Dynamic Stomatal Changes

Abstract

Stomatal pores regulate CO₂ uptake and water loss from leaves. Stomatal responses are dynamic by nature and often lag behind the faster changing environmental conditions as is common in tree canopies. Even under constant conditions, gas exchange of angiosperms occasionally shows cycling fluctuations, called stomatal oscillations. They are interpreted as an effect of feedback control failing to achieve stable regulation and thus demonstrate that stomata not only respond to external factors, but also to the environment inside the leaf. The processes which translate transpiration into turgor are called the physiological gain. The physical processes and environmental conditions which control stomatal aperture, stomatal conductance and transpiration are called the physical gain. More research on the physiological gain is needed in order to understand these processes. In order to overcome the epidermal backpressure, guard cell turgor has to reach a certain threshold level, although guard cell swelling anticipates the opening. When the pore opens, the relation between pore area and stomatal conductance determines the physical gain. In contrast to the Fick’s first law of diffusion, this relation is not linear, but convex shaped, with a rapid increase of conductance just after opening and much less effect of aperture changes at large apertures. The high and abruptly changing gain at smallest pore openings can promote overshooting oscillatory responses, as supported by microscopic observations of stomatal apertures. A review of the literature suggests that stomatal movements are metabolically active responses of guard cells to local water status. A full understanding of the mechanisms, however, is complex because stomatal movements result from the interaction of two processes that are difficult to separate experimentally: hydraulic effects, and active osmotic adjustment of guard cells and epidermal cells. Hydropassive movement, resulting from an unbalance of turgor pressure between guard cells and the surrounding epidermis, may also occur. An example of hydropassive movement is the so-called Iwanoff effect or Wrong Way Response (WWR), i.e. a fast opening response followed by a slow closure, that occurs as a response to a steep increase in the leaf to air difference in water vapor pressure and may last 2.5–38 min depending on the species and the experimental conditions. An additional 10–60 min may be required for completing the closing response. In contrast to the rather slow osmoregulatory negative feedback, hydraulic responses act fast, starting within seconds and completing within minutes, and have been suggested as a key mechanism in stomatal oscillations. In a plant displaying oscillations, movements of individual stomata are more or less synchronized on a very small scale within a leaf (1–2 mm). The nature of the synchronizing mechanism is not clear. Synchronization can also occur among leaves, ultimately leading to concerted cycling of gas exchange of entire plants. Comprehensive models of stomatal behaviour based on the mechanisms operating in and around stomatal guard cells are still missing, and may help explaining gas exchange response to stressors. Studies with the air pollutant of most concern to forests, i.e. ground-level ozone, suggest that stomata show a transient decrease of stomatal conductance upon exposure and are sluggish in responding to further stimuli.