Everything you need to know about storing and preserving vegetable seeds for a long time. Learn about: 1. Vegetable Seed Storage Methods 2. Temperature Conditions 3. Disease and Insect Management 4. Packaging and Labeling.
The seed storage methods used are:
(i) Ordinary seed stores
(ii) Temporary shelters
(iii) Air cooled storage
(iv) Insulated seed storage
(v) Refrigerated storage
(vi) Dehumidification storage and
(vii) Storage in vapour proof containers.
(i) Ordinary Seed Stores:
This method is common in hills especially in the snow bound arid dry temperature area. It is always available near house, simple, cheap, easily constructed by the local mason, effective and safe for storing vegetable seeds.
(ii) Temporary Shelters:
These shelters are meant only for a season at peak harvesting. The graded or ungraded seed produce after proper drying are stored or stacked in such shelter till disposed-off. These are vegetable seed production areas and removed after use. These protect the seed produce from direct sun, rain and other weather vagaries.
(iii) Air Cooled Storage:
These are simply insulated structures above ground and partly underground which are cooled by circulation of colder air. Air cooled stores are cheap easy to install and to operate, which are still widely used for the storage of most vegetable seeds.
(iv) Insulated Seed Storage:
Such type of seed storage have insulated walls, floor and ceiling which helps in maintaining the desired temperature and humidity needed for storing a particular vegetable seeds. The inside temperature and RH remains or regulated constantly throughout the year during hot or cool weather. The initial cost may be high for its installation.
(v) Refrigerated Storage:
The use of refrigeration in controlling vegetable seed store temperatures is generally confined to long-term storage of high value seed material e.g. germplasm collections and breeders stocks. However, refrigeration is also useful in sub-tropical areas for other categories of vegetable seed stocks. Extra care and attention must be given to thermal insulation and structure of the store when refrigeration is to be included in the control systems.
It is an alternative system to refrigeration. For the dehumidification of seed stores a suitable chemical desiccants are used as a dehumidifier. Silica gel, which can absorb up to 40 per cent of its own dry weight of water, is usually used for seed store systems. In the bed system the silica gel is heat dried at about 175° C to drive-off all the absorbed moisture.
When the silica gel is again in moisture equilibrium with the air it is reheated to dry it before further reuse. Some bed systems use two beds per unit, in which one bed is dehumidifying the store’s atmosphere while the other is being dried to reactivate it. The operation of alteration beds is normally controlled by a time clock.
(vii) Storage in Vapour Proof Containers:
There have been major developments in recent years which have led to the storage of relatively small lots (e.g. 1, 2, 5, 10 kg) of vegetable seeds in separate sealed moisture proof containers. Most of the original research and development work with this technique has been done with seed of vegetable species because of the relatively small-lots required by individual farmers.
The principle is that seed-lots are dried to a moisture level slightly lower than they would be prior to normal open storage, and are then sealed in metal cans, packets or other suitable moisture proof containers. As a result of this packaging, each seed lot is in its own environment and may be stored at ambient temperature and RH for 1-2 years, or even longer, with little or no deleterious effect on germination.
Potential viability or longevity of vegetable seed in storage is the combined effect of temperature and relative humidity. Many areas have periods of fluctuating temperatures coupled with periods of high relative humidity, the combine effect of which leads to a fast seed deterioration in relatively short periods of uncontrolled storage.
In some temperate (arid) areas of the hilly regions the seed’s potential germination is not reduced significantly during short periods of uncontrolled storage. This is because the natural drying of seed is satisfactory following seed maturity in the field and there is low relative humidity during the subsequent storage period. Even relatively short-lived seed of species such as onion can be stored with little reduction in its germination capacity between seasons in many arid areas.
Storage temperature and seed moisture content are the most important factors affecting seed longevity. The general effect of temperature on longevity is that longevity increases as temperature decreases. This is true for “orthodox” seeds i.e. most seeds that follow some general “rules of thumb” regarding longevity during storage.
The relationship between temperature and seed longevity is that for each 5.6°C decrease in temperature, longevity doubles. This rule applies to seeds stored between temperatures of 32°F (0°C) and 122°F (50°C). This rule assumes that the moisture content is a constant. This is a general guideline; in reality the longevity of some vegetable species declines more rapidly than suggested by the rule, while the longevity of others declines more slowly in relation to storage temperature.
