Root apoplastic transport and water relations cannot account for differences in Cl− transport and Cl−/NO3 − interactions of two grapevine rootstocks differing in salt tolerance

Grapevine is moderately sensitive to salinity and accumulation of toxic levels of Cl^? in leaves is the major reason for salt-induced symptoms. In this study, apoplastic Cl^? uptake and transport mechanism(s) were investigated in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt tolerant) and K 51–40 (salt sensitive). Increased external salinity caused high Cl^? accumulation in shoots of the salt sensitive K 51–40 in comparison to Paulsen. Measurement of ¹⁵NO₃ ^? net fluxes under high salinity showed that by increasing external Cl^? concentrations K 51–40 roots showed reduced NO₃ ^? accumulation. This was associated with increased accumulation of Cl^?. In comparison to Paulsen, K 51–40 showed reduced NO₃ ^?/Cl^? root selectivity with increased salinity, but Paulsen had lower selectivity over the whole salinity range (0–45 mM). To examine if root hydraulic and permeability characterisations accounted for differences between varieties, the root pressure probe was used on excised roots. This showed that the osmotic Lp_r was significantly smaller than hydrostatic Lp_r, but no obvious difference was observed between the rootstocks. The reflection coefficient (σ) values (0.48–0.59) were the same for both rootstocks, and root anatomical studies showed no obvious difference in apoplastic barriers of the main and lateral roots. Comparing the uptake of Cl^? with an apoplastic tracer, PTS (3-hydroxy-5,8,10-pyrentrisulphonic acid), showed that there was no correlation between Cl^? and PTS transport. These results indicated that bypass flow of salts to the xylem is the same for both rootstocks (0.77 ± 0.2 and 1.05 ± 0.12 %) and hence pointed to differences in membrane transport to explain difference in Cl^? transport to the shoot.     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

CHLORIDE UPTAKE BY ANACYSTIS NIDULANS (CYANOPHYCEAE)1

Anacystis nidulans (Richt.) Drouet & Daily (UTEX 625), grown in batch culture with 0.5% CO₂ in air, was supplied with chloride labelled with ³⁶Cl in light and dark. Uptake in light was stimulated relative to uptake in darkness. A single transport system for Cl^? with an apparent K_m for Cl^? of 0.14 mM was identified. Chloride in the cells reached a maximum value after 30–50 min at 25 C. At this point the internal Cl^? concentration was calculated to be 60-fold the external (0.1 mM) in light and 37-fold in darkness. DCMU (3-[3,4-dichlorophenyl]–1, 1-dime-thylurea), at concentrations which abolished photosynthetic O₂ evolution did not inhibit Cl^? uptake in light. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP), at uncoupling concentrations for photosynthesis and dark respiration, strongly inhibited Cl^? uptake in light and darkness. N,N'-dicyclohexyl carbodiimide (DCCD), an energy transfer inhibitor, inhibited light Cl^? uptake more slowly than photosynthesis but had no effect on dark Cl^? uptake. It is concluded that Cl^? uptake in A. nidulans was active in light and darkness, and that ATP was the probable energy source for transport.     

Phospholipid, galactolipid and free sterol composition of fibrous roots from citrus genotypes differing in chloride exclusion ability

