Study on nutrient stress tolerance of nine Taraxacum microspecies with a contrasting mineral ecology by cultivation in a low nutrient flowing solution

Growth and mineral status of 9 Taraxacum microspecies were studied under mineral stress conditions, using a flowing solution of low nutrient concentration. Relative growth rate of (whole) plant dry weight, leaf area, and (whole) plant tissue water were used to describe growth. For 4 microspecies, specific uptake rates of NO⁻₃, H₂PO⁻₄, K⁺, Mg²⁺ and Ca²⁺ were investigated.
The applied nutrient condition clearly discriminated between the studied Taraxacum microspecies. With respect to relative growth rate, 3 groups of microspecies could be distinguished: T. nordstedtii > T. lancidens, T. adamii, T. hollandicum, T. taeniatum > T. sellandii, T. eudontum, T. ekmanii, T. ancistrolobum . These categories coincided well with the mineral ecology of the microspecies, going from infertile to fertile sites.
T. nordstedtii , a microspecies of infertile sites, was most efficient in absorbing NO⁻₃, H₂PO⁻₄ and K⁺. T. sellandii and T. eudontum , both occurring in fertile grasslands, showed poor uptake performances for all studied ions. In all Taraxacum microspecies studied, except T. eudontum , internal N concentration appeared to limit growth. Efficiencies in N use, at sub-optimal internal N concentrations, varied with the mineral habitat of the microspecies studied. T. nordstedtii , from infertile sites, and T. sellandii , from fertile sites, were established as high and low extremes, respectively.     

Responses to internal potassium ion concentrations of two Taraxacum microspecies of contrasting mineral ecology: The role of inorganic ions in growth

The two microspecies were Taraxacum sellandii Dahlst., which usually occurs in heavily fertilized grasslands, and Taraxacum nordstedtii Dahlst., which on the whole is restricted to undisturbed and mineral-poor habitats. Growth response curves were established, depicting the relative yield of (whole) plant tissue water and the internal K⁺ concentration (on a whole plant basis). The critical K⁺ concentration, i.e. the lowest [K⁺]_i associated with maximal growth, was derived from the response curve. T. nordstedtii , the microspecies with the low maximal growth, showed a distinctly lower critical K⁺ concentration than T. sellandii. A relationship between growth potential and critical K⁺ concentration is proposed. Responses to a declining [K⁺]_i differed between the two microspecies. The roots of T. nordstedtii stopped functioning as a sink for inulin, and mobilized additional carbohydrates for maintaining osmotic potential and growth. The productive strategy of the fast-growing T. sellantlii is lacking such a mechanism to buffer effects of a declining [K⁺]_i.
Various changes were noted as regards the internal concentrations of other inorganic ions, measured as a function of [K⁺]_i, With declining [K⁺]_i, internal NO^-₃ decreased considerably in shoot and roots, especially in T. nordstedtii , while Mg²⁺ accumulated, especially in the roots of T. sellandii. The interactions between growth potential and the accumulation of inorganic ions are discussed.     

Responses to internal nitrogen and phosphorus concentrations of two Taraxacum microspecies of contrasting mineral ecology: critical N and P concentrations of whole plants

Of the two Taraxacum microspecies used. Taraxacum sellandii Dahlst. usually occurs in grasslands with a high nutrient level; Taraxacum nordstedtii Dahlst. is generally restricted to undisturbed and mineral-poor habitats. Growth response curves for internal N and P were established, based on relative yield of (whole) plant tissue water and (whole plant) internal mineral concentration on a tissue water basis. Critical nutrient concentrations of N and P were determined from the response curves derived. For both macroelements, T. nordstedtii showed lower critical nutrient concentrations. The difference in critical N concentrations coincided with differences in internal NO₃^-₃ concentrations between the microspecies. Finally, we discuss the use of tissue water as a (whole) plant growth parameter and internal mineral concentration on tissue water basis as a parameter describing the mineral status.     

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.     

K+ status and ABA affect both exudation rate and hydraulic conductivity in sunflower roots

Twenty‐day‐old sunflower plants ( Helianthus annuus L. cv. Sun‐Gro 380) grown in nutrient solutions with different KCl levels were used to study the effects of K⁺ status of the root and of abcisic acid (ABA) on the exudation rate (J_v), the hydraulic conductivity of the root (L_p), the fluxes of exuded K⁺ and Na⁺ (J_K and J_Na), and the gradient of osmotic pressure between the xylem and the external medium. J_v and L_p increased in direct proportion to the K⁺ starvation of the root. Also addition of ABA (4 µ M ) at the onset of exudation in the external medium made J_v and L_p rise, and this effect also increased with the degree of K⁺ starvation. Similarly, K⁺ starvation and ABA promoted both the flux of exuded Na⁺ and the accumulation of Na⁺ in the root. We suggest that ABA acts as a regulating signal for the radial transport of water across the root, and that potassium may be an effector of this mechanism.     

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.     

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.     

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.     

