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If you grow a plant from seed in your nice warm house, then plant it out in your temperate garden, it will stop growing for a while, possibly die, and if not, subsequently resume growth. Standard gardening practice is to 'harden off' plants, moving them to an area of intermediate temperature for a while before planting them out.

Why does this happen? What is the difference between a plant adapted to a warm environment and one adapted to a cold environment?

Expression of cold-tolerance genes? Levels of some secondary metabolite? Thickness of cell walls? Density of root hairs?

An unidentified author on the Royal Horticultural Society website states that "The effect of hardening off is to thicken and alter the plant's leaf structure and increase leaf waxiness", which is not quite detailed enough to satisfy.

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Cf. Why are some plants frost tender –  Oreotrephes Aug 27 '13 at 4:37
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As far as I know, the main challenges the plants have to face in cold environment are metabolism reduction and membrane fluidity. If the temperature is even lower, they may face freezing.

Increasing the metabolism is quite hard, because plants usually are unable to increase their temperature by its own. Plant may produce more pigments in order to absorb more sunlight; grow trichomes and other structures that may generate an air layer which acts as an insulator or simply reduce its metabolism by, for instance, hibernation. The amount of stomata is also related to temperature, and a plant growing in low temperature needs less stomata.

In bacteria sometimes there exist isoforms of many enzymes that are expressed during different conditions, and since plants usually have large genomes with tons of duplications, it's reasonable that some plants may express different genes for the same processes in high and in low temperature. One group of genes that are certainly expressed in cold exposition are membrane desaturases, which introduce unsaturation in the lipids, increasing the fluidity of the membranes. Some plants also protect themselves of freezing by accumulation of some substances, such as glycerol or proline, avoiding the formation of ice inside the cells.

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Miguel Angel Naranjo Oritz Excellent answer. Could you provide a source for your information that plants hibernate ? –  biogirl Aug 24 '13 at 12:58
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In botany the term hibernation, or winter dormancy (as opposite of estivation, wich is the same for summer months) describes the reduction of metabolism during winter. The most obvious example are trees that lose their leaves during the fall. Maybe I used a term a bit confusing. –  Miguel Ángel Naranjo Ortiz Aug 24 '13 at 15:14
    
Oh! I get it now. Thanks! –  biogirl Aug 24 '13 at 16:23
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