Sun Stalkers

Natural History, May, 1999 by Candace Galen

Solar-tracking flowers bend from the waist.

Morning light arrives early on the high slopes of the Colorado Rocky Mountains. At 5:00 A.M., a climber on a westward route to the summit of a fourteen-thousand-foot peak can count on a cheerful welcome from thousands of snow buttercups (Ranunculus adoneus), their dazzling yellow flowers facing east in the glow of the sun's early light. It's as if, overnight, a blast of wind blew down the slope, all the little yellow umbrellas in its path. Then, as the sun moves across the sky from east to west, the buttercups turn to follow it, and their bright faces greet the climber once again as she makes her descent.

Solar-tracking, or heliotropic, flowers are most common in arctic and alpine environments, where the air is often cool and the growing season is short. The satellite dish-shaped flowers of the snow buttercup, the arctic poppy, and other heliotropic flowers collect the sun's rays so efficiently that they heat up, becoming as much as fourteen degrees Fahrenheit warmer than the air around them. These miniature saunas are enticing to insects, which are by and large unable to generate their own heat and must wait for the sun to warm them up before starting the day's activities. Anyone who has observed bees on a flower early in the morning, when they are so still as to seem drugged or nearly dead, is familiar with insects' dependence on external heat sources. Solar-tracking flowers provide their insect visitors with a warm retreat for basking, foraging, even mating. In return, the visitors pollinate their hosts. The considerable heat absorbed by heliotropic flowers also jump-starts the development of newly fertilized ovules, helping the plants complete seed maturation in as few as eight weeks.

Harsh arctic and alpine conditions provide a "motive" for solar tracking, but what of the means? The mechanisms that leaves use to follow the sun are much better known than those flowers use. Movement in solar-tracking leaves, first written about by Charles Darwin in his 1880 book The Power of Movement in Plants, can occur rapidly and is reversible--two defining features of heliotropism. In nasturtiums, for example, a specialized organ at the base of the leaf--the pulvinus--continually orients the leaf surface at a right angle to the sun's rays, maximizing light interception for photosynthesis. The plant equivalent of a muscle, the pulvinus consists of specialized extensor and flexor cells that swell or shrink with changes in turgor pressure (determined by the amount of water in the cell). As extensor cells swell and flexor cells shrink, the leaf blade is reoriented to track the changing position of the sun.

Experiments with the snow buttercup have begun to reveal the sensory and developmental processes that lead to heliotropism in flowers. My colleague Maureen Stanton, of the University of California, Davis, and I started with a simple yet vital question: How do flowers sense the position of the sun? We knew that light provides plants with information as well as energy. Photomorphogenesis (plants' developmental responses to light) begins with photosensitive molecules in the cells of certain plant organs. Phototropism (one kind of photomorphogenesis) orients growing organs toward a light source. The sunflower, which is the plant kingdom's version of a morning person, shows phototropic stem growth, with the flower at the tip of the main stem always facing east. While phototropism does not exhibit the reversibility seen in heliotropism, we reasoned that similar sensory mechanisms might be involved in true solar tracking.

Anecdote has it that in the nineteenth century, a Frenchman noticed that some plants situated behind bottles of red wine failed to grow toward the sunlight. This early observation suggested that short-wavelength blue light, known to be blocked by red pigment, was necessary for phototropism. Blue light was later shown to play a crucial role in changing turgor in the pulvinus of heliotropic leaves. To determine whether blue light also cues solar-tracking in flowers, we performed our "buttercup in a Wine bottle" experiment. But we had no intention of carrying cases of cabernet up to twelve-thousand-foot elevations in the Rockies. Instead, we constructed lightweight cubes out of red acrylic filters. In the evening, we placed the cubes over snow buttercup plants. The next morning, we discovered that the filters had completely disabled the plants' ability to locate the sun. The confused flowers faced every which way, on average more than thirty degrees away from the sun's rays. Flowers in the two groups of control plants--those surrounded by blue-light-transmitting filters and those simply left in the open--tracked the sun much more successfully, facing, on average, within fifteen degrees of it.

Our experiment with acrylic cubes confirmed that blue light guides the movement of heliotropic flowers. But which organs actually perceive the light signal and translate it into the biochemical language of plant growth? Molecular biologists approach such questions by searching for gene products involved in photomorphogenesis. Rebecca Sherry, formerly a colleague at the University of Missouri-Columbia, and I took a less sophisticated tack, simply removing portions of the buttercup plant and discerning whether what remained could accomplish the task of solar tracking. Inspired by the Queen of Hearts in Alice's Adventures in Wonderland, we began with the edict "Off with its head!" and decapitated a number of innocent snow buttercups, removing the solitary flowers from their supporting stems. Neighboring buttercups were spared, as controls. Surprisingly, we found that the stems of decapitated buttercups continued to move over the course of the day, along with those of the control plants, aligning the ghosts of flowers past with the rays of the sun. Barring a paranormal phenomenon, this result means that the guidance system for flower heliotropism is housed in the stem rather than in the flower.