15.2 Stomatal physiology

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CO2 assimilation necessitates stomatal opening, but transpiration is an inevitable consequence. Effective use of water during leaf gas exchange is thus a strong selection pressure, and terrestrial plants have evolved with stomatal mechanisms that enable gas exchange but forestall leaf desiccation. Evolution of stomata was central to colonisation of terrestrial environments. Life underwater removes any threat of dehydration through evaporation but leaves on terrestrial plants are continually exposed to an evaporative demand. Since evolution has not (yet) resulted in a bio-polymer that is freely permeable to CO2 but not to water, CO2 access to leaf mesophyll cells must occur through pores (stomata) in an impermeable epidermis which is in turn overlain by a waxy cuticle. Stomata thus face a dilemma. To maximise carbon gain, stomata need to open as wide as possible for CO2 entry. However, they must also contain water loss so stomata need to be able to close in response to adverse soil–plant water relations. How plants resolve these conflicting demands is discussed in this section, starting with stomatal structure, function, ionic basis of opening and finally regulation.