How plants "see": plants catch light for the information it carries as well as for its energy. The light helps plants determine when to germinate, when to flower, or how to respond to neighboring plants

Natural History, Sept, 2004 by Marcelo J. Yanovsky, Jorge J. Casal

Two dozen grass plants, each one surrounded by a title circle of red lights, stood in the stillness of the morning air. From a distance they looked just as they had when a team of investigators had left them a few weeks before. But in Fact, they had changed. What the members of the team were about to observe, after a five-hour journey across the hot Argentine Pampas, would come as a generous reward for their labors.

One of us (Casal) was a member of that team, which had come to the plants' natural environment hoping to answer an unusual question: What are the limits of plant "vision"? When a verdant locale becomes overpopulated, and the plants start shading one another, some species display a remarkable strategy: they restrict the development of their own new shoots and accelerate stem growth instead, thus becoming more aggressive competitors for light. Somehow, these plants seemed to be sensing the shadow cast by other plants; but, how were they doing it?

At that time, most plant physiologists thought that mutual plant shading simply reduces the amount of light available for photosynthesis, the process whereby plants build organic molecules with energy from sunlight; shaded plants, after all, grow fewer new shoots than plants growing in the sunlight. A second hypothesis was that plants in the shade detect more specific changes in the light environment caused by the presence of other plants, presumably via an array of molecules called photoreceptors, which work much like a visual system. No one knew which hypothesis was correct.

In recent decades, plant physiologists have become increasingly aware that plants use light for much more than photosynthesis. Plants have sophisticated skills for obtaining and processing the information that light carries about their local environment. Not only can they sense the presence of other plants and react in ways that maximize their chances of-survival. But more, light enables plants to determine that spring is beginning or that winter is ending. Via light, seeds know when to germinate, and adult plants know when to flower.

The flowering process is particularly intriguing because, in most plants, it is precisely synchronized with seasonal changes. Certain plant species do not flower if the days are too long; others flower only when daylight lasts longer than a certain number of hours. This observation suggests that plants can assess the changing length of the day as the seasons come and go, a complex and difficult task. To do so, they must discriminate day from night, measure the passage of time, and integrate the information. Do plants possess the means of carrying out such a complex process? They certainly do. Just as plants measure light with photoreceptors, they measure time, appropriately, with a biological clock. And as for the integration of light and time, we have been able to show, after countless hours of experimentation, that plants are constantly, monotonously making and then destroying molecules that help control flowering. The concentration of those molecules is the key to a seemingly complex decision.

The whole idea that plants can perceive such subtle variations in light as the ones caused by nearby plants, and respond to them with dramatic changes in their own growth patterns, has always fascinated us. Light, of course, carries a wealth of information for those able to decode it. Plants begin decoding light by "dissecting" the ambient white light, which is made up of all the colors of the rainbow. Molecules such as chlorophyll, present throughout the leaves and stems, selectively extract specific colors, or wavelengths, from the white light. Green foliage absorbs most of the red and the blue, and reflects, as well as transmits through its leaves, a color known as "far-red," whose wavelength lies just outside the range of visible light [see illustration on next pace]. And the reason plants look green is that they reflect green light.

Plants can measure not only the amount, or intensity; of each of several colors, but also the ratios of certain pairs of colors. At the time of the experiments in the Pampas, the team had hypothesized that plants can "see" the overshadowing leaves of other plants by measuring the ratio of red to far-red. The idea was that a densely populated, shaded environment is rich in the far-red wavelengths that other plants reflect, but is poor in red, the color that the other plants absorb. Thus, a plant detecting the ratio of red to far-red would immediately "know" if it is being shaded; with other shading plants around, the red-to far-red ratio would be low.

In the field, the uppermost leaves of grass plants are exposed to full sunlight; only their bases are mutually shaded. Hence, only the plant's base is exposed to a low red-to-far-red ratio. Is that light signal enough to modify their growth pattern? That was the big question for Rodolfo A. Sanchez, a plant physiologist, and Victor A. Deregibus, a forage scientist, both at the University of Buenos Aires, in 1980, when Casal joined them as a undergraduate. The three investigators studied the reaction of the grasses Paspalum dilatatum and Sporobolus indicus to the red-to-far-red ratio. Those two grasses, an important source of food for cattle in the Pampas, also turned out to be good experimental models for the studies.