Do flowers smell to repel or to appeal?
Plants don’t like to be eaten and all of us know that therefore they use a number of tricks to minimise losses inflicted on them by herbivores and kin. Spikes, spines, needles, spicules, thorns and barbs as well as toxic substances in leaves, stems, and fruits are some of the better-known measures with which plants increase their unpalatability and thus their survival. However, not only do plants like not to be eaten, but they also have to make sure that their valuable pollen is not stolen by pollen eaters.
The dilemma is that flowering plants need insects to carry pollen from flower to flower. It has been suggested (and there is indeed evidence for this notion) that flowers emitted smells originally to repel pollen thieves and even today strongly smelling inflorescences are noticeably avoided by pollen consuming ants. Bees in particular, but also some other insects, seem to have overcome the smell aversion and now render plants some valuable assistance as pollination aids, but the first odours flowers and plants emitted during their evolutionary history may well have been repulsive rather than pleasant. How many, for example, of us would have imagined that some plants can warn others with wafts of secret perfumes when attacked by insects, or, more cunningly still, emit odours to attract the enemies of those insects that harm the plant? A veritable warfare with odours is going on and until not too long ago we had no idea about it.
A good many years ago, I had to review the book “Insect-plant Interactions and Induced Plant Defence”, edited by D. Chadwick and J. Goode and it was from this book that I learned that for more than 100 plant species in 34 families specific herbivore-induced direct defences are now known and that about 20 species use “indirect defences”. Some as yet undamaged plants can respond to the odour emissions of a damaged plant in several ways: they can toughen their leaves (making it harder for beetles, for instance, to cut holes in them), they can deposit greater amounts of unpalatable chemicals into them, or they can simply drop them, appear unattractive and go into some kind of dormancy, waiting out the danger. Some, however, go a step further and recruit help from further afield. If a maize plant, for example, is nibbled at by Podoptera caterpillars (known infamously as the “armyworm”), the maize begins to produce chemicals like sesquiterpenes and indoles that are highly attractive to small wasps, which lay their eggs into the nasty caterpillar brood. Once parasitized, the caterpillars do not die straight away, but they are doomed and as they get weaker will begin to consume less and less plant tissue; moreover, they will not be allowed to mature (the tiny wasp larvae inside the caterpillar’s body slowly hollow the caterpillar out and eventually kill it).
Very similar behaviours have been observed in some predatory mites, which headed preferentially to plants that had been damaged by the mites’ favourite prey. In all these cases it was not primarily the smell of the plant attacker that the predator detected, but the volatiles, the “help-me-signals”, emitted by the plant in distress. We humans may recognize some flowers by their smell and incorporate some fragrances into our perfumes, but we seem to miss most of the other smell messages plants send to each other as well as to their insect friends and foes. Suffering from hayfever myself, I must admit that I’m not sad that we humans are left out of this nose-irritating business. In fact, I’m a good example of the role of the smell of a flower protecting it, for I would certainly not harm and cut it and take it into my home to place it into a vase on the coffee table! I’m quite happy to have fern leaves instead.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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