The longevity of seeds is generally not affected by subfreezing temperatures provided the moisture content is less than 14% (because ice crystals do not form). This has been established by a number of published reports on storing and germination testing of vegetables, flowers, and herbs seeds at 20°F (-7°C) and below.
Excellent germination have been reported in seed stored for twenty years, which was dried to approximately five percent moisture content. This is the ideal way to store seed, especially small seed that doesn’t require much freezer space. One caveat- seed cycled in and out of the freezer too many times without re-drying may cause degradation of germination.
Seeds of vegetable species stored in containers with calcium chloride at four temperatures and the percentage of germination determined after 11 months (Table – 7.1) showed that as temperature was increased, even in a dry atmosphere, the germination decreased.
Seed Moisture and Humidity:
The moisture percentage of seed in equilibrium with a specific relative humidity varies with crop species. Relative humidity reflects the amount of moisture actually in the air as a percentage of the amount of moisture that the air is capable of holding at the same temperature.
Thus, relative humidity, expressed as a percentage, can be determined as follows- the amount of moisture present in the air divided by the amount of moisture the air is capable of holding at the same temperature and multiplied by 100. Warm air hold more water than cool air. Thus, if the amount of water in the air is held constant and the temperature is increased, the relative humidity will decrease. Conversely, if the temperature of the air is lowered, the relative humidity will increase.
The equilibrium moisture content of several vegetable crops at different relative humidity is given in Table 7.2.
The sigmoid shape of a typical curve relating relative humidity to seed moisture content is shown in Fig. 7.1. The curve was derived from average moisture content and relative humidity of seeds of ten vegetable crops.
Seed moisture has a greater effect than temperature on seed longevity. Most seeds also follow some “rules of thumb” regarding moisture and longevity. The general relationship is that for each one percent increase in seed moisture, longevity decreases by half. This rule applies to seed with moisture content between 5 and 13%. Above 13% moisture content, seed storage fungi and increased heating due to respiration cause longevity to decline at a much faster rate.
Once seed moisture reaches 18 to 20%, increased respiration and the activity of microorganisms cause rapid deterioration of the seed. At 30% moisture content, most non-dormant seeds germinate. At the low end of the moisture range, seed stored at 4 to 5% moisture content is unaffected by seed storage fungi, but such seeds have a shorter longevity than seed stored at a slightly higher moisture content.
Relationship between Relative Humidity and Seed Moisture Content:
When storing commercially grown seed, it is impractical and too costly to use desiccant to dry the seed for storage, unless the seed is small and expensive. Commercial seed is usually packaged for short or long term storage under conditions of ambient humidity (unless special equipment is used). Because relative humidity has a significant effect on seed moisture content, it is important to understand the relationship between humidity and seed moisture.
Regardless of the type of storage conditions, the moisture content of seed eventually comes into equilibrium with the moisture in the surrounding air. For example, seeds of grains (which contain relatively high percentages of carbohydrate) will have a moisture content of 13 to 15% at 75% relative humidity, whereas seeds rich in oils (such as peanuts) can have a moisture content of 9 to 11% at the same humidity.
Note that once the relative humidity reaches 70%, the moisture content of the seed has reached approximately 13%, the point at which increased respiration and seed storage fungi become a significant problem. Above 70% relative humidity the moisture content rises dramatically.
Effect of Temperature and Moisture on Storage Life of Seed:
The effects of temperature, moisture and relative humidity are the factors which affect the longevity of stored seed. In reality, the effects of temperature and relative humidity are highly interdependent in their effect on stored seed.
There is a simple method for calculating the combined effects of relative humidity and temperature on seed longevity i.e. the sum of storage temperature (in degrees F), and relative humidity (in per cent) should not exceed 100. Since seed moisture is the most important concern, the rule stipulates that not more than half the sum should be contributed by the temperature.
The majority of crop seeds lose viability quickly when the humidity approaches 80% at temperatures of 77°F (25°C) to 86°F (30°C), but when stored at a relative humidity of 50% or less, and a temperature below 41°F (5°C), seeds will remain viable for at least ten years. If seeds are taken from a cold or frozen storage and transferred to room temperature, care must be taken to prevent condensation on the seeds.
