Ahh, the weather is finally warming up enough for the plant kingdom to fully come back to life. Splashes of green and jewels of colour are once again making their mark on the winter weary landscape. Along with this new surge of floral vitality, however, comes the bustling activity of life of a different sort, one with which the plant world has contended for millenia: that of the herbivores.
Migrating back to this part of the world as it becomes increasingly hospitable, or awakening from a winter spent in dormancy, the insects, birds, mammals and other creatures that dine on plants return in full swing at this time of the year. Have you noticed the appearance of little clouds of bugs; the presence of the occasional butterfly on one of the really warm days; or the little housefly, tired looking, but there? These are all sure signs that the timeless plant-herbivore relationship is beginning its annual cycle yet again, and that plants will be beefing up their defense systems to combat the onslaught of those who wish to make lunch out of them.
Secondary metabolites: the bane of many a herbivore’s existence. These chemical compounds – phenols, alkaloids, terpenes, and many others – act as plants’ first line of defense against potentially fatal herbivory. Plants such as Poison Ivy, Fragrant Sumac, rhubarb, even the familiar plants of the nightshade family – the tomatoes and potatoes: they have all developed secondary compounds that help to deter herbivory.
What is the mechanism by which these compounds provide plants with protection from their animal predators? The secondary compounds effective in deterring herbivory have toxic properties that can cause everything from a yucky flavour to indigestion, illness, reduced reproductive capacity, and even death. An animal unadapted to dealing with a plant’s particular herbivory deterrent won’t likely leave the dining table feeling too chipper if it decides to throw caution to the wind and try out a new plant flavour.
Of course, the presence of secondary compounds is something of a double-edged sword for some plants. Take the tomato, for instance. The presence of fruit, the home of the plant’s seed, and thus the key to its future, indicates a reliance on animals for dispersal and establishment. Without the help of fruit-loving animals in depositing the seed – primed after a trip through the animal’s digestive tract, and planted lovingly with its very own little supply of fertilizer – the tomato and its relatives would have died out long ago.
But the leaves – the source of life blood for our photosynthetic friends – must remain untouched. So what to do? The allocation of toxic compounds within the plant is a tricky thing indeed, and needs to be well planned, so to speak. Within the plant, toxic compounds must be allocated to the various ‘body’ parts – roots, shoots, leaves, and fruits – selectively, dependent on which is likely to be negatively affected by herbivory.
If some types of herbivory (e.g. fruit-eating and subsequent seed dispersal) will benefit the plant, then the presence of toxic secondary compounds must be low or non-existent in the fruit. If other types of herbivory (e.g. leaf-eating, resulting in an inability to capture the sun’s energy) will negatively affect the plant, then toxic secondary metabolites will likely be found in the leaves in full force. Smart move, until some sneaky little beast like the tomato horn worm figures out a loophole in the plant’s defensive plan, and chows down regardless of the chemical makeup of the leaf. Oh, the best laid plans…
Phenols and alkaloids, plants and their predators: all part of a dynamic relationship that exists between the eaters and the eaten. The resulting interplay of forces is fascinating, as various mechanisms in both the plant and animal realms adapt to the other’s wily defenses. Who will be the next to one-up the other? Stay tuned as plants and herbivores continue for another few million years to battle it out for top spot.
Zoe Dalton is a graduate of York University’s environmental science program, and is currently enjoying working towards a Master of Arts in Integrated Studies with Athabasca U. She can be reached for comments or questions at firstname.lastname@example.org.