Publisher Summary This chapter discusses the potassium estimation, uptake, and its role in the physiology and metabolism of flowering plants. Potassium is one of the vital elements involved in inorganic plant nutrition. A number of methods have been developed for the qualitative and quantitative estimation of potassium. It is now feasible to determine accurately K + content even at the cellular level. Two methods are available for the quantitative estimation of K + in situ: electron probe X-ray microanalysis and potassium-sensitive microelectrodes. Electron probe X-ray microanalysis permits the simultaneous quantitative estimation of several elements in a microstructure. In principle, an electron beam is focused on a section of tissue that excites atoms to emit X-rays whose energy is characteristic of the elements. The X-rays are collected in a tune spectrometer, converted to electrical pulses, and counted. Electron probe X-ray microanalysis can also be used for qualitative localization of ions, a technique called X-ray mapping. The chapter reveals that potassium uptake is mainly metabolic at low external [K + ] ( M ), while it becomes increasingly nonmetabolic at higher (> 1 m M ) external [K + ].

Potassium Estimation, Uptake, and Its Role in the Physiology and Metabolism of Flowering Plants

The xylem is a long-distance transport system that is unique to higher plants. It evolved into a very sophisticated plumbing system ensuring controlled loading/unloading of ions and water and their effective translocation to the required sinks. The focus of this overview will be the intrinsic interrelations between structural and functional features of the xylem. Taken together the xylem is designed to prevent cavitation (entry of air bubbles), induced by negative pressures under transpiration and to repair the cavitated vessels. Half-bordered pits between xylem parenchyma cells and xylem vessels are on the one hand the gates to the vessels but on the other hand a serious ‘bottle-neck’ for transport. Hence it becomes evident that special transport systems exist at the interface between the cells and vessels, which allow intensive fluxes of ions and water to and out of the xylem. The molecular identification and biophysical/ biochemical characterization of these transporters has just started. Paradigms for the sophisticated mechanism of controlled xylem transport under changing environmental conditions are SKOR, a Shaker-like channel involved in K + loading and SOS1, a Na + /H + antiporter with a proposed dual function in Na + transport. In view of the importance

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2003.00930.x

Logistics of water and salt transport through the plant: structure and functioning of the xylem

Publisher Summary The involvement of K+ in a number of plant functions (enzyme activation, maintenance of turgor, protein synthesis), along with the relatively high permeability of plant cell membranes to K+, has been the basis for the study of K+ transport as a model system for plant ion transport Numerous investigations have been conducted over the past 40 years, aimed at furthering the understanding of the mechanism(s), energetics, and cellular location of K+ transport systems in roots Despite the surfeit of literature in these areas, it can be said with some confidence that the processes by which K+ ions are transported into and across the root are still far from being fully resolved This is because of the complex nature of the root, with its various cell types and tissues Additional complications arise from the fact that the individual cells are coupled (both electrically and physiologically) via plasmodesmata, and also simply from the complex nature of individual cells, with their multiple compartments and subcompartments This chapter outlines and summarizes what is known (or at least thought to be known) concerning mechanistic aspects of K+ transport in roots The regulation of these transport processes are discussed in the context of the integration of these cellular processes at the organ and whole plant level

Potassium transport in roots

Growing plant cells increase in volume principally by water uptake into the vacuole. There are only three general mechanisms by which a cell can modulate the process of water uptake: (a) by relaxing wall stress to reduce cell turgor pressure (thereby reducing cell water potential), (b) by modifying the solute content of the cell or its surroundings (likewise affecting water potential), and (c) by changing the hydraulic conductance of the water uptake pathway (this works only for cells remote from water potential equilibrium). Recent studies supporting each of these potential mechanisms are reviewed and critically assessed. The importance of solute uptake and hydraulic conductance is advocated by some recent studies, but the evidence is indirect and conclusions remain controversial. For most growing plant cells with substantial turgor pressure, it appears that reduction in cell turgor pressure, as a consequence of wall relaxation, serves as the major initiator and control point for plant cell enlargement. Two views of wall relaxation as a viscoelastic or a chemorheological process are compared and distinguished.

Water uptake by growing cells: an assessment of the controlling roles of wall relaxation, solute uptake, and hydraulic conductance.

We investigated the mechanism of Na+ reabsorption in exchange for K+ at the xylem/symplast boundary of soybean roots (Glycine max var Hodgson). The xylem vessels of excised roots were perfused with solutions of defined composition to discriminate between entry of ions into or reabsorption from the xylem vessels. In the presence of NaCl, the transport systems released K+ into the xylem sap and reabsorbed Na+. The Na+-K+ exchange was energized by proton-translocating ATPases, enhanced by external K+ concentration, and dependent on the anion permeability. Evidence was presented for the operation of H+/Na+ and H+/K+ antiporters at the xylem/symplast interface.

A. H. Boer

Papers

The role of electrogenic xylem pumps in K+ absorption from the zylem of Vigna unguiculata: the effects of auxin and fusicoccin