where A represents the magnetic vector potential, is an integral of the hydromagnetic equations. This -integral made it possible to formulate a variational principle for the force-free magnetic fields. The integral expresses the fact that motions cannot transform a given field in an entirely arbitrary different field, if the conductivity of the medium isconsidered infinite. In this paper we shall show that the full set of hydromagnetic equations admit five more integrals, besides the energy integral, if dissipative processes are absent. These integrals, as we shall presently verify, are I2 =fbHvdV, (2)

Proceedings of the National Academy of Sciences

Potassium in agriculture - status and perspectives

Salinity is a major threat to modern agriculture causing inhibition and impairment of crop growth and development. Here, we not only review recent advances in salinity stress research in plants but also revisit some basic perennial questions that still remain unanswered. In this review, we analyze the physiological, biochemical, and molecular aspects of Na+ and Cl- uptake, sequestration, and transport associated with salinity. We discuss the role and importance of symplastic versus apoplastic pathways for ion uptake and critically evaluate the role of different types of membrane transporters in Na+ and Cl- uptake and intercellular and intracellular ion distribution. Our incomplete knowledge regarding possible mechanisms of salinity sensing by plants is evaluated. Furthermore, a critical evaluation of the mechanisms of ion toxicity leads us to believe that, in contrast to currently held ideas, toxicity only plays a minor role in the cytosol and may be more prevalent in the vacuole. Lastly, the multiple roles of K+ in plant salinity stress are discussed.

/pdf/plant-salinity-stress-many-unanswered-questions-remain-1mie1ygep4.pdf

Plant Salinity Stress: Many Unanswered Questions Remain.

Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain. While respiratory carbon oxidation pathways, electron transport, and ATP turnover are tightly coupled processes, AOX provides a means to relax this coupling, thus providing a degree of metabolic homeostasis to carbon and energy metabolism. Beside their role in primary metabolism, plant mitochondria also act as "signaling organelles", able to influence processes such as nuclear gene expression. AOX activity can control the level of potential mitochondrial signaling molecules such as superoxide, nitric oxide and important redox couples. In this way, AOX also provides a degree of signaling homeostasis to the organelle. Evidence suggests that AOX function in metabolic and signaling homeostasis is particularly important during stress. These include abiotic stresses such as low temperature, drought, and nutrient deficiency, as well as biotic stresses such as bacterial infection. This review provides an introduction to the genetic and biochemical control of AOX respiration, as well as providing generalized examples of how AOX activity can provide metabolic and signaling homeostasis. This review also examines abiotic and biotic stresses in which AOX respiration has been critically evaluated, and considers the overall role of AOX in growth and stress tolerance.

https://www.mdpi.com/1422-0067/14/4/6805/pdf

Alternative Oxidase: A Mitochondrial Respiratory Pathway to Maintain Metabolic and Signaling Homeostasis during Abiotic and Biotic Stress in Plants

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.

/pdf/the-role-of-na-and-k-transporters-in-salt-stress-adaptation-54zd00mx6p.pdf

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes.

Sodium (Na+) is one of the most intensely researched ions in plant biology and has attained a reputation for its toxic qualities. Following the principle of Theophrastus Bombastus von Hohenheim (Paracelsus), Na+ is, however, beneficial to many species at lower levels of supply, and in some, such as certain C4 species, indeed essential. Here, we review the ion’s divergent roles as a nutrient and toxicant, focusing on growth responses, membrane transport, stomatal function, and paradigms of ion accumulation and sequestration. We examine connections between the nutritional and toxic roles throughout, and place special emphasis on the relationship of Na+ to plant potassium (K+) relations and homeostasis. Our review investigates intriguing connections and disconnections between Na+ nutrition and toxicity, and concludes that several leading paradigms in the field, such as on the roles of Na+ influx and tissue accumulation or the cytosolic K+/Na+ ratio in the development of toxicity, are currently insufficiently substantiated and require a new, critical approach.

https://link.springer.com/content/pdf/10.1007%2Fs11104-013-1801-2.pdf

Sodium as nutrient and toxicant

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 micromol g (root FW)(-1) h(-1)], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only approximately 50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional 'leak-pump' model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes.

