Aluminium interactions with K+(86Rb+) and 45Ca2+ fluxes in three cultivars of sugar beet (Beta vulgaris)

Three cultivars of sugar beet (Beta vulgaris L.), which are sensitive to aluminium (Al) in the order Primahill > Monohill > Regina, were grown in water culture for 2 weeks. Nutrients were supplied at 15% increase of amounts daily, corresponding to the nutrient demand for maximal growth. The 2.4-dinitrophenol (DNP)-sensitive (metabolic) and DNP-insensitive (non-metabolic) uptake of aluminium, phosphate. ⁴⁵Ca²⁺ and K⁺(⁸⁶Rb⁺) in roots were measured as well as transport to shoots of intact plants. All 3 cultivars absorbed more aluminium if DNP was present during the aluminium treatment than in its absence. It is suggested that sugar beets are able to extrude aluminium activity or that they possess an active mechanism to keep Al outside the cell. The presence of Al in the medium during the 1-h experiment affected the metabolic and non-metabolic fluxes of ⁴⁵Ca²⁺ and K⁺(⁸⁶Rb⁺) in different ways. In the presence of DNP, the influx of both ⁴⁵Ca²⁺ and K⁺(⁸⁶Rb⁺) and the efflux of ⁴⁵Ca²⁺ were inhibited by Al in a competitive way. At inhibition of ⁴⁵Ca²⁺ influx, 2 Al ions are probably bound per Ca²⁺ uptake site in cv. Regina (Al-tolerant), but in cvs Primahill and Monohill only one Al ion is bound (more Al sensitive). Aluminium competitively inhibited the active efflux of ⁴⁵Ca²⁺ (absence of DNP) in almost the same way in the 3 cultivars. In contrast, aluminium stimulated the influx of K⁺(⁸⁶Rb⁺) in cvs Primahill, Monohill and Regina in the absence of DNP. Thus, the Al effects on active and passive K⁺(⁸⁶Rb⁺) influx are different. The total influx of K⁺(⁸⁶Rb⁺) increased in the presence of Al and might be connected to an active exclusion of Al. Regina is the least Al-sensitive cultivar, probably because Al interferes less with the Ca²⁺ fluxes and because this cultivar actively excludes phosphate in the presence of Al. Thus Al-phosphate precipitation within the plant could be avoided.     

Mechanism for Cd2+ inhibition of (K++ Mg2+)ATPase activity and K+ (86Rb+) uptake join roots of sugar beet (Beta vulgaris)

Steady state kinetics were used to examine the influence of Cd²⁺ both on K⁺ stimulation of a membrane-bound ATPase from sugar beet roots (Beta vulgaris L. cv. Monohill) and on K⁺(⁸⁶Rb⁺) uptake in intact or excised beet roots. The in vitro effect of Cd²⁺ was studied both on a 12000–25000 g root fraction of the (Na⁺+K⁺+Mg²⁺)ATPase and on the ATPase when further purified by an aqueous polymer two-phase system. The observed data can be summarized as follows: 1) Cd²⁺ at high concentrations (>100 μM) inhibits the MgATPase activity in a competitive way, probably by forming a complex with ATP. 2) Cd²⁺ at concentrations <100 μM inhibits the specific K⁺ activation at both high and low affinity sites for K⁺. The inhibition pattern appears to be the same in the two ATPase preparations of different purity. In the presence of the substrate MgATP, and at K⁺ <5 mM, the inhibition by Cd²⁺ with respect to K⁺ is uncompetitive. In the presence of MgATP and K⁺ >10 μM, the inhibition by Cd²⁺ is competitive. 3) At the low concentrations of K⁺, Cd²⁺ also inhibits the 2,4-dinitrophenol(DNP)-sensitive (metabolic) K⁺(⁸⁶Rb⁺) uptake uncompetitively both in excised roots and in roots of intact plants. 4) The DNP-insensitive (non metabolic) K⁺(⁸⁶Rb⁺) uptake is little influenced by Cd²⁺. As Cd²⁺ inhibits the metabolic uptake of K⁺(⁸⁶Rb⁺) and the K⁺ activation of the ATPase in the same way at low concentrations of K⁺, the same binding site is probably involved. Therefore, under field conditions, when the concentration of K⁺ is low, the presence of Cd²⁺ could be disadvantageous.     

