As common wisdom tells us, plants are blind. It is quite evident that plants do not have eyes and definitely do not have a brain, so obviously they cannot see and wouldn’t know the difference between night and day, right? WRONG! A plant’s very survival is contingent upon its ability to detect and respond to light. Light levels can dictate seed germination, growth responses, senescence, and flowering time, so it is critical that plants can “see.” You have likely noticed this phenomenon when placing a plant next to a window. While plants may not have a specialized organ for light detection or a brain to interpret an image, this is “sight” nonetheless. Broadly, sight is the ability to detect and respond to electromagnetic radiation near the visible light spectrum. Sight is a physical sense in which light stimuli are received by special proteins called photoreceptors which can detect various wavelengths and convey information regarding the position, shape, brightness, and sometimes colour of objects within space. While humans only have 3 photoreceptors to allow colour vision, plants have at least 11, making them more sensitive to light than us!
So, how does it all work? Simply put, plant photoreceptors absorb a light wave of a particular wavelength and use that energy to initiate a physical response! This allows plants to sense the intensity, periodicity, and directionality of light. While there are a multitude of photoreceptor classes, the three most common are: phototropins (detect blue light), phytochromes (detect red and infrared light), and cryptochromes (detect blue light). Phytochromes (red & infrared) are important for the detection of light intensity and regulates processes such as germination, flowering, and growth. Cryptochromes (blue) control a plant's internal clock, which regulates processes like leaf movement and photosynthesis. Lastly, phototropins (blue) are important for plant movement.
Phototropism is the growth or movement towards directional light and is primarily induced by blue light. Plant tropisms remained widely unresearched in the western world until Darwin’s The Power of Movement in Plants (1880). He discovered that the shoot apical meristem was not only responsible for detecting light but somehow transferred this information to the plants’ midsection to tell it to bend towards the direction of the light. The genius Darwin had provided the first evidence to support that plants had rudimentary sight!
Further research has found that a plant’s slow bending towards directional light, evoked by the shoot apical meristem, is due to acid-mediated cell elongation. Acid mediated cell elongation is an irreversible process activated by the plant hormone auxin. Auxin is redirected to the shaded side of the plant's midsection and binds to proton (H+) pumps in the cell membrane making the cell wall acidic. The acidity acts on numerous proteins and cell wall components causing the cell wall to become flexible and distorted through turgor pressure. Cells thus elongate and cause the plant to grow towards the light stimulus.
Light-response research has recently been gaining traction in the plant sciences. In agricultural and horticultural operations, growers have taken advantage of the fact that certain plants are long-day or short-day flowering species. A long-day flowering species will initiate flowering when the nighttime period of darkness exceeds their biologically set minimum. Furthermore, disrupting the night cycle with a flash of red light can trick the plant into flowering or not flowering depending on the type. What’s even more interesting is that a flash of far-red light reverses this effect altogether! Greenhouse operators have taken advantage of this product of plant evolution to control plant flowering in accordance to when it’s convenient for humans.
While plants may not form images like us, their ability to detect light is quite advanced and so are their physiological responses in response to what they “see.” Hence, the next time you stand in the way while a daisy is photosynthesizing in the July Sun, apologize immediately!