Stomatal movements can be induced by electrical signals

Schematic plan of the experimental set-up allowing assessments of leaf gas exchange (CO₂, H₂O), chlorophyll fluorescence, epidermal electrical potential, and stomatal aperture at a pinna of a M. pudica plant.

Rapid hydropassive opening and subsequent active stomatal closure follow heat-induced electrical signals in Mimosa pudica

by Kaiser H., Grams T. E. E. (2006)

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In Journal of Experimental Botany 57(9): 2087–2092 – https://doi.org/10.1093/jxb/erj165 –

https://academic.oup.com/jxb/article/57/9/2087/623683

Time-course of stomatal apertures during leaf folding and images of stomata from the same experiment taken at three different times (A, B, C). Legend symbols in (A) mark three stomata whose responses are plotted in the graph. Time zero is the time of heat stimulation of an adjacent pinna.

Abstract

In Mimosa pudica L., heat stimulation triggers leaflet folding in local, neighbouring and distant leaves. Stomatal movements were observed microscopically during this folding reaction and electrical potentials, chlorophyll fluorescence, and leaf CO₂/H₂O-gas exchange were measured simultaneously. Upon heat stimulation of a neighbouring pinna, epidermal cells depolarized and the stomata began a rapid and pronounced transient opening response, leading to an approximately 2-fold increase of stomatal aperture within 60 s. At the same time, net CO₂ exchange showed a pronounced transient decrease, which was followed by a similar drop in photochemical quantum yield at photosystem (PS) II. Subsequently, CO₂-gas exchange and photochemical quantum yield recovered and stomata closed partly or completely. The transient and fast stomatal opening response is interpreted as a hydropassive stomatal movement caused by a sudden loss of epidermal turgor. Thus, epidermal cells appear to respond in a similar manner to heat-induced signals as the pulvinar extensor cells. The subsequent closing of the stomata confirms earlier reports that stomatal movements can be induced by electrical signals. The substantial delay (several minutes) of guard cell turgor loss compared with the immediate response of the extensor and epidermal cells suggests a different, less direct mechanism for transmission of the propagating signal to the guard cells.

The whole-tree water balance and stomatal oscillations.

 

Whole-tree level water balance and its implications on stomatal oscillations in orange trees [Citrus sinensis (L.) Osbeck] under natural climatic conditions

by Dzikiti S., Steppe K., Lemeur R., Milford J. R. (2007)

in J. Exp. Bot. (2007) 58 (7):1893-1901. – doi: 10.1093/jxb/erm023 – First published online: April 18, 2007

Abstract

Sustained cyclic oscillations in stomatal conductance, leaf water potential, and sap flow were observed in young orange trees growing under natural conditions. The oscillations had an average period of approximately 70 min.

Water uptake by the roots and loss by the leaves was characterized by large time lags which led to imbalances between water supply and demand in the leaves. The bulk of the lag in response between stomatal movements and the upstream water balance resided downstream of the branch, with branch level sap flow lagging behind the stomatal conductance by approximately 20 min while the stem sap flow had a much shorter time lag of only 5 min behind the branch sap flow. This imbalance between water uptake and loss caused transient changes in internal water deficits which were closely correlated to the dynamics of the leaf water potential.

The hydraulic resistance of the whole tree fluctuated throughout the day, suggesting transient changes in the efficiency of water supply to the leaves. A simple whole-tree water balance model was applied to describe the dynamics of water transport in the young orange trees, and typical values of the hydraulic parameters of the transpiration stream were estimated. In addition to the hydro-passive stomatal movements, whole-tree water balance appears to be an important factor in the generation of stomatal oscillations.

 

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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.

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