Control of K+ (Rb+) influx and transport with changed internal K+ concentration and age in two varieties of barley

The influx of Rb⁺ into the roots of two barley varieties (Hordeum vulgare L. cv. Salve and cv. Ingrid) from a K⁺-free ⁸⁶Rb-labelled nutrient solution with 2.0 mM Rb⁺, was checked at intervals from day 6 to day 18. The control plants were continuously grown in complete nutrient solution containing 5.0 mM K⁺, while two other groups of plants were grown in K⁺-free nutrient solution starting on day 6 and between day 6 and day 9, respectively. The pattern of Rb⁺ influx was similar for both varieties, although their efficiencies in absorbing Rb⁺ were different. The relationship between Rb⁺ influx and K⁺ concentration of the root could be interpreted in terms of negative feedback through allosteric control of uptake across the plasmalemma of the root cells. Hill plots were bimodal, but in the opposite direction. The Hill coefficients, reflecting the minimum number of interacting allosteric binding sites for K⁺ (Rb⁺), were low (≤–3.0). It is discussed whether the threshold value, that is the breaking point in the Hill plot, is indicative of a changed efficiency of transporting units for K⁺ (Rb⁺) transport to the xylem. Moreover, feedback regulation might be involved in transport of K⁺ between root and shoot. The variation in K⁺ concentrations in the roots and shoots of control plants were cyclic but in phase opposition despite an exponential growth. The average K⁺ concentration varied only slightly with age.     

Effects of ionic strength and relative humidity on the efflux of K+ (86Rb) and Ca2+ (45Ca) from roots of intact seedlings of cucumber, oat and wheat

Spring wheat (Triticum aestivum L. cv. Svenno), oat (Avena sativa L. cv. Brighton) and glasshouse cucumber (Cucumis sativus L. cv. Bestseller F1) were cultured for a week after germination on complete nutrient solutions of three different dilutions (1, 25 and 50% of the full strength medium). K⁺(⁸⁶Rb) and ⁴⁵Ca were present during the whole culture period. Relative humidity (RH) was 50% except during the last day, when half the material was transferred to 90% RH. Efflux of labelled ions was then followed during eight hours on unlabelled solutions of the same composition as before, and at both 50% and 90% RH in the atmosphere. – Uptake of K⁺(⁸⁶Rb) during growth tended to be saturated in the 25% medium. Contrariwise, the level of Ca²⁺ in the roots increased continuously with strength of the medium. At low concentrations cucumber roots were higher in Ca²⁺ than roots of oat or wheat, whereas all three species showed similar levels of Ca²⁺ in 50% medium. – At the lowest ionic strength, smooth efflux curves were obtained that could be resolved according to the three-compartment theory. At higher ionic strength, irregularities were observed, and more for Ca²⁺ than for K⁺; but for practical purposes compartment analysis with the same time constants could be applied as for the lowest concentration. – Discrimination between K⁺ and Rb⁺ differed between the roots, but not much between the shoots of different species. The roots of oat and wheat took up Rb⁺ preferentially over K⁺ in the 25% and 50% media; whereas K⁺ was preferred over Rb⁺ or little discrimination made in 1% medium and for cucumber. The shoots generally showed less discrimination than the roots. The main variability in discrimination between K⁺ and Rb⁺ thus appears to be localized in the tonoplasts of the roots cells. – Low RH around the shoots increased efflux of K⁺(⁸⁶Rb) from the cytoplasm and vacuoles of the root cells as compared to the efflux at high RH. DNP (2,4-dinitrophenol) in the medium had the same effect as high RH around the shoots. The signal system that must exist between shoots and roots is discussed as a response to “drought” conditions. In relation to investigations of others, it is assumed that the effect of DNP may indicate that part of the chain between roots and shoots consists of metabolically influenced sites, whose output is influenced by the rate of water transport.     