Abstract Fibrous roots of four citrus hybrids and parent rootstocks from which the hybrids were generated, all selected for their different Cl^? exclusion abilities, were assayed for phospholipid, galactolipid and free 4-desmethylsterol content. There was no correlation between a plant's ability to exclude Cl^? and the level of either phospholipid, galactolipid, or total free sterol in the roots of control plants. However, an inverse correlation was established between the ratio of phospholipid to free sterol in control roots and total leaf Cl^? levels of plants treated with 50 mol m^?3 NaCl for 56 d. With the exception of a significant decrease in hybrid 80-05-05, galactolipid levels were unaffected by salt treatment. Phospholipid levels were significantly increased in two parent rootstocks viz. Trifoliate orange (Poncirus trifoliata (L.). Raf.) and Carrizo citrange (Citrus sinensis (L.) Osbeck ×P. trifoliata) and one hybrid (80-02-08) but were otherwise unchanged by salt treatment. Free sterol levels were significantly increased by salt treatment in all of the better Cl^? excluders except Carrizo citrange i.e. in Rangpur lime (Citrus reticulata Blanco var. austera hybrid?), Cleopatra mandarin (Citrus reticulata Blanco) and all hybrids except 80–05–13. In all genotypes examined, salt-treatment resulted in a significant decrease in the ratio of sitosterol to stigmasterol reflecting, primarily, an increase in the stigmasterol level. The two poorer Cl^? excluders (Trifoliate orange and hybrid 80–05–13) both underwent a significant decrease in the ratio of ‘more planar’ to ‘less planar’ sterols. The inverse correlation between the phospholipid to free sterol ratio of control plants and leaf Cl^? level of salt treated plants suggests that this ratio has the potential to be used as a biochemical marker of Cl^? exclusion ability in citrus.     

Rapid Effects of Abscisic Acid on Ion Uptake in Sunflower Roots

Short-term effects of ABA, ABA + kinetin and kinetin on ion (⁸⁶Rb-potassium and phosphate) and water uptake in sunflower plants (Helianthus annuus var. californicus) were examined with a continuous-recording technique. Ion uptake in the roots and transport to the shoots were also investigated by conventional tracer uptake experiments and by sap bleeding experiments with excised roots. After addition of 5 × 10^?6-4 × 10^?5M ABA to the root medium there was an immediate decrease (30–70%) in the rate of ion uptake which lasted 30–70 min. The rate of water uptake was not significantly affected as measured with this method. Ion transport to the shoots and to the bleeding sap of excised roots was decreased by ABA. ABA-induced inhibition of ion uptake was abolished by the presence of kinetin, and uptake was slightly stimulated by 2 × 10^?5M kinetin alone. We suggest that concentration gradients of ABA or rapid changes in the ABA-kinetin balance in the roots affect ion uptake and transport.     

Salt-Induced Hypodermal Transfer Cells in Roots of Prosopis farcta and Ion Distribution within Young Plants

Prosopis farcta was grown on hydroculture with additions of 0.5, 10, 50, and 100 mM NaCl and without salt treatment. In plants from a 0.5 mM NaCl treatment, Cl^? was taken up into stems and leaves, but Na⁺ was withheld from the shoot. At 10 mM NaCl, shoot K⁺ concentration was below that of the control; Na⁺ and Cl^? were taken up to stems and cotyledons in nearly equimolar amounts. However, in the leaves, Na⁺ concentrations were only half of those of Cl^?. With increasing salt stress, Na⁺ and Cl^? were transported to the shoot, but kept at relatively low levels in the roots. Na⁺/ K⁺ ratios in roots did not increase proportionally to those in the solution. At an external Na⁺/K⁺ of > 5 and a root Na⁺/K⁺ of >1 (10 mM NaCl treatment), K⁺ selectivity was induced which rose exponentially with increasing salt stress; and cell wall protuberances were discovered in the hypodermis at the zone of side root formation. These transfer cells were found neither in roots from the 0.5 mM NaCl treatment nor in the controls. Their possible role in the Na⁺/K⁺ selectivity of the roots of Prosopis farcta is discussed.     

