TAMIL NADU AGRICULTURAL UNIVERSTIY COIMBATORE – 641003.

Role of Moisture, Physical Environment – Relative

Humidity, Temperature on Seed Storage.

By

S.SATHISH M.Sc (Agri)

Department of Seed Science and Technology

ROLE OF MOISTURE – PHYSICAL ENVIRONMENT – RELATIVE

HUMIDITY, TEMPERATURE ON SEED STORAGE

Introduction:

Nowadays, whether it’s stored in a clay pot, a hole in the ground, or a highly refined

and controlled storage unit, successful seed storage depends primarily on the percentage of

relative humidity (RH) and temperature in the storage facility and also the seed moisture

content. Picture given below depicts the various factors affecting seed storability and its

relation to seed moisture content.

Among these factors this paper elucidates about the role of seed moisture content,

temperature and relative humidity on seed storability since successful seed storage depends

on these factors.

Moisture content:

The amount of moisture in the seeds is probably the most important factor influencing

seed viability during storage. Over most of the moisture range, the rate of deterioration

increases as the moisture content increases.

Seed moisture content (%) Storage life

11 to 13 ½ year

10 to 12 1 year

9 to 11 2 years

8 to 10 4 years Moisture content and storage at 30° to 32° C of Cereal seeds of high germination and high vigour.

(Harington and Douglas, 1970).

Further, if the seeds are kept at higher moisture contents than mentioned above. The

losses could be very rapid due to mold growth on and in the seed (12 – 14% moisture

content) or due to heating (18 – 20% moisture content). Moreover, with in the normal range,

biological activity of seeds, insects and molds further increases as the temperature increases.

The higher the moisture content of the seeds, the more they are adversely affected by both

upper and lower ranges of temperature.

35-80% Moisture content of developing seed, seed not mature enough for harvest.

18-40%

Seed physiologically mature; respiratory rate high; seed susceptible to

field deterioration; heating occurs if seed bulked without adequate

ventilation; molds and insects very active; seed susceptible to mechanical

damage in harvesting and handling.

13-18% Respiratory rate still high; can get heating at higher levels; molds and

insects can be damaging; seed resistant to mechanical damage.

10-13%

Seed store reasonable well for 6 to 18 months in open storage in

temperate climates; insects can still be a problem in susceptible seed;

seed susceptible to mechanical damage.

8-10% Seed sufficiently dry for 1 to 3 years open storage in temperate climates;

very little insect activity; seed very susceptible to mechanical damage.

4-8% Safe moisture content for sealed storage.

0-4% Extreme desiccation can be damaging to seed; hardness develops in some

kinds of seed.

33-60% Seed germinate when they imbibe water to these levels.

It is important to note that very low moisture content (below 4%) may also damage

seeds due to extreme desiccation or cause hard seededness in some kinds.

Since the life of a seed and its span largely revolves around its moisture content, it is

necessary to dry seeds to safe moisture contents. The safe moisture content, however,

depends upon storage length, type of storage structure, kind/variety of seed, type of

packaging material used. For cereals in ordinary storage conditions for twelve to eighteen

months, seed drying up to 10% moisture content appears quite satisfactory. However, for

storage in sealed containers, drying up to 5 to 8% moisture content, depending upon the

particular kind, may be necessary.

The range of water contents that gave maximum survival varied between species from

7.6-9.7% for wheat to 1.8-2.5% for sesame. It is concluded that there is an optimum water

content for seed storage and, for seeds with high oil contents, the value of the optimum is less

than the benchmark 5% water content recommended for seed storage by the International

Plant Genetic Resources Institute.

Muhammad mumtaz khan and ken Thompson reported that grapefruit (Citrus paradisi

Macf.) and rough lemon (Citrus limon L.) seeds can be dried up to 7% moisture content

without any significant damage and their storage behaviour may be orthodox. Kinnow

mandarin (C. reticulata Blanco) seeds were intolerant of desiccation and showed recalcitrant

storage behaviour.