K+(Rb+) and Ca2+ fluxes in young winter wheat plants exposed to sub-zero temperatures

Ice crystal formation temperature was determined in the region of the crown in one group of 7-day-old intact unhardened high-salt plants of winter wheat (Triticum aestivum L. cv. Weibulls Starke II) with TA (Thermal Analysis) and DTA (Differential Thermal Analysis) methods. After exposure of another group of plants, grown for the first 7 days in the same way as the first group, to various sub-zero temperatures (-1 to 5°C), influx in roots of Rb⁺(⁸⁶Rb⁺) and Ca²⁺(⁴⁵Ca²⁺) and contents of K⁺ and Ca²⁺ were determined at intervals during 7 days of recovery. Ice crystal formation in the crown tissue was probably extracellular and took place at about -4°C. There was a large loss of K⁺ from the roots after treatment at sub-zero temperatures. This loss increased as the temperature of the sub-zero treatment decreased. During recovery, roots of plants exposed to -1, -2 and -3°C gradually reabsorbed K⁺. Reabsorption of K⁺ in roots of plants exposed to -4°C was greatly impaired. Rb⁺ influx decreased and Ca²⁺ influx increased after sub-zero temperature treatments of the plants. Active Rb⁺ influx mechanisms and active extrusion of Ca²⁺ were impaired or irreversibly damaged by the exposure. While Rb⁺ influx mechanisms were apparently repaired during recovery in plants exposed to temperatures down to -3°C, Ca²⁺ extrusion mechanisms were not. The temperature for ice crystal formation in the region of the crown tissue coincides with the temperature at which the plants lost the ability to reabsorb K⁺ and to repair Rb⁺ influx mechanisms during the recovery period. Plants were lethally damaged at temperatures below ?4°C.     

Effects of cadmium and lead on the accumulation of Ca2+ and K+ and on the influx and translocation of K+ in wheat of low and high K+ status

The effects of cadmium and lead on the internal concentrations of Ca²⁺ and K⁺, as well as on the uptake and translocation of K(⁸⁶Rb⁺) were studied in winter wheat (Triticum aestivum L. a. MV-8) grown hydroponically at 2 levels of K⁺ (100 uM and 10 mM). Cd²⁺ and Pb²⁺ were applied in the nutrient solution in the range of 0.3 to 1000 u.M. Growth was more severely inhibited by Cd²⁺ and in the high-K⁺ plants as compared to Pb^z+ and low-K⁺ plants. Ions of both heavy metals accumulated in the roots and shoots, but the K⁺ status influenced their levels. Ca²⁺ accumulation was increased by low concentrations of Cd²⁺ mainly in low-K⁺ shoots, whereas it was less influenced by Pb²⁺. The distribution of Cd²⁺ and Ca²⁺ in the plant and in the growth media indicated high selectivity for Cd²⁺ in the root uptake, while Ca²⁺ was preferred in the radial and/or xylem transport. Cd²⁺ strongly inhibited net K⁺ accumulation in high-K⁺ plants but caused stimulation at low K⁺ supply. In contrast, the metabolis-dependent influx of K⁺(⁸⁶Rb⁺) was inhibited in low-K⁺ plants, while the passive influx in high-K⁺ plants was stimulated. Translocation of K⁺ from the roots to the shoots was inhibited by Cd²⁺ but less influenced in Pb²⁺-treated plants. It is concluded that the effects of heavy metals depend upon the K⁺-status of the plants.     

Long-term effects of plant hormones on K+ levels and transport in young wheat plants of different K+ status

