Ionic activities in the apoplast of the sub-stomatal cavity

Photo credit: Wiley

(b) Photographs of stomata representative of leaves with maximal and with minimal transpiration. The vitality of the closed guard cell was tested with neutral red. Bar = 25 µm.

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

by Felle H. H., Hanstein S., Steinmeyer R., Hedrich R.

(2000)

in The Plant Journal, 2000, 24, 3, 297 – DOI: 10.1046/j.1365-313x.2000.00878.x

Wiley Online Library – 

http://onlinelibrary.wiley.com/doi/10.1046/j.1365-313x.2000.00878.x/full 

Transpiration of Vicia faba leaves and corresponding stomata opening. (a) Light-and ABA-dependent changes in transpiration (mmol H2O m−2 sec−1) of Vicia faba leaves. Increase in water vapour loss (stomatal opening) was induced by light in a CO2-free atmosphere. At the time indicated, 100 µm ABA were added to the transpiration stream. The delay between ABA application to the petiole of the excised leaf and decrease in transpiration (stomatal closure) is mainly due to the time required for xylem transport of ABA. The stomatal apertures of fully transpiring leaves (plateau) and those treated with ABA were monitored on a CLSM. – http://onlinelibrary.wiley.com/store/10.1046/j.1365-313x.2000.00878.x/asset/image_n/TPJ_878_f1.gif?v=1&t=iojs63xw&s=0efeb91939c56a46e187dc830e52952061d101f3

Summary

Stomatal 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^–, Ca²⁺ 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 µm(Ca²⁺) and 89 µm (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 Ca²⁺ and H⁺ activities transiently decreased, and Cl^– and K⁺ activities transiently increased. Thereafter, Ca²⁺, 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 Ca²⁺ activities recovered more slowly. Addition of ABA to dark-adapted leaves evoked responses of Cl^– and Ca²⁺ 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 Ca²⁺ changes, we suggest that anion channels are activated by a rise in cytosolic free Ca²⁺, a process which activates depolarization-activated K⁺ release channels.

Guard cell K+ content and K+-channel activity in poplar stomata

 

The poplar K⁺ channel KPT1 is associated with K⁺ uptake during stomatal opening and bud development.

by Langer K., Levchenko V., Fromm J., Geiger D., Steinmeyer R., Lautner S., Ache P., Hedrich R. (2004)

in Plant Journal 37: 828–838.

Wiley Online Library | PubMed | CAS |

Abstract

To gain insights into the performance of poplar guard cells, we have measured stomatal conductance and aperture, guard cell K+ content and K+-channel activity of the guard cell plasma membrane in intact poplar leaves.

In contrast to Arabidopsis, broad bean and tobacco grown under same conditions, poplar stomata operated just in the dynamic range – any change in conductance altered the rate of photosynthesis. In response to light, CO2 and abscisic acid (ABA), the stomatal opening velocity was two to five times faster than that measured for Arabidopsis thaliana, Nicotiana tabacum and Vicia faba.

When stomata opened, the K+ content of guard cells increased almost twofold, indicating that the very fast stomatal opening in this species is mediated via potassium uptake. Following impalement of single guard cells embedded in their natural environment of intact leaves with triple-barrelled microelectrodes, time-dependent inward and outward-rectifying K+-channel-mediated currents of large amplitude were recorded.

To analyse the molecular nature of genes encoding guard cell K+-uptake channels, we cloned K+-transporter Populus tremula (KPT)1 and functionally expressed this potassium channel in a K+-uptake-deficient Escherichia coli mutant.

In addition to guard cells, this K+-transporter gene was expressed in buds, where the KPT1 gene activity strongly correlated with bud break. Thus, KPT1 represents one of only few poplar genes associated with bud flush.