Dec. 16, 2005 To the ancient Celts and Vikings, mistletoe was a sacred healing plant believed to bestow fertility, bring good luck and avert evil; nowadays, it’s the kissing plant hung in doorways during the winter holiday season.
But there’s a sinister side to mistletoe, explains a Thompson Rivers University researcher.
Dwarf mistletoe (dmt), an evergreen parasitic plant found on conifers in Canada, can significantly reduce a tree’s life expectancy. Even more important in beetle-infested British Columbia, the almost-invisible dwarf mistletoe species that prefers pines can weaken a tree and make it more susceptible to attack by micro-organisms and insects.
Spotting the one-to-two centimetre plants is tricky, but luckily the plant leaves a highly visible calling card: infection by dmt can cause bushy branch distortions called “witches’ brooms.”
That’s not the only Hallowe’en aspect to dwarf mistletoe. Just as fireworks play a part in the fall festival, dwarf mistletoe stages its own little explosions.
Unlike other mistletoe species which depend solely on bird droppings to spread their seeds, when dwarf mistletoe is ready to propagate, it can blast its sticky seeds up to 16 meters away.
This is very unusual, says Dr. Cindy Ross, who’s been studying the plant for the past five years, including a lengthy project studying dmt embryo development on two pine species in Manitoba. Ross collected samples twice a week for three years, strapping on the snowshoes when the going got tough in the prairie winter.
It’s important work, because the more we know about this not-so-innocuous forest parasite, the better we can protect our trees.
“Dwarf mistletoe is a serious forest pest,” says Ross. “Its effects can change habitat, compromise timber quality, create a fire hazard due to dead trees, and weaken a tree to the extent that it dies from other factors.”
Understanding how the plant spreads can help foresters make effective control decisions, she says, explaining that of the five dwarf mistletoe species in Canada, four exist in British Columbia. Of these, only one attacks pines, while the others choose hemlock, Douglas firs or larches as hosts.
While they may have different appetites, all of the dmt species spread their seeds through explosion, an unusual method of propagation unrivalled by any other member of the plant kingdom.
Some plants, like the tomato, have gelatinous seeds, but they don’t explode. Other plants, like caragana, are explosive due to drying. Dwarf mistletoe is unique in the way it explodes gelatinous seeds.
“The unique chemical makeup of the seeds, which contain viscin and vesicular cells, in conjunction with the specific dwarf mistletoe fruit architecture, is what provides a recipe for explosive discharge,” explains Ross.
The day before discharge, the plant draws water into its fruit, saturating the sticky viscin coating. At the same time, the regions between viscin cells undergo a radical change in chemistry which aids pressure buildup.
Essentially, the process works like this: water initially accumulates in the viscin mucilage, creating a steady rise in pressure, then changes in cell chemistry instigate a major influx of water into the mucilage, triggering a rapid increase in pressure. A resistant cell layer makes sure the fruit doesn’t expand in all directions, but rather accumulates pressure in the viscin, so when the cell layer where the fruit joins the stalk dies, pressure is suddenly released, bursting the fruit and dispersing the seed.
“The outer fruit shell is elastic but resistant to pressure build-up, like the skin of a balloon, and the dying cell layer that forms at the fruit stalk mimics the result of putting a pin in the balloon,” says Ross.
It’s a deadly discharge that plays a mean prank on neighbouring trees, and it’s particularly threatening in single-species stands like lodgepole pine plantations.
“It’s ironic that the plant associated with love and well-being at Christmas can be the kiss of death for the trees we’ve come to associate with the same holiday,” says Ross, who will present the results of her study at the Canadian Botanical Association conference next spring.
Dr. Cindy Ross earned her PhD in Botany at the University of Manitoba, and is now an Assistant Professor of Biological Sciences at Thompson Rivers University in Kamloops, BC, where she teaches genetics, biochemistry and cell biology. She has had four papers on dmt published in scientific journals to date, with two others in progress.
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