Long-term effects of 1-naphtaleneacetic acid (NAA), benzyladenine (BA), gibberellic acid (GA₃), abscisic acid (ABA) and ethylene on K⁺ levels, K⁺ uptake and translocation to the shoot were studied in young wheat plants (Triticum aesticum L. cv. Martonvásári-8) grown at different K⁺ supplies. Na⁺ levels and K⁺/Na⁺ selectivity were also investigated. Both in shoots and roots, NAA, BA and ABA decreased K⁺ and Na⁺ levels more effectively in high-K⁺ plants than in low-K⁺ plants. GA, and ethylene did not influence K⁺ and Na⁺ levels. K⁺/Na⁺ selectivity in roots of low-K⁺ plants was increased in favour of K⁺ by BA, NAA and to a lesser extent by ABA. In high-K⁺ plants only BA increased the K⁺/Na⁺ ratio, whereas the effects of the other hormones were the opposite (NAA) or less pronounced (ABA). K⁺(⁸⁶Rb) uptake was inhibited by NAA and BA in low-K⁺ plants but not in high-K⁺ plants. K⁺(⁸⁶Rb) uptake was inhibited throughout by 10 μM ABA. K⁺(⁸⁶Rb) translocation to the shoot was influenced by the hormones similarly to the uptake patterns, with the exception of ABA, which inhibited translocation in low-K⁺ plants but not in high-K⁺ plants. The results show that hormonal effects may quantitatively and qualitatively be modified by K⁺ levels in the plant and that internal K⁺ concentration may play a role in the mechanisms regulating the effects of NAA, BA and ABA but probably not in those of GA₃ or ethylene.     

The Effect of Dichlorophenyldimethylurea on Light-Stimulated 22Na+, 42K+ and 86Rb+ Absorption in Different Tissues of Phaseolus vulgaris Leaves

The light-stimulated absorption of ⁸⁶Rb⁺ by Phaseolus vulgaris L. leaf slices was found to be sensitive to dichlorophenyldimethylurea in air as well as in nitrogen, whereas light-stimulated ²²Na⁺ absorption in nitrogen was not sensitive to this inhibitor. The absorption of ²²Na⁺ is not affected by light in air. The absorption of ⁴²K⁺ is enhanced by a dichlorophenyldimethylurea-insensitive light effect under anaerobic conditions and further increased by light in the absence of the inhibitor. Light-enhanced ⁴²K⁺ absorption in air was also inhibited by dichlorophenyldimethylurea. Previous work showed that light-stimulated ⁸⁶Rb⁺ and ⁴²K⁺ absorption by Phaseolus vulgaris leaf slices is restricted to the guard cells. The present results are discussed with reference to the effect of light on stomatal opening.     

Influence of low temperature on regulation of Rb+ and Ca2+ influx in roots of winter wheat

Influx of Rb⁺(⁸⁶Rb⁺) and Ca²⁺(⁴⁵Ca²⁺) was determined in roots of winter wheat (Triticum aestivum L. cv. Weibulls Starke II) after 14 days at 16°C/16 h light, after 1 and 8 weeks of cold acclimation (2°C/8 h light) and at intervals after deacclimation (16°C/16 h light) for up to 14 days. The plants were cultivated at 3 ionic strengths: 100, 10 and 1% of a full strength nutrient solution, containing 3.0 mM K⁺ and 1.0 mM Ca²⁺. K⁺ concentrations in roots and shoots increased during cold treatment, while Ca²⁺ in the roots decreased. In the shoots Ca²⁺ concentrations remained the same. Influx of Rb⁺ as a function of average K⁺ concentration in the roots of 14-day-old, non-cold-treated plants was high at a certain K⁺ level in the root and decreased at higher root K⁺ levels (negative feedback). The pattern for Ca²⁺ influx versus average concentration of Ca²⁺ in the root was the reverse. Independent of duration of treatment (1–8 weeks), cold acclimation partly changed the regulation of Rb⁺ influx, so that it became less dependent upon negative feedback and more dependent on the ionic strength of the cultivation solution. After exposure to 2°C, Ca²⁺ influx increased at high Ca²⁺ concentrations in the root as compared with influx in roots of 14-day-old non-cold-treated plants. Under deacclimation, Ca²⁺ influx gradually decreased again, and reached the level observed before cold treatment within 7–14 days at 16°C; the number of days depending on the exposure time at 2°C. It is suggested that Rb⁺(K⁺) influx became adjusted to low temperature and that abscisic acid (ABA) may be involved in this mechanism. It is also suggested that extrusion of Ca²⁺ was impaired and/or Ca²⁺ channels were activated at 2°C in roots of plants grown in the full-strength solution and that extrusion was gradually restored and/or Ca²⁺ channels were closed under deacclimation conditions.     

