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.     

Importance of native arbuscular mycorrhizal inoculation in the halophyte Asteriscus maritimus for successful establishment and growth under saline conditions

Background and aims

The biological restoration of saline habitats could be achieved by using halophyte plant species together with adapted arbuscular mycorrhizal fungi (AMF). An interesting plant to be used in restoration of saline environments, Asteriscus maritimus, is highly mycotrophic. The aim of this study was to assess the effectiveness of native and allochthonous AMF to enhance the establishment and growth of the halophyte A. maritimus under saline conditions.

Methods

We studied the symbiotic effectiveness of four AMF strains (three native fungal isolates from a saline soil and one allochthonous, from collection) in A. maritimus subjected to increasing salinity stress. We measured plant physiological parameters by which AMF may ameliorate the detrimental effects of salinity stress.

Results

A. maritimus plants showed a high mycorrhizal dependency, even in absence of salt stress. Plants inoculated with native AMF had higher shoot dry weight, efficiency of photosystem II, stomatal conductance and accumulation of glutathione than those inoculated with the collection AMF at the highest level of salinity. Moreover, at this salt level, only 30 % of A. maritimus plants inoculated with the collection AMF survived, while with the three native AMF, the rate of survival was 100 %.

Conclusions

Results points out the importance of native AMF inoculation in the establishment, survival and growth of A. maritimus plants. Inoculation with these native AMF enhanced A. maritimus salt tolerance by increasing efficiency of photosystem II, stomatal conductance and glutathione content and by reducing oxidative damage. Thus, the use of adequate native AMF inocula could be a critical issue for success in recovering saline degraded areas.     

Response to non-uniform salinity in the root zone of the halophyte Atriplex nummularia: growth, photosynthesis, water relations and tissue ion concentrations

Background and Aims

Soil salinity is often heterogeneous, yet the physiology of halophytes has typically been studied with uniform salinity treatments. An evaluation was made of the growth, net photosynthesis, water use, water relations and tissue ions in the halophytic shrub Atriplex nummularia in response to non-uniform NaCl concentrations in a split-root system.

Methods

Atriplex nummularia was grown in a split-root system for 21 d, with either the same or two different NaCl concentrations (ranging from 10 to 670 mm), in aerated nutrient solution bathing each root half.

Key Results

Non-uniform salinity, with high NaCl in one root half (up to 670 mm) and 10 mm in the other half, had no effect on shoot ethanol-insoluble dry mass, net photosynthesis or shoot pre-dawn water potential. In contrast, a modest effect occurred for leaf osmotic potential (up to 30 % more solutes compared with uniform 10 mm NaCl treatment). With non-uniform NaCl concentrations (10/670 mm), 90 % of water was absorbed from the low salinity side, and the reduction in water use from the high salinity side caused whole-plant water use to decrease by about 30 %; there was no compensatory water uptake from the low salinity side. Leaf Na⁺ and Cl⁻ concentrations were 1·9- to 2·3-fold higher in the uniform 670 mm treatment than in the 10/670 mm treatment, whereas leaf K⁺ concentrations were 1·2- to 2·0-fold higher in the non-uniform treatment.

Conclusions

Atriplex nummularia with one root half in 10 mm NaCl maintained net photosynthesis, shoot growth and shoot water potential even when the other root half was exposed to 670 mm NaCl, a concentration that inhibits growth by 65 % when uniform in the root zone. Given the likelihood of non-uniform salinity in many field situations, this situation would presumably benefit halophyte growth and physiology in saline environments.Key words: Split-root system, salinity heterogeneity, root zone heterogeneity, water potential, water use, stomatal conductance, saltbush, leaf ions, photosynthesis, NaCl     

Effects of heterogeneous salinity on growth,water uptake,and tissue ion concentrations of alfalfa

Aims

Soil salinity varies greatly in the plant rhizosphere. The effect of nonuniform salinity on the growth and physiology response of alfalfa plants was determined to improve understanding of salt stress tolerance mechanisms of alfalfa.

