Potassium transport in wheat seedlings grown with different potassium supplies.

The K⁺(⁸⁶Rb) uptake into the roots and the translocation to the shoots of 11-day-old intact wheat seedlings ( Triticum aestivum L. cv. Martonvásári 8) were investigated using plants grown with different K⁺ supplies. The effects of environmental conditions (darkness, humidity) and of metabolic and transport inhibitors (oligomycin, disalicylidene-propanediamine, 2,4-dinitriphenol, diethylstilbestrol, colchicine) were also studied. Plants with K content of about 0.2 mmol/g dry weight in the root and 0.5 mmol/g dry weight in the shoot (low K status) showed high K⁺ uptake into the roots and high translocation rates to the shoots. Both transport processes were very low in plants with K content of more than 1.5 and 2.2 mmol/g dry weight in the root and shoot, respectively (high K status).
Darkness and a relative humidity of the air of 100% did not influence K⁺ uptake by roots, but did inhibit upward translocation and water transport. Inhibition of photosynthesis and treatments with diethylstilbestrol (10⁻⁵ mol/dm³), as well as with colchicine resulted in inhibition of translocation in plants of low K status, but these inhibitors had little effect on K⁺ uptake by the roots. Oligomycin, 2,4-dinitrophenol and diethylstilbestrol (10⁻⁴ mol/dm³), however, inhibited K⁺ uptake by the roots. In general, K⁺ transport processes were almost unchanged in plants of high K status. It is concluded that only plants of low K status operating with active K⁺ transport mechanisms are responsive to environmental factors. In high K⁺ plants the transport processes are passive and are uncoupled from the metabolic energy flow.     

Free space uptake and influx of Ni2+ in excised barley roots

The effects of switches between high and low nutrient supplies on growth and mineral nutrition of winter wheat ( Triticum aestivum L. cv. Martonvásári-8) were followed in four main developmental phases: tillering, shooting, heading and grain filling. Growth of the shoots was significantly affected by switches. Under low nutrient supply the life cycle was shortened. Root growth was only slightly affected by switches, but an early high nutrient supply followed by low nutrient supply gave an impetus for root development. In general, the growth data indicate that the nutrient status of the plants is determined by the nutrient level supplied during shooting. A high level of nutrients during shooting leads also to high vegetative growth, whereas the best grain yield was obtained by a high dose of nutrients during tillering followed by low nutrient conditions during the shooting stage and later. K⁺(⁸⁶Rb) influx in the roots decreased with age. The potential for K⁺ (⁸⁶Rb) influx was low in plants of high-salt status, but it became high in response to switching to low supply at shooting, whereas later switches had no influence on this function in high-salt plants. The highest K⁺(⁸⁶Rb) influx was found in plants starting with high nutrient supply followed by low-salt conditions; this plant group was outstanding also with respect to its high grain yield.     

Salinity tolerance of Kosteletzkya virginica. I. Shoot growth, ion and water relations

Abstract. Kosteletzkya virginica (L.) Presl., a dicotyledonous halophyte native to brackish tidal marshes, was grown on nutrient solution containing 0. 85, 170 or 255 mol m^-3 NaCl, and the effects of external salinity on shoot growth and ion content of individual leaves were studied in successive harvests. Growth was stimulated by 85 mol m^-3 NaCl and was progressively reduced at the two higher salinities. Growth suppression at high salinity resulted principally from decreased leaf production and area, not from accelerated leaf death. As is characteristic of halophytic dicots. K. virginica accumulated inorganic ions in its leaves, particularly Na⁺ and K⁺. However, the Na⁺ concentration of individual leaves did not increase with time, but remained constant or even declined, seeming to be well-coordinated with changes in water content. A striking feature of the ion composition of salinized plants was the development of a dramatic gradient in sodium content, with Na⁺ partitioned away from the most actively growing leaves. Salt-treated plants exhibited a strong potassium affinity, with foliar K⁺ levels higher in salinized plants than unsalinized plants after an initial decrease. These results suggest that selective uptake and transport, foliar compartmentation of Na⁺ and K⁺ in opposite directions along the shoot axis, and the regulation of leaf salt loads over time to prevent build-up of toxic concentrations are whole-plant features which enable K. virginica to establish favourable K⁺-Na⁺ relations under saline conditions.     

