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III. Methods of Preserving Seeds

III. Methods of Preserving Seeds

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PRESERVATION OF SEED STOCKS



95



humidities to which they are exposed. Moisture equilibria values for most

seeds may be found throughout

the literature. Tables I1 and 111 show

"

the most complete ones, taken from Harrington ( 1960).



s

C

0

.+

0



.-c



E

L



W

(r



W



0

0



L



W



>



a



FIG. 2. Viability of vegetable seeds stored in an office for 17 to 30 years at

Cheyenne, Wyoming.



The type of seed storage will depend on the climate of the area in

which the seeds are to be stored. An aerated room will suffice in some

sections, whereas in others an elaborate, expensive installation will be

required.



B. CONTROL

OF BOTH TEMPERATURE

AND HUMIDITY

The most complicated and expensive method of preserving seeds is

one in which both the temperature and humidity are maintained at low

levels. In an installation of this kind, the storage room requires thorough

moistureproofing and effective insulation. The construction of such a

room has been explained in detail by Munford (1965). Equipment for

maintaining low temperatures and humidities is dependent upon the size

of the installation. A competent engineer should be able to specify proper

equipment. A popular-type article by James (1962), however, would

enable one to estimate his own requirements.



96



EDWIN JAMES



Most seeds will keep well if the temperature is maintained at 40°F.

Air cooled to this temperature will have a high R.H., often above 75

percent, so some provision must be made to reduce the R.H. with which

the seed moisture attains equilibrium. This can be accomplished by

reducing the air temperature to a point below that required in the

storage room and reheating it to the desired point. The lower temperaTABLE I1

Absorbed Moisture Content of Field Seed in Equilibrium with Air of Various Relative

Humidities a t Room Temperature (Approximately 77"F.)a

Relative humidity, percent

Seed

Barley

Buckwheat

Shelled corn, Yd

Shelled corn, Wd

Shelled corn, Pop

Flaxseed

Oats

Peanut

Rice, milled

RYe

Sorghum

Soybeans

Wheat, white

Wheat, durum

Wheat, soft red winter

Wheat, hard red winter

Wheat, hard red spring

a



15



30



45



60



75



90



100



6.0b

6.7

6.4

6.6

6.8

4.4

5.7

2.6

6.8

7.0

6.4

4.3

6.7

6.6

6.3

6.4

6.8



8.4

9.1

8.4

8.4

8.5

5.6

8.0

4.2

9.0

8.7

8.6

6.5

8.6

8.5

8.6

8.5

8.5



10.0

10.8

10.5

10.4

9.8

6.3

9.6

5.6

10.7

10.5

10.5

7.4

9.9

10.0

10.6

10.5

10.1



12.1

12.7

12.9

12.9

12.2

7.9

11.8

7.2

12.6

12.2

12.0

9.3

11.8

11.5

11.9

12.5

11.8



14.4

15.0

14.8

14.7

13.6

10.0

13.8

9.8

14.4

14.8

15.2

13.1

15.0

14.1

14.6

14.6

14.8



19.5

19.1

19.1

18.9

18.3

15.2

18.5

13.0

18.1

20.6

18.8

18.8

19.7

19.3

19.7

19.7

19.7



26.8

24.5

23.8

24.6

23.0

21.4

24.1



-



23.6

26.7

21.9

26.3

26.6

25.6

25.0

25.0



Data from Harrington (1960).



r, Moisture content wet basis, in percent.



ture ranges of a psychrometric chart show that for approximately every

15°F. rise in temperature the humidity is reduced by one-half. If air

leakage is disregarded, in which case the absolute amount of moisture

in the room would remain constant, the initial refrigeration temperature

would have to be about 25". If the R.H. at 25" is 75 percent, reheating

the air to 40" would result in an R.H. of 38 to 40 percent. The additional

heat is generally provided by electric heaters regulated by a humidistat.

