Salt accumulation and depletion in the root-zone of the halophyte Atriplex nummularia Lindl.: influence of salinity,leaf area and plant water use

Background and aims

Salt is known to accumulate in the root-zone of Na⁺ excluding glycophytes under saline conditions. We examined the effect of soil salinity on Na⁺ and Cl^? depletion or accumulation in the root-zone of the halophyte (Atriplex nummularia Lindl).

Methods

A pot experiment was conducted in soil to examine Na⁺ and Cl^? concentrations adjacent to roots at four initial NaCl treatments (20, 50, 200 or 400 mM NaCl in the soil solution). Plant water use was manipulated by leaving plants with all leaves intact, removing approximately 50 % of leaves, or removing all leaves. Daily evapotranspiration was replaced by watering undrained pots to weight with deionised water. After 35-38 days, samples were taken of the bulk soil and of soil loosely- and closely-adhering to the roots.

Results

In plants with leaves intact grown with 200 and 400 mM NaCl, average Na⁺ and Cl^? concentrations in the closely adhering soil were about twice the concentrations of the bulk soil. Ion accumulation increased with final leaf area and with cumulative transpiration over the duration of the trial. By contrast, in plants grown with the lowest salinity treatment (20 mM NaCl), Na⁺ and Cl^? concentrations decreased in the closely adhering soil with increasing leaf area and increasing cumulative water use.

Conclusions

Our data show that Na⁺ and Cl^? are depleted from the root-zone of A. nummularia at low salinity but accumulate in the root-zone at moderate to high salinity, and that the ions are drawn towards the plant in the transpiration stream.     

Analysis of salts transport affected by root absorption capacity in surface — irrigated fields in the upper Yellow River basin

In order to analyze the salt transport affected by roots and its effects on soil salinity in an experimental irrigated field newly established in an alluvial valley of the Yellow River in China, spatial distribution of ions contained in waters, soils and crops relevant to these phenomena were evaluated there. During the intensive surveys conducted in year 2007–2008, the Yellow River water, irrigation canal water, groundwater, field soils and crops, etc. were sampled and their chemical characteristics such as electrical conductivity, concentrations of ions Na⁺, Ca²⁺, Mg²⁺, K⁺, Cl⁻, SO₄²⁻and NO₃⁻ were measured. Irrigation seemed to cause increases in the concentrations of ions Na⁺, Cl⁻ and SO₄²⁻ in the groundwater. Although those were also major ions contained in the field soil, the soil was classed as saline but not sodic according to the standard classification. On the other hand, K⁺, which is one of the major essential nutrients for plant growth, was highly concentrated in the crops, while Na⁺ was not concentrated because of crop’s poor ability to absorb it. The ion concentration within the plant body seemed to be reflected by the active and selective ion uptake by roots and the transpiration stream. Furthermore, salt accumulation in the surface-irrigated field largely depended on the upward transport of water and ions in the soil profile affected by root absorption capacity. The information obtained in this study will contribute to the development of scientific methods for sustainable and effective plant production in irrigated fields.     

Soil solute concentration and water uptake by single lupin and radish plant roots

Application of computer assisted tomography to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes were used to determine the spatial distributions of soil water content and Na⁺ concentrations respectively near single roots of eighteen day old lupin and radish plants. These quantities were monitored at root depths of 3, 6 and 9 cm and at zero, 2, 4, 6, and 8 hour intervals from the diurnal commencement of transpiration. The plants were subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels. Water extraction rates for the lupin and radish roots increased continuously with time but were substantially reduced with increasing Na⁺ concentration in the treatment. Water uptake was uniform along the length of the essentially constant diameter lupin roots but decreased along the tapering radish roots as the diameter and hence the surface area per unit length of the roots decreased. The accumulation of Na⁺ at the root surfaces of both plants increased gradually with time in a near linear fashion and was slightly higher under the higher transpiration demand. These increases were not exponential as would be expected with non-absorption by the roots and this is considered to be due to back diffusion at the relatively high water contents used. At these water contents matric potentials had a much smaller influence on transpiration than osmotic potentials. The relationships between leaf water potentials (Ψ₁) and osmotic potentials at the root surfaces were linear with the decreases in Ψ₁ almost exactly reflecting the decreases in Ψ_π indicating rapid plant adjustment. Leaf water potentials decreased progressively with time and the relationships between leaf water potential and the transpiration rate were also linear supporting the suggestion of constant plant resistances at any given concentration.     

