The stomatal guard cell-environment connection

Update on guard‐cells: The guard cell-environment connection

by Kearns E. V., Assmann S. M. (1993)

Department of Organismic and Evolutionary Biology, Harvard University, The Biological Laboratories, 16 Divinity Avenue, Cambridge, Massachusetts 02138.

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In Plant Physiol. 102: 711–715 – DOI: 10.1104/pp.102.3.711 –

http://www.plantphysiol.org/content/102/3/711

Abstract

This review surveys the signals, intermediate events, and effectors that connect GC swelling and shrinking to environmental conditions.

GCs swell when protons are released, hyperpolarizing the plasma membrane (shifting the membrane potential to more negative voltages). This hyperpolarization drives K+ entry down an electrical gradient through inward K+ channels. Cl- influx also occurs, presumably via Cl-/H+ symport or Cl-/OH- antiport. The entering ions are stored primarily in the vacuole along with malate, which is synthesized during stomatal opening. As intracellular osmotica increase, GCs take up water, balancing their water potential with that of their surroundings, and they begin to swell.

Differential cell wall reinforcement bends the swelling GCs outward and widens the pore between them (Willmer, 1983; Taiz and Zeiger, 1991).

Stomatal closure is not the reversal of opening. Anion release and/or Ca2+ uptake depolarizes the plasma membrane (shifts the membrane potential to more positive values). This depolarization provides the driving force for K+ efflux through outward K+ channels. As levels of K+ and malate drop, GCs release water and shrink, relaxing the outward bend and closing over the substomatal cavity.

Stomatal turgor mechanism

Photo credit: JXB

Fig. 5.

Images of stoma 1 from the experiment shown in Fig. 4 during a peak of the oscillation (a) and in the closed state 6 min later (b). The time of observation is also marked by arrows in Fig. 4. The arrow in (a) points to the slightly open pore.

Stomatal oscillations at small apertures: indications for a fundamental insufficiency of stomatal feedback‐control inherent in the stomatal turgor mechanism

by Kaiser H., Kappen L. (2001)

in J. Exp. Bot. (2001) 52 (359):1303-1313. – doi:10.1093/jexbot/52.359.1303. 

Abstract

Continuous measurements of stomatal aperture simultaneously with gas exchange during periods of stomatal oscillations are reported for the first time. Measurements were performed in the field on attached leaves of undisturbed Sambucus nigra L. plants which were subjected to step‐wise increases of PPFD. Oscillations only occurred when stomatal apertures were small under high water vapour mole fraction difference between leaf and atmosphere (Δ_W). They consisted of periodically repeated opening movements transiently leading to very small apertures. Measurements of the area of the stomatal complex in parallel to the determination of aperture were used to record volume changes of guard cells even if stomata were closed. Stomatal opening upon a light stimulus required an antecedent guard cell swelling before a slit occurred. After opening of the slit the guard cells again began to shrink which, with some delay, led to complete closure. Opening and closing were rhythmically repeated. The time‐lag until initial opening was different for each individual stoma. This led to counteracting movements of closely adjacent stomata. The tendency to oscillate at small apertures is interpreted as being a failure of smoothly damped feedback regulation at the point of stomatal opening: Volume changes are ineffective for transpiration if stomata are still closed; however, at the point of initial opening transpiration rate rises steeply. This discontinuity together with the rather long time constants inherent in the stomatal turgor mechanism makes oscillatory overshooting responses likely if at high Δ_W the ‘nominal value’ of gas exchange demands a small aperture.

Read the full article: JXB