Net sodium fluxes change significantly at anatomically distinct root zones of rice (Oryza sativa L.) seedlings

Casparian bands of endodermis and exodermis play crucial roles in blocking apoplastic movement of ions and water into the stele of roots through the cortex. These apoplastic barriers differ considerably in structure and function along the developing root. The present study assessed net Na⁺ fluxes in anatomically distinct root zones of rice seedlings and analyzed parts of individual roots showing different Na⁺ uptake. The results indicated that anatomically distinct root zones contributed differently to the overall uptake of Na⁺. The average Na⁺ uptake in root zones in which Casparian bands of the endo- and exo-dermis were interrupted by initiating lateral root primordia (root zone III) was significantly greater than that at the root apex, where Casparian bands were not yet formed (root zone I), or in the region where endo- and exo-dermis with Casparian bands were well developed (root zone II). The measurement of net Na⁺ fluxes using a non-invasive scanning ion-selective electrode technique (SIET) demonstrated that net Na⁺ flux varied significantly in different positions along developing rice roots, and a net Na⁺ influx was obvious at the base of young lateral root primordia. Since sodium fluxes changed significantly along developing roots of rice seedlings, we suggest that the significantly distinct net Na⁺ flux profile may be attributed to different apoplastic permeability due to lateral root primordia development for non-selective apoplastic bypass of ions along the apoplast.     

Mechanisms of sodium uptake by roots of higher plants

The negative impact of soil salinity on agricultural yields is significant. For agricultural plants, sensitivity to salinity is commonly (but not exclusively) due to the abundance of Na⁺ in the soil as excess Na⁺ is toxic to plants. We consider reducing Na⁺ uptake to be the key, as well as the most efficient approach, to control Na⁺ accumulation in crop plants and hence to improve their salt resistance. Understanding the mechanism of Na⁺ uptake by the roots of higher plants is crucial for manipulating salt resistance. Hence, the aim of this review is to highlight and discuss recent advances in our understanding of the mechanisms of Na⁺ uptake by plant roots at both physiological and molecular levels. We conclude that continued efforts to investigate the mechanisms of root Na⁺ uptake in higher plants are necessary, especially that of low-affinity Na⁺ uptake, as it is the means by which sodium enters into plants growing in saline soils.     

Role of Arbuscular Mycorrhizal Symbiosis in Proline Biosynthesis and Metabolism of Cicer arietinum L. (Chickpea) Genotypes Under Salt Stress

Salinity causes osmotic stress and negatively impacts plant growth and productivity. Proline is one of the most important osmoprotectants synthesized under stressed conditions. Accumulation of free proline occurs due to enhanced biosynthesis and repressed degradation, and both processes are controlled by feedback regulatory mechanisms. Arbuscular mycorrhizal (AM) fungi are considered to be bioameliorators of salinity stress due to their wide-ranging presence in contaminated soils and their role in modulation of biochemical processes. Chickpea is considered sensitive to salinity. However, reports on AM-induced osmoprotection through regulation of proline biosynthesis in chickpea genotypes are scant. The present study investigated the influence of AM symbiosis on proline metabolism in two chickpea (Cicer arietinum L.) genotypes (PBG-5 and CSG-9505) under salt stress and correlated the same with sodium (Na⁺) ion uptake. Salinity reduced plant biomass (roots and shoots), with roots being more negatively affected than shoots. Mycorrhizal colonization with Glomus mosseae was much stronger in PBG-5 and was correlated with reduced Na⁺ ion uptake and higher growth when compared with CSG-9505 under stressed and unstressed conditions. Mycorrhizal symbiosis with chickpea roots boosted proline biosynthesis by significantly increasing pyrroline-5-carboxylate synthetase (P-5-CS) and glutamate dehydrogenase (GDH) activities with a concomitant decline in proline dehydrogenase (ProDH) activity under salt stress. The enhancement of the activity of these enzymes was higher in PBG-5 than in CSG-9505 and could be directly correlated with the percent mycorrhizal colonization and Na⁺ uptake. The study indicated a strong role of AM symbiosis in enhancing stress tolerance in chickpea by significantly modulating proline metabolism and Na⁺ uptake.     

