A large oak can breathe out 150,000 litres of water a year
Plants drink far more than they need. Of all the water their roots absorb, only a sliver goes into growth and metabolism; somewhere between 97 and 99.5 percent escapes into the air. A single acre of corn releases thousands of gallons daily, and the plant spends no energy of its own doing it.
Transpiration is the movement of water up through a plant and out of its leaves, stems and flowers as vapour. The name joins the Latin trans, across, with spirare, to breathe. The engine is evaporation. As a water molecule leaves a leaf through a tiny pore called a stoma, it tugs on the molecule behind it, because water molecules cling together. That pull runs down through the xylem, the plant's plumbing, all the way to the roots. Scientists call this the cohesion-tension theory, and in tall trees it is the only thing strong enough to beat gravity.
Each stoma is flanked by guard cells that open or close it, and the plant faces a constant trade-off. Open pores let in carbon dioxide for photosynthesis and draw up dissolved minerals, but they leak water. When roots cannot keep up, stomata close, slowing growth. Humidity, temperature, wind and sunlight all change the rate. Evaporation also cools the leaf, since each litre carries away about 2,260 kilojoules of heat.
The volumes are striking. Over a growing season a leaf can transpire many times its own weight, and crops typically lose 200 to 1,000 kilograms of water for every kilogram of dry matter they build. Desert plants fight back with thick waxy coatings, small leaves, sunken pores and hairs. Many cacti photosynthesise in their stems, and plants using a strategy called CAM keep their stomata shut by day and open them at night.
The system can fail. If a plant cannot take up enough water, bubbles of vapour form in the xylem and block it, a problem called cavitation that can push a plant to permanent wilting. Many plants repair it overnight: with stomata closed, root pressure crushes the bubbles and refills the pipes. MRI scans have shown this happening, with vessels that clogged during 20 hours of sunlight refilled after three hours of darkness.
Source: Transpiration