DGT-measured fluxes explain the chloride-enhanced cadmium uptake by plants at low but not at high Cd supply

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal uptake by plants in a wide range of soils. With the DGT technique, diffusion fluxes of trace metals through a diffusion layer towards a resin layer are measured. The DGT technique therefore mimics the metal uptake by plants if uptake is limited by diffusion of the free ion to the plant roots, which may not be the case at high metal supply. This study addresses the capability of DGT to predict cadmium (Cd) uptake by plants at varying Cd supply. To test the performance of DGT in such conditions, we used the chloride (Cl^?) enhancement effect, i.e. the increase in Cd solution concentrations—due to chloride complexation of Cd—and Cd uptake with increasing Cl^? concentrations, as previously characterized in pot, field and solution culture experiments. The uptake of Cd by spinach was assessed in soil amended with Cd (0.4–10.5 mg Cd kg^?1) and NaCl (up to 120 mM) in a factorial design. Treatments with NaNO₃ were included as a reference to correct for ionic strengths effects. The effect of Cl^? on the shoot Cd concentrations was significant at background Cd but diminished with increasing soil Cd. Increasing Cl^? concentrations increased the root area based Cd uptake fluxes by more than a factor of 5 at low soil Cd, but had no significant effect at high soil Cd. Short-term uptake of Cd in spinach from nutrient solutions confirmed these trends. In contrast, increasing Cl^? concentrations increased the DGT measured fluxes by a factor of 5 at all Cd levels. As a result, DGT fluxes were able to explain soil Cl^? effects on plant Cd concentrations at low but not at high Cd supply. This example illustrates under which conditions DGT mimics trace metal bioavailability. If biouptake is controlled by diffusive limitations, DGT should be a successful tool for predicting ion uptake across different conditions.     

Leaf gas exchange,water relations,nutrient content and growth in citrus and olive seedlings under salinity

The effects of salinity on growth, leaf nutrient content, water relations, gas exchange parameters and chlorophyll fluorescence were studied in six-month-old seedlings of citrus (Citrus limonia Osbeck) and rooted cuttings of olive (Olea europaea L. cv. Arbequina). Citrus and olive were grown in a greenhouse and watered with half strength Hoagland’s solution plus 0 or 50 mM NaCl for citrus, or plus 0 or 100 mM NaCl for olive. Salinity increased Cl⁻ and Na⁺ content in leaves and roots in both species and reduced total plant dry mass, net photosynthetic rate and stomatal conductance. Decreased growth and gas exchange was apparently due to a toxic effect of Cl⁻ and/or Na⁺ and not due to osmotic stress since both species were able to osmotically adjust to maintain pressure potential higher than in non-salinized leaves. Internal CO₂ concentration in the mesophyll was not reduced in either species. Salinity decreased leaf chlorophyll a content only in citrus.     

Chloride transport and compartmentation within main and lateral roots of two grapevine rootstocks differing in salt tolerance

Root Cl^? transport was investigated using ³⁶Cl^? flux analysis in two grapevine (Vitis sp.) rootstock hybrids differing in salt tolerance; 1103 Paulsen (salt-tolerant) and K 51–40 (salt sensitive). Initial ³⁶Cl^? influx to the root was greater in Paulsen than K 51–40. This flux, attributed to the Cl^? influx to the cytoplasm (Φ _oc) increased with increasing external concentrations of Cl^? for plants adapted to growth in 30 mM NaCl. The concentration kinetics in this high concentration range could be fit to a Michaeils–Menton equation. There was no significant difference between genotypes in Km (28.68 ± 15.76 and 24.27 ± 18.51 mM for Paulsen and K 51–40, respectively), but Paulsen had greater V _max (0.127 ± 0.042) compared to K 51–40 (0.059 ± 0.026 μm g^?1 FW min^?1). In Paulsen, the main root had greater contribution to ³⁶Cl^? uptake than lateral roots, there being no significant difference in lateral root influx between the genotypes. ³⁶Cl^? transport to the shoot of K 51–40 was greater than for Paulsen. It was estimated that efflux rate from the xylem parenchyma cells to the xylem vessels (Φ _cx) in K 51–40 was twice that of Paulsen. Compartmental analysis from ³⁶Cl^? efflux kinetics confirmed the larger Φ _oc and the higher ratio of main to lateral root Φ _oc for Paulsen. Efflux from the cytoplasm (Φ _co) was higher than 95 % of Φ _oc indicating a high degree of cycling across the plasma membrane in roots at these high external Cl^? concentrations. Paulsen appears to keep the cytoplasmic Cl^? concentration in roots lower than K 51–40 via greater efflux to the vacuole and to the outside medium. The difference in salt tolerance between the genotypes can be attributed to different Cl^? transport properties at the plasma membrane and tonoplast and particularly in Cl^? efflux to the xylem.     

