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II. Theories Regarding Seed Deterioration

II. Theories Regarding Seed Deterioration

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PRESERVATION OF SEED STOCKS



89



nitrogen, and stored at low temperatures, would keep indefinitely. Also,

Nutile (1964), when drying a number of crop seeds to moisture contents

of 0.4 to 1 percent, and storing at room temperatures for 5 years, found

some damage in seeds of celery, eggplant, carrot, pepper, tomato, and

Kentucky bluegrass, Poa pratensis L., but in no case was viability completely lost. Seeds of cabbage, cucumber, lettuce, onion, and Highland

bentgrass, Agrostis tenuis Sibth., were not injured during storage with

moisture contents as low as 0.4 percent.

Crocker (1938) suggested that the loss of seed viability is due to the

coagulation of proteins. Later (1948) he stated: “This theory has the

fault of being very general. There are many different kinds of proteins

in the embryo, and this work does not throw any light on the particular

proteins which coagulate with time. Furthermore, it throws no light on

the possibility of the degeneration of some particular mechanism in the

cell.” Crocker may have had in mind the possibility of chromosome

disintegration, which has received much attention and will be discussed

in a later section.

OF ENZYMES

AND RESPZRATION

C. INACTIVATION



A number of attempts have been made to associate the decline of

enzymatic activity with losses of seed viability. This approach raises the

question of separating causes from effects. Only a few of the numerous

enzymes in seeds have been investigated. Considering our limited knowledge of molecular biology, they are still a fruitful field of research.

Perhaps the statement by Crocker, concerning the degeneration of some

mechanism in the cell, could be interpreted in terms of DNA and RNA,

upon which enzymatic reactions depend.

Much of the early work with enzymes and respiration, as related to

declines in viability, was limited with emphasis on catalase. Although

Davis (1926) was able to show a relationship between a catalase ratio

and viability in lettuce seeds, his observations did not cover a wide

enough range to establish a linear relationship. Leggatt ( 1929-30) also

obtained high correlations between catalase activity and germination of

wheat, Triticum aestivum L. Crocker and Harrington (1918) found that

catalase was active in seeds of Johnsongrass, Sorghum halepense L., that

lost all viability, yet established a relationship with viability. No such

relationship, however, could be established with seeds of Amaranthus

retroflexus L.

The relationship of catalase to seed viability is questionable. Results

have not been consistent, and too few species have been investigated.

The function and formation of catalase have not been clarified. Lantz

(1927) concluded that there was no evidence that catalase was involved



90



EDWIN JAMES



in physiological oxidation. Rhine ( 1924) suggested that the catalase

content of seeds is not stable but created as needed.

Apparently phenolase activity is not an indication of viability. Davis

(1931) found a relationship between phenolase and germination in wheat

but not with age of seeds. On the other hand, the relationship in oats

was with age but not with germination.

In their report on the reduction of 2,3,5-triphenyltetrazolium by dehydrogenases, Thorneberry and Smith ( 1955) concluded: “Loss of viability

appeared more closely related with respiratory failure in most seeds.

Malic dehydrogenase activity was more closely correlated with germination percentage and respiratory capacity than the other two enzymes



FIG. 1. Relationship of glutamic acid decarboxylase (GADA) and germination

in corn as affected by storage conditions. (From Grabe, 1964.)



( alcohol dehydrogenase and cytochrome dehydrogenase ) although considerable malic activity was retained by nonviable seeds. It is doubtful

whether the inactivation of these three enzymes was a major cause of

loss of viability.”

A close relationship appears to exist between glutamic acid dicarboxylase and viability. This was demonstrated by Linko and Sogn (1960),

Bautista and Linko (1962), and Grabe (1964). Figure 1 is taken from

Grabe’s work to show this association. The 12 seed lots of corn used were

under increasingly poor storage conditions, ranging from favorabIe for

Lot 1 to very poor for Lot 12. The correlation coefficient for this relation-



PRESERVATION OF SEED STOCKS



91



ship was found to be 0.901. Germination decreases lag considerably

behind decreases in the activity of the enzyme; Grabe interpreted this

as evidence of incipient damage, particularly in the range of seed lots

with higher germinations. Results seem to be related to the storage of

corn under poor conditions.

