Dynamics of ionic activities in the apoplast of the sub‐stomatal cavity of intact Vicia faba leaves during stomatal closure evoked by ABA and darkness

Abstract

SummaryStomatal movement is accomplished by changes in the ionic content within guard cells as well as in the cell wall of the surrounding stomatal pore. In this study, the sub-stomatal apoplastic activities of K + , Cl ± , Ca 2+ and H + were continuously monitored by inserting ion-selective micro-electrodes through the open stomata of intact Vicia faba leaves. In light-adapted leaves, the mean activities were 2.59 mM (K + ), 1.26 mM (Cl ± ), 64 mM (Ca 2+ ) and 89 mM (H + ). Stomatal closure was investigated through exposure to abscisic acid (ABA), sudden darkness or both. Feeding the leaves with ABA through the cut petiole initially resulted in peaks after 9±10 min, in which Ca 2+ and H + activities transiently decreased, and Cl ± and K + activities transiently increased. Thereafter, Ca 2+ , H + and Cl ± activities completely recovered, while K + activity approached an elevated level of around 10 mM within 20 min. Similar responses were observed following sudden darkness, with the difference that Cl ± and Ca 2+ activities recovered more slowly. Addition of ABA to dark-adapted leaves evoked responses of Cl ± and Ca 2+ similar to those observed in the light. K + activity, starting from its elevated level, responded to ABA with a transient increase peaking around 16 mM, but then returned to its dark level. During stomatal closure, membrane potential changes in mesophyll cells showed no correlation with the K + kinetics in the sub-stomatal cavity. We thus conclude that the increase in K + activity mainly resulted from K + release by the guard cells, indicating apoplastic compartmentation. Based on the close correlation between Cl ± and Ca 2+ changes, we suggest that anion channels are activated by a rise in cytosolic free Ca 2+ , a process which activates depolarization-activated K + release channels.

Studying guard cells in the intact plant: modulation of stomatal movement by apoplastic factors

“…This value is in agreement with the free Ca 2+ concentration measured by Felle et al . (2000), who found an Felle et al (2000) 3 mM 10 mM Mühling and Läuchli (2000) 27 mM* 40 mM* Lohaus et al (2001) 13 mM Cl - Felle et al (2000) 1 mM 1 mM Lohaus et al (2001) 11 mM Ca 2+ Felle et al (2000) 64 µM Lohaus et al (2001) 700 µM pH Felle et al (2000) 5.1 5.5 Mühling and Läuchli (2000) 5.0 5.4…”

“…Using fluorescent dyes Mühling & Läuchli (2000) determined the K + concentration in the apoplast throughout the leaf (Table 1). They found slightly higher concentrations of K + compared with Felle et al , 2000), but again the K + level increased upon a switch from light to dark. The concentration of Cl -, measured by Felle et al .…”

“…The K + concentration of the apoplast bordering the guard cell walls is not a static value, but changes in response to hormones, such as ABA, or altered light conditions (Felle et al , 2000). The latter authors found a K + concentration of 3 mM in the guard cell wall, when measured in the light.…”

“…Information on the apoplastic solute composition of Vicia faba has been obtained by infiltration of intact leaves (Lohaus et al , 2001). For the dynamical changes in the ion concentration this method is less suitable, but rapid changes in ion concentration have been measured using fluorescent dyes (Mühling & Läuchli, 2000) or with ion selective electrodes brought in contact with the cell wall via an open stoma (Felle et al , 2000). The latter measurements have provided an insight in the effect of various signals on ion concentrations in the apoplast.…”

“…The data of Mühling and Läuchli (2000) derive from measurements with fluorescent dyes fed to the leaf via the petiole or by infiltration of the leaf. The ion concentrations obtained by Felle et al (2000) were measured with ion-selective electrodes brought in contact with the apoplast via an open stoma. *K + concentration at the abaxial side of the leaf, the adaxial concentrations were lower.…”