Hydrogen peroxide as a mediator of 5‐aminolevulinic acid‐induced Na+ retention in roots for improving salt tolerance of strawberries

To explore the mechanisms of 5‐aminolevulinic acid (ALA)‐improved plant salt tolerance, strawberries (Fragaria × ananassa Duch. cv. ‘Benihoppe’) were treated with 10 mg l^?1 ALA under 100 mmol l^?1 NaCl stress. We found that the amount of Na⁺ increased in the roots but decreased in the leaves. Laser scanning confocal microscopy (LSCM) observations showed that ALA‐induced roots had more Na⁺ accumulation than NaCl alone. Measurement of the xylem sap revealed that ALA repressed Na⁺ concentrations to a large extent. The electron microprobe X‐ray assay also confirmed ALA‐induced Na⁺ retention in roots. qRT‐PCR showed that ALA upregulated the gene expressions of SOS1 (encoding a plasma membrane Na⁺/H⁺ antiporter), NHX1 (encoding a vacuolar Na⁺/H⁺ antiporter) and HKT1 (encoding a protein of high‐affinity K⁺ uptake), which are associated with Na⁺ exclusion in the roots, Na⁺ sequestration in vacuoles and Na⁺ unloading from the xylem vessels to the parenchyma cells, respectively. Furthermore, we found that ALA treatment reduced the H₂O₂ content in the leaves but increased it in the roots. The exogenous H₂O₂ promoted plant growth, increased root Na⁺ retention and stimulated the gene expressions of NHX1, SOS1 and HKT1. Diphenyleneiodonium (DPI), an inhibitor of H₂O₂ generation, suppressed the effects of ALA or H₂O₂ on Na⁺ retention, gene expressions and salt tolerance. Therefore, we propose that ALA induces H₂O₂ accumulation in roots, which mediates Na⁺ transporter gene expression and more Na⁺ retention in roots, thereby improving plant salt tolerance.     

Ionic and pH signalling from roots to shoots of flooded tomato plants in relation to stomatal closure

Soil flooding damages shoot systems by inhibiting root functioning. An example is the inhibition of water uptake brought about by decreased root hydraulic conductance. The extent of any resulting foliar dehydration this causes is limited by partial stomatal closure that begins within 4 h and is maintained for several days. Root to shoot signals that promote closure in flooded tomato plants have remained elusive but may include changes in solute delivery to the shoot by transpiration. Accordingly, we examined total osmolites and selected mineral ions in samples of xylem sap flowing at rates approximating whole plant transpiration. After 2.5 h flooding,delivery of total osmolites and of PO₄ ^3-SO₄ ^2-Ca²⁺K⁺NO₃ ^– and H⁺strongly decreased while Na⁺ remained excluded. Several hours later, deliveries of osmolites, PO₄ ^3-, SO₄ ^2-, Ca²⁺, and Na⁺ rose above control values, suggesting that, after approximately 10 h, root integrity became degraded and solute uptake de-regulated. Deliveries of NO₃ ^– remained below control values. Reducing or eliminating the supply of K⁺ to detached leaves to test the potential of decreased K⁺ delivery to close stomata proved negative. Decrease in H⁺ delivery was associated with sap alkalisation. However, raising the pH of buffer from 6.0 or 6.5 to 7.0 did not close stomata when tested in the presence of abscisic acid (ABA) at a concentration (10 mol m^–3) typical of the transpiration stream of flooded plants. It is concluded that despite their rapidity and scale, negative messages in the form of increased pH and decreased solute delivery from roots to shoots are, themselves, unlikely initiators of stomatal closure in flooded tomato plants.     