With the possible exception of glutamic acid decarboxylase, there

appears to be no clear-cut evidence that enzymes can be used as an index

of seed viability as affected by respiration. Yet, in storage, seeds do

respire and finally lose viability. There is a strong possibility that the

end products of respiration as mutagens have a lethal effect on seeds

when stored under adverse conditions. A discussion of this theory follows

in Section 11, E.



D. DEVELOPMENT

OF FATACIDITY

In some investigations the development of fat acidity in seeds has

been shown to accompany losses of viability. It has been considered the

cause of these losses. Holman and Carter (1952) associated losses of

viability in soybeans with an increase in fat acidity. KeIly et at. (1942)

found the same association in wheat, and Zeleny and Coleman (1939)

in corn. With peanuts, however, Davis (1961) found that significant

increases in fat acidity occurred only after the stored seeds had lost all

viability. The literature on fat acidity indicates values as high in corn

germinating 70 to 80 percent as in corn with 30 to 60 percent viability.

Barton (1961) concludes her review of the fat acidity problem with the

statement: “The fat acidity test has been applied to several hundred

samples of sound and damaged grain with the result that it has been

possible to establish fat acidity values for grain showing little or no

deterioration. The obvious conclusion regarding fat acidity is that even

though there is often an association with viability, the results are not

consistent enough to use it as a reliable index of viability.”

E. MUTAGENIC

EFFECTS

In recent years the theory that seed deterioration is due to the development of mutagens in stored seed has gained considerable acceptance.

In aging seeds, mutations have been found in onion by Nichols (1941);

in wheat, barley (Hordeum vulgare L. ), rye (Secale cereale L. ), and

peas (Pisum sativum L.) by Gunthardt et al. (1953); in corn, by Pet0

(1933); in Crepis spp. by Navashin (1933); in Datum spp. by Blakeslee

(1954); and in sugar beets by Lynes (1945). Observed aberrations include fusion, fragmentation, bridges, micro or giant nuclei, ring chromosomes, and others.

Many examples of the effect of seed age on chromosome aberrations



92



EDWIN JAMES



could be given. The work of Gunthardt et al. (1953) will serve as an

illustration (Table I). With the exception of 27-year-old seeds, the

number of both chromosome bridges and fragments increases with age.

The abnormally low germination of the 27-year-old seeds may have

eliminated many seedlings in which abnormalities may have been present. A large increase in aberrations is evident in 32- and 33-year-old seeds.

TABLE I

Development of Chromosome Aberrations with Age in Baart Wheat"

Chromosome aberrations

Bridges



Fragments



Germination

Age of seeds

1



11



17

21

26

27

32



Strong



Weak



Dead



SO

74

47

66



9

9

21

17

4

45



11



-



5

15



17

32

17

-



91

40



Total

No.

21

-



30

101

40

222L



No./cell

-



Total

No.



No./cell



-



0.11



81

-



0.41



0.15

0.54

0.31

1.66b



143

212

141

465b



0.72

1.13

1.08

3.4Tb



-



-



Data from Gunthardt et al. (1953)

Seeds 33 years old.



Numerous compounds are known to induce mutations, but most of

these are not normally found in seeds. The review of D'Amato and

Hoffinan-Ostenhof ( 1956) covers this subject quite thoroughly, and

some of their citations may bear repeating. A number of compounds

which can cause chromosome aberrations are given. Among those found

in plants (and quoted by the preceding authors) are adenine; the degradation products of the nucleic acids adenine, uracil, thymine, and adenosine, which can act as chromosome-breaking agents; and even the nucleic

acids themselves, deoxyribonudeic acid and ribonucleic acid.

One explanation in regard to loss of seed viability given in the foregoing review is that accumulations of toxic materials in cells induce

massive mutations in the embryonic tissue, preventing normal cell

division. Attention has also been directed to the fact that in germinating

seeds the first cells most often affected are in the root tips, which are the

first to divide. This possibly accounts for the fact that abnormalities in

germinating seeds are often found in the failure of root primordia to

develop.

Three degrees of mutagenic action are quoted by DAmato and

Hoffman-Ostenhof (1956). They are as follows: (1) the lethal zone,



PRESERVATION OF SEED STOCKS



93



where the accumulations of mutagens become toxic, causing the death

of seeds; (2) the narcotic zone, which results in the inhibition or destruction of the spindle mechanism; (3) the subnarcotic zone, in which mutations develop.

In further support of the mutagenic theory are the following observations. (1) Extracts from aged seeds induce mutations in fresh seeds.