Methods

Plant growth, predawn leaf water potential, water uptake, and tissue ionic content were studied in alfalfa plants grown hydroponically for 9 days using a split-root system, with uniform salinity or horizontally nonuniform salinity treatments (0/S, 75/S, and 150/S corresponding to 0, 75, and 150 mM NaCl on the low salt side, respectively).

Results

Compared with uniform high salinity, 0/S and 75/S treatments significantly increased the alfalfa shoot dry mass and stem extension rate. Compensatory water uptake by low salt roots of 0/S and 75/S treatments was observed. However, decreased leaf Na⁺ concentration, increased leaf K⁺/Na⁺, and compensatory growth of roots on the low salt side were observed only following the 0/S treatment.

Conclusions

Nonuniform salinity dose not enhance plant growth once a threshold NaCl concentration in low salinity growth medium has been reached. Compensation of water uptake from the low-salt root zone and regulation of K⁺/Na⁺ homeostasis in low salt root play more important role than regulation of leaf ions in enhancing alfalfa growth under nonuniform salinity.

     

Ecophysiological response of Crambe maritima to airborne and soil-borne salinity

Background and Aims

There is a need to evaluate the salt tolerance of plant species that can be cultivated as crops under saline conditions. Crambe maritima is a coastal plant, usually occurring on the driftline, with potential use as a vegetable crop. The aim of this experiment was to determine the growth response of Crambe maritima to various levels of airborne and soil-borne salinity and the ecophysiological mechanisms underlying these responses.

Methods

In the greenhouse, plants were exposed to salt spray (400 mm NaCl) as well as to various levels of root-zone salinity (RZS) of 0, 50, 100, 200 and 300 mm NaCl during 40 d. The salt tolerance of Crambe maritima was assessed by the relative growth rate (RGR) and its components. To study possible salinity effects on the tissue and cellular level, the leaf succulence, tissue Na⁺ concentrations, Na⁺ : K⁺ ratio, net K⁺/Na⁺ selectivity, N, P, K⁺, Ca²⁺, Mg²⁺, proline, soluble sugar concentrations, osmotic potential, total phenolics and antioxidant capacity were measured.

Key Results

Salt spray did not affect the RGR of Crambe maritima. However, leaf thickness and leaf succulence increased with salt spray. Root zone salinities up to 100 mm NaCl did not affect growth. However, at 200 mm NaCl RZS the RGR was reduced by 41 % compared with the control and by 56 % at 300 mm NaCl RZS. The reduced RGR with increasing RZS was largely due to the reduced specific leaf area, which was caused by increased leaf succulence as well as by increased leaf dry matter content. No changes in unit leaf rate were observed but increased RZS resulted in increased Na⁺ and proline concentrations, reduced K⁺, Ca²⁺ and Mg²⁺ concentrations, lower osmotic potential and increased antioxidant capacity. Proline concentrations of the leaves correlated strongly (r = 0·95) with RZS concentrations and not with plant growth.

Conclusions

Based on its growth response, Crambe maritima can be classified as a salt spray tolerant plant that is sensitive to root zone salinities exceeding 100 mm NaCl.     

Nocturnal and daytime stomatal conductance respond to root-zone temperature in ‘Shiraz’ grapevines

Background and Aims

Daytime root-zone temperature may be a significant factor regulating water flux through plants. Water flux can also occur during the night but nocturnal stomatal response to environmental drivers such as root-zone temperature remains largely unknown.

Methods

Here nocturnal and daytime leaf gas exchange was quantified in ‘Shiraz’ grapevines (Vitis vinifera) exposed to three root-zone temperatures from budburst to fruit-set, for a total of 8 weeks in spring.