Genotypic difference in salinity tolerance in quinoa is determined by differential control of xylem Na loading and stomatal density

Quinoa is regarded as a highly salt tolerant halophyte crop, of great potential for cultivation on saline areas around the world. Fourteen quinoa genotypes of different geographical origin, differing in salinity tolerance, were grown under greenhouse conditions. Salinity treatment started on 10 day old seedlings. Six weeks after the treatment commenced, leaf sap Na and K content and osmolality, stomatal density, chlorophyll fluorescence characteristics, and xylem sap Na and K composition were measured. Responses to salinity differed greatly among the varieties. All cultivars had substantially increased K⁺ concentrations in the leaf sap, but the most tolerant cultivars had lower xylem Na⁺ content at the time of sampling. Most tolerant cultivars had lowest leaf sap osmolality. All varieties reduced stomata density when grown under saline conditions. All varieties clustered into two groups (includers and excluders) depending on their strategy of handling Na⁺ under saline conditions. Under control (non-saline) conditions, a strong positive correlation was observed between salinity tolerance and plants ability to accumulate Na⁺ in the shoot. Increased leaf sap K⁺, controlled Na⁺ loading to the xylem, and reduced stomata density are important physiological traits contributing to genotypic differences in salinity tolerance in quinoa, a halophyte species from Chenopodium family.     

Organic acids are not specifically involved in the nitrate-enhanced Zn hyperaccumulation mechanism in Noccaea caerulescens

Nitrogen form has been shown to affect Zn uptake, translocation and storage in the Zn-hyperaccumulating plant Noccaea caerulescens but the biochemical processes are not fully understood. Organic acids and amino acids have been implicated in Zn transport and storage. This study aimed to examine the effect of N form on concentrations of organic acids and amino acids and how these metabolites correlated with Zn hyperaccumulation. Plants were grown in nutrient solution with NO₃⁻, NH₄NO₃ or NH₄⁺, supplied with 50 or 300 μM Zn, and buffered at either pH 4.5 or 6.5. The metabolomic profile was determined by gas chromatography mass spectroscopy. The concentration of Zn in shoots, xylem and roots was greatest for the NO₃⁻, pH 6.5 and 300 μM Zn treatments. For all N forms, the lower growth-medium pH raised xylem sap pH but had no influence on Zn concentration or exudation rate of the xylem sap. Nitrate enhanced organic acid production while NH₄⁺ increased amino acid production. Organic acids in the xylem were more responsive to changes in growth-medium pH than N form, and did not correlate with Zn concentration in shoots, roots or xylem. Serine might be directly involved in Zn hyperaccumulation. Phosphoric acid was associated with reduced Zn accumulation in the shoots. Malic acid was not detected in the shoots but responded to cation uptake more than to Zn specifically in the roots. Citric acid responded to cation uptake more than to Zn specifically in the shoots but did not correlate with Zn concentration in the roots or the xylem sap, or any other cations in the roots. In conclusion, organic acids in N. caerulescens are not specifically involved in Zn hyperaccumulation but are involved in regulating pH in the xylem and cation–anion balance in plants.     

Relative importance of osmotic-stress and ion-specific effects on ABA-mediated inhibition of leaf expansion growth in Phaseolus vulgaris

The osmotic and ion-specific components of salt-induced inhibition of leaf expansion growth were investigated in beans grown from 12 h to several days in either NaCl-containing solution cultures, an isosmotic concentrated macronutrient solution, or a vermiculite–compost mixture with low Na⁺ but high Cl^– availability. Inhibition of leaf expansion and leaf ABA increase was more intense in the NaCl than in the isosmotic macronutrient treatment. Root Na⁺ was highly correlated to inhibition of leaf expansion and leaf or xylem sap ABA. When Na⁺ was sequestered in soil, salinized plants showed no reduction in leaf expansion or ABA increase, regardless of the presence of high leaf Cl^– concentrations. Stomatal conductance exhibited an exponential relationship with the reciprocal value of xylem sap ABA. Our results indicate that an ion-specific effect caused by Na⁺ in roots may account for an ABA-mediated reponse of both stomatal closure and leaf expansion inhibition.     