The effect of N‐acetylcysteine on chloride efflux from airway epithelial cells

Defective chloride transport in epithelial cells increases mucus viscosity and leads to recurrent infections with high oxidative stress in patients with CF (cystic fibrosis). NAC (N‐acetylcysteine) is a well known mucolytic and antioxidant drug, and an indirect precursor of glutathione. Since GSNO (S‐nitrosoglutathione) previously has been shown to be able to promote Cl^? efflux from CF airway epithelial cells, it was investigated whether NAC also could stimulate Cl^? efflux from CF and non‐CF epithelial cells and through which mechanisms. CFBE (CF bronchial epithelial cells) and normal bronchial epithelial cells (16HBE) were treated with 1 mM, 5 mM, 10 mM or 15 mM NAC for 4 h at 37°C. The effect of NAC on Cl^? transport was measured by Cl^? efflux measurements and by X‐ray microanalysis. Cl^? efflux from CFBE cells was stimulated by NAC in a dose‐dependent manner, with 10 mM NAC causing a significant increase in Cl^? efflux with nearly 80% in CFBE cells. The intracellular Cl^? concentration in CFBE cells was significantly decreased up to 60% after 4 h treatment with 10 mM NAC. Moreover immunocytochemistry and Western blot experiments revealed expression of CFTR channel on CFBE cells after treatment with 10 mM NAC. The stimulation of Cl^? efflux by NAC in CF airway epithelial cells may improve hydration of the mucus and thereby be beneficial for CF patients.     

The absorption of metal ions by Eichhornia crassipes

Abstract

Uptake of metals by whole Eichhornia crassipes plants and excised roots was studied using a bioassay system. Results indicate that in time-series bioassay at concentration of 10 mg L^?1 of either Zn or Cu, rapid uptake by whole plants occurred during the first 4 h, but subsequently levelled off after 48 h. A greater proportion of metals absorbed remain in the root system rather than being translocated to other parts of plant. When treated with different combinations of both Cu and Zn, some synergistic effect of metals appeared to have occurred as indicated by enhanced uptake when bioassays involved combination of metals. Absorption kinetics of monovalent K⁺ by excised Eichhornia roots after a 24-h treatment, indicated an initial linear trend over the range of 1–10 mg K L^?1, but subsequently levelled off at 15 mg K L^?1 concentration with V_max of 7.5 × 10^–6 M K g^?1 dry tissue h^?1 and K_m of 1.1 × 10^?3 M K. Potentiometric titrations revealed existence of pH-dependent charge densities on root system which have variable affinities for cations and helps explain the ability of Eichhornia roots to absorb and accumulate large amounts of metal ions especially at high pH of media.     

Multiphasic uptake of potassium by barley roots of low and high potassium content: Separate sites for uptake and transitions

In the range 10^?6M - 5 × 10^?2M uptake of K⁺ in excised roots of barley (Hordeum vulgare L. cv. Herta) with low and high K content could in both cases be represented by an isotherm with four phases. Uptake, especially in the range of the lower phases, was reduced in high K roots through decreases in V_max and increases in K_m. Similar data for other plants are also shown to be consistent with multiphasic kinetics. The concentrations at which transitions occurred were not affected by the K status, indicating the existence of separate uptake and transition sites. Uptake was markedly reduced in the presence of 10^?5M 2,4-dinitrophenol, especially at low K⁺ concentrations, but the isotherms remained multiphasic. This contraindicates major contributions from a non-carrier-mediated, passive flux. A tentative hypothesis for multiphasic ion uptake envisions a structure which changes conformation as a result of all-or-none changes in a separate transition site. The structure is “tight” at low external ion concentrations (low V_max. low K_m. active uptake, allosteric regulation) and “loose” at high concentrations (high V_max- high K_m- facilitated diffusion, no regulation).     