If the seeds are in a sealed container, allow them to sit until they reach room temperature before opening the container. If they are stored in paper, place the seeds into a plastic bag with the excess air sucked out seal the bag and wait for the temperature to stabilize before unsealing.
Disease and Insect Management in Seed Storage:
In many parts of the world, testing for seed borne diseases and insects infestation is an integral part of the routine inspection for seed quality. However, in South Asia, pathological & entomological testing has not been as important as purity and germination testing.
However, these testing should be done as seeds can carry a wide variety of fungi, bacteria, viruses, nematodes and insects many of which can cause diseases in seedlings or plants. Some of them live on the seed surface and do not visibly affect the seeds’ appearance. They may become harmful only when environmental conditions favour their growth and’ reproduction.
Management of pests at proper time and stage of vegetable seed crop is very important. The pests can be managed by the use of safe chemicals, sanitation, management practices, growing resistant varieties, safe storage etc. Pest manipulation can be reduced or eliminated by proper drying of vegetable seeds to minimum moisture limit fixed by the ISTA. Insect – pests can be controlled by creating good storage facilities and preventing infestation by adequate use of insecticides, fungicides and fumigants.
The destruction of vegetable seeds by fungi or insects can be controlled by chemical treatments. Almost all the countries require phyto-sanitary certificates for imported seeds to insure that seed-borne pathogens are not introduced from another country. Thus, domestic as well as international forces are contributing to the recognition of seed- health testing as an important part of the seed testing process.
Fungi cause the largest number of plant diseases and occur more commonly in or on seeds than bacteria or viruses. More than 8000 species of fungi have been identified as plant pathogens. Fungi associated with seeds consist of both saprophytic and pathogenic fungi. Saprophytic fungi are not specific to any particular host and may be found on seed of various plants, whereas pathogenic fungi are usually confined to a limited host range.
Both types may occur on the seed surface, in cracks or inside the seed coat, but pathogenic fungi can also occur within the seed itself. While saprophytic fungi may cause problems in the seed testing laboratory by contaminating germination media, pathogenic fungi endanger crop productivity. Spores of saprophytic fungi occur on seed as well as air. They are especially numerous on seed in storage, and will germinate and grow profusely any time the storage environment exceeds 75% relative humidity and 15°C temperature.
Similarly, about 200 plant viruses are known to cause diseases of plants, of which 100 are fairly well known. An additional 500, though nonpathogenic, are thought to be transmissible plant viruses. Of all these, only about 80 different viruses or virus like organisms are considered to be seed transmitted. A few of these, such as Tobacco Mosaic Virus (TMV) on tomato, are carried on the surface of seed or inside the seed but outside the embryo. However, Bean Common Mosaic Virus (BCMV) carried inside the embryo.
Pre harvest control of seed- borne diseases may be accomplished by one of three different methods:
(1) Selection of disease free seed production area
(2) Cultural practices and
(3) Point of origin of infection.
In first case, seed is produced under environmental conditions that restrict the occurrence of diseases. Thus, dry edible bean and snap bean seed produced in dry, irrigated areas are more likely to be free of bacterial blight than humid areas.
Regardless of seed production area, cultural practices are crucial in the prevention and control of seed-borne diseases.
These include the following:
i. Planting disease-free seed.
ii. Treatment of seed with fungicides & antibiotics
iii. Spraying seed fields with fungicides, bactericides and other antibiotics to prevent disease buildup.
iv. Hand rouging of diseased plants
v. Avoiding overhead irrigation which might otherwise create conditions favouring disease build up.
vi. Use of resistant cultivars
vii. Crop rotation
viii. Isolation of seed fields from sources of potential infection
ix. Chemical and biological control of insect vectors.
The third method, pre-harvest control of seed-borne diseases where inspection of seed field is done so that potential problems may be detected and eliminated prior to harvest. Diseased areas may be destroyed or diverted from seed use, or the entire field can be diverted. While these precautions may not completely prevent contamination by surface-borne dusts, they do lower the probability of seed infection.
Postharvest control of seed- borne diseases should be considered only as a last resort, since it is better to prevent the occurrence of seed- borne diseases than to eradicate disease infection that is already present. However, several methods may help upgrade the phyto-sanitary quality of seed after harvest. These include – (1) surface disinfectant by chemical seed treatment (2) separation of diseased seed and foreign materials (3) hot water treatment, and (4) organic solvent infusion of antibiotics.
