Effect of high external NaCl concentrations on ion transport within the shoot of Lupinus albus. I. Ions in xylem sap

Abstract. Xylem sap was collected from individual leaves of intact transpiring lupin plants exposed to increasing concentrations of NaCl by applying pneumatic pressure to the roots. Concentrations of Na⁺ and Cl⁻ in the xylem sap increased linearly with increases in the external NaCl concentration, averaging about 10% of the external concentration. Concentrations of K⁺ and NO₃⁻, the other major inorganic ions in the sap, were constant at about 2.5 and 1.5 mol m⁻³, respectively. There was no preferential direction of Na ⁺ or Cl⁻ to either young or old leaves: leaves of all ages received xylem sap having similar concentrations of Na⁺ and Cl⁻, and transpiration rates (per unit leaf area) were also similar for all leaves. Plants exposed to 120–160 mol m⁻³ NaCl rapidly developed injury of oldest leaves; when this occurred, the Na⁺ concentration in the leaflet midrib sap had increased to about 40 mol m⁻³ and the total solute concentration to 130 osmol m⁻³. This suggests that uptake of salts from the transpiration stream had fallen behind the rate of delivery to the leaf and that salts were building up in the apoplast.     

Na+, K+ and Cl- in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Munns, R. 1985. Na⁺, K⁺ and Cl^– in xylem sap flowing toshoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl^– and K⁺ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m^–3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 5–10 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na⁺]_x, was only 4.0 mol m^–3 at external [NaCl] of25–150 mol m^–3, and increased to 7.0 mol m^–3at 200 mol m^–3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25–150 mol m^–3 externalNaCl and indicates little change even at 200 mol m^-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na⁺ or Cl^–. The fluxes of Na⁺, Cl^– and K^– increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K⁺ increased even when [K⁺]_x wasgreater than external [K⁺], indicating that membrane transportprocesses modify the K⁺ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na⁺, K⁺, Cl^– fluxes, salinity, barley     

Turgor Pressure, Volumetric Elastic Modulus, Osmotic Volume and Ultrastructure of Chlorella emersonii Grown at High and Low External NaCl

Chlorella emersonii (211/11n) was grown at external NaCl concentrationsranging between 1.0 and 335 mM (0.08–1.64 MPa). Previousstudies showed that there was no significant change in the internalconcentrations of Na⁺ or Cl^– over this range, the concentrationsremaining below 35 mM. Relative growth rates of C. emersoniiwere 30–45% lower in 335 mM NaCl than in 1.0 mM NaCl.Turgor pressure varied with the osmotic pressure of the growthmedium. Plots of cell volume versus (external osmotic pressure)^–1indicated that cells grown in 1.0 mM NaCl (0.08 MPa) had turgorpressures ranging from 0.5 to 0.8 MPa, while cells in 335 mMNaCl (1.64 MPa) had turgor pressures of 0.0–0.14 MPa.Estimates of turgor pressure derived from the osmotic pressureof cell sap had a mean value of 0.6 MPa for cells in 1.0 mMNaCl, and 0.3 MPa for cells in 335 mM NaCl. The volumetric elasticmodulus () depended on the osmotic pressure of the growth medium: was 8.5 ± 1.7 MPa for cells grown in 1.0 mM NaCl, and0.9 ± 0.6 for cells in 335 mM NaCl. was measured bychanging turgor pressures over the range 0.0–0.5 MPa,and was found to be independent of turgor. Electron micrographsshowed that the walls of cells grown in 335 mM NaCl were 70%thicker than those grown in 1.0 mM NaCl. Other changes in cellularstructure were small, however, the area occupied by vacuolesincreased from 7% in cells grown in 1.0 mM NaCl to 14% in cellsin 335 mM. The percent osmotic volume of cells grown in 1.0–335mM NaCl (61 ± 17%, v/v) was similar to the percent watercontent (59 ± 13%, w/w). Key words: Chlorella emersonii, Sodium chloride, Osmotic volume, Turgor, Volumetric-elastic-modulus     

Effects of accumulation of 3-O-methylglucose on levels of endogenous osmotic solutes in Chlorella emersonii

