Respiration in Germinating Seeds
Germination can seem almost magical. A dried, wrinkled, brown particle suddenly turns into a rapidly growing seedling. The transformation begins with the absorption of water by the seed, which leads to many other changes in the plant's cells, including an increase in respiration. This process, common to both plants and animals, involves the use of oxygen from either air or water to break down carbohydrates and other food into a usable form.
A seed is a storehouse of carbohydrates, fats, organic acids and amino acids that surround the embryonic plant. The whole thing is covered with a hard, protective seed coat. The embryo contains the beginnings of both roots and shoots plus the cotyledons, the first leaves. Seeds may look dry and lifeless, but they do respire during dormancy, slowly absorbing oxygen and releasing carbon dioxide to maintain life.
The process of respiration, which takes place in the mitochondria and cytoplasm of the cells, converts stored food compounds into energy. Each step is regulated by enzymes. Seed germination and most plant processes require oxygen, usually absorbed from the air, though there are other respiratory operations, called anaerobic respiration, that do not.
Respiration usually increases as the temperature rises and decreases as it falls. This is one reason why dormant seeds have a longer life span at lower temperatures. At the proper temperature, the one preferred by a particular species for germination, respiration is accelerated as the seed germinates. Temperatures that are too high, however, interfere with respiration.
Water is necessary for enzymes to act. At the usual 10 percent water content of dormant seeds, respiration is slow. When water is absorbed, respiration increases dramatically as enzymes begin to activate.
In low-oxygen conditions, such as a bog for instance, respiration decreases. If seeds are stored in a low-oxygen atmosphere, they will be viable for longer.
Germination begins with the swelling of the seed as it absorbs water, enzymes become active, respiration increases and the internal temperature of the seed rises. If, at this point, the seed dries out, freezes or experiences some other mishap, it will die. It cannot go dormant again.
Next, the cells of the embryonic plant multiply, the seed coat splits and the plant emerges, first the root, then the shoot. The root helps absorb more water and nutrients and anchors the germinating seed in place.
Planting Depth and Respiration
Seeds planted too deeply may not get the oxygen they need to sustain respiration, and may not have enough stored food to send the shoot to the surface. Seeds planted too near the surface may dry out after absorbing water or may be heated too much by the sun. In either case, the respiration process will stop.