NaCl effects on 4-desmethylsterol composition of plasma-membrane-enriched preparations from citrus roots

Abstract The free 4-desmethylsterol composition of plasma-membrane-enriched preparations from white fibrous roots of Rangpur lime (Citrus reticulata var. austera hybrid?), Kharna khatta (C. kharna Raf.) and Etrog citron (C. medica L.) seedlings grown in the presence of 0, 50, or 100 mol m^?3 NaCl for 28 d was quantitated by gas chromatography (GC) on analytical capillary (SE-54 fused silica) columns and the sterols were identified by combined gas chromatography-mass spectrometry (GC-MS). Only three 4-desmethylsterols were positively identified by GC-MS, viz. campesterol, stigmasterol and sitosterol. Cholesterol could not be positively identified in any of the membrane preparations. Campesterol levels were generally similar for all treatments and for all three genotypes, approximating 30% of the total free 4-desmethylsterol content of the plasma membranes. At all levels of salinity (0, 50 or 100 mol m^?3 NaCl) sitosterol levels decreased in the order Rangpur lime > Kharna khatta > Etrog citron and stigmasterol levels decreased in the reverse order. The ratio of sitosterol to stigmasterol was highest in Rangpur lime and lowest in Etrog citron at each level of salinity and was reduced by salt treatment in all three genotypes. Salt-induced reductions in the ratio of ‘more planar’ to ‘less planar’ sterols correlated inversely with the accumulation of Cl^? in the leaves of the three genotypes suggesting a role for plasma membrane sterols in the Cl^? exclusion mechanism. A model relating sterol structure, membrane sterol composition and membrane permeability to Cl^? exclusion ability in citrus is presented.     

A kinetic study of phosphorus absorption by excised maize roots from flowing solutions with different phosphorus concentrations

The effect of two different mechanisms of phosphorus ion transport from the nutrient solution volume to the surface areas of excised maize roots was studied under concentrations ranging from 0.01 mM to 50.0 mM KH₂PO₄. A modified technique of study of kinetic ion absorption was used. In the control series, the roots were placed in absorption solution without flow (the dominant mechanism of ion transport to the roots being diffusion), while in the experimental series the absorption solution was flowing round the roots at a rate of 0.162 cm s^?1 (the dominant mechanism of ion transport to the roots being mass flow). The rate of phosphorus absorption by the roots from flowing solutions was highly significantly increased at all concentrations of absorption solution except for the 50.0 mM KH₂PO₄ concentration. The increase in phosphorus absorption in the case of 50.0 mM KH₂PO₄ concentration was non-significant due to the fact that the high concentration of phosphorus together with the diffusion of phosphorus ions ensured a sufficient supply of phosphorus to the roots, covering the requirement for their uptake. The results point to the need for an analysis of environmental factors to be carried out in studying ion absorption kinetics, and reveal the inadequacy of methods usually employed in such investigations, in particular with respect to the homogeneity of the nutrient solution in the whole of its volume and especially round the roots.     

Xylem and cell turgor pressure probe measurements in intact roots of glycophytes: transpiration induces a change in the radial and cellular reflection coefficients

Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σ_r) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σ_r assumed values of the order of about 0·2–0·4. The σ_r values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m^?2 s^?1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σ_r= 1. Further increase of the light intensity to about 400 μmol m^?2 s^?1 resulted in a slight but significant decrease of the σ_r values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m^?2 s^?1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σ_r values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m^?2 s^?1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σ_r values of the respective glycophytes, the σ_c values of the root cortical cells of wheat and barley were close to unity, whereas σ_c for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m^?2 s^?1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σ_r values, the σ_c values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.     