Involvement of Long-Distance Na<Superscript>+</Superscript> Transport in Maintaining Water Potential Gradient in the Medium-Root-Leaf System of a Halophyte <Emphasis Type="Italic">Suaeda altissima</Emphasis>

The aim of this study was to determine the range of NaCl concentrations in the nutrient solution that allow Suaeda altissima (L.) Pall., a salt-accumulating halophyte, to maintain the upward gradient of water potential in the “medium-root-leaf” system. We evaluated the contribution of Na⁺ ions in the formation of water potential gradient and demonstrated that Na⁺ loading into the xylem is involved in this process. Plants were grown in water culture at NaCl concentrations ranging from zero to 1 M. The water potential of leaf and root cells was measured with the method of isopiestic thermocouple psychrometry. When NaCl concentration in the growth medium was raised in the range of 0–500 mM (the medium water potential was lowered accordingly), the root and leaf cells of S. altissima decreased their water potential, thus promoting the maintenance of the upward water potential gradient in the medium-root-leaf system. Growing S. altissima at NaCl concentrations f 750 mM and 1 M disordered water homeostasis and abolished the upward gradient of water potential between roots and leaves. At NaCl concentrations of 0–250 mM, the detached roots of S. altissima were capable of producing the xylem exudate. The concentration of Na⁺ in the exudate was 1.3 to 1.6 times higher than in the nutrient medium; the exudate pH was acidic and was lowered from 5.5 to 4.5 with the rise in the salt concentration. The results indicate that the long-distance Na⁺ transport and, especially, the mechanism of Na⁺ loading into the xylem play a substantial role in the formation of water potential gradient in S. altissima. The accumulation of Na⁺ in the xylem and acidic pH values of the xylem sap suggest that Na⁺ loading into the xylem is carried out by the Na⁺/H⁺ antiporter of the plasma membrane in parenchymal cells of the root stele.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 549–557.Original Russian Text Copyright © 2005 by Balnokin, Kotov, Myasoedov, Khailova, Kurkova, Lun’kov, Kotova.     

Brief pre- and post-irrigation sprinkling with fresh water reduces foliar salt uptake in maize and barley sprinkler irrigated with saline water

Brief pre- and post-irrigation sprinkling treatments using freshwater were tested to determine if these practices could reduce the uptake of salts through leaves when saline water is used to sprinkler irrigate crops. Maize and barley were sprinkler irrigated 2 to 3 times per week for 30 min with saline water (4.2 dS m^–1, 30 mmol L^–1 NaCl and 2.8 mmoles L^–1 CaCl₂ for maize and 9.6 dS m^–1, 47 mmoles L^–1 NaCl and 23.5 mmoles L^–1 CaCl₂ for barley) in separate experiments with plants grown in pots outdoors. The soil surface of all pots was covered to prevent salinization of the soil by the sprinkling water. One half of the sprinkled plants was grown in nonsaline soil to study the effects of pre-wetting and post-washing when ion uptake was primarily through leaves. The other half of the sprinkled plants was grown in soil salinized by drip irrigation, in order to evaluate the effects of pre-wetting and post-washing when Na⁺ and Cl^- uptake was through both leaves and roots.Post-washing with freshwater (5 min) reduced the leaf sap concentrations of Cl^- in saline-sprinkled plants from 56 to 43 mmol L^–1 in maize and from 358 to 225 mmol L^–1 in barley (averages for plants grown in nonsaline and saline soil). Na⁺ concentrations in leaf sap were reduced from 93 to 65 mmoles L^–1 (maize) and from 177 to 97 mmoles L^–1 (barley) by the post-washing. Pre-wetting had a small effect on ion uptake through leaves, the only significant reduction in seasonal means being in leaf Na⁺ concentrations for plants grown in nonsaline soil. Pre-wetting and post-washing, when combined, reduced leaf Cl^- concentrations to levels similar to those of nonsprinkled plants grown in saline soil; however, Na⁺ concentrations in leaves remained 3.5 times (maize) and 1.5 times (barley) higher than those of nonsprinkled plants. When pre-wetting and post-washing were not applied, sprinkled barley plants grown in saline soil had grain yields which were 58% lower than nonsprinkled plants grown in saline soil, but the reduction in grain yield was only 17% when the freshwater treatments were given. We conclude that a brief period of post-washing with freshwater is essential when saline water is employed in sprinkler irrigation. By comparison, the benefits from pre-wetting were small in these experiments. ei]T J Flowers     