Key Results

Despite lower stomatal density, night-time stomatal conductance and transpiration rates were greater for plants grown in warm root-zones. Elevated root-zone temperature resulted in higher daytime stomatal conductance, transpiration and net assimilation rates across a range of leaf-to-air vapour pressure deficits, air temperatures and light levels. Intrinsic water-use efficiency was, however, lowest in those plants with warm root-zones. CO₂ response curves of foliar gas exchange indicated that the maximum rate of electron transport and the maximum rate of Rubisco activity did not differ between the root-zone treatments, and therefore it was likely that the lower photosynthesis in cool root-zones was predominantly the result of a stomatal limitation. One week after discontinuation of the temperature treatments, gas exchange was similar between the plants, indicating a reversible physiological response to soil temperature.

Conclusions

In this anisohydric grapevine variety both night-time and daytime stomatal conductance were responsive to root-zone temperature. Because nocturnal transpiration has implications for overall plant water status, predictive climate change models using stomatal conductance will need to factor in this root-zone variable.     

Development changes in calla lily plants due to salt stress

Soil salinity is mainly caused by excessive use of fertilizers and the use of poor quality water, and adversely affected crop growth especially when grown in protected environments. Soil salinity causes salt stress in plants, which inhibits plant growth, leading to morphological, metabolic and physiological changes. Though it is a major problem occurs more frequently, there is less information on the behavior of calla lily (Zantedeschia aethiopica) under these conditions, and most studies are conducted with other species of the genus Zantesdeschia. Therefore, this study aimed to evaluate ecophysiological, biochemical and anatomical growth responses of calla lily plants to salt stress. Rhizomes were grown in trays containing coconut fiber as a substrate and treated with 0, 25, 50, 75 and 100 mM NaCl to induce stress. A decrease in plant height was observed, as well as in the number of tillers and leaves, main root length, fresh and dry matter of the shoot and root system. A reduction in photosynthetic rate, stomatal conductance and transpiration rate was observed at 60 days. However, after 90 days, the photosynthetic rate was unchanged, with increased stomatal conductance and transpiration rate for plants exposed to 75 mM NaCl. Salt stress caused a higher accumulation of carbohydrates in shoots and roots. Thus, high concentrations of NaCl affect the development of calla lily, indicating that this species is susceptible to salt stress.     

Comparative responses of ‘Gala’ and ‘Fuji’ apple trees to deficit irrigation: Placement versus volume effects

Aims

Climate, soil water potential (SWP), leaf relative water content (RWC), stomatal conductance (g_s), fruit and shoot growth, and carbohydrate levels were monitored during the 2008 and 2009 growing seasons to study the responses of ‘Gala’ and ‘Fuji’ apple trees to irrigation placement or volume.

Methods

Three irrigation treatments were imposed, conventional irrigation (CI), partial root-zone drying (PRD, 50% of CI water on one side of the root-zone, which was alternated periodically), and continuous deficit irrigation (DI, 50% of CI water on both sides of the root-zone).

Results

After each irrigation season, DI generated twice the soil water deficit (SWD_int) than PRD (average of dry and wet sides) and a greater integrated leaf water deficit (LWD_int) than PRD and CI. Both PRD and DI reduced g_s by 9 and 15% over the irrigation period. RWC of both PRD and DI was directly related to SWP and inversely related (non-linear) to vapor pressure deficit (VPD), whereas it was unrelated to g_s. Considering individual sampling days, g_s of ‘Gala’ leaves was inversely related to VPD mainly until early August (fruit at cell expansion phase and high VPD), while it was directly related to VPD in September (no fruit and low VPD). On the contrary, g_s of ‘Fuji’ leaves was inversely related to VPD from late August until mid October (low VPD and fruit at cell expansion phase). Fruit growth was not affected by irrigation, whereas shoot and trunk growth was reduced by DI. Irrigation induced sporadic and inconsistent changes in carbohydrate contents or partitioning, with a general tendency of DI leaves to degrade and PRD to accumulate sorbitol and sucrose in dry periods.

Conclusions

‘Gala’ trees exhibited a more conservative water use than ‘Fuji’ trees due primarily to different timing of fruit growth and crop loads. Different levels of SWD_int, rather than changes in stomatal control and carbohydrate partitioning, seem to play a major role in determining a better water status in PRD than in DI trees.     

