Long distance transport of potassium in cereals during grain filling in detached ears

Ears of wheat plants ( Triticum aestivum L. cv. Kolibri), which were given different and uniform K⁺-nutrition in two experiments, were cut at 2, 4 and 6 weeks after anthesis at 15 cm below the ear. These detached ears were fed 30 m M (experiment 1) or 15, 30, 60 or 90 m M ⁸⁶Rb-K₂ malate (experiment 2) and 146 m M [¹⁴C]-sucrose. After a pulse period of 6 and 4 h, respectively, the ears were transferred to identical non-labeled solutions for additional 0, 4, 8 or 20 h.
About 50% of the K⁺ and sucrose supplied was absorbed by detached ears. This rate declined with plant age and decreasing transpiration. Within the 6 and 4 h uptake period less than 7% of the absorbed K⁺, but 20% of the sucrose taken up were incorporated into the grain. During the chase period labeled K⁺ in the grain increased to 15% and ¹⁴C even to 50% of total tracer uptake. Incorporation of labeled K⁺ into the grain was not affected by the previous K⁺ nutrition of the plant and was proportional to the K⁺ concentration in the uptake solution. Transition of K⁺ from xylem into phloem during its acropetal transport is assumed. No evidence was found that the grain itself could control its uptake of K⁺.     

Membrane and ion transport properties in cereals under acidic and alkaline stress

The effects of pH on the growth and the K⁺ (⁸⁶Rb) uptake and K⁺ content of excised rice ( Oryza sativa L. cv. Dunghan Shali) and wheat ( Triticum aestivum L. cv. GK Szeged) roots were investigated. Rice roots responded to H⁺ stress with an increased K⁺(⁸⁶Rb) influx and a decreased K⁺ content, suggesting an increased exchange between the cytoplasmic K⁺ pool and the external medium. Under the same experimental conditions wheat did not show any anomalous K⁺(⁸⁶Rb) influx. Growth of both rice and wheat was relatively insensitive to pH between 4 to 10.     

Phases in potassium transport and their regulation under near-equilibrium conditions in wheat seedlings

Potassium uptake and release in roots and translocation to the shoots were studied in 14-day-old winter wheat ( Tritictum aestivum L. cv. Martonvásári 8) of different K status. Transport processes were measured in the growth solutions for 5 h ensuring near-equilibrium conditions. The uptake showed three phases: (1) at low external K⁺ concentrations it increased with increasing concentrations and culminated at 0.1 m M : (2) between 0.1 and 1 m M it decreased, and (3) it increased again above 1 m M : The release of K⁺ showed a constant low level below 1 m M while paralleling the uptake above that. The uncoupler 2,4-dinitrophenol inhibited uptake phases (1) and (2), whereas it did not affect either phase (3) or K⁺ release. Translocation showed similar patterns. It is concluded that phases (1) and (2) depend on metabolic energy while phase (3) is mostly passive. It is emphasized that different types of regulation seem to operate in the transport mechanism: i.e. limitation by transport sites, control by negative feedback and by K⁺/K⁺ exchange, respectively.     

Effect of the crown on uptake and transport in embryonic shoots of Picea abies

Embryonic shoots of Picea abies (L.) Karst, isolated from 10-year-old trees, were excised either with or without the crown. Various short-term uptake experiments (3, 6 and 24 h) and one long-term uptake experiment (4 weeks) were performed with these shoots to obtain information about the physiological role of the crown as translocation barrier for different substances. Transport through the embryonic shoots was followed in both acropetal and basipetal directions using radiolabelled substances supplied in an agarified Schenk and Hildebrandt medium. The medium was labelled with [¹⁴C]-IAA and/or [³²P]-phosphate, or with [³⁵S]-sulphate and ⁸⁶Rb (as a tracer for K⁺). The experiments were conducted in light at 20°C, with the exception of one of the short-term experiments, which was carried out at 5°C to evaluate the connection between transport and metabolism. The main observation is that the crown in its collenchymatous stage of development acts as a selective barrier both acropetally and basipetally for transport of substances such as [¹⁴C]-IAA and [³²P]-phosphate or their metabolized forms. This could explain why the embryonic shoot when cultured plus or minus its crown shows different growth and developmental patterns in vitro.     