Control of Na+ and K+ transport in Spergularia marina. III. Relationship between ion uptake and growth at moderate salinity

In this paper, we continue our analysis of Na⁺ and K⁺ uptake by mid-vegetative Spergularia marina (L.) Griseb. plants growing on 0.2x sea water medium, with attention to the relationship of ion uptake and growth. In the first part of the paper, growth analysis techniques are used to compare relative growth rates (RGR) and relative accumulation rates (RAR) for Na⁺ and K⁺. Under constant growth conditions, a high correlation between RGR and RAR indicated that growth and accumulation of both ions were well balanced, resulting in Na⁺ and K⁺ concentrations within the plants which were stable after adjustement to the saline medium. The analysis confirmed the existence of a Na⁺ -related growth stimulation in S. marina and an associated increase in the efficiency of K⁺ utilization for growth. When plants were subjected to more rapid salinization and step changes in the light intensity of the growth chamber, RGR and RAR were again similar, even through the discontinuities in growth conditions, suggesting that growth and ion accumulation were co-regulated rather than simply correlated. The growth analysis data were then transformed to give net uptake rates for Na⁺ and K⁺ and the results were compared to those of isotope studies under similar growth conditions. In roots, the rates estimated by the two techniques differed substantially; net uptake rates reflected primarily growth, while isotope studies indicated a substantial ion exchange rate between mature cells and the growth medium. The rates of transport of either Na⁺ or K⁺ to the shoot were very similar using the two estimation techniques. As the rates measured with isotopes were taken from studies lasting at most a few hours, this suggested a very rapid turnover of the upwardly mobile Na⁺ and K⁺ pools in the roots.     

Interactions between K+ and NH4+: effects on ion uptake by rice roots

Interactive effects of K⁺ and N (principally NH₄⁺) on plant growth and ion uptake were investigated using hydroponically grown rice (Oryza sativa L. cv. M202) seedlings by varying the availability of NH₄⁺ or NO₃^? and K⁺ during an 18d growth period, a 3d pretreatment period and during flux measurements. Plants grew best in media containing 100 mmol m^?3 NH₄⁺ and 200mmolm^?3 K⁺ (N100/K200), followed by N2/K200 < N100/K2 < N2/K2. ⁸⁶Rb⁺(K⁺) fluxes were increased by exposure to N during the 18 d growth period and the 3 d of pretreatment, but decreased by the presence of NH₄⁺ during flux measurements. This inhibition was a function of prior N/K provision and the [NH₄⁺]₀ present during flux determinations. NH₄⁺ was least inhibitory to 86Rb⁺(K⁺) influx in high-N/low-K plants. Pretreatments with K⁺ failed to stimulate NH₄⁺ uptake, and the presence of K⁺ in the uptake solutions reduced NH₄⁺ fluxes only in high-N/low-K plants.     

Short term 22Na+ and 42K+ uptake in intact, mid-vegetative plants

Spergularia marina (L.) Griseb. is. a rapidly growing, annual, coastal halophyte. Because of its small size, it is suitable for isotope studies of ion transport well beyond the seedling stage. The purpose of this report is to establish the similarities and differences between ²²Na⁺ and ⁴²K⁺ uptake in S. marina and in more commonly used mesophytic crop species. Vegetative plants were used 18 days after transfer to solution culture. Plants were grown either on Na⁺-free medium or on 0.2 × sea water. ²²Na⁺ uptake was linear with time for several hours. The rate was relatively insensitive to external concentration between 1 and 180 mol Na⁺ m?³, particularly in Na⁺-free plants. Transport to the shoot accounted for 40 to 70% of the total uptake, dependent on salinity but largely independent of time. ⁴²K⁺ uptake decreased with increasing salinity in Na⁺-free plants and increased in 0.2 × sea water plants. Both uptake and transport to the shoot were non-linear with time, upward concavity suggesting recovery from a manipulative and/or osmotic injury. Steady state root contents were compared with predicted contents based on cortical cell electrical potentials using the Nernst equation. Reasonable agreement was found in all cases except Na⁺ content of 0.2 × sea water plants, in which active efflux was indicated. Uptake studies conducted in the presence of chemical modifiers (dicyclohexylcarbodiimide, dinitrophenol and fusicoccin) showed responses of ⁴²K⁺ uptake as expected from studies on agronomic species, and implied the presence of a similar active uptake here despite the appearance of equilibrium. Active Na⁺ uptake was suggested at low Na⁺ levels. We conclude that S. marina is a promising experimental system combining the rapid nutrient acquisition strategy of agionomically important annuals with a high degree of salt tolerance.     