Physiological and biochemical mechanisms of the ornamental <Emphasis Type="Italic">Eugenia myrtifolia</Emphasis> L. plants for coping with NaCl stress and recovery

Main Conclusion

We studied the response of Eugenia myrtifolia L. plants, an ornamental shrub native to tropical and subtropical areas, to salt stress in order to facilitate the use of these plants in Mediterranean areas for landscaping. E. myrtifolia plants implement a series of adaptations to acclimate to salinity, including morphological, physiological and biochemical changes. Furthermore, the post-recovery period seems to be detected by Eugenia plants as a new stress situation. Different physiological and biochemical changes in Eugenia myrtifolia L. plants after being subjected to NaCl stress for up to 30 days (Phase I) and after recovery from salinity (Phase II) were studied. Eugenia plants proved to be tolerant to NaCl concentrations between 44 and 88 mM, displaying a series of adaptative mechanisms to cope with salt-stress, including the accumulation of toxic ions in roots. Plants increased their root/shoot ratio and decreased their leaf area, leaf water potential and stomatal conductance in order to limit water loss. In addition, they displayed different strategies to protect the photosynthetic machinery, including the limited accumulation of toxic ions in leaves, increase in chlorophyll content, changes in chlorophyll fluorescence parameters, leaf anatomy and antioxidant defence mechanisms. Anatomical modifications in leaves, including an increase in palisade parenchyma and intercellular spaces and decrease in spongy parenchyma, served to facilitate CO₂ diffusion in a situation of reduced stomatal aperture. Salinity produced oxidative stress in Eugenia plants as evidenced by oxidative stress parameters values and a reduction in APX and ASC levels. Nevertheless, SOD and GSH contents increased. The post-recovery period is detected as a new stress situation, as observed through effects on plant growth and alterations in chlorophyll fluorescence and oxidative stress parameters.

     

Linking oxidative and salinity stress tolerance in barley: can root antioxidant enzyme activity be used as a measure of stress tolerance?

Aims

A causal relationship between salinity and oxidative stress tolerance and a suitability of using root antioxidant activity as a biochemical marker for salinity tolerance in barley was investigated.

Methods

Net ion fluxes were measured from the mature zone of excised roots of two barley varieties contrasting in their salinity tolerance using non-invasive MIFE technique in response to acute and prolonged salinity treatment. These changes were correlated with activity of major antioxidant enzymes; ascorbate peroxidase, catalase, and superoxide dismutase.

Results

It was found that genotypic difference in salinity tolerance was largely independent of root integrity, and observed not only for short-term but also long-term NaCl exposures. Higher K⁺ retention ability (and, hence, salinity tolerance) positively correlated with oxidative stress tolerance. At the same time, antioxidant activities were constitutively higher in a sensitive but not tolerant variety, and no correlation was found between SOD activity and salinity tolerance index during large-scale screening.

Conclusion

Although salinity tolerance in barley correlates with its oxidative stress tolerance, higher antioxidant activity at one particular time does not correlate with salinity tolerance and, as such, cannot be used as a biochemical marker in barley screening programs.     

Xylem sap calcium concentrations do not explain liming-induced inhibition of legume gas exchange

Background and aims

Liming is considered normal agricultural practise for remediating soil acidity and improving crop productivity; however recommended lime applications can reduce yield. We tested the hypothesis that elevated xylem sap Ca²⁺ limited gas exchange of Phaseolus vulgaris L. and Pisum sativum L. plants that exhibited reduced shoot biomass and leaf area when limed.

Methods

We used Scholander and whole-plant pressure chamber techniques to collect root and leaf xylem sap, a calcium-specific ion-selective electrode to measure xylem sap Ca²⁺, infra-red gas analysis to measure gas exchange of limed and unlimed (control) plants, and a detached leaf transpiration bioassay to determine stomatal sensitivity to Ca²⁺.

