Plants and winter: they can't run, and there's nowhere to hide


what makes plants remarkable is that they can't move. For from making them boring, this immobility leads to many fascinating adaptations; they must "invent" ways of doing everything that other organisms do (grow, survive, reproduce, etc.) while standing still. One of the major hurdles in the lives of plants in Canadian soil is surviving the winter. They can neither follow the favourable conditions southward, nor can they shelter in the safety of a den, hollow tree, hive, or stream sediment. So, do they simply endure the winter, end of story? Not quite.

Send out the seeds and buds

Different plant species may have vastly different lifestyles, and the different "solutions" to coping with winter depend largely on the overcome the hardships of winter by devoting all available energy to mating and producing seeds in the autumn, and dying. Only the seeds (newly formed embryos) must survive the winter and germinate in spring. This is no small task; an embryo can be very delicate and it must recognize spring's arrival. Seeds of a vast number of annuals and perennials enter a state of dormancy that not only protects them, but is essential for germination.

In a deciduous tree, new leaf bud are formed in the autumn before, and during, the time of leaf fall (see shunpiking, October, 1996); frost hardiness develops in the tree's tissues. Like seeds, buds remain dormant over the winter until they have experienced enough cold, and detect the correct daylength, to break dormancy. A lilac bush in a heated greenhouse will not produce leaves in the spring, but if one branch is sticking outside through a hole, the buds on that branch will "leaf out." To survive fluctuating winter temperatures and "Indian summers," dormancy is often controlled by a combination of temperature and photoperiod, mediated by plant hormones; when the internal concentrations of hormones reach a particular level, the seed's or bud's dormancy is lost, and given suitable conditions, growth will continue. Here's a story about ultimate patience; in 1967, seeds of the Arctic tundra lupine were discovered in a lemming burrow in Yukon. The seeds were estimated by carbon-dating to have been there for 10,000 years; but it was worth the wait, for they germinated almost instantly when their dicoverers placed them in the right conditions!

The root of the solution


Perennials survive winter after winter. They more-or-less shut down their activities and rely on stores of nutrients - primarily starch - in their roots and bulbs (which are part of the stem itself.) The nutrients allow the plants to essentially "hibernate" and resume growth in the spring. In some rare cases, special contractile roots provide further protection by literally pulling the plant underground. Conifers, and hardy little plants like wintergreen, are still able to carry out some limited photosynthesis and growth during favourable days throughout the winter.

Softly go the biennials

Seeds, buds, and trees survive by shutting themselves down. What about small soft plants such as biennials and perennials which, unlike most annuals, must survive as more than just a seed? The biennial lifestyle is comparatively rare (or they just haven't come out of the closet yet). Biennials, strictly speaking, germinate and develop to some extent in the first year, overwinter without having produced flowers and seeds, and then flower in the second year once they have reached a certain "threshold" size. Indian tobacco is quite common here, and is supposed to behave as either an annual, a biennial, or even a monocarpic perennial (meaning that it can live several years but flowers only once).Upon germination, the plant first develops into a many-leafed prostrate thing called a rosette, which is cold-hardy. Only if this rosette becomes large enough early enough in the season will it start its sudden (for a plant) vertical growth in preparation for flowering, a process known as bolting. Once a plant has bolted there is no turning back, and it loses its cold hardiness. The plant must "decide" based on its size and the daylength whether it will overwinter as a frost-hardy rosette or go for it and flower.

Believe it or not, the white and brown fields and forests will soon be turning green. This spring, notice not just the coordination, but also the staggered timing of the emergence of vegetation, as different plants emerge and bloom at different times.



*Andrew Simons is a Ph.D. candidate in evolutionary biology at Dalhousie University.


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Copyright New Media Services Inc. 2004. The views expressed herein are the writers' own and do not necessarily reflect those of shunpiking magazine or New Media Publications.