Read more: 鈥�Rooted in experience: The sensory world of plants鈥�
What do plants see? The obvious answer is that, like us, they see light. Just as we have photoreceptors in our eyes, they have their own throughout their stems and leaves. These allow them to differentiate between red and blue, and even see wavelengths that we cannot, in the far red and ultraviolet parts of the spectrum. Plants also see the direction light is coming from, can tell whether it is intense or dim and can judge how long ago the lights were turned off.
In one of his last studies, Charles Darwin showed that plants bend to the light as if hungry for the sun鈥檚 rays, which is exactly what they are. Photosynthesis uses light energy to turn carbon dioxide and water into sugar, so plants need to detect light sources to get food. We now know they do this using phototropins 鈥� light receptors in the membranes of cells in the plant鈥檚 tip. Phototropins are sensitive to blue light. When they sense it, they initiate a cascade of signals that ends up modulating the activity of the hormone auxin. This causes cells on the shaded side of the stem to elongate, bending the plant towards the light.
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Plants see red light using receptors in their leaves called phytochromes. A phytochrome is a sort of light-activated switch: when irradiated with red light, it changes its conformation so that it is primed to detect far-red light, and when irradiated by far red it changes back to the form that is sensitive to red light. This has two key functions. It allows plants to 鈥渢urn off鈥� at the end of the day 鈥� because far-red light predominates at sunset 鈥� and wake up again next day when the sun is high enough in the sky for red light to switch their phytochromes back on. It also allows them to sense when they are in the shade. Chlorophyll, the main pigment for photosynthesis, absorbs red but not far-red light, so when a plant is being crowded out by other plants it will see more far-red light than when it is growing in full sunshine. This directly influences the level of activated phytochromes, causing the plant to grow rapidly to get better exposure to the sun.
Phototropins and phytochromes are completely different from the photoreceptors found in animals鈥� eyes, although all consist of a protein connected to a chemical dye that absorbs the light. There is one type of photoreceptor, however, that we share. During daylight hours, cryptochromes within cells detect blue and UV light, using this signal to set an organism鈥檚 internal clock or circadian rhythms. In plants, this clock regulates many processes, including leaf movements and photosynthesis. So sight even helps plants tell the time.