Salinity tolerance of Kosteletzkya virginica. II. Root growth, lipid content, ion and water relations

Abstract. Kosteletzkya virginica (L.) Presl., a dicot halophyte native to brackish tidal marshes, was grown on nutrient solution containing 0. 85, 170 or 255 mol m ³ NaCl, and the effects of external salinity on root growth, ion and water levels, and lipid content were examined in successive harvests. Root growth paralleled shoot growth trends, with some enhancement observed at 85 mol m ³ NaCl and a reduction noted at the higher salinities. Root Na⁺ content increased with increasing external NaCl, but remained constant with time for each treatment. K⁺ content, although lower in salt-grown plants after 14 d salinization, subsequently increased to levels comparable to unsalinized plants. A strong K⁺ affinity was reflected in the increased K⁺/Na⁺ selectivity of salt-grown plants and by their low Na⁺/K⁺ ratios. Cl levels rose in salinized plants and values were double or more those for Na⁺, indicating the possibility of a sodium-excluding mechanism in roots. Root phospholipids and sterols, principal membrane constituents, were maintained or elevated and the free sterol/phospholipids ratio increased in salinized K. virginica plants, suggesting retention of overall membrane structure and decreased permeability. This response, considered in light of root calcium maintenance and high potassium levels, suggests that salinity-induced changes in membrane lipid composition may be important in preventing K⁺ leakage from cells.     

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.     

Nutrition of winter wheat during the life cycle. I. Yield and accumulation of dry matter and minerals

Mineral uptake by winter wheat (Trilicum aestivum L. cv. Martonvasari 8) was studied throughout the life cycle. Accumulation of macronutrients (i.e. total nitrogen, phosphorus, potassium, sodium, magnesium and calcium) and the water content of roots and shoots of plants grown in complete nutrient solution were higher than those of plants grown in two types of soils. The supply of macronutrients was in some cases limiting for soil-grown plants as revealed by a comparison of available and accumulated amounts of these nutrients. Their supply was abundant, however, for solution-grown plants. This led to a doubling of grain yield for the latter plants with a three fold increase in accumulation of dry matter and a five-fold increase in fresh weight. The efficiency ratios of solution-grown plants to soil-grown plants were approximately 1 for N and Na, 0.5 for Mg and Ca, and 0.3 for P and 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.     

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.     

Sodium ion transport in Neurospora crassa

Na⁺ influx and efflux in Neurospora crassa RL21a can be studied separately to calculate net Na⁺ movements. In the absence of external K⁺, Na⁺ influx was independent of the K⁺ content of the cells, but when K⁺ was present, the inhibition of Na⁺ influx by external K⁺ was higher the higher the K⁺ content. Efflux depended on the K⁺ and Na⁺ content, and on the history of the cells. Efflux was higher the higher the Na⁺ and K⁺ contents, and, in low-K⁺ cells, the efflux was also higher in cells grown in the presence of Na⁺ than when Na⁺ was given to cells grown in the absence of Na⁺. Addition of K⁺ to cells in steady state with external Na⁺ resulted in a net Na⁺-loss. In cells grown without Na⁺ this loss was a consequence of the inhibition of Na⁺ influx. In Na⁺-grown cells, addition of K⁺ inhibited Na⁺ influx and increased Na⁺ efflux.     

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.     

Puccinellia tenuiflora maintains a low Na+ level under salinity by limiting unidirectional Na+ influx resulting in a high selectivity for K+ over Na+

Puccinellia tenuiflora is a useful monocotyledonous halophyte that might be used for improving salt tolerance of cereals. This current work has shown that P. tenuiflora has stronger selectivity for K⁺ over Na⁺ allowing it to maintain significantly lower tissue Na⁺ and higher K⁺ concentration than that of wheat under short- or long-term NaCl treatments. To assess the relative contribution of Na⁺ efflux and influx to net Na⁺ accumulation, unidirectional ²²Na⁺ fluxes in roots were carried out. It was firstly found that unidirectional ²²Na⁺ influx into root of P. tenuiflora was significantly lower (by 31–37%) than in wheat under 100 and 150 m m NaCl. P. tenuiflora had lower unidirectional Na⁺ efflux than wheat; the ratio of efflux to influx was similar between the two species. Leaf secretion of P. tenuiflora was also estimated, and found the loss of Na⁺ content from leaves to account for only 0.0006% of the whole plant Na⁺ content over 33 d of NaCl treatments. Therefore, it is proposed that neither unidirectional Na⁺ efflux of roots nor salt secretion by leaves, but restricting unidirectional Na⁺ influx into roots with a strong selectivity for K⁺ over Na⁺ seems likely to contribute to the salt tolerance of P. tenuiflora .     