Abstract. Regulation of the concentration of osmotic solutes was studied in Chlorella emersonii grown at external osmotic pressures (II) ranging between 0.08 and 1.64MPa. NaCl was used as osmoticum. The total solute content of the cells was manipulated by applying 2 mol m⁻³ 3- O -methylglucose (MG), which was not metabolized, and accumulated at concentrations ranging between 60 and 230 mol m⁻³ within 4 h after its addition to the medium. Methylglucose uptake resulted in decreases in concentrations of proline and sucrose, the two solutes mainly responsible for osmotic adaptation of C. emersonii to high external II. The responses were consistent with the hypothesis that proline and sucrose concentrations are controlled by a system of osmotic regulation, with turgor and/or volume as a primary signal. Short-term experiments showed that even very small increases in turgor and/or volume, due to accumulation of methylglucose, resulted in large decreases in proline and sucrose. Over the first 30-60 min the total solute concentration in the cells increased by at most 15 osmol m⁻³ which would represent an increase in turgor pressure of at most 0.04 M Pa. Yet, the decreases in proline and sucrose were as fast as those in cells exposed to a sudden decrease of 0.25 MPa in external II, when the turgor pressure would have increased by at least 0.15 MPa. High concentrations of methylglucose in cells grown at high II did not affect the rapid synthesis of proline and sucrose which started when the cells were transferred to yet higher II. Thus, methylglucose had no direct effects on proline and sucrose metabolism, and it has been assumed that it acted solely as an inert osmotic solute within the cell.     

Concentrations and Transport of Solutes in Xylem and Phloem along the Leaf Axis of NaCl-treated Hordeum vulgare

Hordeum vulgare cv. California Mariout was established in sandculture at two different NaCl concentrations (0.5 mol m^–3‘control’ and 100 mol m^–3) in the presenceof 6.5 mol m^–3 K ⁺. Between 16 and 31 d after germination,before stem elongation started, xylem sap was collected by useof a pressure chamber. Collections were made at three differentsites on leaves 1 and 3: at the base of the sheath, at the baseof the blade, i.e. above the ligule, and at the tip of the blade.Phloem sap was collected from leaf 3 at similar sites throughaphid stylets. The concentrations of K ⁺, Na⁺, Mg2⁺ and Ca²⁺were measured. Ion concentrations in xylem sap collected at the base of leaves1 and 3 were identical, indicating there was no preferentialdelivery of specific ions to older leaves. All ion concentrationsin the xylem decreased from the base of the leaf towards thetip; these gradients were remarkably steep for young leaves,indicating high rates of ion uptake from the xylem. The gradientsdecreased with leaf age, but did not disappear completely. In phloem sap, concentrations of K⁺ and total osmolality declinedslightly from the tip to the base of leaves of both controland salt-treated plants. By contrast, Na⁺ concentrations inphloem sap collected from salt-treated plants decreased drasticallyfrom 21 mol m^–3 at the tip to 7.5 mol m^–3 at thebase. Data of K/Na ratios in xylem and phloem sap were used to constructan empirical model of Na⁺ and K⁺ flows within xylem and phloemduring the life cycle of a leaf, indicating recirculation ofNa⁺ within the leaf. Key words: Hordeum vulgare, xylem transport, phloem transport, NaCl-stress     

Effects of accumulation of 3-O-methylglucose on growth and osmotic regulation in Chlorella emersonii

Abstract. The effect of accumulation of 3- O -methylglucose (MG) on growth and steady-stale concentrations of the endogenous osmotic solutes proline and sucrose was studied in Chlorella emersonii grown at external osmotic pressure (II) of 0.08-1.64 MPa. NaCL was used as osmoticum. The total solute content of the cells was manipulated by supplying 2 mol m⁻³ MG for 4 and 48 h. MG accumulated to 50–230 mol m⁻³ within 4h and was not metabolized. Uptake of MG resulted in decreases in concentrations of proline and sucrose, the two solutes mainly responsible for osmotic adaptation of C. emersonii to high II. After 4 or 48 h growth in the presence of MG, the decreases in concentration of proline and sucrose were as predicted from the contribution of MG to the total solute content of the cell.     

Leaf expansion in sunflower as influenced by salinity and short-term changes in carbon fixation

Abstract With a view to defining factors regulating the growth responses of sunflower to salinity, plants were grown in solution culture (0, 50 or 100 mol m⁻³ NaCl) and under natural light, and the areas of every leaf measured once or twice daily from 22 until 38 d after germination. During this period, carbon availability for growth was manipulated by changing light levels and by the use of a photosynthesis inhibitor, DCMU. Salinity reduced relative leaf expansion rates per plant (RLER) by an average of 0.04 (50 mol m⁻³) and 0.08 (100 mol m⁻³) m² m⁻² d⁻¹ compared with control plants of equivalent leaf area: the effects were found in expanding leaves regardless of age or size. Control plants expanded faster during the day than the night, but plants grown in salt had an almost constant RLER throughout the 24 h, indicating that salt influences the rate of utilization of assimilates independently of their production. DCMU reduced RLER considerably in both control and salt-treated plants and reduced the advantage of control plants during the day. Conditions of low light also reduced the differences in RLER between control and salt-treated plants. When salt was removed from the root medium of non-DCMU plants, the expansion rates equalled that of the controls within 24 h and remained at the same levels for the following 3 d measurement period: this recovery applied to leaves of all ages. Salt-grown plants with no photosynthesis (DCMU treatments) also increased their expansion rates upon removal of salt from the root medium, thus providing further evidence that growth was not limited by carbohydrate status, i.e. that salt influences growth primarily via its effects on the rate of utilization of stored assimilates.     