Net uptake and partitioning of nitrogen and potassium in cultivars of barley during ageing

Experiments were carried out with barley cultivars ( Hordeum vulgare L.) grown in both pot- and water-culture. Net uptake of NO3₋ and K⁺ in the roots was followed in two barley cultivars grown on water-culture for 85 days. After an initial period of low net uptake of both ions, uptake increased until a maximum was reached after 30 to 45 days. Thereafter, net uptake of NO₃⁻ and K⁺ steadily decreased. In the pot experiments, effects of different mineral supply on day 4 to 18 upon the development of five barley cultivars of various earliness were investigated. The effect of earliness on fresh weight production was largest when mineral supply on day 4 to 18 was limited. The influence of limited mineral supply on day 4 to 18 on K-economy was independent of earliness of the cultivars. The maximal N-content was reached at the same time as maximal fresh and dry weight in fairly late cultivars; in early cultivars maximum N-level was reached later than maximum fresh and dry weight. Overall, maximal N-content was higher in the fairly late cultivars than in the early cultivars. The highest rate of ¹⁵N-transport was attained later in two of three fairly late cultivars than in early cultivars. Partitioning of dry weight, N and K in the shoots changed during ageing, ears being an important sink. Varietal differences in partitioning depended on the earliness of the cultivars. The largest fraction of recently supplied ¹⁵N, supplied as nitrate, and K⁺ (⁸⁶Rb) were found in the stems. In the oldest plants of the early cultivars the transport to the ears of these isotopes was gradually impaired, reflecting the decreasing function of the long distance transport system.     

Comparison of the effects of auxin and fusicoccin on phosphate absorption by aged potato tuber discs

Auxin (IAA, 5 × 10⁻⁵ M ) partially prevents the increase in the rate of phosphate uptake during ageing of potato tuber discs ( Solatium tuberosum L. cv. Bintje), whereas fusicoccin (FC, 10⁻⁵ M) stimulates it. After the development of enhanced phosphate transport capacity, the response to fusicoccin is greater than with fresh discs. Complementary experiments on K⁺ (⁸⁶Rb) absorption show that FC also slightly enhances the rate of K⁺ uptake, while IAA has no much effect. It is suggested that IAA acts specifically on the development of a mechanism which occurs during the ageing period, while FC action may be more directly linked to the system of phosphate transport itself.     

Nitrate and nitrite uptake and reduction by intact sunflower plants

Nitrogen-starved sunflower plants (Helianthus annuus L. cv. Peredovic) cannot absorb NO ₃ ^– or NO ₂ ^– upon initial exposure to these anions. Ability of the plants to take up NO ₃ ^– and NO ₂ ^– at high rates from the beginning was induced by a pretreatment with NO ₃ ^– . Nitrite also acted as inducer of the NO ₂ ^– -uptake system. The presence of cycloheximide during NO ₃ ^– -pretreatment prevented the subsequent uptake of NO ₃ ^– and NO ₂ ^– , indicating that both uptake systems are synthesized de novo when plants are exposed to NO ₃ ^– . Cycloheximide also suppressed nitrate-reductase (EC 1.6.6.1) and nitrite-reductase (EC 1.7.7.1) activities in the roots. The sulfhydryl-group reagent N-ethylmaleimide greatly inhibited the uptake of NO ₃ ^– and NO ₂ ^– . Likewise, N-ethylmaleimide promoted in vivo the inactivation of nitrate reductase without affecting nitrite-reductase activity. Rates of NO ₃ ^– and NO ₂ ^– uptake as a function of external anion concentration exhibited saturation kinetics. The calculated K_m values for NO ₃ ^– and NO ₂ ^– uptake were 45 and 23 M, respectively. Rates of NO ₃ ^– uptake were four to six times higher than NO ₃ ^– -reduction rates in roots. In contrast, NO ₂ ^– -uptake rates, found to be very similar to NO ₃ ^– -uptake rates, were much lower (about 30 times) than NO ₂ ^– -reduction rates. Removal of oxygen from the external solution drastically suppressed NO ₃ ^– and NO ₂ ^– uptake without affecting their reduction. Uptake and reduction were also differentially affected by pH. The results demonstrate that uptake of NO ₃ ^– and NO ₂ ^– into sunflower plants is mediated by energy-dependent inducible-transport systems distinguishable from the respective enzymatic reducing systems.Abbreviations CHI cycloheximide - NEM N-ethylmaleimide - NiR nitrite reductase - NR nitrate reductase - pHME p-hydroxymercuribenzoate This research was supported by grant PB86-0232 from the Dirección General de Investigatión Científica y Técnica (Spain). One of us (E.A.) thanks the Consejeria de Educación y Ciencia de la Junta de Andalucia for the tenure of a fellowship. We thank Miss G. Alcalá and Miss C. Santos for their valuable technical and secretarial assistance.     

