The twins K and Na in plants

In the earth's crust and in seawater, K⁺ and Na⁺ are by far the most available monovalent inorganic cations. Physico-chemically, K⁺ and Na⁺ are very similar, but K⁺ is widely used by plants whereas Na⁺ can easily reach toxic levels. Indeed, salinity is one of the major and growing threats to agricultural production. In this article, we outline the fundamental bases for the differences between Na⁺ and K⁺. We present the foundation of transporter selectivity and summarize findings on transporters of the HKT type, which are reported to transport Na⁺ and/or Na⁺ and K⁺, and may play a central role in Na⁺ utilization and detoxification in plants. Based on the structural differences in the hydration shells of K⁺ and Na⁺, and by comparison with sodium channels, we present an ad hoc mechanistic model that can account for ion permeation through HKTs.     

Shoot growth in Ilex paraguariensis plants grown under varying photosynthetically active radiation is affected through gibberellin levels

The aim of this work was to analyse the response ofNH₄ ⁺ assimilation in leaves of tobacco plants(Nicotiana tabacum L. cv. Tennessee 86), to different Bapplications (B1, 5 M H₃BO₃; B2, 10M H₃BO₃; B3, 20 MH₃BO₃). The plants were grown under controlledenvironmental conditions and received a complete nutrient solution. In thisexperiment, we analysed the foliar concentrations of B andNH₄ ⁺, as well as the corresponding enzymaticactivities: GS,GOGAT, GDH, PEPC; the end products of this assimilation, aminoacids and proteins; and finally the concentration of non-structural sugars. Ourresults indicated that the different B treatments influenced the utilization ofNH₄ ⁺ by tobacco leaves. The B3 treatment registeredthe lowest NH₄ ⁺ concentration, and B1 the highest,due probably to the higher GS, GOGAT and GDH activities registered at B3.Conversely, a decline in the concentration of non-structural sugars wasrecordedat B3. In addition, the high assimilation rate caused a progressiveaccumulationof amino acids as well as proteins, and boosted biomass production in theleaves.     

Photosynthesis and carbon export in white clover plants grown at various levels of phosphorus supply

Net photosynthesis, concurrent carbon export and starch, sucrose and inorganic phosphorus concentrations were measured in leaves of white clover ( Trifolium repens L. cv. Grasslands Huia) grown at four levels of phosphorus supply in the presence or absence of mineral nitrogen. The nitrogen treatments had no effect on growth, photosyntheis or carbon export. At the three higher levels of phosphorus supply, the amount of carbon exported was about 77% of net fixation. Photosynthesis and export per leaf decreased with phosphorus supply, primarily through the effect of phosphorus supply on leaf area. The rate of photosynthesis was reduced only at the lowest level of phosphorus supply.
Inorganic phosphorus rose with phosphorus supply but starch concentration was unaffected. Sucrose was reduced at the lowest level of phosphorus supply but not significantly affected at higher levels. The ratio between starch and sucrose concentration was also unaffected at the higher levels, but was increased at the lowest level of supply. There thus appeared to be direct effects of phosphorus supply on photosynthesis, partitioning of carbon to carbohydrates and, by implication, export, only at the lowest level of phosphorus supply. As leaf area and plant growth were affected over the whole range of phosphorus supply, factors other than photosynthesis per se must have determined the response of growth to phosphorus supply.     

Some effects of chromium toxicity on bush bean plants grown in soil

Summary Chromium applied to a noncalcareous soil at 50 ppm did not decrease yields of bush beans (Phaseolus vulgaris L. var Improved Tendergreen), but when EDTA (ethylenediamine tetraacetic acid) was added with it, it did. Very little Cr was present in leaves. In solution culture 10^-5 M Cr and higher were toxic. With solution culture the highest level of Cr in leaves was about 30 ppm and in general there was a decreasing gradient in Cr from roots to stems to leaves. EDTA had less effect in solution cultures on Cr toxicity because the Cr was already in solution. Chromium toxicity decreased cation levels in plants.     

Cis-allosteric effects of cytoplasmic Na+/K+ discrimination at varying pH. Low-affinity multisite inhibition of cytoplasmic K+ in reconstituted Na+/K(+)-ATPase engaged in uncoupled Na(+)-efflux.

In liposomes with reconstituted shark Na+/K(+)-ATPase the effect of cytoplasmic K+ was investigated in the absence of extracellular alkali ions. During such conditions the Na+/K(+)-ATPase is engaged in the so called uncoupled Na+ efflux mode in which cytoplasmic Na+ activates and binds to the enzyme and becomes translocated without countertransport of K+ as in the physiological Na+/K+ exchange mode. In this uncoupled flux mode only low-affinity inhibition by K+cyt is found to be present. The inhibition pattern is consistent with a model in which cytoplasmic K+ exhibit mixed inhibition of Na+ activation, probably by binding at the three cytoplasmic loading sites on E1ATP (E1A). With determined intrinsic binding constants for cytoplasmic Na+ to this form of KS1, KS2, KS3 = 40 mM, 2 mM, 2 mM the inhibition pattern can be simulated assuming three K+cyt sites with equal affinity for Ki = 40 mM, similar to KS1 for the first Na+cyt site. The discrimination between cytoplasmic Na+ and K+ is therefore enhanced by allosteric interaction initiated from the cis-side due to binding of the first Na+, as opposed to K+, which induces the positive cooperatively in the successive Na+ bindings. pH is found to influence the pattern of K+cyt inhibition: A lowering of the pH potentiates the K+cyt inhibition, whereas at increased pH the inhibition is decreased and transformed into a pure competitive competition.     

