Friday, December 13, 2013

Winter dormancy period of plants or " is my crab apple tree really sleeping " ?

Evolution has programmed cold climate plants to survive winter by going into a period of dormancy. Tissues gradually hardens as days grow short and temperatures decline in autumn. Chemical activity continues within the plant at a slow pace until the growth inhibiting hormones a fall and winter give way to the growth promoting hormones of spring. This prevents the catastrophe of a seed or bud sprouting too early, only to greet the killing frost. Evolution has also ordained that the winter to spring chemical changeover be completed only after a specific period of chilling. Without enough hours of the right degree of cold aplant cannot fully break it's dormancy. Dormancy last in plants  for varying lengths of time and begins at varying seasons of the year. Spring flowering bulbs, for example, start to become dormant when the foliage dies back in Midsummer and do not emerge from their period of rest until mid winter, when the chilling of the soil coupled with the increasing amount of sunlight stimulates them to resume their annual cycle of growth. But however long it lasts dormancy always occurs in several stages the first few of which are more or less invisible.
To prepare for the rest period ,plants enter a period of intense activity. First they manufacture and store extra proteins, sugars and fats in their cells to tide them over the lean months and give them the energy they need to burst forth in the spring. Then if they are to bloom early in the following year, they will set flowerbulbs so they will be ready to move quickly as soon as their dormant period is over. Leaf buds too are formed many of which contained in miniature all of the preformed leaves and stems for the next years growth only after these preliminaries do they begin to show signs of slowing down: stems stop growing, and deciduous plants shed their leaves. As winter sets in some perennial plants vanish from site only leaving the crown to indicate the continued presence in the garden. Underlying these various physical maneuvers is a series of chemical changes in the plant. Some of these changes are central to the hardiness and help to explain why certain plant can survive subfreezing temperatures while others, the tender plants, cannot. Like much else in nature this ability, or lack of it, is a product of evolution. Scientist think that the ancestors of all today's plants originated when the was climate was warm and humid and temperatures varied little in the course of the year. In the eons that followed the climate cooled and the plants that could adapt to cyclical changes in temperatures were the ones that persisted and reproduced. Some plants, the annuals, survived the winter in the form of seeds, which the parent plants produced and distributed in a single season before they died. Others persisted in the form of bulbs into which were packaged in embryonic form of all the flower stems and leaves and other plant parts Still others kept their woody structure intact but went to a hardening process that protected them from being injured by the cold. Certain favorable conditions a moist well-drained soil, for instance, and shelter from the wind will occasionally combine to let a plant grow in temperatures that drop below that usual heartiness limits. It may live through seven winters, with protection, but then succumb during especially severe one. The term half hardy is sometimes used to describe such plants, but strictly speaking is erroneous because a plant is either hardy in a particular climate zone or it is not.If a plant is not hardy, the correct term for it is tender. A plants ability to endure the cold also varies with its condition at the time the cold strikes. If you  subjected a supposedly hardy plant to freezing temperatures in Midsummer, it might not be hardy at all but could die. Plants become hardy only when they have had a chance to acclimate themselves gradually to the cold a process that takes a long time. For most plants the prime trigger that sets the hardening process in motion is not the falling temperatures, but the shortening of days and lengthening of nights. Even if an autumn is unusually warm, these plants will prepare themselves for winter. This is clearly a better arrangement for plants since the seasonal change in the length of days is constant year in and out, while the temperatures are not.

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