Reversible Responses to Climatic Factors
Stomata open with increasing light intensity. They are activated by a blue light receptor (Zeiger et al. 1987). Thus stomatal conductance in the morning increases earlier than photosynthesis, which is only activated when the sun has risen further and with a larger proportion of red light. Therefore, stomata open wide and do not limit the flow of C02 during the early morning when the humidity is high.
At low temperatures (freezing point) stomata are closed and open as temperature increases. This opening is exponential at temperatures above 40 °C, so leaf temperature decreases below that of the air because of the strong transpirational cooling (Raschke 1979).
Stomata close with increasing vapour pressure deficit between leaf and air, and this can also be observed on isolated epidermis (Lange et al. 1971); where closing is faster than opening (Fig. 2.2.17 A). This response can be so strong that transpiration decreases despite an increasing water vapour gradient between leaf and air (Fig. 2.2.17 B; Schulze et al. 1972). It is still unclear how stomata "measure" humidity. Research on Tradescantia suggests that stomata react to local water deficits in the epidermis caused by transpiration from the stomata themselves and not from water loss of the epidermis (Nonami et al. 1990) and this depends on the time constants of water flows.
In high trees the reaction to local deficits probably plays an important role in the regulation of stomata during the course of the day. In the canopy of a forest, turbulence of air movement occurs with fast exchanges of air packages with different humidity (see Chap. 2.2.7). This correlates with fast changes in the xylem flow. Neighbouring trees, exposed to the same air masses, show synchronous changes in xylem flow (Hollinger et al. 1994). Stomatal closure is induced (Kostner et al. 1992) by short-term change in transpiration and the associated change of the water state of the epidermis. As closure is faster in dry air than is opening in moist air, a continuous decrease in stomatal conductance is the consequence. Under ambient
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