Relative Humidity and Temperature during storage

Relative humidity and temperature are the most important factors determining the

storage life of seeds. Seed attain rather specific and characteristic moisture content when

subjected to given levels of atmospheric humidity. This characteristics moisture content is

referred to as equilibrium moisture content. Equilibrium moisture content, for a particular

kind of seed at a given relative humidity, tends to increase as temperature decreases and as

deterioration progresses.

Equilibrium moisture content varies among seed kinds. In general, the equilibrium

moisture content of “oily” seed is lower than that of “starchy” seed at the same relatively

humidity and temperature. This phenomenon can be accounted for by the fact that fats and

oils do not mix with water. Thus, in a seed with 50% oil content, the moisture has to be

concentrated in half the seed, while in a seed containing 10% oil, the moisture is distributed

throughout 90% of the seed.

Crop Relative Humidity (%)

15 30 45 60 75 90 100

Barley 6.0 8.4 10.0 12.1 14.4 19.5 26.8

Maize 6.5 8.5 9.8 12.2 13.6 18.3 23.0

Oats 5.7 8.0 9.6 11.8 13.8 19.5 24.1

Rice 6.8 8.6 10.7 12.6 14.4 18.4 23.6

Rye 7.6 8.7 10.5 12.2 14.8 20.6 26.7

Sorghum 6.4 8.6 10.5 12.0 15.2 18.8 21.9

Wheat (aestivum)

(durum)

6.7

6.6

8.6

8.5

9.9

10.0

11.8

11.5

15.0

14.1

19.7

19.3

26.3

26.6

Groundnut 2.5 4.2 5.6 7.2 9.8 13.0 -

Soybean 4.3 6.5 7.4 9.3 13.1 18.0 -

Sunflower - 5.1 6.5 8.0 10.0 15.0 -

Cotton - 6.0 7.5 9.1 11.5 18.0 -

Flax - 5.6 6.3 7.9 10.0 15.2 21.4

Brinjal - 6.3 8.0 9.8 11.9 - -

Cucumber - 5.6 7.1 8.4 10.1 - -

Cucurbits - 5.6 7.4 9.0 10.8 - -

Lettuce - 5.1 5.9 7.1 9.6 - -

Lima bean - 7.7 9.2 11.0 13.8 - -

Radish - 5.1 6.8 8.3 10.2 - -

Tomato - 6.3 7.8 9.2 11.1 - -

Turnip - 5.1 6.3 7.4 9.0 - -

Watermelon - 5.1 6.3 7.4 9.0 - -

Seed Moisture Equilibrium at Various Levels of RH at 25°C (Roberts, 1972)

Thus the maintenance of seed moisture content during storage is a function of relative

humidity and to a lesser extent of temperature. At equilibrium moisture content, there is no

net gain or loss in seed moisture content. Seed placed in an environment with a relative

humidity higher or lower than with which its moisture content is in equilibrium, will gain or

loss moisture until an equilibrium is established with the new environment. In sealed storage,

seed moisture content determines the relative humidity of the environment in the containers.

Crop Relative humidity (%)

30 45 60 75 90

Wheat 20-25 25-35 45-60 55-65 45-55

Maize 15-20 15-20 50-60 45-65 45-55

Barley 15-20 20-25 25-30 25-30 55-60

Bajra 15-20 20-25 25-30 25-30 20-25

Period (Days) required to attain EMC in different Relative Humidity.

Establishment of moisture equilibrium in seeds is a time dependent process. It does

not occur instantaneously. A period of time is required, the length of which varies with the

seed kind, initial moisture content, the average relative humidity and the temperature. Under

open storage conditions, seed moisture content, fluctuates with changes in relative humidity.

However, normal diurnal fluctuations in relative humidity have little effect on moisture

content.

Jaap Ooms

Graph provided above depict the moisture uptake by the seeds of leek, tomato,

pepper, cauliflower at different relative humidity ranging from 40% to 70% and it is clear

that moisture absorption by seeds increases with increase in relative humidity of the storage

environment. Thus it shows the linear pattern of moisture absorption over relative humidity.

