 60 | 75 | 90 | 100 | |||||
| Seed moisture content (%) | ||||||||||
| Rice(milled) | 6.8 | 10.7 | 12.6 | 14.4 | 18.1 | 23.6 | ||||
| Wheat(white) | 8.5 | 10.1 | 11.8 | 14.8 | 19.7 | 25.8 | ||||
| Soybean | 6.5 | 7.4 | 9.3 | 13.1 | 18.8 | --- | ||||
| Peanut | 4.2 | 5.6 | 7.2 | 9.8 | 13.0 | --- | ||||
| Mustard | 4.6 | 6.3 | 7.8 | 9.4 | --- | --- | ||||
Thum Rule for seed moisture:
Between 5-14% seed moisture, each 1% reduction in seed moisture doubles the life of seed.
Temperature:
Temperature indirectly influences the relative humidity and consequently affects the seed moisture content. For every 20o F increase in temperature, the seed moisture content reduces by 1%.Again with the increase of temperature the rate of metabolic activities of seed increases and consequently the seed deteriorate rapidly.
Thum Rule for Temperature:
Between 0-5o C temperatures, each 5o C reduction in temperature doubles the life of seed.
Because of the interdependency of temperature and relative humidity Harrington(1973) proposed that the sum of the percentage of relative humidity plus the temperature in degrees Farenheit should not exceed 100 for safe storage.
B) Mechanical damage:
Seeds that have broken, cracked, or even bruised deteriorate more rapidly than undamaged seeds. The immediate effect of mechanical damage on seed quality is generally not serious; the delayed effects of such damage on seed longevity are much more troublesome and much greater of economic significance. Direct injuries on the embryonic tissues are much more detrimental to seed longevity than are large injuries to the nonembryonic tissues.
C) Presence of pests:
There are five types of pest found to attract seed in storage.
Bacteria-grows only above 90%RH of air
Fungi-growth is reduced below 75%RH of air and stopped below 62%RH
Mites- growth is stopped below 60%RH.
Insect-can infest at above 10% seed moisture and survive 8-80o C temperature.
Rodents-can cause damage at wide range of temperature and RH
D) Genetical fector:
Genetically some seeds possess higher storability than others.
E) Seed Maturity:
The greatest storage potential is attained at the time of physiological maturity. Immature seeds have been shown to be inferior to mature seeds in viability and vigor.
F) Seed structure and composition:
Seed structure and composition influence the storability of seed as they affect water adsorption or desorption.
G) Hardness of seed coat:
Seeds having hard seed coat possess higher storability than that of normal ones.
H) Seed Dormancy:
Seeds having higher dormancy possess higher storability than that of normal ones.
I) Seed Vigor:
Seeds having higher vigor possess higher storability than that of low vigor ones.