/pdf/futile-na-cycling-at-the-root-plasma-membrane-in-rice-oryza-34nbxg7vc0.pdf

Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance

The interaction of sodium and potassium ions in the context of the primary entry of Na + into plant cells, and the subsequent development of sodium toxicity, has been the subject of much recent attention. In the present study, the technique of compartmental analysis with the radiotracers 42 K + and 24 Na + was applied in intact seedlings of barley (Hordeum vulgare L.) to test the hypothesis that elevated levels of K + in the growth medium will reduce both rapid, futile Na + cycling at the plasma membrane, and Na + build-up in the cytosol of root cells, under saline conditions (100 mM NaCl). We reject this hypothesis, showing that, over a wide (400fold) range of K + supply, K + neither reduces the primary fluxes of Na + at the root plasma membrane nor suppresses Na + accumulation in the cytosol. By contrast, 100 mM NaCl suppressed the cytosolic K + pool by 47–73%, and also substantially decreased low-affinity K + transport across the plasma membrane. We confirm that the cytosolic [K + ]:[Na + ] ratio is a poor predictor of growth performance under saline conditions, while a good correlation is seen between growth and the tissue ratios of the two ions. The data provide insight into the mechanisms that mediate the toxic influx of sodium across the root plasma membrane under salinity stress, demonstrating that, in the glycophyte barley, K + and Na + are unlikely to share a common low-affinity pathway for entry into the plant cell.

/pdf/non-reciprocal-interactions-between-k-and-na-ions-in-barley-1cdqyf89gs.pdf

Non-reciprocal interactions between K+ and Na+ ions in barley (Hordeum vulgare L.)

The heavy metals silver, gold, and mercury can strongly inhibit aquaporin-mediated water flow across plant cell membranes, but critical examinations of their side effects are rare. Here, the short-lived radiotracer 42 K is used to demonstrate that these metals, especially silver, profoundly change potassium homeostasis in roots of intact barley (Hordeum vulgare L.) plants, by altering unidirectional K + fluxes. Doses as low as 5 mM AgNO3 rapidly reduced K + influx to 5% that of controls, and brought about pronounced and immediate increases in K + efflux, while higher doses of Au 3+ and Hg 2+ were required to produce similar responses. Reduced influx and enhanced efflux of K + resulted in a net loss of >40% of root tissue K + during a 15 min application of 500 mM AgNO3, comprising the entire cytosolic potassium pool and about a third of the vacuolar pool. Silver also brought about major losses of UVabsorbing compounds, total electrolytes, and NH4 . Co-application, with silver, of the channel blockers Cs + , TEA + ,o r Ca 2+ , did not affect the enhanced efflux, ruling out the involvement of outwardly rectifying ion channels. Taken together with an examination of propidium iodide staining under confocal microscopy, the results indicate that silver ions affect K + homeostasis by directly inhibiting K + influx at lower concentrations, and indirectly inhibiting K + influx and enhancing K + efflux, via membrane destruction, at higher concentrations. Ni 2+ ,C d 2+ , and Pb 2+ , three heavy metals not generally known to affect aquaporins, did not enhance K + efflux or cause propidium iodide incorporation. The study reveals strong and previously unknown effects of major aquaporin inhibitors and recommends caution in their application.

/pdf/silver-ions-disrupt-k-homeostasis-and-cellular-integrity-in-1tc3nfhcok.pdf

Silver ions disrupt K+ homeostasis and cellular integrity in intact barley (Hordeum vulgare L.) roots

Soil sodium, while toxic to most plants at high concentrations, can be beneficial at low concentrations, particularly when potassium is limiting. However, little is known about Na(+) uptake in this 'high-affinity' range. New information is provided here with an insight into the transport characteristics, mechanism, and ecological significance of this phenomenon. High-affinity Na(+) and K(+) fluxes were investigated using the short-lived radiotracers (24)Na and (42)K, under an extensive range of measuring conditions (variations in external sodium, and in nutritional and pharmacological agents). This work was supported by electrophysiological, compartmental, and growth analyses. Na(+) uptake was extremely sensitive to all treatments, displaying properties of high-affinity K(+) transporters, K(+) channels, animal Na(+) channels, and non-selective cation channels. K(+), NH(4)(+), and Ca(2+) suppressed Na(+) transport biphasically, yielding IC(50) values of 30, 10, and <5 μM, respectively. Reciprocal experiments showed that K(+) influx is neither inhibited nor stimulated by Na(+). Sodium efflux constituted 65% of influx, indicating a futile cycle. The thermodynamic feasibility of passive channel mediation is supported by compartmentation and electrophysiological data. Our study complements recent advances in the molecular biology of high-affinity Na(+) transport by uncovering new physiological foundations for this transport phenomenon, while questioning its ecological relevance.

https://minerva-access.unimelb.edu.au/bitstream/handle/11343/214443/A pharmacological analysis of high-affinity sodium transport in barley (Hordeum vulgare L.): a 24Na+/42K+ study.pdf

Lasse M. Schulze

Papers

Sodium as nutrient and toxicant

Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance

Non-reciprocal interactions between K+ and Na+ ions in barley (Hordeum vulgare L.)

Silver ions disrupt K+ homeostasis and cellular integrity in intact barley (Hordeum vulgare L.) roots

A pharmacological analysis of high-affinity sodium transport in barley (Hordeum vulgare L.): a 24Na+/42K+ study