Effects of Helminthosporium carbonum Toxin on Absorption of Solutes by Corn Roots

Susceptible corn roots exposed to the host-selective toxin of Helminthosporium carbonum took up and retained more NO₃⁻, Na⁺, Cl⁻, 3-o-methylglucose, and leucine than did control roots. Stimulatory effects on uptake were more pronounced with freshly cut roots than with roots that were washed and aged. Solutes were accumulated against a concentration gradient, and toxin-treated tissues developed a steeper gradient than did control tissues. Toxin affected both the low and high affinity uptake systems for Na⁺ and Cl⁻. Toxin did not affect uptake of Na₂⁻, K⁺, Ca²⁺, phosphate ion (H₂PO₄⁻ and HPO₄⁻), SO₄⁻, and glutamic acid. No toxin-induced leakage of any solute tested was detected within 5 to 6 hr after initial exposure to toxin. The data suggest that toxin from H. carbonum does not cause the general plasma membrane derangement caused by other host-selective toxins. Instead, H. carbonum toxin may cause specific changes in characteristics of the plasmalemma, which result in increased uptake of certain solutes.     

Potassium and sodium absorption kinetics in roots of two tomato species : lycopersicon esculentum and lycopersicon cheesmanii

Excised roots of the tomato species, Lycopersicon esculentum Mill. cv Walter (the commercial species) and of Lycopersicon cheesmanii ssp. minor (Hook.) C.H. Mull. (a wild species from the Galapagos Islands), were used in comparative studies of their absorption of K⁺ and Na⁺. Uptake of ⁸⁶Rb-labeled K⁺ and ²²Na-labeled Na⁺ by excised roots of `Walter' and L. cheesmanii varied as a function of genotype and tissue pretreatment with or without K<sup>+</sup>. Excised roots of `Walter' consistently absorbed more ⁸⁶Rb-labeled K⁺ than those of L. cheesmanii. Absorption of K⁺ from solutions ranging from 0.01 to 0.2 millimolar KCl showed saturation kinetics in both K⁺-pretreated and K⁺-depleted roots of `Walter,' and for K<sup>+</sup>-depleted roots of L. cheesmanii. K<sup>+</sup>-pretreated roots of L. cheesmanii had exceedingly low rates of K<sup>+</sup> uptake with strikingly different, linear kinetics. Pretreatment with K<sup>+</sup> caused a decrease in rates of K<sup>+</sup> uptake in both genotypes. Potassium depleted roots of L. cheesmanii absorbed Na<sup>+</sup> at a greater rate than those of `Walter,' whereas K⁺-pretreated roots of `Walter' absorbed Na⁺ at a greater rate than those of L. cheesmanii. The results confirm and extend previous conclusions to the effect that closely related genotypes may exhibit widely different responses to the two alkali cations, K⁺ and Na⁺.     

Reconstitution and characterization of a sodium-stimulated active aminoisobutyric acid transport system derived from partially purified plasma membranes from mouse fibroblasts transformed by simian virus 40: comparison of reconstituted vesicles with native membrane vesicles

A Na⁺-specific and Na⁺-stimulated active α-aminoisobutyric acid transport system was reconstituted from plasma membranes isolated from mouse fibroblast BALB/c 3T3 cells transformed by simian virus 40. The plasma membranes were treated with dimethylmaleic anhydride and then extracted with 2% cholate. The cholate-solubilized supernatant proteins were combined with exogenous phospholipids and eluted through a Sephadex G-50 column. This yielded reconstituted vesicles which in the presence of Na⁺ could actively transport α-aminoisobutyric acid as shown by the transient accumulation above the equilibrium level (overshoot). The overshoot was not obtained with other monovalent cations such as K⁺, Li⁺, and choline⁺. The electrochemical effect of the lipophilic anion, SCN^?, led to greater α-aminoisobutyric acid uptake as compared to that observed with Cl^? or SO₄^2?. The Na⁺-stimulated transport of a-aminoisobutyric acid was a saturable process with an apparent K_m of 2 mm. Studies of the inhibition of α-aminoisobutyric acid transport by other amino acids showed that methylaminoisobutyric acid [specifically transported by A system (alanine preferring)]had a pronounced inhibitory effect on a-aminoisobutyric acid uptake in contrast to the slight inhibitory effect produced by phenylalanine [primarily transported by L system (leucine preferring)]. The results show that the reconstituted vesicles, prepared from partially purified membrane proteins and exogenous phospholipids, regained the same important transport properties of native membrane vesicles, i.e., Na⁺-specific and Na⁺-stimulated concentrative α-aminoisobutyric acid uptake.     

