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Susanne von Caemmerer, Neil Baker, The Biology of Transpiration. From Guard Cells to Globe, Plant Physiology, Volume 143, Issue 1, January 2007, Page 3, https://doi.org/10.1104/pp.104.900213
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Transpiration in higher plants accounts for about three-quarters of the water that is vaporized at the global land surface and one-eighth of that vaporized over the entire globe. The availability of water is one of the major factors restricting terrestrial plant production on a global scale. Since plants do not have membranes that are both permeable to CO2 and impermeable to water, transpiration is an inevitable consequence of photosynthesis. To control water loss, plants are covered with relatively water-impermeable surfaces that are punctuated by stomatal pores. Almost all of the CO2 fixed by terrestrial plants and most of the water transpired pass through these stomatal pores. The degree of opening of these pores is modulated by variation in the turgor status of the two surrounding guard cells. The regulation of stomatal aperture determines the compromise between increasing CO2 fixation and reducing transpiration to prevent desiccation. At the same time, plant transpiration provides evaporative cooling, forming a major component of the leaf energy balance. Transpiration also provides the driving force for transport of water and nutrients from roots to shoots. Consequently, transpiration processes affect the yield and survival of agricultural species, and impact on the global carbon and hydrological cycles. These in turn feed back on climate and have a direct effect on global warming and climate change.
