The Secret Senses of Plants

In an extreme dramatization in the film, The Happening, trees emit neurotoxins that cause humans to kill themselves. In Tobago, the Main Ridge Forest Reserve is treasured for the benefits to human well-being.  Do plants really see, smell and respond to stimuli. Dr. Anjani Ganase, marine scientist, looks at the evidence.

Plants have been around for about 465 million years with some of the oldest plant species existing for over 100 million years ago, including species of pine, cycads and gingko. Humans have only been around for 250 thousand years. The evolution of bacteria in the ocean to carry out photosynthesis, where light energy from sun and carbon dioxide were converted to food and oxygen, dramatically altered the atmospheric conditions of the planet. This was known as the Great Oxidation Event. Increasing levels of oxygen formed the ozone layer that made living outside the water possible; and about 500 million years ago, plants began evolving on land along with the animals. The silent evolution of plants paralleled the development of the animal kingdom, diversifying and adapting to physical environments but also co-evolving with animals. However, to this day, we still don’t know exactly the mechanisms for how plants detect and respond to environmental cues for survival, a system that seems very distinct from our own sensory system.

Reaching towards the light. Plants compete for space under the sun. Photo courtesy Pat Ganase

Do plants see?

Plants have receptors to detect light for photosynthesis. They use these receptors to grow toward the light and some plant species are able to move their leaves and flowers daily or seasonally to mirror the sunlight for maximum light absorption. Light may be important and utilised by all the exposed parts of the plant, including the leaves and stems; but scientists have discovered that light may also be used directly in the root system akin to fibre optic cables. Some plant species are able to redirect light through the stem and into the roots, where cells could receive and use the (red or long wave) light energy directly rather than relying on the transport of nutrients. While changes in light conditions may indicate changes in food availability, it may also be an indication of other ecological conditions such as competition. Scientists have found functional eyespots on certain plant species that may reveal a more sophisticated form of ‘seeing’. This sophisticated vision has even been used to theorise for cases of plant mimicry. A woodland vine, called Boquila trifoliolata, is able modify its leaves to mimic the shape and colour of the host plant it is growing over to blend in and avoid predation.

Do plants feel?

We all know the children’s verse, “Ti Mari, Ti Mari, shut your door,” and the stories of the Venus Fly Trap that attracts and closes over prey insects. These plants display the closest thing to animal behaviour seen in plants. Ti Mari is a species of plant called Mimosa pudica also known as ‘Touch Me Not” common to the South American and Caribbean regions. Leaves of the Mimosa plant would collapse upon contact as a defence mechanism to avoid being damaged or trampled on. Another species of Mimosa (microphyllia) displays the same strategy but there are thorns between the leaves. When the leaves collapse, the thorns are exposed to the predator. Yet, most plant species respond to touch in less obvious ways. While the plants grow towards the light, they also need to detect obstructions as they grow. This form of thigmotaxis – the orientation of the plant in response to touch - is how vines are able to grow along a tree branch or a fence. Plants can also “feel” changes the environment and weather including daily and seasonal changes in temperatures; and can even anticipate rainfall.

Do plants hear?

Plant species have also been shown to be able to pick up on certain unique sound vibrations. For example, scientists were able to show that a species of plant was able to differentiate a grazing caterpillar on its leaves from the rustling leaves from wind and other buzzing insect sounds. The plant responded to the recorded chewing vibrations by releasing a chemical defence mechanism into its tissues that would deter caterpillars. Plant roots are also able to hear the sound of water, even through pipes, and it may be why the root system of certain trees, such as the Ficus species, are notorious for wrapping their roots around underground water pipes and eventually penetrating the pipes. Plants’ hearing can be even more sophisticated. Experiments done on a species of primrose (Oenothera drummondii), revealed that the flowers increased the sugar concentrations of the nectars in response to low frequency buzzing of nearby bees. The higher sugar content would hopefully encourage the bees to visit these flowers and increase the chances of cross-pollination. It is even thought that the flower shape acts as an ear for detecting and channelling the buzzing sounds through the plant.

Do plants smell?

Plants can detect changes in the air and the presence of odour molecules. A parasitic plant species called Dodder vines (Cuscuta pentagona) lacks chlorophyll and is known for wrapping around tomato plants and eventually strangling and feeding on them. The Dodder vines were used in experiments to determine how the vines were able to detect and find a neighbouring tomato by sniffing out the odour molecules released by the host. Other studies have shown plants ‘smelling’ fear or detecting threat. Field studies showed that plants that were being infested by herbivores would release a stress hormone that would be picked up by neighbouring plants and result in their beefing up their own security by increasing their chemical resistance to herbivory. A more common example of plants using their sense of smell is through synchronous ripening fruits. Ethylene is a plant hormone that is released by plants to trigger the ripening of fruits. Having fruits that ripen at the same time increases the odds of attracting animals to feed and disperse seeds.

It is easy for us to talk about the senses of smell, sight and touch to describe how plants function, because we can relate to our animal sensory systems. But using the same terminology, we also limit our understanding of the sensory systems of plants, considering that they use unique modes to carry out their functions and the mechanisms are still unknown. As we continue to learn more about how plants utilise and shape the environments around them, there will be an increasing appreciation for the secret lives of plants, especially where they occur as communities and collectives, in rainforests and even our own Main Ridge Forest Reserve.

REFERENCES:

https://www.scientificamerican.com/article/veggies-with-vision-do-plants-see-the-world-around-them/

http://www.bbc.com/earth/story/20170109-plants-can-see-hear-and-smell-and-respond

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