BIOLOGY Volume 3 - A Guide to General Biology - 2004

22. GROWTH AND DEVELOPMENT

22.4. Growth and Development in Flowering Plants

22.4.1. Seed Dormancy

The Development of an embryo within a seed requires specific environmental conditions, namely moisture, optimal Temperature, and oxygen. However, even when these conditions are met, certain seeds fail to germinate; such seeds are referred to as dormant. For germination to commence, specific internal changes must take place within the seed, a collective process known as after-ripening. This requirement for after-ripening prevents premature germination. For instance, if seeds shed onto the soil in summer or autumn were to germinate immediately, the resulting seedlings might not survive the winter. In other words, mechanisms exist to synchronize germination timing with the onset of a season favorable for growth. Several such mechanisms are described below.

Barrier Mechanisms

These mechanisms typically involve the outer seed coats, which are impermeable to Water or oxygen, or possess sufficient mechanical strength to restrict embryo growth, as seen in many legumes. Such barriers are sometimes overcome by mechanical damage (scarification) to the seed coat; this damage can also be inflicted artificially by removing the seed coat or simply piercing it with a pin. In natural conditions, a similar effect may be brought about by soil Bacteria or by the passage of the seed through an animal's gut. The germination of certain seeds is stimulated by fire. More commonly, however, dormancy is broken by some physiological change involving the factors discussed below.

GROWTH INHIBITORS. Many fruits and seeds contain chemical growth inhibitors that prevent germination. Abscisic acid frequently acts in this capacity, as seen in ash seeds. Upon sufficiently prolonged soaking, the inhibitor is leached out of the seeds, or its effect is counteracted by an increase in growth stimulants such as Gibberellins. Tomato seeds serve as a good example; they contain high levels of abscisic acid, which prevents the seeds from germinating while still inside the fruit.

LIGHT. Seed dormancy can be broken by exposure to light following imbibition in water—a response controlled by Phytochrome. This stimulation is associated with an increase in the gibberellin content of the seed, as observed in certain lettuce varieties. Less commonly, light inhibits germination, as in Phacelia and Nigella (the corresponding experiment is described in Table 16.5, Section 16.4.2). This can be advantageous by ensuring that a seed does not germinate until it has been buried in a suitable environment, such as the soil.

TEMPERATURE. To germinate, certain seeds must first be subjected to low temperatures (a process known as stratification). This is characteristic of cereals and members of the rose family, such as plums, cherries, and apples. Stratification increases gibberellin activity and sometimes reduces the concentration of growth inhibitors. Consequently, seeds require a period of low-temperature exposure; furthermore, this mechanism reduces the likelihood of seeds germinating during a winter thaw.

The exact MECHANISM OF ACTION of light and low temperatures remains unclear; it may involve not only alterations in the levels of plant growth regulators, but also an increase in seed coat permeability.

22.2. Seeds that require light stimulation for germination are typically relatively small. What might be The Significance of this?

22.3. Light that has passed through leaves is enriched in green and far-red Components of the spectrum compared to light incident upon the leaf surface.

a) What causes this?

б) What ecological significance might this have for the seeds of plants such as lettuce, whose germination is controlled by phytochrome?

(Refer to Section 16.4.2 if necessary.)