Effects of nitrate on influx, efflux and translocation of potassium in young sunflower plants

Influx, efflux and translocation of K⁺(⁸⁶Rb) were studied in the roots of sunflower seedlings ( Helianthus annuus L. cv. Uniflorus) treated with 0–4.0 m M NO₃⁻ during a 9 day growth period or a 24 h pretreatment period. Roots treated with high levels of NO₃⁻ absorbed and translocated more K⁺(⁸⁶Rb) than seedlings treated with low levels of NO₃⁻. The content of K⁺ in the shoots was, however, higher in seedlings treated with low levels of NO₃⁻, indicating a low rate of retranslocation of K⁺ in those plants. K⁺(⁸⁶Rb) efflux was highest into the low-NO₃⁻ solutions. All effects on K⁺(⁸⁶Rb)-fluxes were more obvious in high-K plants than in low-K plants. The results are discussed in relation to the Dijkshoorn-Ben Zioni hypothesis for K⁺+ NO₃⁻-uptake and translocation in plants.     

2,4-Dichlorophenoxyacetic acid inhibition of potassium transport in barley seedlings

Growth, potassium uptake and translocation as well as transpiration rates were measured in intact low-salt barley seedlings ( Hordeum vulgare L. cv. Union) in the presence of different 2,4-D concentrations at pH 6.5. Growth was only affected at 10^-3 M .
Above 10^-7 M 2,4-D both uptake by the roots and transport to the shoots were inhibited. The inhibition at 10^-5 M remained constant for at least 24 h. Furthermore inhibition of uptake was measurable within 1 h. Excised roots and roots of intact plants showed the same uptake pattern.
It is suggested that the observed effects were caused by 2,4-D-induced changes in uptake and translocation systems in the roots. Pre-treatment with 10^-5 M 2,4-D had no effect upon subsequent potassium uptake. Transpiration was reduced within 1 h in 10^-4 or 10^-3 M 2,4-D, probably due to changes in water transport or root permeability.     

Long-term effects of abscisic acid on K+ transport in young wheat plants of different K+ status

The effects of abscisic acid (ABA) on growth, uptake and translocation of potassium ions, K⁺,Mg²⁺-ATPase activity and transpiration were investigated in young wheat ( Triticum aestivum L. cv. Martonvásári-8) plants grown at different K⁺ supplies. Long-term treatment with ABA (10 μ M ) reduced growth in high-K⁺ plants, but had less effect under low-K⁺ conditions. K⁺(⁸⁶Rb) uptake was inhibited by about 70 and 40% in low- and high-K⁺ plants, respectively. The stimulation by K⁺ of the Mg²⁺-ATPase activity in the root microsomal fraction was lost with ABA treatment. It is suggested that the inhibitory effect of ABA on K⁺ uptake may be related to this effects on the K⁺,Mg²⁺-ATPase. Translocation of K⁺ to the shoot was inhibited in low-K⁺ plants only, and it was not affected in high-K⁺ plants. In parallel to this, ABA treatment reduced transpiration by about 50% in low-K⁺ plants, whereas a much smaller effect was seen in high-K⁺ plants. These observations suggest that the regulation by ABA of the stomatal movements is strongly counteracted by high-K⁺ status.     