Erythrocyte cations and Na+,K+-ATPase pump activity in athletes and sedentary subjects

The chronic effect of training on intraerythrocyte cationic concentrations and on red cell Na+,K+-ATPase pump activity was studied by comparing well-trained athletes with sedentary subjects at rest. Also the acute effect of a 50-min cross-country run on these erythrocyte measurements was studied in the athletes. At rest the intraerythrocyte potassium concentration was increased (P less than 0.01) in the athletes compared to that of the control subjects. The intraerythrocyte concentrations of sodium and magnesium and red cell Na+,K+-ATPase pump activity were, however, similar in the trained and the untrained subjects. As compared with the resting condition, the intraerythrocyte potassium concentration was decreased (P less than 0.05) after exercise in the athletes, and this was accompanied by a minor increase in the intraerythrocyte sodium concentration. Red cell Na+,K+-ATPase pump activity was slightly increased (P less than 0.05) after exercise.     

Sodium and buffer cations inhibit dephosphorylation of (Na+ + K+)-ATPase

Effects of various cations on the dephosphorylation of (Na+ + K+)-ATPase, phosphorylated by ATP in 50 mM imidazole buffer (pH 7.0) at 22 degrees C without added Na+, have been studied. The dephosphorylation in imidazole buffer without added K+ is extremely sensitive to K+-activation (Km K+ = 1 microM), less sensitive to Mg2+-activation (Km Mg2+ = 0.1 mM) and Na+-activation (Km Na+ = 63 mM). Imidazole and Na+ effectively inhibit K+-activated dephosphorylation in linear competitive fashion (Ki imidazole 7.5 mM, Ki Na+ 4.6 mM). The Ki for Na+ is independent of the imidazole concentration, indicating different and non-interacting inhibitory sites for Na+ and imidazole. Imidazole inhibits Mg2+-activated dephosphorylation just as effective as K+-activated dephosphorylation, as judged from the Ki values for imidazole in the two processes. Tris buffer and choline chloride, like imidazole, inhibit dephosphorylation in the presence of residual K+ (less than 1 microM), but less effectively in terms of I50 values and extent of inhibition. Tris inhibits to the same extent as choline. This indicates different inhibitory sites for Tris or choline and for imidazole. These findings indicate that high steady-state phosphorylation levels in Na+-free imidazole buffer are due to the induction of a phosphorylating enzyme conformation and to the inhibition of (K+ + Mg2+)-stimulated dephosphorylation.     

Na+ and K+ binding by glycerinated muscle fibers at equal concentrations of the chlorides of these cations in the medium

The Na+ and K+ equilibrium distribution between the medium and glycerinated muscle fibres of the frog has been investigated under equal concentrations of NaCl and KCl in solutions. Concentrations of NaCl and KCl varied from 0.5-1.5 mkM till 50 mM. Ion strength (0.11) was constant owing to the imidazol--HCl buffer. The binding of Na+ and K+ by model fibres occurred in accordance with the Langmur equation. Two kinds of cation-binding sites were found. The one with a low limiting ion sorption (A infinity approximately 1.3 mmol/kg dry weight of fibres) and high affinities (-delta F0 approximately 4.3 kcal/mol) was saturated at 0.5 mM concentrations (Na+ = K+) in the medium, and the other--with A infinity exceeding the previous one by an order and low -delta F0 (2.5 kcal/mol) was discovered at Na+, K+-1-10 mM. At ion concentrations equal to 0.5-1 mM the Langmur-binding is disturbed. At Na+-K+ less than or equal to 1 mM Na+ bound:K+ bound approximately to 1:1. At higher concentrations of cations Na+ bound:K+ bound approximately equal to 3:2. It is concluded that at least part of the sites in model fibres is capable of interacting only with Na+, but not with K+. It is supposed that at equal concentrations of Na+ and K+ in the medium the cations are bound by Na+, K+-ATPase of glycerinated muscle fibres.     