Temperature also plays an important role in life of seed, although it does to be a

controlling one. Within the normal range of biological activity of seeds, insects and moulds

increase as temperature increases. The higher the moisture content of the seeds, the more

they are adversely affected by temperature. Decreasing temperature and seed moisture,

therefore, is an effective means of maintaining seed quality in storage.

Low temperatures are very effective in maintaining seed quality, even though relative

humidity might be quite high. Good cold storage for seeds should not exceed sixty percent in

relative humidity. However in hot pepper or chilly (Capsicum annuum L.) browning of seeds

were identified in the seeds stored at low temperature (5°C).

Harrington’s rule:

The following simple rules put forth by Harrington are a useful guide as well as

measure of the effect of moisture content, temperature and relative humidity on seed ageing.

1) A one per cent decrease in moisture content nearly doubles storage potential of seed.

2) A 10° F decrease in temperature nearly doubles storage potential of seed.

3) Good seed storage is achieved when the percentage of relative humidity in storage

environment and the storage temperature in degrees Fahrenheit add up to one

hundred.

These simple rules are reasonably accurate, particularly in the middle ranges of seed

moisture content and temperature. For effective sealed storage two to three per cent lower

moisture content, as compared to open storage, is required.

Effects of fluctuating environmental conditions on viability:

There have been a few reports to the effect that fluctuating conditions are harmful.

However, at present there is not a priori reason to suppose that change in temperature, or

moisture content, would in itself be deleterious save, possibly, for very rapid changes in seed

moisture content.

Special effect of extreme storage conditions on viability:

There sets of storage conditions such as

i. Temperature and moisture content, say about 30% in cereals, provided the

temperature is suitable, germination will take place and thus the seeds will be lost.

ii. If temperature is sufficiently low a special type of damage, freezing injury, will result

in loss of viability when seeds are very moist.

iii. If seeds are subjected to extreme desiccation, the period of viability may be less than

expected.

Gaseous environment

Increase in pressure of oxygen tends to decrease the period of viability. The little

work carried out on the use of antioxidants shows that heat injury to kidney bean embryos

was decreased in reduced oxygen pressures, and that the application of cysteine overcame the

injury to some extent. Onion and okra seeds treated with either starch phosphate or

alphatocopherol suggest that starch phosphate is very effective in prolonging the viability of

both spp., and alphatocopherol had some beneficial effect on onion seeds.

General prescriptions for seed storage:

The general prescription for seed storage is a dry and cool environment. At this point,

the question naturally arises: How dry and how cool? It is difficult to answer this question

unless three factors are known:

1) kinds of seed to be stored

2) desired period of storage and

3) Physiological quality of the seed.

Seed of most grain crops, e.g., corn, wheat, sorghum, barley, rye, oats, rice will

maintain germination for 8-9 months period from harvest to planting, at a moisture content

12-13% Seed of most grain crops, e.g., corn, wheat, sorghum, barley, rye, oats, rice will

maintain germination for 8-9 months period from harvest to planting, at a moisture content

12-13% under normal warehouse temperature except possibly in southern coastal areas. For

maintenance of vigour as well as germination, moisture content should not exceed 12%

(relative humidity below 60%) and temperature in the warehouse should not exceed 65° F. In

the case of carry-over seed, which means a storage period of 20-21 months, the moisture

content of seed of grain crops should be less than 11% and temperature should be exceed

65°F. Since the period of carry-over storage encompasses at least one summer period,

temperatures and humidity control during the period is most important.

Cotton seed stores well, as seed of grain crops, at the conditions mentioned above but

soybeans and peanut seed are poor storers. For one year’s storage (actually 8-9 months),

moisture content should be 11 to 12% and the warehouse temperature should not exceed as

65° F. shelled peanuts may have to be stored in a cold room. Carry-over storage should not

be attempted unless conditioned storage facilities are available: 65° F and 50% relative

humidity.