Na+ and K+ transport in excised soybean roots

Uptake, accumulation and xylem transport of K⁺ and Na⁺ in excised roots of soybean were investigated by use of a perfusion technique. This technique permitted independent quantification of, on the one hand, entry of ions into the roots and their transport through the cortex to the xylem vessels, and on the other hand reabsorption from the xylem vessels to the neighbouring cells and the external medium. Data are consistent with a low degree of selective uptake of K⁺ over Na⁺. However, Na⁺ depletion of the xylem stream by reabsorption limits, although weakly, its translocation to the shoots. Na⁺ reabsorbed is for a great part reexcreted into the external medium. The low efficiency of these processes is discussed in relation to the Na⁺ sensitivity of soybean.     

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     

An assessment of the relative effects of adverse physical and chemical properties of sodic soil on the growth and yield of wheat (Triticum aestivum L.)

Two wheat varieties were grown in artificially created sodic soils in pots at a range of sodicity levels (exchangeable sodium percentage (ESP) 15–52), with and without an anionic polyacrylamide soil conditioner (PAM) to stabilise soil aggregates. Increasing sodicity decreased the % water stable aggregates (% WSA) in soil and survival, grain and straw yield of wheat. Plants grown at high sodicity also had higher Na⁺, lower K⁺ and Ca²⁺ concentrations and lower K⁺/Na⁺ ratio in flag leaf sap than plants grown in control (non-sodic) soil. Sodicity had no effect on the concentrations of Cu²⁺, Fe²⁺, Mn²⁺ and Zn²⁺ in grains and straw, but total uptake of these micronutrients was deceased due to lower dry weight of these tissues per plant. At all sodicity levels treatment of sodic soil with PAM increased the % WSA to values greater than in the non-sodic control soil, and slightly lowered ESP. Over the range ESP 15–44 the effects of PAM on wheat grain yield increased as sodicity increased, so that at ESP 44 grain yield in the treatment with PAM was only 25% lower than in the non-sodic control. However at ESP 52 the effects of PAM were smaller, and grain yield was 86% lower than in the control. At this sodicity level the decreases in grain yield due to sodicity and the increases in reponse to treatment of sodic soil with PAM were similar in the two varieties tested. At high sodicity levels (ESP 44 and 52) treatment of sodic soil with PAM decreased the concentration of Na⁺ and increased K⁺ and K⁺/Na⁺ ratio in flag leaf sap. However, at the highest sodicity level (ESP 52), flag leaf Na⁺ concentration remained above the level (100 mol m^-3) at which it has been found to be toxic. Concentrations of Cu²⁺, Fe²⁺, Mn²⁺ and Zn²⁺ in grain and straw were unaffected by PAM. These results suggest that at ESP up to 40–50 adverse physical characteristics are the major cause of low wheat yield in sodic soils, either due to their direct effects in decreasing growth, or their indirect effects in increasing uptake of Na⁺ and decreasing uptake of K⁺. Above ESP 50, roots are less able to exclude Na⁺, even in the presence of improved soil physical conditions, so that at these sodicity levels, both adverse physical and adverse chemical properties contribute to the decreased yield. This revised version was published online in June 2006 with corrections to the Cover Date.     