Sodium and potassium compartmentation and transport in the roots of intact lettuce plants

In this report, we consider the accumulation in roots, and transport to the shoot, of Na⁺ and K⁺ in intact lettuce plants (Lactuca sativa cv Black-seeded Simpson). Plants were grown in modified Hoagland medium supplemented with 10 moles NaCl per cubic meter. At this salinity, significant levels of Na⁺ were accumulated in roots and shoots, but there was no reduction in plant growth. Transport characteristics for both Na⁺ and K⁺ were qualitatively similar to those previously reported, for Spergularia marina, indicating that the results obtained with these experimental protocols are not limited to one unconventional experimental plant. The most pronounced difference in transport of the two ions was evident when transport was followed in a chase period after a 10 minute uptake pulse. For Na⁺, there was an initially rapid, but small, loss of label to the medium, and very little movement to the shoot. For K⁺, little label was lost from the plants, but translocation to the shoot proceeded for at least 60 minutes. The transport systems were further distinguished by treating the roots during labeling with 20 micrograms per milliliter cycloheximide. For K⁺, both uptake and translocation were reduced by about 50%. For Na⁺, root accumulation was stimulated more than five-fold, while transport to the shoot was reduced about 20%. Cycloheximide also modified the Na⁺ transport characteristics such that continued translocation occurred during the chase period of pulse-chase studies.     

A cooperative system of silicon transport in plants

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Uptake and long-distance transport of phosphate,potassium and chloride in relation to internal ion concentrations in barley: evidence of non-allosteric regulation

The extent to which uptake and transport of either phosphate, potassium or chloride are controlled by the concentration of these ions within the root, perhaps through an allosteric mechanism, was investigated with young barley plants in nutrient solution culture. Plants were grown with their roots divided between two containers, such that a single seminal root was continuously supplied with all the required nutrient ions, while the remaining four or five seminal roots were either supplied with the same solution (controls) or, temporarily, a solution lacking a particular nutrient ion (nutrient-deficient treatment). Compared with controls, there was a marked stimulation of uptake and transport of labelled ions by the single root following 24 h or more of nutrient dificiency to the remainder of the root system. This stimulation, which comprised an increased transport to the shoot and, for all ions except Cl^-, increased transport to the remainder of the root system, took place without appreciable change in the concentration of particular ions within the single root. However, nutrient deficiency quickly caused a lower concentration of ions in the shoot and the remaining roots. The results are discussed in relation to various mechanisms, proposed in the literature, by which the coordination of ion uptake and transport may be maintained within the plant. We suggest that under our conditions any putative allosteric control of uptake and transport by root cortical cells was masked by an alternative mechanism, in which ion influx appears to be regulated by ion efflux to the xylem, perhaps controlled by the concentration of particular ions recycled in the phloem to the root from the shoot.     

Modulation of K+ translocation by AKT1 and AtHAK5 in Arabidopsis plants

Root cells take up K⁺ from the soil solution, and a fraction of the absorbed K⁺ is translocated to the shoot after being loaded into xylem vessels. K⁺ uptake and translocation are spatially separated processes. K⁺ uptake occurs in the cortex and epidermis whereas K⁺ translocation starts at the stele. Both uptake and translocation processes are expected to be linked, but the connection between them is not well characterized. Here, we studied K⁺ uptake and translocation using Rb⁺ as a tracer in wild‐type Arabidopsis thaliana and in T‐DNA insertion mutants in the K⁺ uptake or translocation systems. The relative amount of translocated Rb⁺ to the shoot was positively correlated with net Rb⁺ uptake rates, and the akt1 athak5 T‐DNA mutant plants were more efficient in their allocation of Rb⁺ to shoots. Moreover, a mutation of SKOR and a reduced plant transpiration prevented the full upregulation of AtHAK5 gene expression and Rb⁺ uptake in K⁺‐starved plants. Lastly, Rb⁺ was found to be retrieved from root xylem vessels, with AKT1 playing a significant role in K⁺‐sufficient plants. Overall, our results suggest that K⁺ uptake and translocation are tightly coordinated via signals that regulate the expression of K⁺ transport systems.     

Kinetics of high-affinity K+ uptake in plants, derived from K+-induced changes in current-voltage relationships

