Seeds can delay germination if they detect the presence of plants overhead.

For a small seed that has few stored reserves, germinating in the shade of another plant could be fatal. But how can a seed detect the presence of plants overhead?

As sunlight passes through leaves, the red wavelengths are absorbed by chlorophyll but the far-red wavelengths are not. The ratio of red to far-red light is thus much higher under an open sky than it is in the shade of another plant. When scientists first began to study the effects of light on seed germination, they found that red light is particularly good at stimulating germination, while far-red light inhibits germination. A simple “flip-flop” experiment by H. A. Borthwick and colleagues showed that the inhibitory effect of far-red light could be overcome by a subsequent exposure to red light (Fig. 31.22). Ultimately, it was shown that only a single photoreceptor was needed to detect both far-red and red light.

HOW DO WE KNOW?

FIG. 31.22

How do seeds detect the presence of plants growing overhead?

BACKGROUND To study the effect of light on seed germination, scientists exposed lettuce seeds that had been kept continually in the dark to different wavelengths of light and then counted what fraction of the seeds germinated. Red light had the greatest ability to stimulate germination, but surprisingly, far-red light inhibited germination such that fewer seeds germinated than in the control seeds, which were kept in darkness. In the rush to conduct more experiments, petri dishes with the light-treated seeds piled up by the sink until someone noticed that the seeds that had been experimentally treated with far-red light were now germinating.

HYPOTHESIS This observation suggested that the inhibitory effect of far-red light can be overcome by a subsequent exposure to red light.

EXPERIMENT The scientists exposed lettuce seeds to red and far-red light in an alternating pattern, ending with either red light or far-red light. They then placed the seeds in the dark for two days and afterward counted the number of seeds that had germinated.

image
FIG. 31.22

RESULTS When the lettuce seeds were exposed to red light last, nearly 100% of the seeds germinated. By contrast, when the last exposure of the seeds was to far-red light, the percentage of seeds that germinated was dramatically reduced.

CONCLUSION Seed germination in lettuce is triggered by exposure to red light and is inhibited by exposure to far-red light in a reversible fashion. As a result, plants are able to track changes in the relative amount of red and far-red light, which provides information on the presence or absence of plants overhead (see Fig. 30.23).

FOLLOW-UP WORK Additional studies showed that this result was due to a single pigment in a photoreceptor, now known as phytochrome, that is converted into an active form by red light and reversibly converted into an inactive form by far-red light.

SOURCE Borthwick, H. A., et al. 1952. “A Reversible Photoreaction Controlling Seed Germination.” Proceedings of the National Academy of Sciences 38: 662–666.

Phytochrome is a photoreceptor that switches back and forth between two stable forms depending on its exposure to light (Fig. 31.23a). Red light causes phytochrome to change into a form that absorbs primarily far-red light (Pfr); far-red light causes phytochrome to change back into the form that absorbs red light (Pr). Because red light stimulates seed germination, we know that Pfr is the active form of phytochrome. That is, red light causes phytochrome to change into its Pfr form and Pfr sends a signal that triggers seed germination.

image
FIG. 31.23 Detection of shading. Phytochrome detects the depletion of red light under the canopy.

660

Phytochrome allows dormant seeds to detect the presence of plants overhead (Fig. 31.23b). Because it responds to the ratio of red to far-red light, the proportion of phytochrome that is in the active Pfr form is much greater in open environments than in the forest understory. Furthermore, because the ratio of red to far-red light is independent of light intensity, it provides a reliable signal that leaves are overhead even for seeds covered by a thin layer of soil. Independent of whether light is present or not, Pfr slowly converts back into the inactive Pr form. The light-independent conversion of Pfr to Pr prevents seeds that are too deep in the soil, yet still receive some light, from germinating.