After-ripening, Rest Period, and Dormancy

BRUCE M. POLLOCK AND VIVIAN KEARNS TOOLE.

SEED MATURITY and seed germination follow in direct sequence in the life of a plant, but normally they are separated in time and space.

The interval may be a few hours or many years. It may be a few inches or thousands of miles.

The function of the seed is to carry its embryo plant through the hazards of time and space to a time and place where the new plant can grow, flower, and in its turn produce seeds.

It is of advantage to the seed to remain in an inactive condition until it reaches a favorable time and place for germination: A young plant is vulnerable to lack of water and extremes of heat and cold hazards that the embryo plant within the seed is adapted to withstand. In the nongrowing condition, the water content of the tissue is relatively low, the protoplasm of the cells is protected from damage, and the metabolic rate is low. Thus the seed can survive on its nutrient reserves for a long period.

DELAYED GERMINATION is not accidental. It is the result of physiological mechanisms that keep the seed in a nongerminating state.

The term "dormancy" is used to describe two inactive conditions. One results from an unfavorable environment. The other is due to internally imposed blocks. For example, germination may be delayed by inadequate water supply or unfavorable temperature. In some seeds, however, germination is prevented by blocking mechanisms within the seed. They must be removed before the germination can occur.

The terms "rest" and "rest period" also have been used to describe seeds and buds that are inactive because of these internal blocks.

One should be aware of this confusion in terminology when he reads about seeds and germination, but he need not think the dual terminology is an exercise in scientific semantics. Our scientific terminology has to be precise. Often in writings it is difficult to tell whether the investigator was working with seeds that were "dormant" because they were dry or cold, or with seeds that were "dormant" because of blocks. It is simple to write on paper a definition of "dormancy" or "rest," but it is more difficult to apply the definitions to a seed or a seed lot.

An example illustrates the problem. The seed of silver maple (Acer saccharinum) can germinate as soon as it falls from the parent tree. You are familiar with the appearance of seedlings in early summer under the trees. This seed is "inactive dormant" at the time of maturity, but it is not "resting" or "blocked dormant," because it germinates as soon as it reaches the water supply in the soil.

At the other extreme, the seed of the apple tree (Pyrus malus) is "resting" or "blocked dormant" at maturity. It will not grow even under good conditions for germination until it has undergone changes, known as after-ripening, to remove the germination blocks.

If one defines these two extremes as "dormant" and "resting" and then attempts to apply these definitions to other seeds, difficulties arise. Take the lettuce seed (Lactuca sativa). It germinates promptly in total darkness if it is planted in moist soil at 57 F. It does not contain a germination block. If the same seed is planted at 84 , however, it remains inactive. If, following 84 for a few days, the temperature drops to 57 , the seed still cannot germinate. Exposure of the imbibed seed to the high temperature induced the formation of a block that did not exist previously. This block may be removed by an exposure to red light.

Can one then easily apply a rigid definition of dormancy or rest to describe a lettuce or similar seed?

Obviously not, without qualifying the definition by listing carefully the conditions under which germination was attempted. The variety of the lettuce and the previous history of the seed also are important not all varieties behave in the way we described, nor do all lots of one variety.

GERMINATION BLOCKS are relative, not absolute.

Close examination of some seeds, such as the sour cherry (Prunus cerasus), has disclosed that growth is not completely stopped, even in a blocked seed at a low temperature. Cells of the root and shoot can divide, and the whole embryonic axis grows slightly at a time when the seed cannot germinate even under good conditions.

This observation and the fact that something obviously does occur during after-ripening to permit subsequent germination show that the blocks are only relative.

A blocked seed is like an automobile with its motor running at idling speed but with the gears disengaged there is no motion.

Germination blocks are variable. Gardeners and farmers know that all the viable seeds they plant do not germinate. The proportion of those that do germinate varies with conditions of germination. A major reason for the variability is that all seeds are not genetically identical.

Conditions required for germination are the expression of the seed's heredity as influenced by environment during seed formation, maturity, and germination.

We do not know, even for a single kind of seed, exactly what are the critical environmental factors, when they act, or how they may be controlled experimentally or in commercial practice.

The result of the interaction of genetic and environmental factors is extreme variability in the rate at which germination of different kinds of seeds and different seeds of one kind begins.

The germination of seeds has a continuous range from prompt growth over a wide range of environmental conditions to sluggish growth over a narrow range of environmental conditions. Most farmers, scientific workers, seedsmen, and gardeners recognize this variability. It is the variability of Nature.

A species survives because of blocks that delay germination. They tend to spread germination over a period of years. One unfavorable growing season does not obliterate a species.

Consider weeds. All farmers and gardeners see how weeds emerge in soil clean cultivated for many years. Some may have been introduced recently by animal carriers or the wind, but most were already present in the soil from previous years. These seeds had germination blocks that previously prevented germination.

Blocked seeds are more obvious and more extreme in wild plants than in most of our cultivated forms. One inherited difference between seeds is the ability to develop germination blocks.

Through the years, man has tended to select seeds that give relatively prompt germination. The result is that some of our cultivated plants cannot survive without man to protect the seeds by proper storage until a favorable season for germination. In a sense, man has substituted himself for the germination blocks that probably were present in the ancestors of our common cultivated plants.

The term "block" is a convenient name for a mechanism that restricts germination. Blocks act through a number of different physiological mechanisms. Some blocks are simple and well understood. Others are complex and almost completely unknown.

The end result is the same in all instances: The seed is held in a g non-growing condition.

THE MOST complete block to germination is in seeds that have seedcoats impermeable to water. These hard seeds are common in the waterlily, mallow (cotton, okra), and legume (beans, clovers) families.

During ripening and drying of red and white clover seeds (Trifolium pratense and T. repens), the seedcoat becomes impermeable to water when the moisture content of the seed has reached a low level. The impermeable seedcoat has a fissure along the groove of the hilum, which functions as a hygroscopic valve. When the seeds are surrounded by dry air, the fissure opens and permits water vapor to escape. The fissure closes in moist air. Thus the seeds can dry further through outward diffusion of water vapor, while reentry of water is prevented. Such seeds remain impermeable to water until the seedcoat is somehow broken. If water cannot enter, the first steps toward germination are prevented.

Some germination blocks are localized. Many blocks affect the whole embryo. Seeds of the apple are blocked and germinate only after undergoing a period of after-ripening while moist at a temperature around 40 . Imbibed seeds that have been chilled for 2 to 3 months germinate promptly and produce normal seedlings.