NaCl胁迫对圆柏幼苗生长和离子吸收及分配的影响

以当年生圆柏幼苗为实验材料,采用温室调控盆栽土培法研究了不同浓度NaCl(0、100、200、300mmol·L-1)胁迫21d对其生长情况及不同器官(根、茎、叶)中K~+、Na~+、Ca~(2+)和Mg~(2+)的吸收和分配的影响,以探讨圆柏幼苗对盐环境的生长适应性及耐盐机制。结果表明:(1)随着NaCl胁迫浓度的增加,圆柏幼苗生长,包括株高、地径、相对生长量以及生物量的积累均呈下降趋势,而其根冠比却增加。(2)在各浓度NaCl胁迫处理下,圆柏幼苗根、茎、叶中Na~+含量较对照均显著增加,而且叶中Na~+含量显著高于茎和根,叶中Na~+含量是根中的5倍。(3)随着NaCl胁迫浓度的升高,圆柏幼苗各器官中K~+、Ca~(2+)和Mg~(2+)含量以及K~+/Na~+、Ca~(2+)/Na~+及Mg~(2+)/Na~+比值均呈下降趋势。(4)在NaCl胁迫条件下,圆柏幼苗根系离子吸收选择性系数SK,Na、SCa,Na、SMg,Na显著提高,茎、叶离子转运选择性系数SCa,Na、SMg,Na则逐渐降低,叶中离子转运选择性系数SK,Na则随着NaCl胁迫浓度的升高显著降低,大量Na~+进入地上部,减缓了盐胁迫对根系的伤害。研究认为,圆柏幼苗的盐适应机制主要是通过根系的补偿生长效应及茎、叶对Na~+的聚积作用来实现的,同时也与根对K~+、Ca~(2+)、Mg~(2+)的选择性运输能力增强和茎、叶稳定的K~+、Ca~(2+)、Mg~(2+)的选择性运输能力有关。     

Growth,ion composition,and stomatal conductance of peas exposed to salinity

Availability of irrigation water of appropriate quality is becoming critical in many regions. Excess salt in irrigation water represents a risk for crop yield, crop quality, and soil properties. During the short vegetation period, field peas require high amounts of water, and irrigation is often indispensable for successful production. Steady presence of NaCl (0.1, 0.2, 0.6 or 1.2 g NaCl L⁻¹ in 1/2 strength Hoagland nutrient solution) under semi-controlled conditions reduced growth and resulted in shorter vegetation. Disturbances in the peas’ water regime were provoked by NaCl, as water content in pea tissues was reduced and stomatal density and stomatal diffusive resistance increased in the presence of higher NaCl concentrations. Concentration of Na⁺ increased in all pea tissues with increased NaCl concentration in the nutrient medium. In the presence of NaCl, concentrations of K⁺, Ca²⁺ and P_i increased in roots, stems and leaves, and decreased and in pods and grains. Concentration ratios Na⁺/K⁺, Na⁺/Ca²⁺, K⁺/Ca²⁺ and (Na⁺+K⁺)/Ca²⁺ in various plant parts were affected as well, but magnitudes of changes were variable. Continuous presence of NaCl in concentrations frequently met in irrigation waters significantly reduced pea growth, impaired the water regime, and altered plant chemical composition.     

Seasonal variation of water uptake of a Quercus suber tree in Central Portugal

Hydraulic redistribution (HR) is the phenomenon where plant roots transfer water between soil horizons of different water potential. When dry soil is a stronger sink for water loss from the plant than transpiration, water absorbed by roots in wetter soil horizons is transferred toward, and exuded into dry soil via flow reversals through the roots. Reverse flow is a good marker of HR and can serve as a useful tool to study it over the long-term. Seasonal variation of water uptake of a Quercus suber tree was studied from late winter through autumn 2003 at Rio Frio near Lisbon, Portugal. Sap flow was measured in five small shallow roots (diameter of 3–4 cm), 1 to 2 m from the tree trunk and in four azimuths and at different xylem depths at the trunk base, using the heat field deformation method (HFD). The pattern of sap flow differed among lateral roots as soil dried with constant positive flow in three roots and reverse flow in two other roots during the night when transpiration ceased. Rain modified the pattern of flow in these two roots by eliminating reverse flow and substantially increasing water uptake for transpiration during the day. The increase in water uptake in three other roots following rain was not so substantial. In addition, the flux in individual roots was correlated to different degrees with the flux at different radial depths and azimuthal directions in trunk xylem. The flow in outer trunk xylem seemed to be mostly consistent with water movement from surface soil horizons, whereas deep roots seemed to supply water to the whole cross-section of sapwood. When water flow substantially decreased in shallow lateral roots and the outer stem xylem during drought, water flow in the inner sapwood was maintained, presumably due to its direct connection to deep roots. Results also suggest the importance of the sap flow sensor placement, in relation to sinker roots, as to whether lateral roots might be found to exhibit reverse flow during drought. This study is consistent with the dimorphic rooting habit of Quercus suber trees in which deep roots access groundwater to supply superficial roots and the whole tree, when shallow soil layers were dry.     