Treating seed with antibiotics is usually effective only against surface-borne pathogen, but in some cases systemic antibiotic can penetrate into the seed and eradicate internal infection. Sometimes penetration of antibiotics can be improved by organic solvent infusion. Separation measures are effective for eliminating seeds or foreign material in a seed lot that is disease infested.
Damage to vegetable seeds mostly occurs in storage and transit if not managed properly. However, damages may take place in the field also. When the seed crop is nearing maturity no apparent injury to seed is noticed until critically observed and the pest is found feeding inside the fruit e.g. cucurbits and garden pea.
This type of infestation is known as hidden one where it is very difficult to separate infested vegetables seeds from healthy ones and may pass them through the seed cleaning process. Insects can be controlled by creating good storage facilities and by using insecticides and fumigants properly.
The following preventive measures should be adopted:
i. Before arrival of new produce, all processing and storage structure should be thoroughly cleaned, white – washed and disinfected with sprays of insecticides e.g., malathion 50 EC (1 in 100 parts of water). Seed containers should also be treated.
ii. Seed should be cleaned and their moisture content should be reduced to below 9%.
iii. Processed seeds should be treated with malathion 5% dust @ 0.5g/kg seed.
iv. The previously used bags must be thoroughly cleaned and treated with either malathion sprays or fumigants like aluminium phosphide @ one tablet or 3 gm/m3 in an air tight space for 5 to 7 days for reuse.
v. Processed seeds should be stored and stacked properly and not alongwith unprocessed or carry over seeds.
vi. It is advisable to store different types of seeds separately. This helps in better management of insect pests.
vii. Seed should be inspected fortnightly. Seeds must be fumigated under air tight condition if any living insect or infestation is detected with aluminium phosphide 2-3 tablets of 3 g each/ton of seed.
viii. After fumigation, seed godowns should be aerated and thoroughly cleaned with brush or hard broom sticks to remove all dead insects. To prevent reinfestation, surface treatment with malathion 50 EC or fenitrothion 50 EC (1 part in 100 parts of water) @ 4 to 5 litre/m3 area or malathion dust 5% @ 3 to 4 kg/100m2 should be given.
ix. Surface treatment of seed godowns and processing shed should be carried out at 2 to 4 weeks interval, depending on the severity of pest occurrence. Alternate use of malathion and fenitrothion gives effective check on reinfestation and prevents insect resistance to insecticides.
x. Intense care should be taken to use the insecticide as they are highly toxic to human beings.
Packaging and Labeling of Vegetable Seeds:
Seed packaging or bagging is essentially the last and important operation in vegetable seed industry. A wide range of materials and types of containers are in use for seed packaging. Some small vegetable seed stocks are kept for relatively longer periods in controlled storage conditions before packaging in vapour proof containers.
Other vegetable seeds especially the relatively bulky species like leguminous vegetables are distributed by the seed companies without vapour proof containers. The main objectives of packaging are to facilitate handling in storage, transit and distribution. Other important objectives are maintaining seed viability, proper identification and protection of seeds from pathogens, insects, rodents and mechanical injury.
Modern packaging uses various materials and methods to sustain the original quality of seed during storage and transportation. Proper labelling of the package indicating all the details of seed quality standard on appropriate tag attached to the container or stamped directly on the container is compulsory prior to transportation and distribution of the seeds.
The choice of packaging materials and amount of seeds to be packed depends on kind of seeds to be packed, duration of storage, storage environment, seed moisture content, cost of seed, cost of packaging materials and the geographical area where the seed will be stored.
Packaging Materials and Containers for Seed Storage:
The seed packaging materials should be chosen not only for its cheapness, but also for capability to control the moisture of seed. The choice of packaging will vary depending on the temperature and humidity during storage and duration of the storage.
1. Materials for Short-Term Storage:
There are a wide variety of materials that can be used to store seed for short- term storage. Most of these are non-rigid materials such as cotton, paper, and composite materials such as multi-wall paper and plastic film, or polyethylene bags. Materials used for short-term storage are generally porous. They adequately contain and protect the seeds from mixing, but do not provide protection from moisture or loss of seed viability.