Effect of high external NaCl concentration on ion transport within the shoot of Lupinus albus. II. Ions in phloem sap

Abstract. Phloem sap was collected from petioles of growing and fully expanded leaves of lupins exposed to 0–150 mol m⁻³ [NaCl]_ext, for various periods of time. Sap bled from growing leaves only after the turgor of the shoot was raised by applying pneumatic pressure to the root. Increased pressure was also needed to obtain sap from fully expanded leaves of plants at high [NaCl]_ext. Exposure to NaCl caused a rapid rise in the Na⁺ concentration in phloem sap to high levels. The Na⁺ concentration reached 20 mol m⁻³ within a day of exposure and reached a plateau of about 60 mol m⁻³ in plants at 50–150 mol m⁻³ [NaCl]_ext, after a week. There was a slower, smaller increase in the Cl⁻ concentration. K⁺ concentrations in phloem sap were not affected by [NaCl]_ext. Cl⁻ concentrations in phloem sap collected from growing leaves were similar to those from old leaves while Na⁺ concentrations were somewhat increased, suggesting that there was no reduction in the salt content of the phloem sap while it flowed within the shoot to the apex. Calculations of ion fluxes in xylem and phloem sap indicated that Na⁺ and Cl⁻ fluxes in the phloem from leaves of plants at high NaCl could be equal to those in the xylem. This prediction was borne out by observations that Na⁺ and Cl⁻ concentrations in recently expanded leaves remained constant.     

The Role of the Stem in the Partitioning of Na+ and K+ in Salt-Treated Barley

Hordeum vulgare cv. California Mariout was grown for 50 d insand culture at 100 mol m^–3 NaCl. Xylem sap was collectedthrough incisions at the base of individual leaves along thestem axis by applying pressure to the root system. K⁺ concentrationsin the xylem sap reaching individual leaves increased towardsthe apex, while concentrations of Na⁺, NO₃^–, and Cl^–declined. Phloem exudate was obtained by collecting into Li₂EDTAfrom the base of excised leaves. K/Na ratios of phloem exudatesincreased from older to younger leaves. K/Na ratios in xylem sap and phloem exudate were combined withchanges in ion content between two harvests (38 and 45 d aftergermination) and the direction of phloem export from individualleaves, to construct an empirical model of K⁺ and Na⁺ net flowswithin the xylem and phloem of the whole plant. This model indicatesthat in old leaves, phloem export of K⁺ greatly exceeded xylemimport. In contrast, Na⁺ export was small compared to importand Na⁺ once imported was retained within the leaf. The direction of export strongly depended on leaf age. Old,basal leaves preferentially supplied the root, and most of theK⁺ retranslocated to the roots was transferred to the xylemand subsequently became available to the shoot. Upper leavesexported to the apex. Young organs were supplied by xylem andphloem, with the xylem preferentially delivering Na⁺ , and thephloem most of the K⁺ . For the young ear, which was still coveredby the sheath of the flag leaf, our calculation predicts phloemimport of ions to such an extent that the surplus must havebeen removed by an outward flow in the xylem. Within the culm,indications for specific transfers of K⁺ and Na⁺ between xylemand phloem and release or absorption of these ions by the tissuewere obtained. The sum of these processes in stem internodes and leaves ledto a non-uniform distribution of Na⁺ and K⁺ within the shoot,Na⁺ being retained in old leaves and basal stem internodes,and K⁺ being available for growth and expansion of young tissues. Key words: Hordeum vulgare L., K⁺, Na⁺, stem, salt stress     

Physiological processes limiting plant growth in saline soils: some dogmas and hypotheses

Recent progress in improving the salt tolerance of cultivated plants has been slow. Physiologists have been unable to define single genes or even specific metabolic processes that molecular biologists could target, or pinpoint the part of the plant in which such genes for salt tolerance might be expressed. While the physiological might be expressed. While the physiological processes are undoubtedly complex, faster progress on unraveling mechanisms of salt tolerance might be made if there were more effort to test hypotheses rather than to accumulate data, and to integrate cellular and whole plant responses. This article argues that salts taken up by the plant do not directly control plant growth by affecting turgor, photosynthesis or the activity of any one enzyme. Rather, the build-up of salt in old leaves hasten their death, and the loss of these leaves affects the supply of assimilates or hormones to the growing regions and thereby affects growth.