15.2.3 Light, CO2 and stomatal aperture

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Leaves occupy highly variable environments. Daylength, irradiance, temperature, water availability, wind speed, water vapour pressure deficit and atmospheric CO2 concentration vary continually. All of these factors can influence assimilation, growth and stomatal conductance. Stomata must be able to sense and respond to these changes.

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Figure 15.8 Stomatal conductance in well-nourished plants (either C3 or C4) increases rapidly each morning and reaches a peak prior to maximum daily irradiance. In unstressed plants, stomata remain open all day although a small dip is often observed around midday when light and temperature loads are maximum. In CAM plants, stomata may open entirely at night or may open only partially both day and night, according to the degree to which CAM metabolism has been engaged (water and temperature driven) (Generalised values compiled by D. Eamus)

In a typical diurnal pattern of stomatal conductance (Figure 15.8) stomata remain predominantly closed at night and open at or around dawn, as soon as light levels begin to increase. Entrained diurnal rhythms can also lead to stomatal opening and closing occurring despite exposure to continuous light for several days. Crassulacean acid metabolism (CAM) plants represent a special photosynthetic adaptation where stomata open at night for CO2 fixation via PEP carboxylase with attendant economies in water use (Section 2.1.6, Table 2.1).

Stomatal aperture usually peaks early each day, before light levels are saturating. In the absence of stresses, stomata remain open throughout the day and close rapidly as light levels fall towards evening. Light is a direct factor for stomatal opening and also drives photosynthesis, but a distinction exists between response to light per se and light responses that are mediated via assimilation of CO2.

As photon irradiance increases, photosynthesis increases. Intercellular CO2 partial pressure (pi) decreases as CO2 is fixed. Stomata respond to pi, rather than the CO2 partial pressure of ambient air (pa). It is a property of the guard cells themselves and is not the result of a signal derived from mesophyll cells since the stomata of isolated epidermal strips or guard cell protoplasts respond to variations in CO2 concentration. Furthermore stomata of leaves and epidermal strips open in response to reduced CO2 concentration in darkness.

 

 

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