Seed Longevity and Aging

One of the most interesting characteristics of seeds is their wide variability in length of life, ranging from a few days to several decades or even centuries. It was reported, for example, that seeds of the arctic tundra lupine that had been buried for at least 10,000 years in frozen silt germinated readily in the laboratory (Porsild et al., 1967). Seeds of albizia retained viability for at least 150 years and those of Hovea sp. for 105 years (Osborne, 1980). Seeds of some forest trees of the temperate zone can be stored for a long time. For example, viability of slash and shortleaf pine seeds was 66 and 25%, respectively, after 50 years of cold storage (Barnett and Vozzo, 1986). There was some loss of vigor during the storage period but probably not enough to influence the genetic makeup of the next generation of plants.

Longevity of seeds in the soil has important ecological implications, particularly with respect to weed control and plant succession. Seeds of many species may remain viable in the soil for 50 years or longer. Oosting and Humphreys (1940) concluded that seeds of weed species in North Carolina soils retained viability for several decades. Livingston and Allessio (1968) reached similar conclusions in Massachusetts. Hill and Stevens (1981) reported that seeds survived for 30 to 50 years under forests in Wales and Scotland. Fivaz (1931) concluded that Ribes seeds could retain viability in the soil for as long as 70 years.

Short-lived seeds of woody plants include those high in water content (e.g., those of Taxus, Populus, Ulmus, Salix, Quercus, Carya, Betula, and Ae-

sculus). Mature seeds of many tropical species deteriorate rapidly. Tropical genera whose seeds characteristically have a short life span include The-obroma, Coffea, Cinchona, Erythroxylum, Litchi, Montezuma, Macadamia, Hevea, Thea, and Cocos. However, by temperature and humidity adjustments during storage, the life of many tropical seeds can be prolonged from a few weeks or months to at least a year.

With respect to desiccation, seeds have been classified as orthodox or recalcitrant (Roberts, 1973). Recalcitrant seeds cannot be dried below some relatively high moisture content without rapidly losing vitality. Even when fully hydrated, recalcitrant seeds usually lose viability in a few weeks or months (E. H. Roberts et al., 1984). Unlike recalcitrant seeds orthodox seeds can be dried to low moisture contents (usually to 5% and often to 2% on a fresh weight basis) without injury. The seeds of most herbaceous crop plants are orthodox, whereas recalcitrant seeds are mainly fleshy seeds produced by woody plants of both the temperate zone (e.g., chestnut, hazel, horse chestnut, oak, walnut) and the tropics (e.g., avocado, cacao, mango, rubber) and forest trees of the families Aracauriaceae and Diptero-carpaceae.

For a given genotype, the deterioration of orthodox seeds in storage is a function of time, moisture content, and temperature. Over a relatively narrow range of temperature and moisture contents, there is an approximately linear relation between storage temperature, moisture content, and the logarithm of viability period (Roberts, 1972). These considerations led to the following rules for storage of orthodox seeds (Murray, 1984):

1. For each 1% decrease in seed moisture content, the storage life of the seed is doubled.

2. For each 5.6°C decrease in storage temperature, the life of the stored seed is doubled.

3. The sum of the storage temperature (°F) and relative humidity (%) should not be greater than 100, with not more than half of the sum contributed by the temperature.

For detailed information on the storage conditions needed to prolong the life of seeds of many woody plants, the reader is referred to the chapter by Harrington in Volume 3 of Seed Biology (Kozlowski, 1972c) and the book by Young and Young (1992).

Considerable attention has been paid to the vigor of different seed lots. High seed vigor commonly is associated with rapid seed germination and seedling growth under field conditions. Expression of seed vigor is influenced by heredity; seed development, harvest, and storage conditions; and the seed germination environment. In general, high seed vigor is associated with high anabolic enzyme activity, respiration, ATP pool size, and synthesis of proteins, RNA, and DNA (Ching, 1982).

The vigor of aging seeds declines progressively, and the seeds change imperceptibly from one stage of deterioration to the next. Initial symptoms of aging include a decrease in capacity to germinate as well as an increase in susceptibility to attacks by microorganisms. As seed deterioration continues, the emerging radicles are progressively shorter, and cotyledons do not emerge from the seed coat. Finally, the seed dies.

As seeds age they undergo several structural, biochemical, and genetic alterations. These include reduction in capacity for synthesis of proteins, lipids, and RNA; injury to membranes and chromosomes; and decreased synthesis of labile enzymes and repair systems (Fig. 2.14) (Osborne, 1980, 1982; Priestley, 1986).

Because of worldwide concern for conservation of germplasm, much attention has been given to aging-induced genetic changes in seeds and their progeny. Induction of genetic variation in aging seeds includes changes in DNA, cytoplasm, RNA, and chromosomes. Most commonly, the chromosome aberrations in roots of plants produced from old seeds involve breakage of chromosomes. Because they are easily detected, chromosome aberrations have received primary attention. However, pollen abortion mutations and chlorophyll mutations also are associated with seed aging (Roos, 1982).

Free radical damage

/ Lipid peroxidation

Stochastic ^changes

Continuous nuclease activity is

Free radical damage

/ Lipid peroxidation

Increasing activity of endodeoxyri-bonuleases

Loss of DNA integrity and template activity

Stochastic ^changes

Continuous nuclease activity is

Failure of synthesis of functional RNA

Nonviable

Figure 2.14 Changes during progressive senescence of dry seeds in storage. Reprinted with permission from Osborne, D.J. (1980). Senescence in seeds. In "Senescence in Plants" (K. V. Thimann, ed.), pp. 13-37. Copyright CRC Press, Boca Raton, Florida.

Increasing activity of endodeoxyri-bonuleases

Loss of DNA integrity and template activity

Failure of synthesis of functional RNA

Nonviable

Figure 2.14 Changes during progressive senescence of dry seeds in storage. Reprinted with permission from Osborne, D.J. (1980). Senescence in seeds. In "Senescence in Plants" (K. V. Thimann, ed.), pp. 13-37. Copyright CRC Press, Boca Raton, Florida.

Seeds of Italian stone pine tnat were stored for more than 1 year lost capacity for germination and for increasing their protein and nucleic acid levels during germination (DeCastro and Martinez-Honduvilla, 1982). Although most synthetic events are initiated within minutes after dry embryos are rehydrated, initial DNA replication occurs rather late during metabolic reactivation. In old seeds, activation of DNA synthesis is further delayed, and phenotypic abnormalities and chromosomal aberrations at first mitosis become more numerous. After prolonged storage of dry seeds, the capacity to synthesize protein after imbibition may fail completely.

The lack of an effective DNA repair system in old seeds is associated with loss of membrane integrity and with leakage of cell contents. Decrease in germination capacity after many years of storage of Italian stone pine seeds was correlated with increased loss of reducing sugars (DeCastro and Martinez-Honduvilla, 1984). The greater leakage of solutes from old than young seeds implies that the integrity of the plasmalemma and/or the tonoplast is lost during aging. Ultrastructural studies provide additional convincing evidence of disruption of membrane integrity in old seeds (Priestley, 1986). Phospholipid degradation and peroxidation of unsaturated fatty acids, followed by membrane destruction, played an important role in aging of Norway maple seeds (Pukacka and Kuiper, 1988).

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