In the normal operation of an evaporator (refrigerating coil), frost accumulates on the fins and has to be removed at regular intervals. The

defrosting schedule at the National Seed Storage Laboratory is every 4



97



PRESERVATION OF SEED STOCKS



hours in summer and every 6 hours in winter. The use of electric coils is

probably the simplest of all defrosting methods.

The foregoing is only a brief explanation of providing low temperatures coupled with low humidities, and I wish to emphasize that a novice

TABLE I11

Approximate Moisture Content of Vegetable Seeds in Equilibrium with Air a t Different

Relative Humidities at Room Temperature (Approximately 77°F.)"

Relative humidity, percent

Seed

Bean, broad

Bean, lima

Bean, snap

Beet, garden

Cabbage

Cabbage, Chinese

Carrot

Celery

Corn, sweet

Cucumber

Eggplant

Lettuce

Mustard, leaf

Okra

Onion

Onion, Welsh

Parsnip

Pea

Pepper

Radish

Spinach

Squash, winter

Tomato

Turnip

Watermelon

a



10



20



30



45



60



75



4.2"

4.6

3.0

2.1

3.2

2.4

4.5

5.8

3.8

2.6

3.1

2.8

1.8

3.8

4.6

3.4

5.0

5.4

2.8

2.6

4.6

3.0

3.2

2.6

3.0



5.8

6.6

4.8

4.0

4.6

3.4

5.9

7.0

5.8

4.3

4.9

4.2

3.2

7.2

6.8

5.1

6.1

7.3

4.5

3.8

6.5

4.3

5.0

4.0

4.8



7.2

7.7

6.8

5.8

5.4

4.6

6.8

7.8

7.0

5.6

6.3

5.1

4.6

8.3

8.0

6.9

7 .O

8.6

6.0

5.1

7.8

5.6

6.3

5.1

6.1



9.3

9.2

9.4

7.6

6.4

6.3

7.9

9.0

9.0

7.1

8.0

5.9

6.3

10.0

9.5

9.4

8.2

10.1

7.8

6.8

9.5

7.4

7.8

6.3

7.6



11.1

11.0

12.0

9.4

7.6

7.8

9.2

10.4

10.6

8.4

9.8

7.1

7.8

11.2

11.2

11.8

9.5

11.9

9.2

8.3

11.1

9.0

9.2

7.4

8.8



14.5

13.8

15.0

11.2

9.6

9.4

11.6

12.4

12.8

10.1

11.9

9.6

9.4

13.1

13.4

14.0

11.2

15.0

11.o

10.2

13.2

10.8

11.1

9.0

10.4



Data from Harrington (1960).

Moisture content wet basis, in percent.



in seed storage should not attempt to design his own equipment. Even

when the equipment is set up in consultation with a refrigeration engineer, the factor of humidity control should be emphasized. I have been

contacted by many people regarding deficient equipment for humidity

control, which had to be corrected with additional dehumidifiers.



98



EDWIN JAMES



C. CONTROL

OF HUMIDITY

ONLY

Using the rule of thumb mentioned in Section 11, E, it should be

possible to preserve seeds satisfactorily at 80°F. and 20 percent relative

humidity. This can be accomplished through the control of humidity

alone. In designing such a storage, special provisions must be made for

the construction of the storage room. Insulation is unnecessary but an

efficient moisture barrier to prevent the ingress of external air should be

included in the structure, Otherwise, moisture will enter the storage area

and dehumidification will be relatively ineffective. It is practically impossible, unless a storage room is hermetically sealed, to prevent air leakage,

but it can be reduced to a minimum through the use of appropriate

materials.

The least expensive method of providing a moisture barrier in room

construction is to coat the inside and outside walls with a good waterproofing paint. Ten-mil polyethylene of high density has good moisturebarrier properties and may be used as a lining or in the periphery of the

construction. The joints should be heat-sealed where possible. Aluminum

foil or foil laminated with paper could be used in place of polyethylene,

but this would increase construction costs. The use of foil, correctly

applied, would result in a room with almost the same moisture-barrier

properties as a room lined with sheet metal. Also, the door or doors to the

room would require adequate sealing. Rapid air changes through

entrances can be minimized by the construction of a small anteroom

entrance so that each door may be opened and closed in sequence. If the

construction of the room is adequate, the moisture within can easily be

reduced to the desired R.H. by using a silica gel dehumidifier.