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.     

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.     

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.     

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 .     

Effects of aluminium on growth and kinetics of K+(86Rb) uptake in two cultivars of wheat (Triticum aestivum) with different sensitivity to aluminium

Two cultivars of wheat (Triticum aestivum L. cvs Kadett and WW 20299) were grown for 9 days with 20% relative increase in nutrient supply per day at pH 4.1. Aluminium at 50 μ M retarded the growth of roots more than that of shoots in both cultivars, thus decreasing the root/shoot ratio. The inhibition was largest in WW 20299. With long term Al treatment (9 days), K_m for K⁺(⁸⁶Rb) influx increased five times in both cultivars and V_max decreased in WW 20299. Efflux of K⁺(⁸⁶Rb) was little affected. When the roots were treated with aluminium for two days, only relative growth rate of roots was retarded, while growth of shoots was unaffected and influx of K⁺(⁸⁶Rb) adjusted to the actual K⁺ demand of the plants. It is concluded that the effects of aluminium on K⁺ uptake in these wheat cultivars are not primary factors contributing to aluminium sensitivity. However, in soil with Al the demand for a comparatively high concentration of K⁺ to maintain an adequate K⁺ uptake rate, in combination with a slow growth rate of the roots, may secondarily lead to K⁺ deficiency in the plants.     

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.     

Relation of source and sink during grain filling period in wheat and some aspects of its regulation

The effect of Mg²⁺, Na⁺, K⁺, ouabain and pH on ATPase activity of purified membrane fractions enriched in plasmalemma fragments from Hordeum vulgare L. (glycophyte) and Halocnemum strobilaceum L. (halophyte) was studied. Membrane ATPases from both plants were synergistically activated by K⁺ and Na⁺ in the presence of Mg²⁺. The maximum activity of the enzymes were observed at the ratio Na/K = 2–3. Ouabain (10^-4 M) almost completely eliminated the (Na⁺+ K⁺)-stimulated component of the ATPase activity. The Na, K, Mg-ATPase of Hordeum had a single pH optimum (pH 8), but that of the Halocnemum had two optima(pH 6 and 8). It appears that similar enzymes operate in the cells of both plants studied. The higher Na, K, Mg-ATPase activity of the halophyte compared to that of the glycophyte suggests the involvement of the enzyme in the extrusion of Na⁺ from the cytoplasm of cells of both plants.     

Cation-stimulated H+ efflux by intact roots of barley

Abstract. Rates of proton extrusion and potassium (⁸⁶Rb) influx by intact roots of barley ( Hordeum vulgare cvs . Fergus, Conquest and Betzes) plants were simultaneously measured in short-term (15min) experiments. The nature and extent of apparent coupling between these ion fluxes was explored by manipulating conditions of temperature, pH and cation composition and concentration during flux determinations. In addition, the influence of salt status upon these fluxes was examined. At low K⁺ concentrations (0.01 to 1 mol m⁻³), H⁺ efflux and K⁺ influx were strongly correlated in both low- and high-K⁺ roots, although K⁺: H⁺ exchange stoichiometries were almost consistently greater than 2:1. At higher concentrations (1 to 5 mol m⁻³), H⁺ efflux was either reduced or remained unchanged while K⁺ influxes increased. In the presence of Na₂SO₄, rates of H⁺ extrusion demonstrated similar cation dependence, although below 10 mol m⁻³ Na₂SO₄, H⁺ fluxes were generally 50% lower than in equivalent concentrations of K₂SO₄. These observations are considered in the context of current hypotheses regarding the mechanisms of k⁺/H⁺ exchange.