Effects of temperature and dinitrophenol on the uptake of potassium and sodium ions in Ricinus communis roots

Summary Detopped root systems of Ricinus communis plants were used for the study of the effects of temperature and DNP on the uptake of K and Na ions supplied as KNO₃ and NaNO₃.When K and Na ions were offered together in equivalent concentrations, the steady state uptake rates for K⁺ and Na⁺ at 23 to 25° gave a K⁺/Na⁺ ratio of 3. Increasing the Na⁺ concentration relative to K⁺ 3-fold did not alter the preferential uptake of K⁺. The uptake of K⁺ was more sensitive to temperature in the range 10 to 40° and to the application of DNP at 1.5x10^-4 M than was the uptake of Na⁺. When NaNO₃ was the only salt supplied Na⁺ uptake became more sensitive to DNP than when both K⁺ and Na⁺ nitrates were supplied. Prolonged application of DNP led to net K⁺ efflux from the roots, even when no K⁺ was being supplied to the roots. Net Na⁺ efflux under the influence of DNP occurred only in roots previously grown on Na-containing nutrient medium.The different responses of the K⁺ and Na⁺ uptake processes to temperature and DNP suggest the operation of different uptake mechanisms for K⁺ and Na⁺ These results have been considered in relation to the recent concept of dual mechanisms for the absorption of alkali cations by plant tissues.     

Factors Associated with Determination of Root <Superscript>22</Superscript>Na<Superscript>+</Superscript> Influx in the Salt Accumulation Halophyte <Emphasis Type="Italic">Suaeda maritima</Emphasis>

Salinity is an increasing problem for agricultural production worldwide. The result of low-affinity Na⁺ uptake is toxic to the cytoplasm of most crop plants. Nevertheless, the pathways for this low-affinity Na⁺ uptake are still uncertain. In this work we used ²²Na⁺ isotope tracing technology to investigate factors associated with determination of root ²²Na⁺ influx in the salt accumulation halophyte Suaeda maritima. We found that a 2 min of exposure to the ²²Na⁺ labeled uptake solution was optimal for determining ²²Na⁺ influx into excised roots of S. maritima and that 7 min of blotting is suitable in ²²Na⁺ influx experiments. ²²Na⁺ influx did not increase linearly with the increasing external Na⁺ concentration, in the range tested, of 2 to 300 mM NaCl. But root ²²Na⁺ influx and root Na⁺ concentration were well correlated. ²²Na⁺ influx into excised roots of S. maritima was not, however, well correlated with the plant size. All the above results indicated further that this ²²Na⁺ isotope influx procedure is a good method for quantify Na⁺ uptake rate by the roots of the salt accumulation halophyte.     

Inhibition by white light of rb absorption in the root apex of corn

Measurements of cell lengths made at 0.5 millimeter intervals in median longitudinal sections of the primary roots of corn (Zea mays) were used to construct a growth curve. The region 1.5 to 4.0 millimeters from the apex contained the largest number of elongating cells. Absorption of ⁸⁶Rb⁺ was measured using intact, dark-grown corn seedlings. Following uptake and exchange, the terminal 8.0 millimeters of each root was cut into four 2.0 millimeter segments. Maximum ⁸⁶Rb⁺ uptake occurred in the region from 0.0 to 4.0 millimeter from the root tip. Washing the intact primary root in fresh 2.0 millimolar CaSO₄ for 2 hours prior to uptake augmented the rate of ⁸⁶Rb⁺ uptake in all regions. Illumination with white light during washing caused a reduction of ⁸⁶Rb⁺ uptake as compared with controls washed in darkness, and the region of greatest light response was the region of elongation. Removal of the coleoptile prior to washing did not prevent the light inhibition of subsequent ⁸⁶Rb⁺ uptake. Removal of the root cap prior to washing in light partially reversed the light-induced inhibition of the washing response.     