Results

Liming reduced shoot biomass, leaf area and leaf gas exchange in both species. Root xylem sap Ca²⁺ concentration was only increased in P. vulgaris and not in P. sativum. Detached leaves of both species required 5 mM Ca²⁺ supplied to via the transpiration stream to induce stomatal closure, however, maximum in vivo xylem sap Ca²⁺ concentrations of limed plants was only 1.7 mM and thus not high enough to influence stomata.

Conclusion

We conclude that an alternative xylem-borne antitranspirant other than Ca²⁺ decreases gas exchange of limed plants.     

Alfalfa plant-derived biostimulant stimulate short-term growth of salt stressed Zea mays L. plants

Background & Aims

The effects of an alfalfa plant (Medicago sativa L.) hydrolysate-based biostimulant (EM) containing triacontanol (TRIA) and indole-3-acetic acid (IAA) were tested in salt-stressed maize plants.

Methods

Plants were grown for 2 weeks in the absence of NaCl or in the presence (25, 75 and 150 mM). On the 12th day, plants were supplied for 48 h with 1.0 mg L^?1 EM or 11.2 μM TRIA.

Results

EM and TRIA stimulated the growth and nitrogen assimilation of control plants to a similar degree, while NaCl reduced plant growth, SPAD index and protein content. EM or TRIA increased plant biomass under salinity conditions. Furthermore, EM induced the activity of enzymes functioning in nitrogen metabolism. The activity of antioxidant enzymes and the synthesis of phenolics were induced by salinity, but decreased after EM treatment. The enhancement of phenylalanine ammonia-lyase (PAL) activity and gene expression by EM was consistent with the increase of flavonoids.

Conclusion

The present study proves that the EM increases plant biomass even when plants are grown under salinity conditions. This was likely because EM stimulated plant nitrogen metabolism and antioxidant systems. Therefore, EM may be proposed as bioactive product in agriculture to help plants overcome stress situations.     

Isolation of ACC deaminase producing PGPR from rice rhizosphere and evaluating their plant growth promoting activity under salt stress

Aims

Bacteria possessing ACC deaminase activity reduce the level of stress ethylene conferring resistance and stimulating growth of plants under various biotic and abiotic stresses. The present study aims at isolating efficient ACC deaminase producing PGPR strains from the rhizosphere of rice plants grown in coastal saline soils and quantifying the effect of potent PGPR isolates on rice seed germination and seedling growth under salinity stress and ethylene production from rice seedlings inoculated with ACC deaminase containing PGPR.

Methods

Soils from root region of rice growing in coastal soils of varying salinity were used for isolating ACC deaminase producing bacteria and three bacterial isolates were identified following polyphasic taxonomy. Seed germination, root growth and stress ethylene production in rice seedlings following inoculation with selected PGPR under salt stress were quantified.

Results

Inoculation with selected PGPR isolates had considerable positive impacts on different growth parameters of rice including germination percentage, shoot and root growth and chlorophyll content as compared to uninoculated control. Inoculation with the ACC deaminase producing strains reduced ethylene production under salinity stress.

Conclusions

This study demonstrates the effectiveness of rhizobacteria containing ACC deaminase for enhancing salt tolerance and consequently improving the growth of rice plants under salt-stress conditions.     

Effects of short-term salinity on leaf gas exchange of the fig (Ficus carica L.)

The influence of short-term salinity (day 1–day 2: 50 mol m^–3 NaCl, day 3–day 7: 100 mol m^–3 NaCl in the nutrient solution) on leaf gas exchange characteristics were studied in two fig clones (Ficus carica L.), whose root mass had been varied in relation to the leaf area. The stomatal conductance was diminished by NaCl in the first week of treatment. NaCl slightly reduced the calculated intercellular partial pressure of CO₂. The net photosynthetic rate of plants with many roots was stimulated by NaCl on some days of the first week of treatment, whereas the net assimilation rate of the plants with few roots remained unaltered or decreased by NaCl. Only the assimilation of the salt-treated plants of one clone for some days during the first week of treatment seemed to be influenced by stomatal conductance. Nonstomatal factors were primarily responsible for the changes in CO₂ uptake in response to salt and/or root treatment. The water use efficiency increased during several days of the first week of NaCl treatment. Decreased stomatal conductance, increased water use efficiency and stimualtion of the net CO₂ assimilation rate appear to enhance salt tolerance during the first few days of salinity. ei]H Lambers     