Cl− transport in apical plasma membrane vesicles isolated from bovine tracheal epithelium

The Cl^? transport properties of the luminal border of bovine tracheal epithelium have been investigated using a highly purified preparation of apical plasma membrane vesicles. Transport of Cl^? into an intravesicular space was demonstrated by (1) a linear inverse correlation between Cl^? uptake and medium osmolarity and (2) complete release of accumulated Cl^? by treatment with detergent. The rate of Cl^? uptake was highly temperature-sensitive and was enhanced by exchange diffusion, providing evidence for a carrier-mediated transport mechanism. Transport of Cl^? was not affected by the ‘loop’ diuretic bumetanide or by the stilbene-derivative anion-exchange inhibitors SITS (4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid) and DIDS (4,4′-diisothiocyanostilbene-2,2′-disulfonic acid). In the presence of the impermeant cation, tetramethylammonium (TMA⁺), uptake of Cl^? was minimal; transport was stimulated equally by the substitution of either K⁺ or Na⁺ for TMA⁺. Valinomycin in the presence of K⁺ enhanced further Cl^? uptake, while amiloride reduced Na⁺-stimulated Cl^? uptake towards the minimal level observed with TMA⁺. These results suggest the following conclusions: (1) the tracheal vesicle membrane has a finite permeability to both Na⁺ and K⁺; (2) the membrane permeability to the medium counterion determines the rate of Cl^? uptake; (3) Cl^? transport is not specifically coupled with either Na⁺ or K⁺; and, finally (4) Cl^? crosses the tracheal luminal membrane via an electrogenic transport mechanism.     

Sodium and Chloride Sensitivity in Alfalfa (Medicago sativa L.): Growth,Photosynthesis, and Tissue Ion Regulation in Contrasting Genotypes

Alfalfa (Medicago sativa L.) is a moderately salt-tolerant plant. This study was conducted to evaluate responses of two contrasting alfalfa genotypes (OMA-84-salt sensitive and OMA-285-salt-tolerant) to components (Na⁺, and/or Cl^?) of salt stress. Alfalfa genotypes were exposed to Na⁺???salts (without chloride), Cl^????salts (without sodium), and NaCl (sodium chloride) stresses with two concentrations (30 and 150 mM). The treatments, involving macronutrients, with the same osmotic potentials, were taken as control. Salt stress, irrespective of type and intensity, caused a significant reduction in plant biomass, physiological (net photosynthetic rate, photosystem II efficiency, chlorophyll fluorescence, water use efficiency, maximum yield of primary photochemistry, and electron transport rate), and shoot mineral (calcium, magnesium, phosphorus, and potassium) contents compared to control; however, this reduction was in the order of NaCl (150 mM)?>?Na⁺ (150 mM)?>?Cl^? (150 mM). The alfalfa genotype OMA-285 sustained growth under both types of salt stresses than the genotype OMA-84 due to less accumulation of Na⁺ and Cl^? ions, maintenance of higher K⁺/Na⁺ ratio, and better photosynthetic activities. In conclusion, salt stress caused a significant reduction in alfalfa growth, this reduction was more under NaCl stress and the effect was mainly additive. The alfalfa genotype OMA-285 sustained growth under salt stresses than the genotype OMA-84 due to ionic homeostasis. However, the tested genotypes were more sensitive to Na⁺ toxicity than the Cl^? toxicity, and the contrasting genotypes differed in tissue tolerance of high Na⁺ and Cl^?. Further research is needed to evaluate tissue tolerance in a diverse and large group of alfalfa genotypes to elucidate the general salt tolerance mechanism in alfalfa.

