Color Perception in Plants
For experimental philosopher and artist Jonathon Keats — the world is a playground. Unbound by conventional thinking, he pushes at the limits of our experience by creating physical manifestations of imaginative thought experiments. Such experiments challenge us to consider and contemplate our world in a way that is both concrete and tangible, not detached and academic like most forms of philosophy. One of these experiments, the Photosynthetic Restaurant, is literally a “restaurant” for plants. We all know plants need light to survive. You could even say they “eat” light, as photosynthesis is the process whereby plants convert sunlight to energy and food. The Photosynthetic Restaurant is an attempt to play with this notion of light as food by exploring and imagining the different effects and possibilities of spectral light. To make his restaurant, Keats positioned a series of colored acrylic filters in the garden at the Crocker Art Museum in Sacramento, California. These transparent gels, mounted on copper poles, manipulated the portion of the electromagnetic spectrum the plants received throughout the day as the sun moved across the sky. In an interview in The Atlantic, Keats maintained that, “For nearly a half billion years, plants have subsisted on a diet of photons haphazardly served up by the sun and indiscriminately consumed, without the least thought given to culinary enjoyment.”
It has been known for some time that plants respond selectively to different wavelengths of light. For Keats, this was an opportunity to explore parallels to culinary practices in human culture, while at the same time imagining plant functions from an aesthetic point of view rather than just a biological one. For instance, plants use far-red light (between red and infrared) to detect other plants nearby, as this type of light is often reflected off of leaves. The presence of other plants is not necessarily a good thing. In an interview in the Atlantic, Keats said, “If you’re a plant, you don’t want others getting too close because then your sunlight is occluded, and so plants have a sort of fear response to the far-red part of the spectrum.” For Keats, the dietary equivalent in humans might be something like a habanero pepper or spicy food. “We get an enjoyment out of eating them, in spite of the physiological panic that they cause, because spicy cuisine brings about that sort of panic in a controlled way.” At the Photosynthetic Restaurant, the red filters became the culinary equivalent of spicy food for the plants!
At the other end of the spectrum, one way that plants often discern the end of the day is through short-wave light (deep blue to ultraviolet) along with far-red light. To this end, Keats employed violet filters late in the day to provide the plants with a sunlight version of a digestif — a nightcap of sorts, to “aid digestion.” To mix things up a bit, he also developed what he called an avant-garde menu for some plants. He did this by juxtaposing the violet digestif with an orange mid-day light in order to play a sensory trick on the plants. He describes this menu as “playing against the expectations of the organisms that are my patrons. Just as in the case of human cuisine, when it becomes avant-garde, there’s an element of surprise and disrupted expectations, and what comes naturally is put out of order.”
Though Keats’ projects often use animals or plants to contemplate human behavior, the photosynthetic restaurant does raise some interesting points about color and plants. Plants don’t see, in the sense that we understand the term, but they do “perceive” color. Their sensory apparatus is incredibly sophisticated. Being sessile (unable to move), they have developed some amazing abilities to adapt to and exploit their environments. One of the key components of this ability is their response to light. For most people, the obvious example is phototropism — the ability of plants to bend towards the light. Experiments by Darwin and others, in the latter half of the 19th century, demonstrated that this has nothing to do with photosynthesis but rather with a type of “rudimentary sight” (photoreceptors) on the tip of new shoots that is responsive only to blue light! Cover these “eyes” on a plant and it will not move toward the light; it becomes blind to this possibility. But phototropism is only one of many plant behaviors that depend on specific wavelengths of light — others, like we have seen, involve detecting the presence of other plants, knowing the changing length of the day (and hence the season), knowing when to germinate, or grow, or flower, and knowing when not to grow or blossom. All of this is achieved through the presence of a wide variety of photoreceptors in the plant that respond to the specific wavelengths of sunlight.
Humans have four types of photoreceptors in the eye; rhodopsin in the rods and three types of photopsin in the cones, one each for detecting short, medium, and long-wavelength light. Most plants, however, have many more than four. For example, Arabidopsis thaliana, a type of wild mustard, has eleven types of photoreceptors that fall into a variety of different classes, such as phototropins, phytochromes, and cryptochromes. At the level of these chemical photoreceptors, sensitive to the spectral composition of light, one might speculate that color perception in plants is even more sophisticated than humans!
Keats’ Photosynthetic Restaurant is more than just a speculative exercise in “solar gastronomy,” it is an opportunity to reimagine our relationship to the non-human world. An opportunity to contemplate the intelligence and sophistication of all living things.
In the end, however, all is not haute cuisine. For that particular class of domesticated plants, limited to the confines of the house, Keats has developed the ultimate TV dinner (literally), a DVD of colored light that you literally play on the TV for your houseplants. “I filmed the sky through different color filters and then made an hour-long movie that plants are able to consume.”