To investigate coupled, charge-translocating transport, it is imperative that the specific transporter current-voltage (IV ) relationship of the transporter is separated from the overall membrane IV relationship. We report here a case study in which the currents mediated by the K⁺-H⁺ symporter, responsible for high-affinity K⁺ uptake in Arabidopsis thaliana (L.) Heynh. cv. Columbia roots, are analyzed with an enzyme kinetic reaction scheme. The model explicitly incorporates changes in membrane voltage and external substrate, and enables the derivation of the underlying symport IV relationships from the experimentally obtained difference IV data. Data obtained for high-affinity K⁺ transport in A. thaliana root protoplasts were best described by a 1:1 coupled K⁺-H⁺ symport-mediated current with a parallel, outward non-linear K⁺ pathway. Furthermore, the large predictive value of the model was used to describe symport behaviour as a function of the external K⁺ concentration and the cytoplasmic K⁺ concentration. Symport activity is a complex function of the external K⁺ concentration, with first-order saturating kinetics in the micromolar range and a strong activity reduction when external K⁺ is in the millimolar range and the membrane depolarises. High cytoplasmic K⁺ levels inhibit symport activity. These responses are suggested to be part of the feedback mechanisms to maintain cellular K⁺ homeostasis. The general suitability of the model for analysis of carrier-mediated transport is discussed. Received: 23 November 1996 / Accepted: 22 April 1997     

Abscisic acid and ethylene interact in wheat grains in response to soil drying during grain filling

Grain filling is an intensive transportation process regulated by soil drying and plant hormones. This study investigated how the interaction between abscisic acid (ABA) and ethylene is involved in mediating the effects of soil drying on grain filling in wheat (Triticum aestivum). Two wheat cultivars, cv. Yangmai 6 and cv. Yangmai 11, were field-grown, and three irrigation treatments, well-watered, moderately soil-dried (MD) and severely soil-dried (SD), were imposed from 9 d post anthesis until maturity. A higher ABA concentration and lower concentrations of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) were found in superior grains (within a spike, those grains that were filled earlier and reached a greater size) than in inferior grains (within a spike, those grains that were filled later and were smaller), and were associated with a higher filling rate in the superior grains. An increase in ABA concentration and reductions in ethylene and ACC concentrations in grains under MD conditions increased the grain-filling rate, whereas much higher ethylene, ACC and ABA concentrations under SD conditions reduced the grain-filling rate. Application of chemical regulators gave similar results. The results did not differ between the two cultivars. The grain-filling rate in wheat is mediated by the balance between ABA and ethylene in the grains, and an increase in the ratio of ABA to ethylene increases the grain-filling rate.     

Zinc effects on cadmium accumulation and partitioning in near-isogenic lines of durum wheat that differ in grain cadmium concentration

Here, we examined the effectiveness of two approaches for reducing cadmium (Cd) accumulation in durum wheat (Triticum turgidum L. var durum) grain: the application of supplemental zinc (Zn), and the use of cultivars exhibiting reduced grain Cd concentrations. Two durum wheat near-isogenic lines (NIL) that differ in grain Cd accumulation were grown to maturity in solution culture containing a chelating agent to buffer the free activities of Zn and Cd at levels approximating those of field conditions. The low Cd accumulating (L-Cd) isoline had Cd concentrations, in grains and shoot parts, which were 60-70% lower than those of the high Cd accumulating (H-Cd) isoline. Increasing the Zn activities in the nutrient solution from deficient to sufficient levels reduced the concentration of Cd in grains and vegetative shoot parts of both isolines. The results suggest that supplemental Zn reduces Cd tissue concentrations by inhibiting Cd uptake into roots. Cd partitioning patterns between roots and shoots and between spike components suggest that the physiological basis for the low Cd trait is related to the compartmentation or symplasmic translocation of Cd.     

Adenosine Receptors in Myelin Fractions and Subtractions: The Effect of the Agonist (R)-Phenylisopropyladenosine on Myelin Membrane Microviscosity

In this article the existence of A1 adenosine receptors and the absence of A2 adenosine receptors in myelin membranes purified from pig brain white matter are demonstrated. The characterization of (R)-[3H]phenylisopropyladenosine ([3H]R-PIA) binding to purified myelin fractions was performed. The distribution of high- and low-affinity species of the A1 adenosine receptor was different in heavy, medium, and light myelin. The fluidity of myelin subfractions and of pig brain cortical membranes was estimated; the microviscosity of heavy myelin (5.4 poises) and of cortical membranes (5.1 poises) was similar and less than that of medium (7.8 poises) and light (8.2 poises) myelin. It was also demonstrated that the agonist R-PIA modifies the microviscosity of myelin membranes and that the degree of modification depends on the fluidity of the membrane assayed. These results suggest that adenosine receptors may have an important role in the functionality of myelin membranes.