Seed of most forage grass and legume crops will store well for one year at moisture

content of 10-11% at normal warehouse temperatures. When “carried over”, moisture content

should be about 10% and temperature should not exceed 65%.

Vegetable seed vary considerably among kinds in their storage requirements.

Generally, however, most kinds will store well for one year at a moisture content of 9-11%

and a temperature that does not exceed 65° F. when a storage period longer than 19-21

months is required, conditioned storage is essential for all kinds of seed. Most kinds of seed

will maintain quality for 2-3 years when stored at 60° F and 50-55% relative humidity. For

storage longer than 3 years, conditions should be 50° F and 50% relative humidity.

Management for successful storage:

Control of temperature

Control of seed moisture

Control of temperature

Temperature is one of the most important environmental factors which influence seed

viability and vigour during storage, the lower the temperature; the longer the seeds maintain

germination capacity. Thus, temperature control is an important consideration in building a

seed storage.

Temperature control may be achieved in one of the following ways:

Ventilation

Insulation

Refrigeration

These methods are not mutually exclusive, and are normally used to supplement each other.

Ventilation

Ventilation could be used to reduce seed temperature and seed moisture content, if

used judiciously. In addition, it also helps prevent hot spots from developing; the formation

of convection air currents and maintenance of uniform seed moisture content and

temperature.

Time of ventilation: Whenever the outside temperature of air and relative humidity are low

enough to benefit the seeds, either by reducing seed temperature or seed moisture content, the

ventilating fans (exhaust fans) can be turned on.

Storage

condition

Outside temperature °F

95° 90° 85° 80° 75° 70° 65° 60° 55° 50° 45°

Temp

(°F)

RH

(%)

Ventilate at these temperatures when the relative humidity % is

less than the figure given

100 90 100 100 100 100 100 100 100 100 100 100

80 93 100 100 100 100 100 100 100 100 100 100

70 81 95 100 100 100 100 100 100 100 100 100

60 69 80 94 100 100 100 100 100 100 100 100

50 58 67 79 92 100 100 100 100 100 100 100

40 47 55 63 75 89 100 100 100 100 100 100

90 90 - 90 100 100 100 100 100 100 100 100 100

80 - 80 94 100 100 100 100 100 100 100 100

70 - 70 82 96 100 100 100 100 100 100 100

60 - 60 70 83 98 100 100 100 100 100 100

50 - 50 58 69 81 95 100 100 100 100 100

40 - 40 47 54 65 77 90 100 100 100 100

80 90 - - - 90 100 100 100 100 100 100 100

80 - - - 80 94 100 100 100 100 100 100

70 - - - 70 82 98 100 100 100 100 100

60 - - - 60 70 83 100 100 100 100 100

50 - - - 50 59 70 82 99 100 100 100

70 90 - - - - - 90 100 100 100 100 100

80 - - - - - 80 96 100 100 100 100

70 - - - - - 70 82 99 100 100 100

60 - - - - - 60 72 85 100 100 100

50 - - - - - 50 59 70 83 100 100

To use this table, first determine the temperature and relative humidity of air inside

and outside the storage. The temperature is measured with thermometers and the relative

humidity with a psychometer. After determining these values it could readily be determined

from the table when it is safe to ventilate storage in order to cool it without increasing seed

moisture.

Precautions to be taken ventilation

The moisture content of the seed should not be allowed to increase to a value in

equilibrium with air relative humidity above 65%.

The seed temperature should not increase above 33°C for more than few hours.

Insulation

Insulation of seed storage is done to reduce the flow of heat from the warmer exterior,

through the walls, roof and floor of the storage, to the cooler air and seeds in storage. Heat

flow depends upon:

Temperature difference between the two places in the material. Heat flow is twice as

fast with a 20° temperature difference as with a 10° difference.

Distance the heat must flow. Heat flows twice as fast through one inch of insulation

as through two inches of the same material.

Air is the best insulator. However, it has serious defect in that if the air space is more than

a fraction of an inch, a convection current occurs moving the heat from one surface to

another. Therefore, the air, to be a useful insulator, must be so trapped that it cannot

move.