Influence of Helminthosporium maydis, Race T, Toxin on Potassium Uptake in Maize Roots

The effect of a toxin extract of Helminthosporium maydis, race T on K⁺ (⁸⁶Rb) uptake by excised root segments of normal (N) and Texas cytoplasmic male-sterile (T) versions of corn inbred W64A was investigated. The uptake of K⁺ was inhibited in both N and T roots by the toxin. This was true for both basal (freshly excised) and augmented (pretreated with aeration) K⁺ uptake. Augmented uptake was more toxin-sensitive than basal uptake (irrespective of cytoplasm type), and the augmented uptake in T roots was seven to eight times more toxin-sensitive than in N roots.     

Effect of Exogenous Glycinebetaine on Na+ Transport in Barley Roots

Ahmad, N., Wyn Jones, R. G. and Jeschke, W. D. 1987. Effectof exogenous glycinebetaine on Na⁺ transport in barley roots.—J.exp. Bot. 38: 913–921. A comparison has been made of the kinetics of 22Na⁺ uptake intoexcised barley roots and roots pre-loaded with glycinebetaine.The elevated intracellular glycinebetaine or a metabolic consequencethereof increased the Na⁺ influx, and the effect was relatedto the level of internal glycinebetaine and or Na⁺ [Cl^–].The quasi-steady-state Na⁺ influx at the tonoplast rather thanthe plasmalemma influx was apparently influenced by glycinebetaineloading. The tonoplast fluxes and vacuolar Na⁺ content wereconsistently higher in glycinebetaine-loaded roots than unloadedroots. A membrane-modifying role of glycinebetaine in relationto ion compartmentation is discussed. Key words: Excised roots, glycinebetaine, Na⁺, ion fluxes, barley     

The Effect of Calcium on the Uptake and Distribution of Sodium in Excised Barley Roots

The total uptake and relative distribution of Na⁺ along the root was investigated in excised barley roots (Hordeum vulgare L. cv. Union) of 6-day-old plants cultured on 0.25 mM CaSO₄. One technique involved cutting the roots at harvest time after different uptake periods into different segments each 2 cm long. These segments were ashed and sodium was measured by flame photometry. For each experiment two treatments were done, one containing calcium in the uptake solution and one without calcium. A second technique involved the measurement of sodium transport, using labelled ²²Na⁺ solution. Although no significant differences could be found between the calcium containing and the calciumless treatments at 24 h, there were clearcut differences for the first 6 to 8 h. The effect of calcium on the first stages of the sodium uptake was a large accumulation of sodium in the part close to the apex with a translocation close to nil; whereas the sodium taken up in the absence of calcium did not accumulate in such large amounts in the apex region but was immediately transported basipetally.     

Involvement of na in active uptake of pyruvate in mesophyll chloroplasts of some c(4) plants : na/pyruvate cotransport

