WORD OF THE MONTH – IMMUNITY
By Bruce Palmer
So far, we appear to be lucky in our area with relatively few cases of COVID-19 circulating in our community. Immunity might be a good topic for this month. Immunity comes from the Latin immunitas, exemption (originally from public service). We tend to apply the term to animals, including humans, where antibodies at various locations in the body act to neutralize antigens (bacteria, viruses, toxic substances and protein fragments called prions) that cause major problems. Some animals, cockroaches for example, appear to have a natural immunity to all antigens. Most do not. Often having a disease confers immunity to having it again, but not always. At this point we don’t know whether getting COVID-19 makes you immune, although current studies done with small numbers of hamsters and ferrets seems to indicate that immunity might result from being sick with the disease. Let’s hope so and hope that we can get a viable vaccine in super record time. Let’s hope so and hope that it doesn’t drag on for two or three years as did the “Spanish Flu” a hundred years ago. Using that term for the Great Influenza is a misnomer – it appears to have originated at the post hospital at Fort Riley, Kansas (where I was stationed in the Army Veterinary Corps in the nineteen fifties) and been spread to Europe by U.S. troops in World War I.
We are a plant organization, so what about immunity in plants? Used in a broad sense, immunity would include defenses against predators as well as disease. In that sense, plants have an impressive arsenal. Plants do not have kidneys, livers or sweat glands to get rid of toxic byproducts of metabolism, so they often store them in their trunks, as in camphor and sandalwood or in leaves, as in sage and rosemary. Often, plants use these byproducts as defenses against pests, most often against insects, but also against browsing animals. Some plants can use these substances in ways that we usually associate with animal behavior. A well-documented case is giraffes and African acacias. When a leaf of an acacia tree is bitten by a giraffe, the leaf does two things. First, it fills its cells with tannic acid to irritate the giraffe’s tongue and stop it from eating more leaves. Then the attacked tree sends out ethylene (the chemical that ripens fruit) into the air to warn neighboring acacias about the giraffe. The neighboring acacias then secrete tannic acid to prevent attack. The giraffes have adapted to this tactic and move either upwind or a hundred yards or more away. There are many other tactics employed by plants, mostly to repel insects. Tan oaks locally use tannic acid to repel insects and we in turn harvested tan oak bark in the early nineteen hundreds to sell to animal hide tanners. Willows store salicylic acid, the precursor of aspirin, to repel insects. In the tropics, the sensitive plant takes a more physical approach to escape being eaten by grazing animals. When one leaf is touched by anything, all leaves on the plant wilt immediately and the plant effectively disappears.
What about rhododendrons? Several members of the Ericaceae family, including Rhododendron and Pieris, use toxins to fight insect attacks. These toxins are named Grayanotoxins (after the American botanist Asa Gray) and are deadly to many insects and toxic but not usually deadly to humans. They kill honeybees but not bumblebees, which may provide more pollen to rhodies as the big bumblebees rummage around in the flowers. Grayanotoxins don’t usually kill people but, as neurotoxins, they can have interesting neurological effects. “Mad Honey”, produced from the nectar of Rhododendron ponticum is used in Turkey to produce hallucinations. Many of us are aware of the famous incident in 69 B.C., when a Greek king defeated an invading Roman army by tricking the soldiers into eating mad honey or drinking mead made from it. You may have noticed that some of your rhodies are more susceptible to weevil damage than others. That is because some rhodies produce more Grayanotoxin than others. The weevil that attacks our plants came from Central Europe and Asia Minor. So did R. ponticum. If you have R. ‘Fastuosum Flore Pleno’ (a very early hybrid, ± 1846), you might have noticed that it seldom gets attacked by weevils. That may be because one its parents is R. ponticum, which has very high levels of Grayanotoxin and immunity to weevils.