Choice of insulation material

Low thermal conductivity

Durability

Cost and availability

Ensure that moisture will not be trapped

It should not settle

Resistance to rats, insects and molds

Easy to install, to attach and fit

Low combustibility

Dimensional stability

Low water absorptivity (will itself absorb little water)

Mechanical strength

Low weight per cubic feet

No material used for insulation has all these advantages. The natural materials are

cheap and easily available, but they settle leaving upper areas uninsualted. They harbour

insects and rats, and once the natural insulation is infested, it is difficult to rid it of these

pests. Moreover, they absorb moisture readily and may mold and rot. Neither are they fire-

proof, nor do they have any mechanical strength. Thus, although they are cheap, they have

some severe restrictions.

Foam polystyrene has the advantage of being light in weight. Because the air bubbles

are enclosed, no convection currents are possible. It has considerable resistance to moisture

penetration. It is easy to install, has mechanical strength and dimensional stability. However,

it is inflammable. It is eaten by rats and cockroaches. It can become water-logged, losing its

insulation value if the storage develops leaks. Certain fumigants may affect it. It does

deteriorate slowly with age.

Glass wool is rat proof, but insects can harbour in it. It is odourless. It has a low water

absorptivity. It is fire proof. It is unaffected by fumigants. However, it allows a free flow of

moisture, so an additional moisture barrier is imperative. People can be allergic to it.

Convection currents are possible particularly in the lighter grades. It has less dimensional

stability and mechanical strength than rigid insulation material, but is more easily worked

around the curves and corners.

Refrigeration

The basic objective of refrigeration is to keep the storage temperatures below the

usual ambient temperatures. An alternative to refrigeration is storing the seeds dry, either by

using dehumidification or by drying and storing in sealed containers. Refrigeration often

becomes necessary for carry-over seeds, special kind of seeds, foundation seed and nucleus

seed / breeder seed.

Heat leakage: This is the flow of heat through the floor, walls and roofs into the storage,

when the outside temperature is higher than the temperature in the storage. This is directly

proportional to the temperature difference and the distance heat must travel.

Insulation as discussed above reduces heat leakage but does not eliminate it. The

better and thicker the insulation, the less refrigeration is required because leakage of heat is

reduced.

Field heat: Field heat refers to the heat of seeds, packaging material, pallets, bales of

bags and anything else brought in to and put in the storage in excess of the heat at the storage

temperature. Refrigeration capacity must be sufficient to remove field heat in a reasonably

short time.

Heat of respiration: Heat of respiration in seed storages is not a serious heat input

because dry seeds have an extremely low respiration rate. Below 11% seed moisture, seed

respiration is so low as to be almost immeasurable. Respiration, however, is higher at higher

temperatures and decreases with decreasing temperatures in the seeds of the same moisture

content. At higher seed moisture contents, the respiration jumps astronomically.

Incidental heat: The incidental heat includes heat from electric lights and external heat

that enters a storage when the door is opened. Care should be taken not to leave the door

open.

The amount of refrigeration needed is usually arrived at by calculating heat leakage,

field heat, heat of respiration and then arbitrarily adding 10% more as incidental heat.

Although theoretically, it is possible to obtain lower relative humidity by

refrigeration, the practical minimums depending on temperature are as follows:

Temperature Minimum relative humidity

90°F (32°C) 30

81°F (27°C) 35

74°F (23°C) 40

70°F (21°C) 45

67°F (19°C) 50

62°F (16°C) 60

57°F (14°C) 70

Below about 15°C, the relative humidity possible with refrigeration alone is too high

for proper seed storage. Hence refrigeration alone is not considered sufficient for storage of

seed. Refrigeration storages must be used in combination with dehumidification, or with

scaling the dried seeds in moisture proof containers before they are placed in a refrigerated

storage.