     

CHLORIDE AND SODIUM UPTAKE POTENTIAL OVER AN ENTIRE ROTATION OF POPULUS IRRIGATED WITH LANDFILL LEACHATE

There is a need for information about the response of Populus genotypes to repeated application of high-salinity water and nutrient sources throughout an entire rotation. We have combined establishment biomass and uptake data with mid- and full-rotation growth data to project potential chloride (Cl^?) and sodium (Na⁺) uptake for 2- to 11-year-old Populus in the north central United States. Our objectives were to identify potential levels of uptake as the trees developed and stages of plantation development that are conducive to variable application rates of high-salinity irrigation. The projected cumulative uptake of Cl^? and Na⁺ during mid-rotation plantation development was stable 2 to 3 years after planting but increased steadily from year 3 to 6. Year six cumulative uptake ranged from 22 to 175 kg Cl^? ha^?1 and 8 to 74 kg Na⁺ ha^?1, while annual uptake ranged from 8 to 54 kg Cl^? ha^?1 yr^?1 and 3 to 23 kg Na⁺ ha^?1 yr^?1. Full-rotation uptake was greatest from 4 to 9 years (Cl^?) and 4 to 8 years (Na⁺), with maximum levels of Cl^? (32 kg ha^?1 yr^?1) and Na⁺ (13 kg ha^?1 yr^?1) occurring in year six. The relative uptake potential of Cl^? and Na⁺ at peak accumulation (year six) was 2.7 times greater than at the end of the rotation.     

Mechanism of sulfate uptake by excised maize roots

The mechanism of uptake of sulfate by excised maize roots (Zea mays L. cv. Ganga-5) was investigated. It was found that in the concentration range l × 10^?9 M ? 5 × 10^?2 M, the sulfate uptake could be represented by a single multiphasic isotherm having four phases. Each phase covered a limited concentration range and obeyed Michaelis-Menten kinetics, which indicates mediation in sulfate uptake by a structure which changes characteristics (kinetic constants) at certain discrete salts concentrations (transition points).     

γ-Aminobutyric acid-stimulated chloride permeability in crayfish muscle

γ-Aminobutyric acid selectively increased Cl^? permeability in isolated strips of crayfish abdominal muscle. Muscle fibers incubated in VAn Harreveld's solution at room temperature took up ³⁶Cl^? to the extent of 700 ml/kg wet weight with a halftime of 2.5 min. During 15-s incubations, the control ³⁶Cl^? uptake space was 131 ± 4 ml/kg (n = 60) and this was significantly increased by γ-aminobutyric acid at 200 μM or higher concentrations to 177 ± 4 ml/kg (n = 48, P < 0.05). This effect was specific for chloride since γ-aminobutyric acid did not increase the uptake by crayfish muscle of radioactive sucrose, inositol, or propionate. γ-Aminobutyric acid stimulation of ³⁶Cl^? uptake is mediated by receptor-ionophore function since the process shows pharmacological properties virtually identical to those observed by electrophysiological techniques. The γ-aminobutyric acid stimulation of Cl^? permeability is dose dependent with 50% of the maximal effect at 40 μM γ-aminobutyric acid and the dose vs. response curve is somewhat sigmoid. The γ-aminobutyric acid agonist muscimol causes the same maximal effect on Cl^? uptake as γ-aminobutyric acid, but acts at 5-fold lower concentrations, i.e. is more potent. However, the partial agonist γ-amino, β-hydroxybutyric acid produced little or no stimulation of ³⁶Cl^? flux. The response to γ-aminobutyric acid was blocked by 2 mM β-guanidinopropionate or γ-guanidinobutyrate, 0.5 mM bicuculline, and 10 μM picrotoxinin. Picrotoxinin inhibition was dose dependent with 50% inhibition occurring at 4 μM. Antagonists did not affect control ³⁶Cl^? uptake. These results confirm electrophysiological observations that the postsynaptic response to the inhibitory neurotransmitter γ-aminobutyric acid involves a rapid increase in membrane permeability to Cl^?