Effect of tentoxin on K+transport in winter wheat seedlings of different K+-status

Klotz, M. G. and Erdei, L. 1988. Effect of tentoxin on K⁺ transport in winter wheat seedlings of different K⁺-status. The influence of the phytoeffective mycotoxin, tentoxin, [cyclo-(L-leucyl-N-methyltrans-dehydronhenyl-alanyl-glycyl-N-methyl-L-alanyl)] (in K⁺ uptake and on translocation of K⁺ from roots to shoot was studied in 14-day-old winter wheat plants (Triticum aestivum L. cv. Martonvásári-8) grown at different levels of K⁺ supply. For comparison, the effects of 2,4-dinilrophcnol and valinomycin were also investigated. In I-h experiments I pM tentoxin reduced K⁺ influx in the routs over the external K⁺ concentration range 0.1 to 1 m^M (low-K⁺ plants), whereas stimulation was observed al lower and higher K⁺ concentrations. On the other hand, in plants grown at 0.3 m^M K⁺, tentoxin stimulated the translocation of K⁺ from roots to shoots in 5-h experiments. Valinomycin affected K⁺ transport only al high K⁺-status (slight stimulation). In low-K⁺ plants 2,4-dinitrophenol (DNP) caused drastic inhibition of K⁺ uptake, but in high-K⁺ plants uptake was only slightly inhibited and translocation slightly stimulated, It is concluded that the opposite effects of tentoxin on K⁺ uptake and translocation agree1 with the directions of the H⁺-ATPases pumping H⁺ towards the apoplast and located at the cortex plasmalemma and the xylem parenchyma plasma-membrane, respectively. These effects should probably be attributed to the interaction between tentoxin and the K⁺-carrier protein rather than to a direct influence of tentoxin on H⁺-ATPase.     

Effects of interrupted K+ supply on growth and uptake of K+, Ca2+, Mg2+ and Na+ in spring wheat

Effects of interrupted K⁺ supply on different parameters of growth and mineral cation nutrition were evaluated for spring wheat (Triticum aestivum L. cv. Svenno). K⁺ (2.0 mM) was supplied to the plants during different periods in an otherwise complete nutrient solution. Shoot growth was reduced before root growth after interruption in K⁺ supply. Root structure was greatly affected by the length of the period in K⁺ -free nutrient solution. Root length was minimal, and root branching was maximal within a narrow range of K⁺ status of the roots. This range corresponded to cultivation for the last 1 to 3 days, of 11 in total, in K⁺ -free nutrient solution, or to continuous cultivation in solution containing 0.5 to 2 mM K⁺. In comparison, both higher and lower internal/external K⁺ concentrations had inhibitory effects on root branching. However, the differing root morphology probably had no significant influence on the magnitude of Ca²⁺, Mg²⁺ and Na⁺ uptake. Uptake of Ca²⁺ and especially Mg²⁺ significantly increased after K⁺ interruption, while Na⁺ uptake was constant in the roots and slowly increased in the shoots. The two divalent cations could replace K⁺ in the cells and maintain electroneutrality down to a certain minimal range of K⁺ concentrations. This range was significantly higher in the shoot [110 to 140 μmol (g fresh weight)^?1] than in the root [20 to 30 μmol (g fresh weight)^?1]. It is suggested that the critical K⁺ values are a measure of the minimal amount of K⁺ that must be present for physiological activity in the cells. At the critical levels, K⁺ (⁸⁶Rb) influx and Ca²⁺ and Mg²⁺ concentrations were maximal. Below the critical K⁺ values, growth was reduced, and Ca²⁺ and Mg²⁺ could no longer substitute for K⁺ for electrostatic balance. In a short-term experiment, the ability of Ca²⁺ to compete with K⁺ in maintaining electroneutrality in the cells was studied in wheat seedlings with different K⁺ status. The results indicate that K⁺, which was taken up actively and fastest at the external K⁺ concentration used (2.0 mM), partly determines the size of Ca²⁺ influx.     

