THE DISCERNING TASTE OF TREES

Dec 19, 2016: I've just finished a fascinating book, called "The Hidden Life of Trees: What They Feel, How They Communicate" written by Peter Wohlleben, a German forester. He takes some of the latest scientific research on plant communication and delivers it in a very understandable and easily readable fashion. One of the areas of research that was new to me and utterly mindboggling was that plants (or at least some of them) have to ability to taste. And that their taste palettes may be able to discern between the "saliva" of the different insect herbivores that are eating them.

It's been know for a couple of decades that plants have a very complex defense communication system. We know that some trees, such as acacia, will emit a phytochemical (plant chemical) into the air that warns other acacias that there are predators about feeding on their leaves. Other acacias then have time to prepared toxic, bitter substances in their leaves that deter feeding. One of the biggest herbivore threats to acacia in Africa is the giraffe and in the eons old battle between predator and prey, the giraffe has learned after feeding on one acacia to either move upwind of their last target or at least 100 meters away. The defense aroma can't tip off those unsuspecting acacias.

What's more recent is that botanists, or more specifically chemical ecologists, believe that some plants may be able to identify who is attacking them. There are two possible ways they may do that. By recognizing the saliva of the herbivorous insect or by recognizing the vibration patterns of the leaf munching of that insect. Why does a plant care what's feeding on it? This is what is truly evolutionary genius.

The acacia I mentioned above doesn't distinguish between whether it's a giraffe or whether it's a caterpillar. It's reaction and those of its buddies close enough to "smell" is to produce bitter toxins in their leaves. That requires a fair bit of energy to do on the part of the plant. What if a plant could instead enlist others to protect it? Maybe other insects that will feed on the insects that are feeding on them?

That's what some elms and pines . . . and brussel sprouts do. They are able to detect the species that isSawfly parasitic waspSawfly parasitic wasp feeding on them and then secrete a fragrance that attracts a tiny parasitic wasp that then attacks the attackers. The parasite - host relationship is very species specific, so the leaves must produce a very specific fragrance that only attracts the specific parasitic wasp for that attacker. If another species attacks then it has got to produce a different fragrance to attract a different beneficial parasite or predator. The advantage is that the plant is not taking up a lot its time and energy building up defensive chemicals in it's leaves. The disadvantage is that there are a limited number of fragrance options. Over evolutionary time the fragrance palette develop to the most commonly occurring attacking species. Get a new species in the area or environmental pressures cause a minor pest to become a major pest and there's no alarm fragrance for it.

Again, for every plant defense strategy its predators try to develop a counter strategy and it's been found that some species of caterpillars secrete a enzyme inhibitor when feeding that masks the true nature of its saliva. Copenhagen for insects.

For more detailed information, I highly recommend ( and I absolutely love the title of this article!) '"The Silent Scream of the Lima Bean" from the Max Planck Institute of Chemical Ecology. Google it.
As with all good research the attempts to unravel mysteries lead to the exhumation of many more. For instance, if plants can taste the saliva of different predators does that mean they have a memory? Don't ask me. To quote the Great Bard, "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."

And hey, the next time your kid turns up their nose when you serve them brussel sprouts . . . there might just be a perfectly good phytochemical reason.

F & P