The SIDECAR POLLEN gene encodes a microspore-specific LOB/AS2 domain protein required for the correct timing and orientation of asymmetric cell division

Control of ion loading into the xylem has been repeatedly named as a crucial factor determining plant salt tolerance. In this study we further investigate this issue by applying a range of biophysical [the microelectrode ion flux measurement (MIFE) technique for non‐invasive ion flux measurements, the patch clamp technique, membrane potential measurements] and physiological (xylem sap and tissue nutrient analysis, photosynthetic characteristics, stomatal conductance) techniques to barley varieties contrasting in their salt tolerance. We report that restricting Na⁺ loading into the xylem is not essential for conferring salinity tolerance in barley, with tolerant varieties showing xylem Na⁺ concentrations at least as high as those of sensitive ones. At the same time, tolerant genotypes are capable of maintaining higher xylem K⁺/Na⁺ ratios and efficiently sequester the accumulated Na⁺ in leaves. The former is achieved by more efficient loading of K⁺ into the xylem. We argue that the observed increases in xylem K⁺ and Na⁺ concentrations in tolerant genotypes are required for efficient osmotic adjustment, needed to support leaf expansion growth. We also provide evidence that K⁺‐permeable voltage‐sensitive channels are involved in xylem loading and operate in a feedback manner to maintain a constant K⁺/Na⁺ ratio in the xylem sap.     

Evaluation of water stress control with polyethylene glycols by analysis of guttation

The water relations of pepper plants (Capsicum frutescens L.) under conditions conducive to guttation were studied to evaluate the control of plant water stress with polyethylene glycols. The addition of polyethylene glycol 6000 to the nutrient solution resulted in water relations similar to those expected in soil at the same water potentials. Specifically, xylem pressure potential in the root and leaf became more negative during a 24-hour treatment period, while osmotic potential of the root xylem sap remained constant. The decrease in pressure potential was closely correlated with the decrease in osmotic potential of the nutrient solution. In contrast, the addition of polyethylene glycol 400 to the nutrient medium resulted in a reduction of osmotic potential in the root xylem sap; this osmotic adjustment in the xylem was large enough to establish an osmotic gradient for entry of water and cause guttation at a nutrient solution osmotic potential of −4.8 bars. Pressure potential in the root and leaf xylem became negative only at nutrient solution osmotic potentials lower than −4.8 bars. About half of the xylem osmotic adjustment in the presence of polyethylene glycol 400 was caused by increased accumulation of K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the root xylem. These studies indicate that larger polyethylene glycol molecules such as polyethylene glycol 6000 are more useful for simulating soil water stress than smaller molecules such as polyethylene glycol 400.     

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h⁻¹) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.     

Naphthenic acids affect plant water conductance but do not alter shoot Na+ and Cl− concentrations in jack pine (Pinus banksiana) seedlings

Solution culture-grown, six-month old jack pine (Pinus banksiana Lamb.) seedlings were treated with naphthenic acids (NAs) (150 mg l^–1) and sodium chloride (45 mM NaCl) which were applied together or separately to roots for four weeks. NAs aggravated the effects of NaCl in inhibiting stomatal conductance (g _s) and root hydraulic conductance (K_r). Naphthenic acids did not affect needle and root electrolyte leakage in the absence of NaCl. However, in plants treated with NaCl, NAs further increased electrolyte leakage from needles and NaCl induced electrolyte leakage from needles, but not from roots. Both NaCl and NAs treatments resulted in a reduction in root respiration. The measured Na⁺ and Cl^– concentrations in the shoots for combined NaCl + NAs treatments were lower than in NaCl-only treatments. These decreases were correlated with a reduction in water conductance. The accumulation of Na⁺ and Cl^– in shoots was accompanied by an increased in needle electrolyte leakage. However, greater concentrations of Cl^– compared with Na⁺ were present in shoots and in the xylem sap suggesting that roots had relatively lower capacity for Cl^– storage compared with Na⁺.