Stimulation or inhibition of K+(86Rb+)influx in wheat roots depending on different ABA treatments

The regulatory role of abscisic acid (ABA) and kinetin on influx of K+(86RB+) IN tools of 7day old intact winter wheat which plant (Fritieun aestivum I ass starke 1 and 11) Was studied the inhibitory effect of 40,80 μM ABA in the uptake solution on K+(86RB+)influx was transiently stipulated pretreatment of the plants with ABA kinetin content enacted inhibitors effect caused by ABA. At low water potential in the uptake solution (05MPa)K+(86RB+) influx was slights higher in the presence of ABA than in is absence High humidity 123kpa ca 100% relative humidity (RID)around the shoots counteracted the inhibitory effect on k+(86RB+) influx caused by A,B,A IN the uptake solution the present data contain the hypothesis that when plants are subjected to conditions such as low water potential and low temperature. ABA stimulates K influx to facilitate water uptake.     

Changes in Rb+ and Ca2+ influx in winter wheat roots before, during and after exposure to low temperature and short days

Influx of Rb⁺(⁸⁶Rb⁺) and Ca²⁺ (⁴⁵Ca²⁺) in roots of intact winter wheat (Triticum aestivum L. cv. Weibulls Starke II) was determined at intervals before, during and after exposure to cold acclimation conditions (2°C and 8 h light period). The plants were grown in nutrient medium of two ionic strengths. During the initial two weeks of growth at 16°C and 16 h light period, Rb⁺ influx into roots decreased with increasing age, probably as a consequence of a decreasing proportion of metabolically active roots. The presence of 10^?4M 2,4-dinitrophenol (DNP) reduced Rb⁺ influx to a low and constant level, indicating that metabolic influx was the dominant process. In contrast, Ca²⁺ influx in plants grown in full strength nutrient solution was higher in the presence than in the absence of DNP. This effect may have been due to an active extrusion mechanism mediating re-export of absorbed Ca²⁺(⁴⁵Ca²⁺) during the uptake experiment. With the metabolic uncoupler inhibiting such extrusion the Ca²⁺(⁴⁵Ca²⁺) influx mesured would increase. During cold treatment, Rb⁺ influx remained at a low level, and was further decreased when DNP was present in the uptake solution. This effect may have been due to inhibition of residual active influx of Rb⁺ at 2°C by the uncoupler and/or to a decrease in membrane permeability. In contrast to Rb⁺, Ca²⁺ influx increased during cold treatment, which could again be explained as inhibition of re-export. The presence of DNP reduced Ca²⁺ influx at 2°C, indicating decreased membrane permeability by DNP at low temperature. After transfer of plants from cold acclimation conditions to 16°C, Rb⁺ and Ca²⁺ influx increased in plants grown at both ionic strengths. Influx levels were independent of the length of the cold acclimation period (1, 6 and 8 weeks), but the patterns were different for the two ions. After each of the cold acclimation periods, Rb⁺ influx increased during the first week and decreased or remained at the same level during the second week, while Ca²⁺ influx always decreased during the second week of post-cold treatment.     

2,4-D-Induced Changes in the K+ Uptake of Wheat Roots at Different pH Values

The effects of an auxin herbicide, 2,4-D, at a concentration of 0.01 mM, on the K⁺ uptake and efflux of excised roots of wheat (Triticum aestivum L. cv. Rannaya) were investigated at different pH values. The K⁺ movement was monitored with a K⁺ (⁸⁶Rb) tracer. In parallel experiments the ATPase activities of microsomal fractions were determined by the inorganic phosphate liberation method. 2,4-D inhibited the K⁺ uptake especially at low pH, irrespective of whether Ca²⁺ was present or not. No marked changes were observed in the K⁺ efflux properties at pH values above 4. The inhibitory effect on K⁺ uptake exhibited a correlation with the hydrocarbon solubility of the herbicide, but not with the 2,4-D-induced decrease of the ATPase activity. It is suggested that 2,4-D exerts a non-specific effect on the lipid-protein interactions, giving rise to a generalized alteration of the transport barrier properties of the plasma membrane even at as low a concentration as 0.01 mM.     