Dehumidifiers required to meet most needs can be determined

through the use of a psychrometric chart. Figure 3 shows a portion of

such a chart. The amount of moisture per pound of dry air for combinations of temperature and humidity is shown in grains at the right of the

chart. If we assume that the initial condition of the room is 80°F. and

80 percent R.H., following the intersection of the dry bulb temperature

and R.H. curve horizontally to the right (dotted line), we will find that

there are approximately 124 grains (or 0.0177 pound) of moisture per

pound of dry air (1grn. = 0.0001429 pound). Following the same procedure for 80" and 20 percent R.H., a pound of dry air will hold 30 grains

(or 0.0043 pound) of moisture. Therefore, to reduce the R.H. to 20 percent, we will have to remove 94 grains (or 0.0134 pound) of moisture

per pound of air. For estimation purposes, we may assume that a pound

of dry air for the conditions being considered will occupy a volume of

approximately 13.8 cu. ft., and the actual amount of moisture to be



99



PRESERVATION OF SEED STOCKS



removed will depend on the total cubic feet in the room. For example,

a room 20 x 20 x 8 feet would have a volume of 3,200 cu. ft. The

amount of moisture to be removed, to reduce the relative humidity from

80% to 20% at &O", would be 3.11 pounds (3200/13.8 x 0.0134). The

130



I20

110

L

.-



a



70



100 ,"



9



e



90;

a



;

L



YI



?!



80



>



265

0

L



70



0



n



E



s



?



60;



L



50'i



n

360

m



el

C



3



W



40



55



30



Dry Bulb T e m p e r a t u r e Degrees F:



FIG. 3. Psychrometric chart.



dehumidifier must be large enough to take care of this amount plus any

leakage, which may amount to as high as ten complete air changes in 24

hours. Approximations for any set of conditions may be determined by

folIowing this procedure.

D. STORAGEIN MOISTUREPROOF

CONTAINERS



In Section 111, A I mentioned that commercial seedsmen market

their seeds in moistureproof containers. Some seedsmen use hermetically

sealed cans and others use envelopes with moisture-barrier properties.

Envelopes of this type are relatively inexpensive, are adaptable to the

preservation of valuable seeds, and have the advantage of being reusable,

which is not the case with hermetically sealed cans. The envelopes are

easily sealed with heat after they are filled. The heat-seal may be cut

off, a portion of the contents removed, and the envelopes resealed.

Bass et al. (1961) and Harrington (1963) have reported on the types



100



EDWIN JAMES



of materials suitable for packaging seeds. Materials containing aluminum

foil have been found superior. The foil is usually laminated with other

materials to give added strength. The laminates may be polyethylenefoil-polyethylene, paper-foil-polyethylene, or similar combinations.

Storing seeds in moistureproof containers may be disastrous if the

seeds are not dry when enclosed. Moist seeds deteriorate more rapidly

in closed containers than in ordinary envelopes. In our work at the

TABLE IV

Moisture of Seed Packaged



Family

Gramineae



Liliaceae

Chenopodiaceae

Cruciferae



Leguminosae

Umbelliferae

Solanaceae

Cucurbitaceae

Compositae



Kind

Sweet corn

Kentucky bluegrass

Creeping red fescue

Perennial ryegrass

Onion, leek, chive, Welsh onion

Beet, chard

Spinach

Cabbage, broccoli, cauliflower,

collards, Chinese cabbage, kale,

turnip, rutabaga, kohlrabi,

Brussels sprouts, mustard, radish

Snap bean, lima bean, pea

Crimson clover

Carrot, celery, celeriac

Parsnip

Parsley

Tomato

Pepper

Eggplant

Cucumber, muskmelon, squash,

pumpkin

Watermelon

Lettuce



Maximum

percent seed

moisture

8.0

6.0

3.0

8.0

6.5

7.5

8.0

5.0



7 .O

8.0

7.0

6.0

6.5

5.5

4.5

6.0

6.0



6.5

5.5



National Seed Storage Laboratory we have found that seeds with a high

oil content, sealed with 7 percent moisture and stored at room temperature, do not maintain high viability, while those with 4 percent moisture

store well. Starchy seeds, such as sorghum, have kept well for 4 years

when sealed with 7 and 10 percent moisture.