An artificial Na⁺ gradient across the envelope (Na⁺ jump) enhanced pyruvate uptake in the dark into mesophyll chloroplasts of a C₄ plant, Panicum miliaceum (NAD-malic enzyme type) (J Ohnishi, R Kanai [1987] FEBS Lett 219:347). In the present study, ²²Na⁺ and pyruvate uptake were examined in mesophyll chloroplasts of several species of C₄ plants. Enhancement of pyruvate uptake by a Na⁺ jump in the dark was also seen in mesophyll chloroplasts of Urochloa panicoides and Panicum maximum (phosphoenolpyruvate carboxykinase types) but not in Zea mays or Sorghum bicolor (NADP-malic enzyme types). In mesophyll chloroplasts of P. miliaceum and P. maximum, pyruvate in turn enhanced Na⁺ uptake in the dark when added together with Na⁺. When flux of endogenous Na⁺ was measured in these mesophyll chloroplasts preincubated with ²²Na⁺, pyruvate addition induced Na⁺ influx, and the extent of the pyruvate-induced Na⁺ influx positively correlated with that of pyruvate uptake. A Na⁺/H⁺ exchange ionophore, monensin, nullified all the above mutual effects of Na⁺ and pyruvate in mesophyll chloroplasts of P. miliaceum, while it accelerated Na⁺ uptake and increased equilibrium level of chloroplast ²²Na⁺. Measurements of initial uptake rates of pyruvate and Na⁺ gave a stoichiometry close to 1:1. These results point to Na⁺/pyruvate cotransport into mesophyll chloroplasts of some C₄ plants.     

Sodium and potassium compartmentation and transport in the roots of intact lettuce plants

In this report, we consider the accumulation in roots, and transport to the shoot, of Na⁺ and K⁺ in intact lettuce plants (Lactuca sativa cv Black-seeded Simpson). Plants were grown in modified Hoagland medium supplemented with 10 moles NaCl per cubic meter. At this salinity, significant levels of Na⁺ were accumulated in roots and shoots, but there was no reduction in plant growth. Transport characteristics for both Na⁺ and K⁺ were qualitatively similar to those previously reported, for Spergularia marina, indicating that the results obtained with these experimental protocols are not limited to one unconventional experimental plant. The most pronounced difference in transport of the two ions was evident when transport was followed in a chase period after a 10 minute uptake pulse. For Na⁺, there was an initially rapid, but small, loss of label to the medium, and very little movement to the shoot. For K⁺, little label was lost from the plants, but translocation to the shoot proceeded for at least 60 minutes. The transport systems were further distinguished by treating the roots during labeling with 20 micrograms per milliliter cycloheximide. For K⁺, both uptake and translocation were reduced by about 50%. For Na⁺, root accumulation was stimulated more than five-fold, while transport to the shoot was reduced about 20%. Cycloheximide also modified the Na⁺ transport characteristics such that continued translocation occurred during the chase period of pulse-chase studies.     

NaCl胁迫下沙枣幼苗生长和阳离子吸收、运输与分配特性

沙枣(Elaeagnus angustifolia L.)耐盐性强,是我国北方生态脆弱地区造林绿化的一个先锋树种。为探讨沙枣的盐适应机制,研究了不同浓度NaCl(0、100和200 mmol/L)胁迫30d对其水培幼苗生物量累积以及不同组织(根、茎、叶)K+、Na+、Ca2+和Mg2+吸收、运输与分配的影响。结果表明:盐胁迫不同程度地促进了沙枣苗根系生长;100 mmol/L NaCl胁迫对幼苗生物量累积无明显影响,而200 mmol/L则显著抑制了生物量累积;盐胁迫幼苗根、茎、叶中Na+含量以及K+-Na+选择性运输系数(S K,Na)和Ca2+-Na+选择性运输系数(S Ca,Na)显著或大幅度增加,而K+、Ca2+、Mg2+含量以及K+/Na+、Ca2+/Na+和Mg2+/Na+比值则显著或大幅度下降;200 mmol/L NaCl胁迫沙枣根Na+含量和根Na+净累积量分别为22.15 mg/g干重和1.87 mg/株(是对照的16.20倍和20.06倍),根成为Na+净累积量增加幅度最大的组织和Na+含量最高的组织;200 mmol/L NaCl胁迫沙枣茎、叶中的Na+含量以及冠组织Na+净累积量分别高达5.15、7.71 mg/g干重和3.29 mg/株(是对照的7.22倍、9.58倍和5.45倍),但幼苗仍能正常生长。综合分析认为,沙枣的盐适应机制是根系拒盐和冠组织耐盐,主要通过根系的补偿生长效应、根系对Na+的聚积与限制作用以及冠组织对Na+的忍耐来实现的,同时也与根、茎和叶对K+、Ca2+选择性运输能力显著增强有关。     

