No abstract

A, 1984, Ammonium uptake in Lemna gibba G 1, related membrane potential changes, and inhibition of anion uptake, -Physiol, Plant, 61;[369][370][371][372][373][374][375][376] In N-starved (-N) fronds oi Lemna gibba L, G 1, NHJ uptake rates were several-fold those of NOi-supplied ( + N) fronds, NOj uptake in +N-plant,s was slow and not inhibited by addition of NHJ, However, in-N-plants with higher NO^ and still higher NH| uptake rates, addition of NH| immediately reduced the NO5 uptake rates to about one third until the NHJ was consumed. The membrane potential (E,,,) decreased immediately upon addition of NH4+ in all fronds, but whereas depolarisation was moderate and transient in -t-N-plants, it was strong, up to 150 mV, in N-starved plants, where E,,, remained at the level of the K' diffusion potential (Ep) until NHJ was removed. In N-starved plants NHJ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH4+ similarly to NO5 uptake, but only in P-and N-starved plants, not after mere P starvation. Influx of NO5 and H:PO4 into the negatively eharged cells of Lemna is mediated by anion/H* cotransport, but NH4+ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NHJ uniport, the linear component diffusion of NH4+ or NH,, Inhibition of anion/H+ eotransport by high NH4+ influx rates may be due to loss of the proton-driving force, AflH% across the plasmalemma. Reversible inhibition by NHJ of the H+ extrusion pump may contribute to the finding that A|:iH+ cannot be reconstituted in the presence of higher NHJ concentrations.Additional key words -H+ extrusion pump, nitrate uptake, phosphate uptake, W. R. Ullrich (reprint requests) and S. Lesch, Insl. fiir Boianik, Teclmische Hochschule, Sclmittspahnstras.se 3, FRG; M, Botaniska In.st., Stocklwlms Univ., Sweden; A. Novacky,

Phosphate uptake was studied by determining [(32)P]phosphate influx and by measurements of the electrical membrane potential in duckweed (Lemna gibba L.). Phosphate-induced membrane depolarization (ΔE m ) was controlled by the intracellular phosphate content, thus maximal ΔE m by 1 mM H2PO 4 (-) was up to 133 mV after 15d of phosphate starvation. The ΔE m was strongly dependent on the extracellular pH, with a sharp optimum at pH 5.7. It is suggested that phosphate uptake is energized by the electrochemical proton gradient, proceeding by a 2H(+)/H2PO 4 (-) contransport mechanism. This is supported also by the fusicoccin stimulation of phosphate influx. Kinetics of phosphate influx and of ΔE m , which represent mere plasmalemma transport, are best described by two Michaelis-Menten terms without any linear components.

Electrical potentials in cell walls ( Wall ) and at plasma membrane surfaces ( PM ) are determinants of ion activities in these phases. The PM plays a demonstrated role in ion uptake and intoxication, but a comprehensive electrostatic theory of plant-ion interactions will require further understanding of Wall . Wall from potato (Solanum tuberosum) tubers and wheat (Triticum aestivum) roots was monitored in response to ionic changes by placing glass microelectrodes against cell surfaces. Cations reduced the negativity of Wall with effectiveness in the order AlThis order resembles substantially the order of plant-root intoxicating effectiveness and indicates a role for both ion charge and size. Our measurements were combined with the few published measurements of Wall , and all were considered in terms of a model composed of Donnan theory and ion binding. Measured and model-computed values for Wall were in close agreement, usually, and we consider Wall to be at least proportional to the actual Donnan potentials. Wall and PM display similar trends in their responses to ionic solutes, but ions appear to bind more strongly to plasma membrane sites than to readily accessible cell wall sites. Wall is involved in swelling and extension capabilities of the cell wall lattice and thus may play a role in pectin bonding, texture, and intercellular adhesion.The cell wall (CW) is composed of various crosslinked units (macrofibrils, microfibrils, micelles, cellulose units, and linked agents such as neutral sugars, pectin, proteins, and ions; Cosgrove, 1997; Buchanan et al., 2000). The CW behaves as an ion exchanger where the fixed CW charges interact with exchangeable ions in the surrounding solution (Briggs and Robertson, 1957; Gillet and Lefèbvre, 1981; Sentenac and Grignon, 1981; Irwin et al., 1985; Richter and Dainty, 1989a, 1989b, 1990a, 1990b Grignon and Sentenac, 1991). The net CW charge is negative and results from weakly dissociating acidic groups having pK a values similar to those of polyGalUA, the principal origin of the negative charges (Ritchie and Larkum, 1982; Saftner and Raschke, 1981; Richter and Dainty, 1989a; Buchanan et al., 2000). Some positive charges occur too, mainly associated with CW proteins (Cassab and Varner, 1988; Buchanan et al., 2000).The CW determines cell dimensions (Taiz, 1984) and intracellular volume. The volume of the CW is a consequence of the dimensions of its internal spaces, i.e. the distances between the intra-CW units (Shomer et al., 1984; Shomer and Levy, 1988), which are determined by the repulsive strengths of diffuse double layers (Shomer et al., 1991). The swelling of parenchyma CW is restrained by the increase of valence and concentration of the exchangeable ions and with the decrease of the dielectric constant of the bulk solution (Shomer et al., 1991; Shomer, 1995). Although cell expansion and CW extension have been studied comprehensively (Cosgrove, 1997), the properties governing the volume of the CW, from the point of view of the dimensions of its internal spaces...