Life Processes of the Living Seed

R. G. STANLEY AND W. L. BUTLER.

THE EMBRYO in a seed draws on its endosperm for the nutrients it needs to germinate and grow. A delicate balance of internal conditions regulates its life processes.

The living seed is able to incorporate small molecules and simple substances such as glucose, phosphorus, and sulfur into complex chemical units of a cell. These organized parts are the cell wall and the protoplasm, which contains the cell nucleus. Enzymes act as the go-between in these conversions and building processes.

The energy for this work comes from the breakdown, or catabolism, of some of the cell's chemical components, usually by combining them with oxygen in the process called respiration.

Most of the seed components from which the enzymes of the protoplasm and cell walls form new cells can be classed as proteins, fats, carbohydrates, organic acids, and amino acids.

Thus the seed lives as long as its outside environment (against which its seedcoat helps protect it) and internal environment maintain active enzymes and a good balance of chemical substances. Only under these conditions can the embryo, the result of the fusion of the sperm and egg nuclei, produce new cells and a healthy plant.

Moisture, temperature, and gases, particularly carbon dioxide and oxygen, can affect markedly the enzymes and chemical components of the living seed. Fungi, insects, bacteria, chemicals, or light can diminish or destroy the seed's power to germinate. Many of the same factors, in the right concentration or combination, can enhance the life processes of the seed.

THE WATER CONTENT of the developing seed is similar to that of any actively growing tissue about 70 to 80 percent. As the seed reaches maturity and the stage at which it is shed from the plant, however, its moisture drops rapidly.

How much water remains at maturity depends on the species of the plant and the environment in which the seed matures. When seeds are artificially extracted from the fruit, their moisture content is affected by the method of extraction and the storage conditions.

Seeds of the maple, wildrice, and orange illustrate the critical role of the water level in seeds at their time of harvest and during storage.

The silver maple (Acer saccharinum) sheds its seeds in June at a moisture content of about 58 percent. The seed dies if the water content falls below 3o percent. Seeds of sugar maple (Acer saccharum), however, mature in September, contain less than 3o percent water, and can be air dried to about 5 percent without lowering germination.

Some seeds, such as wildrice (Zigania aquatica), must actually be stored in water at 32 F. for maximum germination. They lose their ability to germinate if they are exposed to air for a few days.

Lela V. Barton, working at the Boyce Thompson Institute of Plant Research, Yonkers, N.Y., showed that citrus seeds stored at room temperature are injured if their high content of water drops. Orange seeds are injured by drying to 25 percent moisture. Grapefruit seeds became inactive when dried below 51 percent.

How the external environment in which a seed develops affects its moisture content is illustrated by a study of seed of ponderosa pine. N. T. Mirov, of the U.S. Forest Service, found that seed from trees growing at 2 thousand to 3 thousand feet contained 18 percent more moisture than those at 6 thousand to 7 thousand feet, although he dried all the seeds under the same conditions.

This lower water-holding capacity of seeds from higher elevations supports Nicolai A. Maximov's theory that plant tissues that must survive severe cold usually contain less water than those of warmer climates. The mechanism by which the living seed is protected under such varying conditions of development can be related to their chemical composition.

Research at the Boyce Thompson Institute compared seeds high in fat content (pine and peanuts) with those low in fat and high in carbohydrate (tomato and onion). The amount of water held by seeds stored at the same temperature and relative humidity fell as the amount of fat and oil increased. Thus the ability of tissues of a low water content to withstand cold was related to their chemical composition, especially the amount of fat in them.

High temperatures may kill the seed, too. The seeds with a high content of water are less tolerant of high temperatures. Ponderosa pine and Douglas-fir seeds remained viable after heating at 150 F. for 3 hours when they contained only 7 percent of water. With a water content of 60 percent, the seeds were rapidly killed by temperatures above 110 .

If seeds are stored with a high moisture content, internally produced heat may raise the temperature in the storage container and shorten the lifespan of the seed. This damage, as that from externally applied heat, results from changes in cell metabolism. The breakdown and conversion of chemical components and seed protein by enzymes is made easier by an abundance of water and is accelerated at high temperature.