( 2 ) There is a gradual increase in mutations with age up to a point

where there i s a rapid increase coincident with loss of viability. ( 3 )

Spontaneous mutations arising in dormant seeds become evident in the

pre-split phase and in the development of mutations in the adult plant.

(4) The difference in reaction of the shoot and root tips in aged seeds

closely parallels the reaction of the same kind of seeds when treated

with X-rays.

There is no evidence that mutagens develop in seeds stored under

favorable conditions. All observations in this respect have been made

with seeds affected by high storage temperatures, humidities, or both.

The classical work with the genus Datum by Blakeslee (1954) demonstrated that age of seeds was not alone responsible for the development

of mutations. When seeds were stored at room temperatures, mutations

in Datum did develop, but an extremely low frequency was found in

plants grown from seeds buried in the ground for 39 years, where cool

conditions apparently prevailed. The final answer to the question of the

relationship of mutations to seed age may now repose in the National

Seed Storage Laboratory at Fort Collins, Colorado, where highly favorable conditions for seed preservation exist. The answer will not be found

in the immediate future, however, because storage conditions at the

Laboratory are such that respiration in the stored seeds is near the

minimum.

The reasons proposed for seed deterioration, with the possible exception of fat acidity, are all related to respiration, which may be the

primary cause with all others related thereto. Respiration increases in

proportion to the amount of moisture in seeds but is extremely low in

seeds with moisture contents between 4 and 11 percent, as reported by

Bailey (1940) and Harrington (1963). Respiration rates up to approximately 50°C. are also directly proportional to temperatures. With high

moisture contents and high temperatures, seed deterioration progresses

at a rapid rate. At 90°F. and 90 percent relative humidity (R.H.), most

seeds will have lost viability in 3 months. On the other hand, seeds stored

at 50°F. or below, with a low R.H., will last indefinitely. Peanuts are

recognized as poor keepers, yet I have stored peanuts at the Southern

Regional Plant Introduction Station, Experiment, Georgia, for 8 years

at 50°F. and 50 percent R.H. without a significant drop in germination.



94



EDWIN JAMES



The requirements for satisfactory seed preservation can apparently be

attained in one of three ways, all of which inhibit respiration. We can

store seeds in air with a low R.H., maintain seeds at low temperatures,

or both. Harrington (1960) states that if the sum of the degrees F. and

percent R.H. is 100 or less, conditions for long-time storage of seeds are

good. He also states that the storage life of seeds is doubled for every 1

percent drop in seed moisture content or for each 10°F. drop in temperature. Ways of satisfying these conditions will be discussed in the

next section.

111.



Methods



of Preserving Seeds



A. EFFECTOF CLIMATE

Seed storage is more difficult in warm, humid climates than in areas

of moderate temperatures and low humidities. Seed deterioration progresses rather slowly in the Western Great Plains and some of the intermountain regions of the United States. Such is the case, also, in some of

the desert areas, where temperatures are often high and humidities

extremely low. In a 10-year study of seeds stored under atmospheric

conditions at Fort Collins, Colorado, Robertson and Lute (1937) reported viability losses of only 7 percent for wheat and 14 and 13 percent,

respectively, for oats and barley. James et al. (1964) found that some

vegetable seeds stored in an office at Cheyenne, Wyoming, had long life.

The comparative lives of the species investigated are shown in Fig. 2.

Many of the seeds tested had germinations of 80 to 90 percent after

storage of 25 to 30 years. Differences in longevities of the various crops

are apparent, but differences within species have additional implications.

All the seeds were stored under identical conditions, and differences in

longevities may be due either to cultural or processing factors preceding

storage or to inheritance of longevity within species. There is considerable evidence that longevity of seeds is an inherited characteristic, but

damage during processing is often the first stage in the degeneration of

seeds. Moore (1963) concluded that damage to parts of the embryo

accelerates respiration and that the accumulation of metabolites affects

the surrounding tissue, resulting in the death of the seed.

In some areas of the United States it is difficult to maintain seeds at

high viabilities from one season to the next under ordinary shelf storage.

Seed wholesalers have usually replaced year-old seeds on the merchants’

shelves with fresh stocks so that customers would receive seeds of high

viability. More recently, however, commercial seed producers have

marketed their seeds in moistureproof containers.

The moisture contents of all seeds are dependent on the relative



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.



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