Regulation of cation transporter genes by the arbuscular mycorrhizal symbiosis in rice plants subjected to salinity suggests improved salt tolerance due to reduced Na<Superscript>+</Superscript> root-to-shoot distribution

Rice is a salt-sensitive crop whose productivity is strongly reduced by salinity around the world. Plants growing in saline soils are subjected to the toxicity of specific ions such as sodium, which damage cell organelles and disrupt metabolism. Plants have evolved biochemical and molecular mechanisms to cope with the negative effects of salinity. These include the regulation of genes with a role in the uptake, transport or compartmentation of Na⁺ and/or K⁺. Studies have shown that the arbuscular mycorrhizal (AM) symbiosis alleviates salt stress in several host plant species. However, despite the abundant literature showing mitigation of ionic imbalance by the AM symbiosis, the molecular mechanisms involved are barely explored. The objective of this study was to elucidate the effects of the AM symbiosis on the expression of several well-known rice transporters involved in Na⁺/K⁺ homeostasis and measure Na⁺ and K⁺ contents and their ratios in different plant tissues. Results showed that OsNHX3, OsSOS1, OsHKT2;1 and OsHKT1;5 genes were considerably upregulated in AM plants under saline conditions as compared to non-AM plants. Results suggest that the AM symbiosis favours Na⁺ extrusion from the cytoplasm, its sequestration into the vacuole, the unloading of Na⁺ from the xylem and its recirculation from photosynthetic organs to roots. As a result, there is a decrease of Na⁺ root-to-shoot distribution and an increase of Na⁺ accumulation in rice roots which seems to enhance the plant tolerance to salinity and allows AM rice plants to maintain their growing processes under salt conditions.     

Physiological adaptation and gene expression analysis of <Emphasis Type="Italic">Casuarina equisetifolia</Emphasis> under salt stress

Casuarina equisetifolia is widely planted in coastal areas of tropical and subtropical regions as windbreaks or to stabilize dunes against wind erosion due to its high salt tolerance and nitrogen-fixing ability. To investigate the mechanisms responsible for its salt tolerance, we examined growth, mineral composition, expression of genes for sodium (Na⁺) and potassium (K⁺) transport proteins, and antioxidant responses under NaCl treatments. Increasing NaCl concentrations inhibited lateral root elongation and decreased plant height, length of internodes, and numbers of branches and twigs. The Na⁺ content significantly increased whereas the K⁺ content significantly decreased in both shoots and roots with increasing external NaCl concentration, resulting in a significant increase in Na⁺/K⁺ ratio. Most of the Na⁺/H⁺ antiporter genes (NHXs) were obviously upregulated in roots after 24 and 168 h of salt stress, and NHX7 was especially induced after 168 h. Almost all salt overly sensitive (SOS) genes were induced after 168-h treatment. Additionally, activities of superoxide dismutase, glutathione peroxidase, and catalase were significantly changed in shoots and roots under salt stress. Hence, we conclude that salinity tolerance of C. equisetifolia mainly relied on sequestering excess Na⁺ into vacuoles and on induced expression of NHX and SOS genes in roots and thus the maintenance of sufficient K⁺ content in shoots.     