Control of seed moisture

It has been known that the lower the seed moisture content (down to equilibrium with

20 to 25% relative humidity), the longer the seed will survive. Therefore, control of seeds

moisture content is extremely important. Such control can be achieved by using several

methods listed below:

i. Ventilation

ii. Moisture-proofing

iii. Dehumidification

iv. Sealed containers

v. Desiccants

Ventilation

As discussed earlier, if used judiciously this is an effective technique for reducing the

temperature of storage and of the seeds in it. It could also help in further drying of seeds.

However, under any circumstances, the ventilation alone will not be adequate to improve the

storage.

Moisture-Proofing

It is useful to moisture-proof a storage which is relatively less expensive than

insulating a storage. The three most common moisture-proofing materials are polyethylene,

asphalt and aluminium foil. To be effective, polyethylene should be 10 mm thick, asphalt

should be 3 mm thick, and aluminium foil should be bonded by moisture resistant plastic to

some surface (such as paper) that will keep the foil from cracking. Whichever material is

selected, the entire storage must be moisture proofed. This includes placing a layer of the

moisture proofing underneath the concrete floor. This should be overlapped with the wall

moisture proofing, which in turn should overlap the ceiling moisture proofing. All seams

should be overlapped, of course. The door, or doors, should be moisture proofed and

gasketed like cold storage doors. If ventilation openings are included, there should be

moisture proof covering over them when they are not in use. All nails and openings for pipe

and wire should be sealed against moisture penetration. A storage that is properly moisture

proofed will allow essentially no moisture leakage form outside.

To prevent moisture form entering the room as seeds are brought in or removed, an

antechamber is desirable at the entrance.

In constructing a seed storage, moisture proofing and insulation are done together. It

is essential that insulation be on the dry side of the moisture proofing. Water is a good

conductor of heat, and if the insulation becomes damp it loses much of its insulating value.

Normally, the floor is insulated only if the storage is a room above ground level. The orders

of the material in the wall from outside to inside are: the structural materials, moisture

proofing from physical damage. It is possible to bond the insulation to the structural wall

with moisture proof mastic such as asphalt, but care must be taken to prevent skips or thin

spots. If the storage has only moisture proofing there as the outside layer, e.g., an asphalt felt

or hot tat roof. The most important consideration is to be sure that there are no cracks or

pinholes where moisture can penetrate.

Dehumidification

The importance of low relative humidity in a seed storage room cannot be over

emphasised. If the relative humidity within the room averages above 60% special

dehumidification becomes necessary.

There are two major types of dehumidification:

i. Refrigeration type

ii. Chemical or adsorption type.

Refrigeration type: The refrigeration type dehumidifier operates by draining warm

moist air over a metal coil, through which a refrigerant such as feron is circulated. A part of

the atmospheric moisture condenses on this cooling coil and is collected in a pan or bucket,

or is drained off. The cooled air coming from over the coil, which now has a low temperature

and a high relative humidity, is reheated by the condenser coil of the refrigeration system.

This raises the temperature and lowers the relative humidity.

The water removal capacity of this type of system is dependent on the difference in

temperature between the entering air and the cooling coil. While these units are quite

effective at high temperatures, they lose efficiency below 70°F or 50% relative humidity.

Heat from the electric motors that drive the compressors and fans add sensible heat to the

atmosphere.

Adsorption type: The adsorption type dehumidifier operates by draining moist air over a

solid drying agent (desiccant), which has the ability to extract and retain moisture on its

surface by a phenomenon known as adsorption. The air is filtered and dried to a very low

dew point in the process, and the desiccant is periodically regenerated by means of heated

outside of the conditioned space. Continuous operation of these machines is achieved by

either using two desiccant beds which switch back and forth automatically, or by using

rotating beds of desiccant, a portion of which is always dehumidifying the air, while the

remainder is being regenerated.

Desiccant dehumidifiers provide maximum efficiency at low temperatures, and are

able to maintain constant relative humidity even below 10%. A factor that should not be

overlooked is that heat is added to the controlled atmosphere even though the unit is placed

outside the storage room. The latent heat of vapourization, of the moisture that is removed, is

converted to sensible heat. There is also a certain amount of residual heat left in the desiccant

after reactivation which increases the air temperature.