Although enzymes are present in dry seed, they are activated only on movement of water into the seed. As the temperature increases, the rate of metabolism enzyme activity also increases.

One measurable product of this metabolism is the amount of carbon dioxide given off and oxygen taken up. A rapidly metabolizing seed has a higher gas exchange rate than a quiescent seed. If the energy made available by respiration is not used in growth, it is liberated as heat, and the temperature of the stored seed goes up. Water content, one of the most important factors in seed viability, therefore cannot be considered alone.

Variations in water content influence the seed's metabolic activities, including respiration, its temperature, and its ability to germinate.

Increasing the amount of water in a seed above 10 to 15 percent strongly activates the cell enzymes. In stored sunflower and flax seeds, respiration rates increased with increasing water content up to 50 percent. The temperature inside the seeds increased after the increase in respiration. On heating at temperatures greater than 120 , the living cell protein coagulates irreversibly, just as an egg hardens on heating.

If the water content is too high, large amounts of the chemicals required for growth will be used up. The seeds will then be unable to germinate when they are placed under proper conditions.

Removal of too much water from the seed also causes death. The optimal water content of stored seed at 32 to 40 has been determined for most agriculturally important species.

The moisture content of the storage container is usually regulated by the use of chemical desiccants, such as calcium chloride or solutions of sulfuric acid, which maintain a constant relative humidity in the closed compartment or storage jar.

To maintain maximum viability, most seeds are stored at a fixed moisture content and at a fixed temperature, usually between 32 and 41 . In this temperature range, water in the seeds does not freeze, but enzyme activities are retarded.

Although the optimum storage water content and water-holding capacity may differ in seeds of different species, the universal nature of enzymes controlling metabolism in all living cells establishes the narrow temperature limits.

When enzyme activities are drastically reduced at low temperatures, the chemical substrates are preserved in a form essential to maintaining the maximum germinative capacity of the stored seed.

It is possible to store seeds at temperatures below 32 if the water content is low enough. Temperatures as low as minus 320 , the temperature of liquid nitrogen, do not injure wheat embryos with less than i o percent of water, but kill embryos containing 50 percent of water.

So far, the technique of storing seeds at very low temperatures is merely an experimental laboratory procedure, which is neither necessary nor economically feasible.

Regardless of storage temperature or moisture content of the seed, as long as the protoplasm remains alive the enzymes continue some chemical activities, and respiratory changes occur.

Only the most sensitive instruments can detect these changes.

Changes in organic compounds also occur with the uptake of oxygen and release of carbon dioxide in living, but nongrowing, seeds. If these seeds are germinated, the rate of respiration increases, and the chemical changes, uptake of oxygen, and release of carbon dioxide are easy to detect.

By following respiratory changes over extended periods in the quiescent seed and comparing them to changes in the early phases of germination, we can compute more precisely the amount of gas exchange in stored seed.

THE GAS ATMOSPHERE surrounding mature seeds can determine if the seeds remain alive. If a container of seeds is evacuated and the oxygen pressure is reduced, the seeds keep better than in air. Lack of oxygen retards respiration. Some seeds are short lived in air even at low temperatures. Often they can be kept alive for many years in an atmosphere of nitrogen or hydrogen at temperatures near 40 .

Seeds planted too deeply in soil, where little oxygen is present, will not live. As the depth of planting increases, the available oxygen and seed survival decrease. Wet or poorly drained soils also lack oxygen and inhibit the living processes of the seed. Most seeds immersed in water will die unless air is bubbled through the water.

A shortage of oxygen usually kills the seed when the temperature or respiration is high. This happens because enzymes need oxygen to produce energy for growth of the embryo. The energy is released when the enzymes combine oxygen with various cell compounds.

Sometimes, however, high levels of oxygen are not required by the living cell to obtain energy from its chemical compounds. Some seeds have an abundance of the anaerobic enzymes, which function without oxygen. These enzymes produce enough energy for certain life processes.

Rice seeds (Oryza), for example, do not require much oxygen to function. The cells of the embryo and seedling have a system of anaerobic enzymes and a special kind of respiration that requires little oxygen. Seeds of rice, and of a few other plants, therefore can remain viable and germinate under water that contains too little oxygen for the survival of most seeds.