Estimations of apoplastic concentrations of K+ and Ca2+ in the vicinity of stomatal guard cells.

A method which determines the null point for stomatal aperture has been used to estimate the apoplastic concentrations of potassium and calcium adjacent to the stomatal guard cells of Commelina communis. These two ions are contributors to important aspects of stomatal physiology: the determination of guard cell turgor (K+) and intracellular signalling (Ca2+) when the guard cells respond to the various stimuli that effect changes in stomatal aperture. We obtained estimates of apoplastic K+ concentrations in the range of 50–75 mol m-3, which are in general agreement with those of Bowling (1987). Ca2+ concentrations appear to be in the region of 0·05 mol m-3 adjacent to the guard cells even though much higher concentrations (up to 4 mol m-3) may be delivered to the leaf in the xylem sap. Thus it is suggested that the gradient in apoplastic Ca2+ may be very large over short distances, and may be strictly controlled by cells within the epidermis

Methods and techniques to measure molecular oxygen in plants

Designing and developing sensors for molecular oxygen (O2) has turned into a large, interdisciplinary field of research, with significant progress seen in the past decades. Until the early 1980s, the field of O2 sensing was dominated by polarographic electrode sensors, among which the most popular Clark-type electrode found wide application in plant science. Nevertheless, the great demand for more sophisticated, intracellularly applicable O2 sensors for real-time measurements in plants cannot be satisfied by the predominant techniques. Thus, optical sensors applying an O2-specific reduction of luminescent probes or dyes provide novel, promising tools and open new perspectives on the cellular or even subcellular level of O2 measurements. This chapter aims to give a comprehensive overview on the variety of methods and systems available in the field of O2 sensing with respect to application in plant tissue. Different types of the earlier polarographic electrode technique as well as emerging alternatives will be discussed, including fluorescent proteins as potential, genetically encoded intracellular O2 sensors. Due to the tremendous variety of materials and formats, the young field of optical O2 sensing will receive particular attention directing the focus towards the progress that has been made in developing new probes and dyes. Moreover, the current state of fluorescence measurements will be explored, particularly novel, plant-specific measurement modalities that mask plant autofluorescence. For the potential user, important practical aspects are also presented, revealing the limitations of the existing methods and further encouraging more interdisciplinary research in O2 sensing