Passive uptake of K+(86Rb+) in sunflower roots at low external K+ concentrations

Passive fluxes of K⁺ (⁸⁶Rb) into roots of sunflower ( Helianthus annuus L. cv. Uniflorus) were determined at low K⁺ concentration (0.1 and 1.0 mM K⁺) in the ambient solution. Metabolic uptake of K⁺ was inhibited by 10⁻⁴M 2,4-dinitrophenol (DNP). K⁺ (⁸⁶Rb) fluxes were studied both continuously and by time differentiation of uptake. In high K⁺ roots passive uptake was directly proportional to the K⁺ concentration of the uptake solution, indicating free diffusion. This assumption was supported by the fact that passive Rb⁺ uptake was not affected by high K⁺ concentrations. In low K⁺ roots the passive uptake of K⁺ was higher than in high K⁺ roots. The increase was possibly due to carrier-mediated K⁺ transport. As K⁺ effluxes were quantitatively similar to influxes, it is suggested that passive K⁺ fluxes represent exchange diffusion without relation to net K⁺ transport.     

Nutrient translocation pattern and accumulation of free amino acids in rice coleoptile elongation under anoxia

Comparing nutrient translocation to the rice ( Oryza sativa L. var. Arborio ) shoot during anoxia with the aerobic situation, it was found that anoxia reduced the translocation of K⁺, phosphorus, Mg²⁺ and Ca²⁺ with progressive intensity; Ca²⁺ translocation was practically zero in the absence of oxygen. The translocation of K⁺ and phosphorus under anoxia was still considerable and contributed to the maintenance of a high osmotic potential while the blocking of Ca²⁺ translocation caused a decrease in its concentration in the anoxic coleoptile, possibly favouring high cell wall plasticity in that organ. As anoxia proceeded, amino acids, no longer employed in protein synthesis, accumulated in the coleoptile, reaching spectacular levels [51 mmol kg of tissue-water)⁻¹] and, after 48 h of anoxia, their contribution to the osmotic potential was 80% of that of K⁺, as against less than 20% in all aerobic treatments. Anoxia caused a reduction in soluble hexose concentrations which, however, were more than compensated osmotically by the accumulation of amino acids.     

Effect of potassium status on K+ (Rb)+ uptake and transport in sunflower roots

Young sunflower plants ( Helianthus annuus L. cv. Halcón), grown in nutrient solution at two K⁺ levels (0.25 and 2.5 m M ) were used to study the effect of K⁺ content in the root on uptake and transport of K⁺ to the exuding stream of decapitated plants. Roots of plants grown in low K⁺ gave higher exudation flux, higher K⁺ concentration in exudate and higher K⁺ flux than high K⁺ roots. After 6 h of uptake the K⁺ flux in low K⁺ roots was about three times that in high K⁺ roots. When the roots were kept in a nutrient solution in which Rb⁺ replaced K⁺, low K⁺ roots exuded much more Rb⁺ than K⁺ after the first 2 h, whereas high K⁺ roots exuded about similar amounts of K⁺ and Rb⁺. In intact plants grown at three different K⁺ levels (0.1, 1.0 and 10.0 m M ), there was an inverse relationship between the K⁺ level in the nutrient solution and the Rb⁺ accumulated in the roots or transported to the shoot. The results suggest that the transport of ions from xylem parenchyma to stele apoplast may be controlled by ions coming down from the shoot in sieve tubes.     

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.     

Influence of light and darkness and the experimental design on kinetics of K+ (86Rb) influx in roots of intact spring wheat

Seedlings of spring wheat ( Triticum aestivum L. cv. Svenno) were cultivated at 20°C in continuous light or darkness with the roots in nutrient solutions for six days. The plants were starved for K⁺ during different periods of time to produce plants with various K⁺ status. In one cultivation light-grown plants were pretreated in darkness, and vice versa, before the uptake experiment. In all experiments, roots were put in a complete nutrient medium containing 2.0 m M K⁺ radiolabelled with ⁸⁶Rb. The uptake time was varied (5, 60 or 120 min).
The K⁺ concentration in the roots, [K⁺]_root, increased during the course of the uptake experiments, especially in light and at initially low [K⁺]_root, At the same time K⁺ (⁸⁶Rb) influx in the roots decreased. The simoidal relationship obtained between K⁺ (⁸⁶Rb) influx and [K⁺]_root was affected by these changes, and Hill plots gave various Hill coefficients, n_H, depending on the duration of the uptake experiments. n_H from three apparently straight line segments of the same plot, in different [K⁺]_root - intervals, indicated a falling degree of interaction between the binding sites as [K⁺]_root increased. For the dark-grown plants negative cooperativity could not be demonstrated.     

