Aspectos éticos

58

60 LIGHT AND SEED GERMINATION

The instructions on the packet of lettuce seeds are clear: cover the seed with no more than 3 mm (1/8 inch) of soil. But what would happen if the seeds were planted too deeply? In all like-lihood, many seeds would not germinate. This implies that a lettuce seed is able to sense its position in the soil and use that information to “determine” whether or not germination is appropriate. Now we know seeds do not have a nervous system or conscious thought, so how can a seed sense its position in the soil and “know” whether or not to germinate? The answer lies in the fact that the lettuce seed requires a light treatment before it will germinate and light does not penetrate very far into most soils—a few millimeters at best. If the seed is planted too deeply, it will not be able to detect the light that is required to stimulate germination. Lettuce seed is not alone; the same is true of many other seeds as well. Natural soils, for example, may contain many thousands of weed seeds that fail to germinate because they are buried too deeply and are unable to detect the light. It has been estimated that a hectare (about two acres) of farmland contains aseed bankwith more than two million weed seeds that will not germinate until the soil is disturbed and the seeds are brought closer to the surface where they are exposed to light.

There is good reason for this behavior. Seeds that require light for germination are usually very small (a single one ounce or 28-gram packet of lettuce seeds may contain 20,000 seeds) and, consequently, have limited nutrient reserves. When the seed germi-nates, the seedling has only the store of nutrients in the seed to draw upon until it breaks through the soil and can begin photosynthesis.

The light requirement is a way of telling a seed whether it is in the right position relative to the soil surface and that reserves will be sufficient to ensure the seedling is able to reach the surface. For weed seeds, there is an additional advantage; the large bank of seeds in the soil ensures survival of the species by spreading germina-tion through successive disturbances over several years.

The control of seed germination is just one example of the many ways plants use light to regulate their development.

Collectively, these responses are called photomorphogenesis(from photo, meaning light; morphology, meaning form; and genesis, meaning to give rise to). However, before we delve further into photomorphogenesis, it would help to review some fundamen-tal principles of light.

LIGHT AS A SOURCE OF INFORMATION

The 18^th century English poet, essayist, and lexicographer Samuel Johnson once said, “We all know what light is; but it is not easy to tell what it is.” Light is a form of energy that has some interesting and perplexing properties. The problem arises from the observation that light consists of discrete bundles of energy that have both wave-like and particle-like properties. When light is transmitted through space it is described by regular and repetitive changes in its electrical and magnetic properties; it behaves like a wave. On the other hand, when light is emitted—as from the sun or a light bulb—or absorbed by a pigment, it behaves like a stream of particles called quantaor photons.

The peak-to-peak distance on successive waves is called the wavelength, which is normally measured in nanometers (nm = 10^-9m). The dual nature of light has two important and related consequences for organisms. First, different wavelengths of light are recognized by the eye and brain as different colors (Figure 4.1). Second, the energy carried by a photon is inversely proportional to its wavelength. In other words, a photon of long wavelength red light carries less energy than a photon of shorter wavelength blue light. In a process such as photosynthesis, the

quantumnature of light is most important. The photosynthetic pigment chlorophyll consumes photons of energy that are eventually stored in sugar products. Up to a point, the more energy that is absorbed in photosynthesis, the more products can

be formed. In photomorphogenesis, the wave nature of light is more important. It is not the energy of light that drives photo-morphogenesis so much as the information that is conveyed by different wavelengths of light.

Here is one example of the kind of information that may be conveyed by light. In full sunlight—such as in the middle of a field—the ratio of red light (R, nominal wavelength = 660 nm) to far red light (FR, nominal wavelength = 730 nm) is about 1.05 to 1.25. Under a canopy of trees—such as on the floor of an oak forest—the R/FR ratio drops to something around 0.12 to 0.17. Can you think of why the proportion of red light drops so dramatically in the shade? The reason is that the chlorophyll in the leaves of the canopy filter out the red light for use in

Figure 4.1 Visible light is that small portion of the electromagnetic spectrum that causes the sensation of color in the human brain.

photosynthesis, but chlorophyll is virtually transparent to far red light. Consequently, the canopy has changed the balance of wavelengths or spectral composition of the light reaching the forest floor by enriching it with far red light. A plant that has a mechanism to detect this change in red/far red ratio could use this information to determine whether it was in full sun or in the shade of a canopy and adjust its physiology and development accordingly.

In addition to canopy shade, a variety of atmospheric factors and the time of day can influence the spectral composition of light. Sunlight thus satisfies two very important needs of plants:

(1) energy to drive photosynthesis and (2) providing critical information about the environment that is used by plants to regulate movement, trigger developmental events, and measure the passage of time.