The relationship between stomatal resistance and abscisic-acid levels in leaves of water-stressed bean plants

Leaf water potentials of Phaseolus vulgaris L. plants exposed to a -3.0 bar root medium were reduced to between -7 and -9 bars within 25 min and remained constant for the next several hours. This treatment led to considerable variation between leaves in both abscisic-acid (ABA) content and R_s, although the two were well correlated after a 5-h treatment. There was an apparent 7-fold increase in leaf ABA levels necessary to initiate stomatal closure when plants were exposed to a -3.0 bar treatment, but when plants were exposed to a -5.0 bar stress R_s values increased prior to any detectable rise in ABA levels. To explain these seemingly contradictory results, we suggest that the rate of ABA synthesis in the leaf, rather than the total ABA content, determines the status of the stomatal aperture.Abbreviations ABA abscisic acid - PEG polyethylene glycol - R_s stomatal diffusion resistance of lower leaf surface - leaf water potential     

Reaction of faba bean plants to soil and foliar N application and K nutrition

The effect of different rates and methods of fertilizer nitrogen application and potassium nutrition on the root nodule formation of faba bean plants var. Nadwiślański and on their nitrogenase activity was studied. It was found that fertilizer N depressed the nodule formation and nitrogenase activity, but inhibitory effect of N was smaller when it was supplied to the leaves instead to the soil. Plants growing at higher K level were in a position allowing on better development of nodules and consequently higher N₂- fixation.     

Effect of calcium levels on redistribution of Sr85 in bush bean plants

Summary When bush bean plants previously exposed to Sr⁸⁵ were transferred to solutions containing different levels of calcium without further addition of Sr⁸⁵, and calcium had relatively little, but some, effect on the redistribution of the Sr⁸⁵ in the plants. Calcium did tend to restrict movement of strontium. Sr⁸⁵ tended to concentrate in stems or old leaves, usually more so than calcium. New leaves particularly had lower Sr/Ca ratios than other plant parts indicating some discrimination in favor of calcium.     

The Distribution of Na+, K+ and Glycinebetaine in Salicornia bigelovii

Stems of young actively growing Salicornia bigelovii were dissectedinto the three major tissue layers: vascular, spongy mesophylland palisade. Each layer was analysed for chlorophyll, ash (salt),protein and glycinebetaine content. When glycinebetaine contentwas based on protein content, the vascular and spongy mesophylllayers had nearly identical values. Correction for probableRuBP carboxylase content in the palisade layer gave a glycinebetaine/proteinratio similar to that of the other tissues. All three tissuelayers were found to contain significant amounts of salt. Key words: Salicornia bigelovii, Salt distribution     

K+-dependent net Na+ efflux in roots of barley plants

Summary The influence of K⁺ ions on the net Na⁺ fluxes in cells of excised barley roots (Hordeum distichon L.) and roots of whole barley plants was investigated. The fluxes were determined by flame photometry in the external solution. In both cases a transient net Na⁺ efflux against the external Na⁺ concentration was observed upon addition of K⁺. The results stress the effectiveness of the K⁺-dependent Na⁺ efflux mechanism residing at the plasmalemma, and its involvement in K–Na-selectivity in whole barley plants.     

Divalent cations and the phosphatase activity of the (Na + K)-dependent ATPase

Phosphatase activity of a kidney (Na + K)-ATPase preparation was optimally active with Mg²⁺ plus K⁺. Mn²⁺ was less effective and Ca²⁺ could not substitute for Mg²⁺. However, adding Ca²⁺ with Mg²⁺ or substituting Mn²⁺ for Mg²⁺ activated it appreciably in the absence of added K⁺, and all three divalent cations decreased apparent affinity for K⁺. Inhibition by Na⁺ decreased with higher Mg²⁺ concentrations, when Ca²⁺ was added, and when Mn²⁺ was substituted for Mg²⁺. Dimethyl sulfoxide, which favorsE ₂ conformations of the enzyme, increased apparent affinity for K⁺, whereas oligomycin, which favorsE ₁ conformations, decreased it. These observations are interpretable in terms of activation through two classes of cation sites. (i) At divalent cation sites, Mg²⁺ and Mn²⁺, favoring (under these conditions)E ₂ conformations, are effective, whereas Ca²⁺, favoringE ₁, is not, and monovalent cations complete. (ii) At monovalent cation sites divalent cations compete with K⁺, and although Ca²⁺ and Mn²⁺ are fairly effective, Mg²⁺ is a poor substitute for K⁺, while Na⁺ at these sites favorsE ₁ conformations. K⁺ increases theK _m for substrate, but both Ca²⁺ and Mn²⁺ decrease it, perhaps by competing with K⁺. On the other hand, phosphatase activity in the presence of Na⁺ plus K⁺ is stimulated by dimethyl sulfoxide, by higher concentrations of Mg²⁺ and Mn²⁺, but not by adding Ca²⁺; this is consistent with stimulation occurring through facilitation of an E₁ to E₂ transition, perhaps an E₁-P to E₂-P step like that in the (Na + K)-ATPase reaction sequence. However, oligomycin stimulates phosphatase activity with Mg²⁺ plus Na⁺ alone or Mg²⁺ plus Na⁺ plus low K⁺: this effect of oligomycin may reflect acceleration, in the absence of adequate K⁺, of an alternative E₂-P to E₁ pathway bypassing the monovalent cation-activated steps in the hydrolytic sequence.     

Nonstructural carbohydrates of soybean plants grown in subambient and superambient levels of CO2

Photosynthesis Research - Elevated carbon dioxide (CO2) concentration increases plant photosynthesis, biomass and carbohydrate accumulation. Since plants have grown in low CO2 (200 to 300 µmol...