Effect of K+ and H+ stress and role of Ca2+ in the regulation of intracellular K+ concentration in mung bean roots

The effects of external K⁺, H⁺ and Ca2⁺ concentrations on the intracellular K+ concentration, [K⁺]i, and the K⁺-ATPase activity in 2-day-old mung bean roots [ Vigna mungo (L.) Hepper] were investigated. [K⁺]i, in mung bean roots was markedly decreased by external K⁺ or H⁺ stress and did not recover the initial value even after the stress was removed. This decrease in [K⁺]i, gradually disappeared with the addition of (Ca2⁺. Ca2⁺ may offset the harmful effects of ion stress. Ca2⁺ seems to have two effects on K+ transport; control of K⁺ permeability and activation of K⁺ uptake, although K⁺-ATPase activity was inhibited by Ca2⁺ concentrations higher than 10–4 M. We suggest that Ca2⁺ activates K⁺ uptake indirectly through the acidification of the cytoplasm.     

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.     

Lack of active K+ uptake in aeroponically grown wheat seedlings

The K⁺ (⁸⁶Rb⁺) uptake and the growth of intact wheat seedlings ( Triticum aestivum L. cv. GK Szeged) grown in 0.5 m M CaCl₂ solution and of seedlings grown on wet filter paper in Petri dishes were compared under different experimental conditions. Aeroponic (AP) and hydroponic (HP) conditions brought about striking differences in the growth of the roots, whereas the shoot growth was not influenced. The dry weight of the roots was higher for the AP plants than for the HP plants. The AP grown seedlings exhibit a low rate of K⁺ uptake, which seems to be a passive process. The effect of 2, 4–dinitrophenol (2, 4–DNP) clearly shows the absence of an active component of the K⁺ uptake in roots grown in air with a high relative humidity. In plants grown under AP conditions the effect of Ca²⁺ on the K⁺ uptake is unfavourable, i.e. there is an inhibition (negative Viets effect). Results relating to the effect of 2,4–DNP suggest that the "negative Viets effect" is a feature of the passive K⁺ uptake. The data suggest that the AP growth conditions play a very important role in the induction and/or development of the ion transport system(s), which becomes impaired under the AP conditions.     

Nutrition of winter wheat during the life cycle. II. Influx and translocation of potassium and phosphorus

Changes in the K, Na and P content of solution-grown and soil-grown winter wheat ( Triticum aestivum L. cv. Martonvásári 8) were followed during the life cycle. In parallel experiments the influx of K⁺(⁸⁶Rb) and H₂³²PO-₄ and the translocation of these ions to the shoot were also measured. The K content increased during the seedling and tillering stages (autumn and winter period), but then decreased rapidly as the temperature rose. The influx and translocation of K⁺ increased during vegetative growth and declined in the generative phase. Na⁺ replaced K⁺ when K⁺ uptake was limited. The P content changed less than the K content, but influx was maximal during elongation. Both influx and translocation slowed greatly in the grain-filling period. For both minerals the ratio of influx to net uptake was estimated for the life cycle. This ratio was high for the soil-grown plants but low for the solution-grown plants, suggesting that the supply of K and P was limiting the growth of the former but not of the latter plants. It is suggested that the transport of K and P during the life cycle is regulated by metabolism-dependent direct routes (i.e. negative feedback mechanisms) during vegetative growth and by passive, indirect routes in the generative stage. The possibility of hormone-directed transport processes is also discussed.     