Such materials are usually used for mechanically separating seed lots, and for transporting and shipping seed until the seed can be placed in environmentally controlled conditions for longer-term storage. Each type of packaging material has its own advantages and disadvantages. Burlap bags have the greatest strength, can be re-used many times, and can be stacked high without slipping of the stack. The strength of cotton bags is dependent on the thickness of the weave and thread, and the quality of the seams.
Though not as strong as tear- resistant burlap, cotton bags can often be re-used, depending on the quality of the fabric. Bags made of woven plastic material also are fairly strong but tend to slip when piled high and are harder to close securely after the bag has been opened. Multi- wall bags are made of several layers of paper in a variety of types of construction. They have poor bursting strength when piled high or accidentally dropped, and if used repeatedly, they tend to become brittle along the folds and wear points.
Cardboard boxes and cans, though expensive, are re-usable, good for stacking, and provide some protection against mechanical injury to seed and to infestation by seed storage insects. Flexible packaging that has a weave, whether it is burlap, cotton, or plastic offers little protection against seed storage pests such as grain moths. Consequently, seed stored in such woven bags may have to be inspected if an outbreak is detected in a particular seed bag.
Seed storage insects are very good at locating small openings in bags or containers that are not well sealed. For small lots of seed, paper bags are inexpensive and adequate for storing seed, but the seams are not always reliable, and when used, the bags should be double or triple bagged to ensure integrity to prevent bursting.
Other materials such cellophane, acetate, and 2- to 4-mil polyethylene zip lock bags may be used (provided the seed has dried adequately). If using polyethylene, it is best to use the 4-mil thickness, especially for heavy seeds. Plastics and thin films are not reliable moisture barriers, though such materials offer better moisture protection than paper.
2. Materials for Long-Term Storage:
Metal and glass containers, properly sealed to prevent the exchange of moisture and gases, are the most commonly used containers. They are the only reliable means of protecting seeds against humidity, insects, rodents, floods, and mechanical damage. Plastic should not be used for long-term storage.
For storing large quantities of seed, metal five-gallon cans fitted with a rubber gasketed lid and pressure ring are ideal for storing large seeds such as peas and beans. One-gallon jars are also excellent, provided that the lid has a gasketed seal. Some one-gallon jars have plastic lids that are flexible enough to form a tight seal when the lid is screwed tightly, but metal lids need a rubber gasket.
Gaskets can be cut from sheets of neoprene rubber (available at hardware stores) or used automobile inner tubes. Though glass is breakable, it has the advantage of being transparent so that the contents can be easily inspected for insect damage (especially useful for detecting bean weevils, grain moths, and other seed pests). Metal and glass storage containers are available from a number of sources.
Another material, often used for seed banks, is a heat-sealable barrier pouch. Barrier pouches are a triple-laminate material made from paper, plastic and foil. They are sealed with a heat sealer or an iron set on “wool” setting. When deciding on the type of storage container, it is often helpful to settle on a minimum number of standard sizes. If using canning jars, it is best to choose a particular brand name and style so that same size and same-style containers have the same weight.
This is very helpful in inventory control of seeds being held for long-term storage. Standardization of container style and weight makes it easy to obtain the weight of stored seed without actually removing the seed from the container and exposing to humid air. Once the weight of standard containers is known, it is easy to subtract that weight from the container of seed to obtain the weight of the seed.
Containers for Shipping:
All seed must be thoroughly dried prior to shipment. Typically the minimum recommended time between harvest and shipment is six weeks. During the last three weeks of this period, the seed should be stored in a climate-controlled environment to allow the seed to cure. This means that seeds shipped in the late summer should be cured in an air-conditioned environment, and seeds shipped in winter should be stored in a heated environment prior to shipment.
Small lots of seeds can be packed in ordinary envelopes, provided that all edges of the envelope are sealed with tape, and this envelope placed inside a second envelope. The inner envelope must be sealed so that seeds do not come out of the envelope during shipment. Zip-lock bags are available in various sizes, but it is best to use 4 mm thick bags. Zip-lock bags are fine to use, but if the bag is to hold several Kgs of seed, especially heavy or sharp seed (for example, cucumber seed) it is best to double these bags.
