What a Plant Knows: Do They Smell or See?

This is part two following my last write-up What a Plant Knows: Do they Feel or Hear?, which discussed whether plants feel or hear (In a nutshell, they’re pretty deaf but they do react to touch – think venus flytrap, the plant that traps insects that walk onto it). I recently finished What a Plant Knows by Daniel Chamovitz, a plant scientist who comes from a family of physicians. He decided to go another route and became a plant scientist, but has been surprised to see many similarities between the plant world and the animal world – prompting the angle of his book.

So, do plants see and smell?

My takeaway: Plants have a more complex vision than humans, which they translate into light signals and cues for growth. And plants certainly ‘smell’ in their own way. When injured they produce salicylate (aspirin) and produce chemicals distasteful to insects.

What a Plant Smells: This was a 21-page chapter so I won’t do it justice. But there are plenty of examples that plants ‘smell’ in their own unique ways.

To compare to humans, we have hundreds of different receptors to detect unique chemicals. For example, putrescine is a chemical from dead flesh while menthol is the major component in peppermint. Only one receptor, an ethylene receptor, has been identified in plants. BUT, plants do detect volatile chemicals and convert these signals, although not via an olfactory nerve as humans do, into a physiological response.

Some examples:

• Ethylene Ripens: Ancient Egyptians slashed open a few figs to get a bunch of figs to ripen, most know to put a banana in an avocado to ripen it, and farmers used kerosene to ripen citrus, not knowing why it worked. The answer was ethylene gas induces ripening (kerosene smoke contains small amounts of the gas). Ripening fruits produce ethylene gas. For reproduction purposes, ethylene signaling occurs when environmental stresses such as drought or injury signals plants to get active and produce their fruits. Also, fruits of plants that ripen at the same time ensures heavier seed dispersal by animals.
• Methyl salicylate:To put it simply, plants like to produce aspirin when they’re ill and even when they might become ill. Methyl salicylate is similar to salicylic acid, the chemical precursor for aspirin. Plants produce it when they’ve been attacked by bacteria or viruses. The healthy parts of the plant respond with a number of steps that either kill the bacteria or virus, or stop its spread.

  

• Caterpillars and willow trees: Scientists have noticed that caterpillars don’t infest willows next to ones already infested. Why? The leaves of the resistant trees contained chemicals (phenolic and tannic) that tasted bad to insects. I’ve interviewed Jack Schultz from the University of Missouri who specializes in plant disease communication, and his numerous studies continue to show plants emit a gaseous signal – “Defend Yourselves!” – to communicate with the undamaged ones. The attacked trees send airborne messages to defend themselves. There are other examples of this, for example, lima beans attacked by beetles showed the same response.

What a Plant Sees: “Plant vision is much more complex than human sight at the level of perception,” Chamovitz writes. Humans have four types of photoreceptors in their eyes, plus a fifth for regulating our internal clocks. One plant studied – Arabidopsis – has at least 11 different photoreceptors.

But while plants have a much larger spectrum than humans, they lack a nervous system that translates light into pictures. Rather they translate the light signals into cues for growth. We’ve all witnessed plants growing towards light. Some of these photoreceptors tell a plant when to germinate, others when to bend, or when to flower and or when it’s nighttime.

One way to think of it is an animal needs food so moves towards the food it requires. For a stationary plant – light is food. Plants have to seek out and grow toward light.

U. of Missouri’s Jack Schultz says that plants interpret a green environment as a signal to escape. “Surrounded by green, it turns on a plant’s genetic programs that change growth patterns and go looking for light and escape from their competitors,” he says.

So where are most of these photoreceptors in the plant. It’s probably not a surprise that research goes back to the 1800s researcher Charles Darwin, when one of his many plant studies showed the most photoreceptor sensitivity lays at the tip of a plan. Light hits the tip, which sees the light & transfers the information to the plant so it will bend in that direction. This is why cutting off the tip of a seedling or mature tree is often detrimental.

Both Plants and Humans have Internal Clocks: One thing we have in common is plants and humans both have cryptochrome, the blue-light receptor responsible for resetting our circadian clocks by light. The same one that makes us grumpy after jet lag. For plants this internal clock regulates leaf movements and photosynthesis.

Chamovitz writes about how these internal clocks developed early in single-celled organisms, before the animal and plant kingdoms split off. (Even bacteria and fungi have these clocks.) So why was this needed so early in evolutionary life? “The original clocks probably functioned to protect the cells from damage induced by high UV radiation in the early Earth atmosphere.

Overall, plants are pretty amazing. They smell, see and feel touch. But it does not appear that that thought influences plants. As Schultz says, “that remains unsupported by either data or logic.”