Growth, contents of K+ and kinetics of K+(86Rb) uptake in barley cultured at different low supply rates of potassium

Plants of barley ( Hordeum vulgare L. cv. Salve) were grown with 6.5–35% relative increase of K⁺ supply per day (RKR) using a special computer-controlled culture unit. After a few days on the culture solution the plants adapted their relative growth rate (RGR) to the rate of nutrient supply. The roots of the plants remained in a low salt status irrespective of the rate of nutrient supply, whereas the concentration of K⁺ in shoots increased with RKR. Both V_max and K_m for K⁺(⁸⁶Rb) influx increased with RKR. It is concluded that with a continuous and stable K⁺ stress, the K⁺ uptake system is adjusted to provide an effective K⁺ uptake at each given RKR. Allosteric regulation of K⁺ influx does not occur and efflux of K⁺ is very small.     

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.     

Long distance transport of potassium in cereals during grain filling in intact plants

Translocation of labeled potassium (K⁺) from the root to the ear and its distribution within the culm during 4, 8 and 12 h of uptake was studied in intact wheat plants ( Triticum aestivum L. cv. Kolibri) 3 and 5 weeks after anthesis at 0.5 and 5.0 m M K⁺ concentration in the uptake solution. Uptake of labeled K⁺ into the shoot was proportional to the K⁺ concentration applied. After 4 h of uptake about 2% and after 12 h about 7% of labeled K⁺ applied to the roots were taken up into the shoot at both K⁺ concentrations. After 12 h of uptake only 6% of the total label in the culm had reached the ear, while about 40% of the label was found in the upper three internodes. In spite of an increasing concentration of labeled K⁺ during 12 h in the uppermost internode (peduncle), translocation of K⁺ into the rachis was low. The low and uniform K⁺ content found generally in grain dry weight seems therefore to be due to a controlled K⁺ supply to the ear.     

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.     

Ontogenetic changes in potassium transport in xylem of tomato

The influence of plant ontogeny on xylem exudate K⁺ concentrations and K⁺ transport to the shoot was studied in both nutrient-solution and field-grown tomato plants ( Lycopersicon esculentum ).
K⁺ concentrations in xylem exudate from decapitated plants decreased during tomato plant development from a high of 12 m M to a low of 5 m M . In the nutrient-solution plants, the most rapid decline occurred during the vegetative growth phase, while in field-grown plants, the xylem K⁺ concentrations remained high during an-thesis and then subsequently declined. The rapid decline in nutrient-solution plants might be related to a decrease in the absorptive efficiency of the root system. In field-grown plants, a reduction in the availability of assimilates to the root might account in part for the decrease in xylem exudate K⁺ concentrations. The volume (ml h⁻¹ plant⁻¹) and the net rates of K⁺ exudation (mmol h⁻¹ plant⁻¹) decreased dramatically as the fruits approached maturity. Since only a small reduction in xylem exudate K⁺ concentrations occurred during fruiting, the hydraulic conductivity of the root system decreased as the tomato plants aged. It is proposed that the ontogenetic changes in xylem transport of K⁺ contribute to a reduction in leaf free space K⁺ concentration which would explain the decline in tomato leaf K⁺ concentrations.     

K/Na selectivity at the plasmalemma of cortical root cells and preferential release of sodium to the xylem vessels in roots of Atriplex hortensis

Using excised roots of Atriplex hortensis L., cv. Gelbe Gartenmelde, the uptake, accumulation and xylem transport of K⁺ and Na⁺ have been measured. Influx as well as xylem transport proved to discriminate little between K⁺ and Na⁺, when considered in relation to the external solution. Both K⁺ and Na⁺ inhibited the uptake and xylem transport of each other to about the same degree. Measurements of intracel-lular Na⁺ fluxes by means of compartment analysis indicated that the low degree of K/Na discrimination during uptake was due to low influx selectivity. Moreover, K⁺/Na⁺ exchange at the plasmalemma was not very efficient in Atriplex roots. In order to establish the basis of the low K/Na discrimination in xylem transport, the rates of K⁺ and Na⁺ transport were related to the cytoplasmic K⁺ and Na⁺ concentrations to yield the selectivity ratio of transport, S(transport) = (φ_cx(K) × [Na⁺]_c)/(φ_cx(Na) × [K⁺]_c). Under all conditions this ratio was far below one indicating that Na⁺ was favoured during xylem release in excised roots of Atriplex at low external concentrations. The implications of this discrimination in favour of Na+ are discussed with respect to salt tolerance of A. hortensis .     