海岸不同生态断带植物根叶抗逆生理变化与其Na+含量的关系

以烟台海岸生态断带滨麦(Leymus mollis)和肾叶打碗花(Calystegia soldanella)为材料,在远离高潮线不同位置上取土样和植物材料,通过测定土壤Na+和两植物根叶Na+含量、丙二醛(MDA)含量、抗氧化酶(SOD、POD、CAT)活性和渗透调节物含量,以揭示滨麦和肾叶打碗花根叶中Na+在其适应海岸盐环境中的生理调控机制。结果表明,在高潮线土壤Na+含量最高,滨麦根叶Na+含量较高,两植物根叶中MDA和水分含量、抗氧化酶活力均较低,但渗透调节物含量均较高。随远离高潮线土壤Na+含量下降,滨麦根叶Na+含量下降,而肾叶打碗花根中Na+含量上升,其根叶Na+含量较滨麦分别高637%和319%。同时两植物根叶MDA含量、叶片含水量增加;两植物根中POD和SOD活力增加;两植物根叶可溶性糖和脯氨酸含量下降。但不同生态断带滨麦叶片平均含水量相对较低,MDA含量、POD和CAT和SOD活力、脯氨酸和可溶性糖含量相对较高。在盐土环境中滨麦通过降低Na+的吸收和提高抗氧化酶活力和有机渗透调节物含量维持氧自由基代谢平衡和水分平衡。而肾叶打碗花是泌盐植物,在不同生态断带其叶片Na+含量、平均含水量相对较高,叶MDA含量、POD和CAT活力、脯氨酸和可溶性糖含量均相对较低。泌盐植物的肾叶打碗花依赖根叶中积累的Na+作为无机渗透调节剂维护其离子平衡和水分平衡及正常生长。因此,积累在根叶中的Na+离子既作为无机渗透调节剂维护细胞离子平衡和水分平衡,又引发细胞生理干旱促进有机渗透调节物合成;另外还作为氧自由基诱发剂促使活性氧自由基(ROS)积累,通过积累的ROS激活抗氧化保护酶系统抑制膜脂过氧化、维护氧自由基代谢平衡。海岸沙地土壤中高浓度Na+是海滨滨麦和肾叶打碗花能长期在盐土环境中生存的依靠元素,其对植物的生理调控作用可能是滨麦和肾叶打碗花适应盐土生存的重要生理适应机理。     

Controls on na influx in corn roots

We have investigated the effects of hyperpolarization and depolarization, and the presence of K⁺ and/or Ca²⁺, on ²²Na⁺ influx into corn (Zea mays L.) root segments. In freshly excised root tissue which is injured, Na⁺ influx is unaffected by hyperpolarization with fusicoccin, or depolarization with uncoupler (protonophore), or by addition of K⁺. However, added Ca²⁺ suppresses Na⁺ influx by 60%. In washed tissue which has recovered, Na⁺ influx is doubled over that of freshly excised tissue, and the influx is increased by fusicoccin and suppressed by uncoupler. This energy-linked component of Na⁺ influx is completely eliminated by low concentrations of K⁺, leaving the same level and kind of Na⁺ influx seen in freshly excised roots. The K⁺-sensitive energy linkage appears to be by the carrier for active K⁺ influx. Calcium is equally inhibitory to Na⁺ influx in washed as in fresh tissue. Other divalent cations are only slightly less effective. Net Na⁺ uptake was about 25% of ²²Na⁺ influx, but proportionately the response to K⁺ and Ca²⁺ was about the same.

The constancy of K⁺-insensitive Na⁺ influx under conditions known to hyperpolarize and depolarize suggests that if Na⁺ transport is by means of a voltage-sensitive channel, the rise or fall of channel resistance must be proportional to the rise or fall in potential difference. The alternative is a passive electroneutral exchange of ²²Na⁺ for endogenous Na⁺. The data suggest that an inwardly directed Na⁺ current is largely offset by an efflux current, giving both a small net uptake and isotopic exchange.

     

Effects of ouabain and low temperature on the sodium efflux pump in excised corn roots

Ouabain (0.05 millimolar) and low temperature (4 C) both caused the tissue Na⁺ content of excised 5-day-old corn roots to increase, indicating that there is an inhibition of the Na⁺ efflux pump. Na⁺ efflux was measured utilizing three different methods. Each method gave similar results in terms of rate and ouabain sensitivity. With one of these methods, the compartmental efflux method, it was demonstrated that rates for Na⁺ efflux increase as the external Na⁺ concentration is increased; e.g. the efflux rates are 0.529, 1.78, and 3.64 microequivalents per gram fresh weight per hour for external NaCl concentrations of 1, 10, and 30 millimolar, respectively. The data indicate that the Na⁺ efflux pump is located in the plasmalemma of root cells.