A newly germinated seedling must reach the soil surface before its stored resources are used up or it will die. Seeds that germinate in the dark produce completely white seedlings with thin, highly elongated internodes and small leaves (Fig. 31.24). By putting all their resources into elongation, seedlings grown in the dark maximize their chances of reaching the light. If and when they do, internodal elongation slows markedly and green leaves expand. These dramatic changes are triggered by photoreceptors that signal the presence of light. In its transformation from a dark-grown to a light-exposed seedling, the seedling receives signals from several types of photoreceptor, including one that absorbs blue wavelengths and one, phytochrome, that absorbs both red and far-red wavelengths.

Even after a plant emerges above the soil, its ability to sense and respond to light can enhance its chances for survival. Some species are adapted for shade, producing thin leaves that efficiently use low levels of light; others grow best in direct sunlight. When such sun-adapted species are grown in the shade of another plant, they direct their resources primarily into growing tall. These plant species produce stems that have long, thin internodes and fewer branches, and they invest less in root growth.

Interestingly, the effect is much less if the shade is produced by anything other than another plant. Experiments have shown that the taller, thinner stems are the result of a plant’s sensing the presence of its neighbors. The reason a plant is able to sense neighboring plants is that chlorophyll absorbs red, but not far-red, wavelengths of light. Thus, as light passes through leaves, it becomes depleted in red wavelengths. As a result, phytochrome is converted from the P_fr to the P_r form. Furthermore, light reflected off the leaves of a plant’s neighbors is also depleted in red wavelengths, so phytochrome allows a plant to detect not only when it is underneath another plant, but also when it is surrounded by other plants that could grow to shade it in the future. Thus, phytochrome provides an early-warning system that competitors are nearby.

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