An effect of Na+ on K+ uptake in intact seedlings of Zea mays

Models for the regulation of K⁺ uptake in higher plant roots have become more complex as studies have moved from the level of excised low-salt roots to that of intact plants grown under fully autotrophic conditions. In this paper we suggest that some of the differences between the conditions are qualitative, possibly requiring fundamental changes to the model, rather than simply quantitative.
The uptake of K⁺ by low-salt roots of Zea mays L. [(A619 x Oh 43) x A632], was independent of Na⁺ concentration over a wide range. However, independence of Na⁺ was not the case in plants grown on complete nutrient medium in the light: inclusion of Na⁺ in the uptake medium enhanced K⁺ uptake. In the presence of Na⁺, K⁺ uptake rates were similar in whole plants with high root K⁺ contents to rates in excised or intact, low-salt roots.     

Effects of desiccation on the 77°K fluorescence properties of the liverwort Porella navicularis and the isolated lichen green alga Trebouxia pyriformis

The uptake of the auxin type herbicide 2,4-D into rice seedlings ( Oryza sativa L. cv. Dunghan Shali) and its effects on the K⁺, NH⁺₄ and NO₃ ion uptake and the K⁺ content were investigated at different pH values. A short incubation of the roots in 0.01 m M 2,4-D caused a marked ion uptake inhibition only at low pH. The non-auxin type herbicide benthiocarb did not produce such an inhibitory effect. Lowering of the pH in the external medium led to an increased 2,4-D uptake by the roots. These results can be explained by the increased H⁺ permeability of the membranes, allowing a more rapid entrance of 2,4-D into the root cells, thereby inhibiting the active ion uptake. Rice roots not subjected to 2,4-D treatment responded to H⁺ stress with an increased anomalous K⁺ uptake and a decreased K⁺ content. With reference to the effects of pH changes on the ion and 2,4-D uptake, possible transport mechanism of NH⁺₄ and 2,4-D are briefly discussed.     

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.     

Calcium transport and ATPase activity in a microsomal vesicle fraction from corn roots

Abstract. An investigation has been made of methods for isolating membrane vesicles from corn ( Zea mays L.) roots active in calcium transport and K⁺-stimulated ATPase. Pretreating and grinding the roots at room temperature with EGTA and fusicoccin increases basal ATPase activity. Improvement in Ca²⁺ uptake requires isolation of a scaled vesicle fraction by the method of Sze(1980). Sorbitol is superior to sucrose as an osmoticant. The pH optimum for Ca²⁺ uptake is 7.5. whereas that for associated ATPase activity is 6.5. Calmodulin strongly stimulates Ca²⁺ uptake in a process little affected by uncouplers and ATPase inhibitors, but blocked by chlorpromazine. Fusicoccin gives less stimulation of Ca²⁺ uptake which is sensitive to uncouplers, and is dependent upon isolation with fusicoccin present. It appears that the sealed vesicle fraction may possess two Ca²⁺ transport systems: a calmodulin-activated Ca²⁺-transporting ATPase, and a Ca²⁺/H⁺ antiport coupled through the protonmotive force to a fusicoccin-stimulated H⁺-ATPase.     

Effect of gibberellic acid on K+ (Rb+) uptake and transport in sunflower roots

Four-week-old sunflower plants ( Helianthus annuus L. cv. Halcón), grown in different nutrient solutions, were used to study the effects of gibberellic acid (GA₃) on K⁺ (Rb⁺) uptake by roots or transport to the shoot. Gibberellic acid application to the nutrient solution did not affect the exudation process of excised roots. When GA₃ was sprayed on leaves 2 to 6 days before excising the roots, the rate of exudation and the K⁺ flux increased. When the exudation study was done keeping the roots in a nutrient solution in which Rb⁺ replaced K⁺, the GA₃ effects were evident also on Rb⁺ uptake and transport. In intact plants, GA₃ increased the Rb⁺ transported to the shoot but did not affect Rb⁺ accumulation in the root. It is suggested that these GA₃ effects can be explained if it is assumed that GA₃ acts on the transport of ions to the xylem vessels.