A complete listing of moisture limits for sealed seeds is not available,

but a guideline for drying is given in Table IV, showing the requirements

of the California Seed Law and Regulations.



PRESERVATION OF SEED STOCKS



101



Caution must be used when drying seeds. High drying temperatures

will damage seeds with high moisture contents, and drying temperatures

must be adjusted accordingly. Harrington (1960) gives the temperature

limits for drying as follows: seeds with more than 18 percent moisture,

90°F; seeds with 10 to 18 percent moisture, 100"; seeds with less than 10

percent moisture, 110".

Some seeds, when dried to 4 to 5 percent moisture, may exhibit an

induced dormancy which results in slower germination even though

viability is high. When exposed to a humid atmosphere, so that moisture

is slowly absorbed, many of these apparently dormant seeds will germinate as readily as those with higher moisture contents.

If we assume that the primary cause of seed deterioration is respiration, any of the three methods discussed would be effective in limiting

this process. Advantages of each, however, are not equal. Cold storage

with low humidity prevents the growth of fungus, and any insects present in the seeds become dormant and die. In sealed storage the accumulation of carbon dioxide in the container will kill insects in 1to 4 weeks,

but some damage to the seed may have resulted by that time. Where

humidity alone is controlled, it may be necessary to treat the seeds at

intervals to prevent insect infestation. For small lots of seed composed

of genetic collections or breeding lines, storage in moistureproof envelopes provides an inexpensive, dependable method of preservation.

IV.



The National Seed Storage laboratory



A. HISTORICAL

Since early colonial times the introduction of seeds from worldwide

sources has added to the agricultural economy of our country. With very

few exceptions the crops now grown in the Western Hemisphere

originated in various parts of the Old World. A systematic method of

numbering introductions began with the establishment of the U.S.

Department of Agriculture as an executive department in 1898. Prior to

1898 no method of recording introductions was used, but since this date

over 300,000 accessions have been introduced. Many of these have been

lost, partially because of lack of foresight and also the absence of

adequate storage facilities. Approximately two-thirds of the oats, 90

percent of the soybeans, and 98 percent of the clovers introduced into the

United States are no longer in existence. This loss may not be serious,

but it is possible that some very valuable germ plasm is no longer

available to plant breeders.

Plant introductions have contributed materially to the development

of our improved varieties. From these introductions and through the



102



EDWIN JAMES



efforts of plant breeders, thousands of varieties, breeding lines, and

genetic collections have resulted. A large portion of these has also

disappeared because breeders often had no further interest in an old

variety that had been superseded by a better one. Yet there is no assurance that a variety of breeding line having a poor rating under prevailing circumstances would not have value in the future as new races of

pathogens develop. Potentially valuable breeding materials have often

been stored in a haphazard manner on office shelves, in desk drawers,

or in boxes in attics or basements, and eventually discarded.



FIG.4. Front view of National Seed Storage Laboratory.



Recognizing the inadequacy of seed preservation, the National

Research Council in 1946 recommended the construction of a National

repository for the preservation of valuable seeds. After 10 years of

groundwork by representatives of Federal agencies, State Experiment

Stations, and interested private concerns, justification for a National

facility was presented to the Congress. Congress appropriated funds for

the construction of the Laboratory, which began operations in the fall

of 1958. A front view of the Laboratory is shown in Fig. 4.

B. OPERATION

The Laboratory is a three-level structure. The ground floor houses

all the mechanical equipment. A standby compressor can be put into



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