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Plant’s poison, man’s potion

For years I've been feeding used coffee grounds and tea leaves to a couple of birds'-nest ferns in our garden. It was better than clogging the drain or mucking up the garbage can. Besides, it is good organic mulch for plants that need plenty of it. Visitors often remark about the bright green, three-foot-long, luxuriant fronds.

In my imagination, the plants are nervous wrecks, insomniacs on a caffeine overdose. Unable to rest, perhaps they don't have a choice but to keep growing. I then began to wonder why coffee plants produce caffeine. Why do they need it?

Plants, like other living beings, don't want to be eaten. Even if they can't run away from their predators, they are far from helpless. Some acacias draft ants to fight their battles. They secrete sweet-tasting sap to encourage the insects to take up residence in their branches. Should a herbivore attack the tree, the bite-happy creatures mount a formidable defense and chase the animal away.

Some plants grow thorns and spines that deter browsing animals. Nettles, for instance, are covered in sharp minute hairs loaded with histamine. Any hungry herbivore has to merely brush against the plant to learn a lasting lesson. Some plants have become sophisticated chemistry labs, synthesising bitter compounds to make their leaves and seeds distasteful, even poisonous. This is a battle of adaptations spanning millennia and thousands of species.

Some animals have the upper hand, able to digest a toxic salad of their favourite plants. In the ninth century, Ethiopian shepherds noticed their flocks acting unusually frisky after eating wild red berries in the highlands. Those plants were domesticated and coffee is now cultivated in 80 countries. Today it is said to be the most traded commodity after crude oil. Every day 400 billion cups of the beverage are drunk by people seeking a caffeine fix. Others prefer caffeinated tea or soft drinks for the same reason: to attain a heightened state of alertness. Ironically, the cup we drink to refresh ourselves when our energies flag is an alkaloid produced by plants to put to sleep any insects that have designs on their seeds. In other words, we are addicted to an insecticide that evolved to paralyse and kill.

In the late 1990s, scientists at NASA illustrated the effects of various drugs on the web-spinning abilities of spiders. The eight–legged creature fared disastrously under the influence of caffeine. It strung a few threads in a chaotic, random pattern that bore little resemblance to a web it would have spun under normal circumstances. Caffeine affects even insect predators dramatically. Generally, insects leave the plant alone, except coffee borer beetles, which have developed immunity.

A lot of humans are addicted to caffeine but many more are addicted to tobacco. The nicotine found in the leaves is another powerful neurotoxic insecticide. Besides protecting themselves, the plants have another use for the drug. Their nectar is tainted with it, but they are not trying to poison their pollinators; that would be self-destructive. The plants want them to be brief and move on, and the nicotine discourages insects from drinking long and deep. So the pollinators have to visit more plants to get a full meal. In doing so, they traffic pollen among many plants increasing their genetic diversity. And the insects are apparently hooked to the nicotine. In experiments, insects demonstrated their preference for caffeine or nicotine-tainted nectar to the plain, safe kind. But they like just enough for a buzz, not too much to paralyse them, not too little to be uninteresting.

All the substances that give us a high, such as opium in poppy, cocaine in coca leaves and arecoline in areca nut, are also substances that plants produce to keep predators at bay. I was curious, are we dependent on any other chemicals they concocted?

…to be continued

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Printable version | May 10, 2021 2:27:26 AM | https://www.thehindu.com/news/cities/chennai/chen-columns/plants-poison-mans-potion/article2798455.ece

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