The zipped edge should be folded over and taped or banded to help prevent accidental opening of the zipped edge. All air should be sucked out of the bags to avoid popping. The seeds should then be packed in a cardboard box or a padded mailer. Large seeds, such as peas, beans, and corn can be packed in grocery bags, but must be triple-bagged, and taped securely to prevent breakage of the bag during shipment.
The rule of thumb for shipping is that the container and contents should be able to survive a ten-foot drop without damage to the container or contents. This is about the approximate maximum distance a package might be thrown by a package handler. Remember, if bags break during shipment it may be impossible to separate one variety from another and the seed will be worthless. There should be enough padding inside the container so that there is no rattling or shifting of contents.
During the process of delivery, the packing material will become compacted. Newspaper tends to compact too much, so stiffer materials such as crumpled grocery bags are recommended. One of the most enlightening lessons of running a mail-order business is opening a package that has been shipped across country and back. Mark each seed container with owner name and the variety name on a slip of paper, both inside the container and on the outside of the seed container.
Mark on the outside of the shipping package in large letters “Perishable – Live Seeds Keep Cool and Dry.” Include shipping address and a packing list with the package. Address labels should always be covered with clear packing tape; otherwise the package conveyor belt may snag the edge of the label and rip it off the package. All seams need to be securely taped.
Commonly Used Packaging Materials:
Vegetable seeds are packaged into suitable attractive containers of specified net weight after processing and treating operations are done.
The packaging materials commonly in use are:
i. Conventional Packaging:
Jute bags, cloth bags, paper bags, multi paper bags etc.
ii. Moisture Resistant Packaging:
Polythene bags with less than 700 gauze thickness, jute bag laminated with thin (200-300 gauze) polythene film.
iii. Moisture Proof Containers:
Sealed steel bins, aluminum foils pouches, sealed polythene bags more than 700 gauze thickness, tin cans etc.
The choice of materials and size of containers depends on the development of the seed industry, quantum of seed, value of seed, packaging machines, the destination, type of market, mode of distribution, protection required from hazards such as rain, high relative humidity, rodents, insects, pathogens and amount of handling during transit. Preferably vegetable seeds should be packed in smaller units being costly to avoid risks of physical gradients, particularly vapour pressure, which arise in large bulks.
Packaging in smaller units makes identification, transportation, handling, marketing and distribution easier. Storage & packaging under normal conditions, the seed should be air dried to the minimum moisture (7-8%) content and stored in conventional packaging or in moisture resistant packages.
The relative humidity should also be low, but for valuable vegetable seeds which are to be kept for longer periods and also in wet areas, moisture proof containers are more safe. The initial moisture content in these should be low and must not exceed beyond a critical level, which varies for different vegetable seeds (table 13.1).
For instance cole crops seed with 7-8 per cent moisture content if sealed in moisture proof container, the enclosed packet/storage atmosphere will equilibrate at a relative humidity of approximately 55 per cent, which is too high for safe storage and some storage pathogen will become active and the seeds respiration rate will be relatively high. Majority of species grown as vegetable have seed with a safe moisture content during sealed storage/ package of between 8 and 9 per cent, but in some species it is less than 8 per cent.
During the growing season, through planting, harvest, and processing it is essential that all crops be properly labeled. This is especially important once the crop is harvested, as many seeds may look alike. Labels should be on every container and should travel through every step of processing and cleaning. While this may seem obvious, it is very easy to remember what was in a particular container.
When seed is harvested at different times from the same variety, it is also useful to put a date on the label. This can be important later on if there are questions about purity due to improper isolation, disease, or other issues. The detail information regarding seed materials is either printed on the containers/packages/bags alongwith companies or organizations insignia.
Information about planting times and location for each variety, lot number and source of seed planted, germination percentage or comments about germination and vigour, number of plants transplanted (useful for calculating seed yields later), maturity data, yield data, insect problems, organic fertilizers and amendments, and other data which may become useful later. Detailed records should be kept in a guard file or computer database regarding variety characteristics and performance.
Such data should include an accession number and information about seed source, lot number, year last grown, maturity data, plant characteristics (growth habit, yield, productivity, colour and shape of fruit), disease resistance or susceptibility, and taste comparisons with other varieties. Record keeping can be time consuming, but it is important to keep in mind and reuse the real value of information.