5-Hydroxytryptamine stimulates 86Rb+ efflux from pancreatic β-cells

The effects of 5-hydroxytryptamine and 5-hydroxytryptophan on ⁸⁶Rb⁺ efflux from prelabelled ob/ob-mouse islets were studied to better understand the cellular mechanisms underlying the effects of 5-hydroxytryptamine and 5-hydroxytryptophan on insulin release. 5-Hydroxytryptophan (4 mM) had no effect on ⁸⁶Rb⁺ efflux either at a low (3 mM) or at a high (20 mM) d-glucose concentration, whereas 5-hydroxytryptamine (4 mM) stimulated ⁸⁶Rb⁺ efflux at both glucose concentrations. These results indicate that 5-hydroxytryptamine may reduce glucose-induced insulin release by inhibiting early steps in the β-cell stimulus-secretion coupling.     

Characterization of csi52, a Cs+ resistant mutant of Arabidopsis thaliana altered in K+ transport

Plant roots accumulate potassium from a wide range of soil concentrations, utilizing at least two distinct plasma membrane uptake systems with different affinities for the cation. Although details on the structure and function of these transporters are beginning to emerge many prominent questions remain concerning how these proteins function in planta. Such questions can be addressed through the use of well-defined transport mutants. Csi52, a caesium-insensitive mutant of Arabidopsis thaliana which is defective in potassium transport, is further characterized here using conventional electrophysiology, patch-clamp and radiometric approaches to identify the nature of the potassium transport lesion. Rb⁺ uptake experiments reveal a reduced uptake in csi52 in both the high- and low-affinity uptake range. Patch-clamp analysis indicates that the activity of the predominant inward rectifying channel observed in wild-type cells is extremely low in root protoplasts isolated from csi52, whereas outward rectifying channel activity is comparable between wild-type and mutant. Rb⁺ uptake studies show that in both wild-type and csi52 the high-affinity uptake pathway is considerably less sensitive to Cs⁺ than the low-affinity pathway with K_1/2 values for Cs⁺ of around 1.3 and 0.2 mM, respectively. Furthermore, K⁺ starvation leads to a larger relative increase in high-affinity K⁺ uptake in the mutant than the wild-type. The results demonstrate the Cs⁺ sensitivity of each individual uptake pathway is comparable in wild-type and csi52 but the high-affinity pathway is less Cs⁺ sensitive (in both wild-type and csi52). Therefore, the larger shift toward high-affinity uptake in the mutant compared with the wild-type under K⁺-starvation conditions will endow the mutant with a higher degree of overall Cs⁺ resistance. The data supply evidence for the hypothesis that the csi52 mutation lies within a gene that regulates the activity of several potassium transport systems and coordinates their relative contribution to overall root K⁺ uptake.     

Varietal variation in uptake and utilization of potassium (rubidium) in high-salt seedlings of barley

Seedlings of eleven varieties of barley (Hordeum vulgare L.) showed differences in utilization of K⁺ from a full nutrient solution containing 3.0 mM K⁺. The K⁺ content of both roots and shoots was proportional to the fresh weights and dry weights after a week in the nutrient solution. The K⁺ use-efficiency ratio, which indicates the efficiency of nutrient utilization (mg dry weight produced per mg K⁺ absorbed), differed significantly among the varieties. There was no correlation between influx of Rb⁺ and the content of K⁺. It is suggested that there are wide varietal differences in such genetically-determined properties as ion influx and efflux and net ion transport to the shoot. Further-more, the influx of Rb⁺ was closely linked to transpiration, probably due to a variety-specific non-metabolic part of Rb⁺ influx. Varietal differences in influx of Rb⁺ were more pronounced in high-K⁺ roots than in low-K⁺ roots with maximum rate of Rb⁺ uptake, but the rank of varieties was the same in each case. – Criteria for the selection of K⁺ use-efficient varieties of barley are discussed.