Variation in growth and accumulation of N, K+, Ca2+ and Mg2+ among barley cultivars exposed to various nutrient regimes and root/shoot temperatures

Six cultivars of barley ( Hordeum vulgare L., cvs Salve, Nürnberg II, Bomi, Risø 1508, Mona and Sv 73 608) were exposed for three weeks to combinations of high and low mineral supply and differential root/shoot temperature. For all the parameters tested [fresh and dry weights, contents and levels of N, K⁺, Ca²⁺ and Mg²⁺, and influx of Rb⁺(⁸⁶Rb)] the cultivar differences were influenced by the mineral supply, the root temperature and the age of the plants.
The cultivar differences in N nutrition of three-week-old plants could partly be attributed to variation in root size, uptake of N and in use-efficiency of the element. The cultivar variation in root-shoot partitioning of N was small, except when low mineral supply was combined with a low root temperature. Similarly, cultivar differences in contents of K⁺, Ca²⁺ and Mg²⁺ were influenced by variation in uptake, use-efficiency and root/shoot partitioning of the elements. Low root temperature increased cultivar variation in K⁺, Ca²⁺ and Mg²⁺ partitioning.
The modern cultivar Salve was compared with Nürnberg II, which is derived from a German land race. Nürnberg II performed better than Salve when low root temperature and restricted mineral supply were combined. Otherwise Salve grew better, partly due to a more efficient use of N.
Two high-lysine lines, Risø 1508 and Sv 73 608, were compared with their mother lines Bomi and Mona. The differences obtained revealed no general effect of the high-lysine genes on growth and mineral nutrition of up to three-week-old barley 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.     

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.     

Regulation of K+ and NO3− fluxes in roots of sunflower (Helianthus annuus) after changes in light intensity

Shoot activity has been reported to affect rates of ion uptake by plant roots in other ways than merely through supply of assimilates. To elucidate the mechanisms by which a signal from the upper part of the plant controls the rate of K⁺ and NO₃⁻ uptake by roots, both uptake of K⁺ and NO₃⁻ and secretion into the xylem of young sunflower plants ( Helianthus annuus L.) were measured after changes in light intensity.
No close correlation was observed between the uptake of NO₃⁻ and that of K⁺; an increase in light intensity produced a much greater stimulation of NO₃⁻ uptake than of K⁺ uptake. On the other hand, secretion of NO₃⁻ into the xylem was tightly coupled to that of K⁺, and this coupling was strongly disturbed by excision of the root. The results suggest the involvement of the K₂-malate shuttle on the regulation by the shoot of K⁺ and NO₃⁻ secretion in the xylem, which is linked to NO₃⁻ uptake, while K⁺ uptake is independent of this regulation mechanism.     

Depolarization and Activation of Dihydropyridine-Sensitive Ca2+ Channels Stimulate Inositol Phosphate Accumulation in Photoreceptor-Enriched Chick Retinal Cell Cultures

Abstract: Elevated concentrations of extracellular K⁺ increased inositol phosphate accumulation in primary cultures of chick retinal photoreceptors and multipolar neurons. K⁺-evoked stimulation of inositol phosphate accumulation was greater in photoreceptor-enriched cell cultures than in cultures where multipolar neurons were the predominant cell type. Destroying multipolar neurons, but not photoreceptors, with kainic acid and N -methyl- d -aspartate did not reduce the K⁺-evoked stimulation of inositol phosphate accumulation. Both of these observations indicate that the observed effects occur in photoreceptor cells. The K⁺-evoked stimulation of inositol phosphate accumulation was blocked by omitting Ca²⁺ from the incubation medium or by adding the dihydropyridine-sensitive Ca²⁺-channel antagonists, nitrendipine and nifedipine. Bay K 8644, a dihydropyridine agonist, stimulated inositol phosphate accumulation and enhanced the effect of K⁺. ω-Conotoxin GVIA, an inhibitor of N-type Ca²⁺ channels, had no significant effect on K⁺-stimulated inositol phosphate accumulation. Pretreatment with pertussis toxin neither blocked K⁺-evoked inositol phosphate accumulation nor altered the inhibitory effect of nifedipine. K⁺-evoked inositol phosphate accumulation appears to reflect activation of phosphatidylinositol-specific phospholipase C, as it is inhibited by U-73122. These results indicate that Ca²⁺ influx through voltage-gated, dihydropyridine-sensitive channels activates phospholipase C in photoreceptor inner segments and/or synaptic terminals.     

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.     

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.     

Salt tolerance of the reed plant Phragmites communis

Reed plants ( Phragmites communis Trinius) were grown at NaCl concentrations up to 500 m M and their growth, mineral contents and leaf blade osmotic potential were determined. Addition of NaCl up to 300 m M did not affect growth significantly. Sucrose, Cl^-and Na⁺ concentrations in the shoots increased with the salinity of the medium and the shoot water content decreased. K⁺ always contributed most to the leaf osmotic potential. Even in the presence of 250 m M NaCl in the rooting medium, the leaf blade contained only 50 mM Na⁺, suggesting that the plants have an efficient mechanism for Na⁺ exclusion. ²²Na⁺ uptake experiments suggested that the retranslo-cation of absorbed Na⁺ from shoots to the rooting medium lowered the uptake of Na⁺.     

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.     

Outward-rectifying K+ channel activities regulate cell elongation and cell division of tobacco BY-2 cells

Potassium ions (K⁺) are required for plant growth and development, including cell division and cell elongation/expansion, which are mediated by the K⁺ transport system. In this study, we investigated the role of K⁺ in cell division using tobacco BY-2 protoplast cultures. Gene expression analysis revealed induction of the Shaker -like outward K⁺ channel gene, NTORK1 , under cell-division conditions, whereas the inward K⁺ channel genes NKT1 and NtKC1 were induced under both cell-elongation and cell-division conditions. Repression of NTORK1 gene expression by expression of its antisense construct repressed cell division but accelerated cell elongation even under conditions promoting cell division. A decrease in the K⁺ content of cells and cellular osmotic pressure in dividing cells suggested that an increase in cell osmotic pressure by K⁺ uptake is not required for cell division. In contrast, K⁺ depletion, which reduced cell-division activity, decreased cytoplasmic pH as monitored using a fluorescent pH indicator, SNARF-1. Application of K⁺ or the cytoplasmic alkalizing reagent (NH₄)₂SO₄ increased cytoplasmic pH and suppressed the reduction in cell-division activity. These results suggest that the K⁺ taken up into cells is used to regulate cytoplasmic pH during cell division.