Water acts as a SOLVENT for minerals and stored food that must be moved through the plant. Water evaporation on the leaf surface helps cool the plant during very hot weather in the same way that perspiration keeps us cool.
Plants even require water during winter. Water is absorbed through and stored in roots for winter. When soil is frozen, water movement is very restricted. On windy winter days, leaves of broadleaf evergreens, such as rhododendrons, may curl to reduce water loss. Curling reduces the amount of leaf surface exposed to drying winds, reducing evaporation. Continued water loss may cause tissue to die and turn brown. This kind of plant injury is called desiccation. It is critical that broadleaf evergreens have an adequate supply of soil moisture going into winter.
http://www.hcs.ohio-state.edu/mgonline/botany/env03/03env03a.htm
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