Heat removal: Use of either type of dehumidification results in a heat build up. In a well

insulated room, this is as much as 20°F, above the outside temperature, during the periods of

continuous operation of the dehumidifier while removing the maximum quantities of water.

This should be countered by installing a cooling device in the air stream, between the

dehumidifier and the storage. If abundant cool water is available, the air can be passed

through a water cooled radiator. The air may also be passed through a window type air

conditioner, or if large quantities of air are involved, through the cooling coils of a

refrigeration unit.

Desiccant used for dehumidification: For dehumidification, either solid or liquid spray

desiccants can be used. The common solid desiccants are silica gel and activated alumina.

Both are non-toxic, reasonably indestructible, cheap and efficient. Silica gel can absorb water

up to 40% of its dry weight.

The other type of desiccant are saturated salt solutions with a low relative humidity

equilibrium (e.g., Lithium chloride). The advantage of the liquid spray desiccant is that the

air returning to storage is of a uniformly low relative humidity, compared with that of the

solid desiccant system, in which the relative humidity of the air is reduced less and less as the

desiccants become more nearly saturated. In further contrast, air from the liquid spray

desiccant returns to the room cold, and its relative humidity drops still further as it warms.

Whereas the air from a solid desiccant must be cooled, increasing the relative humidity.

Therefore, drawbacks, however, must be considered.

Corrosion is a serious problem and care must be taken to replace corroded parts

before a breakdown occurs.

Lithium chloride is toxic.

Sealed containers:

Storage of well dried seed (moisture contents not exceeding safe moisture contents),

in sealed containers is one of the most effective methods of controlling seed moisture. If

seeds are first dried to safe moisture levels and then stored in sealed moisture vapour proof

containers, the low moisture content of the seed will be maintained even under storage

conditions of high relative humidity. Also, the storage temperatures are less critical for such

seeds, because the germination capacity of such packaged seeds remains high under adverse

temperature and humidity. Harrington (1963) demonstrated the value of moisture proof

containers in increasing seed longevity under high humidity conditions, and showed the

relative resistance of various containers to moisture vapour penetration. Moisture proof

containers include sealed tins or aluminium cans, glass mason jars with gasketed lids and

pouches of aluminium foils laminated to mylar or polyethylene.

Moisture resistant containers include various heat sealed plastic pouches, or bags of a

thickness equivalent to 3 mm high density polyethylene. Cloth and paper bags have no

moisture resistance, unless laminated to aluminium foil or some moisture resistant plastic.

Larger containers such as fibre board drums, properly laminated with aluminium foil, and

steel boxes or bins, all with gasketed lids, are moisture proof.

Use of desiccants:

Since moisture proof containers are difficult to open and reseal, they are not very

useful for plant breeders or seed control officials who store many small samples that must be

readily accessible. Such samples could be stored in metal boxes with gasketed snap on lids or

with a desiccant (e.g. silica gel) enclosed with the seed samples. Silica gel is available with

all or some of the granules treated with cobalt chloride. The usual cobalt chloride treated

silica gel turns from blue to pink at about 45% relative humidity. Thus a quantity of silica gel

(1 kg per 10 kg of seeds and packets) is dried and enclosed with the seeds in metal box when

the indicator granules turn pink, the silica gel is removed, reactivated by drying in an oven at

175°C, cooled in a sealed container and returned to the metal box. The seeds are thus kept

below equilibrium with 45% relative humidity, a moisture content desirable for several years

of storage In a temperature range of 20 to 25°C. the metal box has other advantages. It is

rodent and insect proof as well as moisture proof. The boxes, which are not very expensive,

are easily stacked on shelves in a small area. Also, seeds in equilibrium with relative

humidity of 45% will not be damaged by storage fungi. The only care required is periodic

inspections to make certain that the indicator silica gel remains blue.

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