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.     

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.     

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.     

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

Six cultivars of spring barley ( Hordeum vulgare L. cvs Salve, Nümberg II, Bomi, Risø 1508, Mona and Sv 73 608) were grown in water culture for three weeks with various combinations of mineral supply and differential roots/shoot temperatures during the growth period. Most important for growth and accumulation of N, K⁺, Ca²⁺ and Mg²⁺ was the mineral supply, followed by the root temperature and the choice of cultivar. Treatments with low mineral supply or low root temperature induced a uniform reduction in growth and accumulation of the ions studied. The effects of low mineral supply and low root temperature on growth and N accumulation was additive, which indicates that these factors exert their influence independently of each other.
Roots grown at 10°C were smaller and Rb⁺(⁸⁶Rb) influx was higher than in roots grown at 20°C. It is suggested that the control of Rb⁺(⁸⁶Rb) influx is affected by the root temperature and the age of the plants. The higher ⁸⁶Rb⁺ (⁸⁶Rb) influx into the low temperature roots could not compensate for the smaller root size. However, the lower total mineral accumulation made up for the needs of the smaller plants and cannot explain the reduction in growth.     

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.     

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.     

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.     

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.     

Nitrendipine is a potent inhibitor of the Ca(2+)-activated K+ channel of human erythrocytes.

Nitrendipine, a classical blocker of L-type Ca2+ channels, is shown to be a potent inhibitor of the Ca(2+)-activated K+ channel of human erythrocytes. In erythrocytes suspended in a solution with physiological Na+ and K+ concentrations and in which the channel was activated using the Ca2+ ionophore ionomycin, nitrendipine inhibited K+(86Rb+) influx with an I50 of around 130 nM. Similar results were obtained for K+(86Rb+) efflux, and for K+(86Rb+) influx into cells suspended in a high-K+ medium.     

Multiple pathways of Pb2+ permeation in rat cerebellar granule neurones

The pathways of lead (Pb(2+)) uptake were studied in fura-2-loaded cerebellar granule cells from 8-day-old rats. In a nominal Ca-free external bath, Pb(2+) (5-50 microM) determined an increase of the fluorescence emission ratio (R = E(340)/E(380)) even in the absence of any specific stimulus. This rise was dose-dependent, was not significantly affected by mM Mg(2+) or Ca(2+), but it was readily reversed by the membrane-permeant heavy metal chelator tetrakis(2-pyridylmethyl) ethylene-diamine (TPEN, 100 microM), indicating that it was due to Pb(2+) influx. The rate of rise, dR/dt, was increased up to a factor of 5 by depolarizing high-KCl solution, indicating a sizeable permeation through voltage-dependent channels. This effect was neither antagonized by nimodipine, nor enhanced by BayK8644, but it was slackened by omega-agatoxin IVA (200 nM), suggesting an involvement of non-L-type calcium channels. Pb(2+) influx was also stimulated by glutamic acid or NMDA in the presence of 10-30 microM glycine, but only in Mg-free solution, suggesting that glutamate channels of the NMDA type are an additional pathway of Pb(2+) uptake. Pb(2+) caused a time-, dose- and stimulus-dependent saturation of the dye, whose intracellular concentration is approximately 10 microM, indicating that intracellular Pb(2+) can readily reach a concentration in the micromolar range. These results indicate that the particular vulnerability of neurones to Pb(2+) poisoning is linked to the presence of specific transport systems, which mediate the rapid uptake of Pb(2+) into the neurone.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Chronic inhibition of cortex microsomal Mg2+/Ca2+ ATPase-mediated Ca2+ uptake in the rat pilocarpine model following epileptogenesis

In the rat pilocarpine model, 1 h of status epilepticus caused significant inhibition of Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in cortex endoplasmic reticulum (microsomes) isolated immediately after the status episode. The rat pilocarpine model is also an established model of acquired epilepsy. Several weeks after the initial status epilepticus episode, the rats develop spontaneous recurrent seizures, or epilepsy. To determine whether inhibition of Ca(2+) uptake persists after the establishment of epilepsy, Ca(2+) uptake was studied in cortical microsomes isolated from rats displaying spontaneous recurrent seizures for 1 year. The initial rate and total Ca(2+) uptake in microsomes from epileptic animals remained significantly inhibited 1 year after the expression of epilepsy compared to age-matched controls. The inhibition of Ca(2+) uptake was not due to individual seizures nor an artifact of increased Ca(2+) release from epileptic microsomes. In addition, the decreased Ca(2+) uptake was not due to either selective isolation of damaged epileptic microsomes from the homogenate or decreased Mg(2+)/Ca(2+) ATPase protein in the epileptic microsomes. The data demonstrate that inhibition of microsomal Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in the pilocarpine model may underlie some of the long-term plasticity changes associated with epileptogenesis.