Change in uptake, transport and accumulation of ions in Nerium oleander (rosebay) as affected by different nitrogen sources and salinity

Background and Aims

The source of nitrogen plays an important role in salt tolerance of plants. In this study, the effects of NaCl on net uptake, accumulation and transport of ions were investigated in Nerium oleander with ammonium or nitrate as the nitrogen source in order to analyse differences in uptake and cycling of ions within plants.

Methods

Plants were grown in a greenhouse in hydroponics under different salt treatments (control vs. 100 mm NaCl) with ammonium or nitrate as the nitrogen source, and changes in ion concentration in plants, xylem sap exuded from roots and stems, and phloem sap were determined.

Key Results

Plant weight, leaf area and photosynthetic rate showed a higher salt tolerance of nitrate-fed plants compared with that of ammonium-fed plants. The total amount of Na⁺ transported in the xylem in roots, accumulated in the shoot and retranslocated in the phloem of ammonium-fed plants under salt treatment was 1·8, 1·9 and 2·7 times more, respectively, than that of nitrate-treated plants. However, the amount of Na⁺ accumulated in roots in nitrate-fed plants was about 1·5 times higher than that in ammonium-fed plants. Similarly, Cl⁻ transport via the xylem to the shoot and its retranslocation via the phloem (Cl⁻ cycling) were far greater with ammonium treatment than with nitrate treatment under conditions of salinity. The uptake and accumulation of K⁺ in shoots decreased more due to salinity in ammonium-fed plants compared with nitrate-fed plants. In contrast, K⁺ cycling in shoots increased due to salinity, with higher rates in the ammonium-treated plants.

Conclusions

The faster growth of nitrate-fed plants under conditions of salinity was associated with a lower transport and accumulation of Na⁺ and Cl⁻ in the shoot, whereas in ammonium-fed plants accumulation and cycling of Na⁺ and Cl⁻ in shoots probably caused harmful effects and reduced growth of plants.Key words: Mineral cycling, Nerium oleander, nitrogen source, salinity, xylem and phloem transport     

Tolerance of Hordeum marinum accessions to O2 deficiency, salinity and these stresses combined

Background and Aims

When root-zone O₂ deficiency occurs together with salinity, regulation of shoot ion concentrations is compromised even more than under salinity alone. Tolerance was evaluated amongst 34 accessions of Hordeum marinum, a wild species in the Triticeae, to combined salinity and root-zone O₂ deficiency. Interest in H. marinum arises from the potential to use it as a donor for abiotic stress tolerance into wheat.

Methods

Two batches of 17 H. marinum accessions, from (1) the Nordic Gene Bank and (2) the wheat belt of Western Australia, were exposed to 0·2 or 200 mol m⁻³ NaCl in aerated or stagnant nutrient solution for 28–29 d. Wheat (Triticum aestivum) was included as a sensitive check species. Growth, root porosity, root radial O₂ loss (ROL) and leaf ion (Na⁺, K⁺, Cl⁻) concentrations were determined.

Key Results

Owing to space constraints, this report is focused mainly on the accessions from the Nordic Gene Bank. The 17 accessions varied in tolerance; relative growth rate was reduced by 2–38 % in stagnant solution, by 8–42 % in saline solution (aerated) and by 39–71 % in stagnant plus saline treatment. When in stagnant solution, porosity of adventitious roots was 24–33 %; salinity decreased the root porosity in some accessions, but had no effect in others. Roots grown in stagnant solution formed a barrier to ROL, but variation existed amongst accessions in apparent barrier ‘strength’. Leaf Na⁺ concentration was 142–692 µmol g⁻¹ d. wt for plants in saline solution (aerated), and only increased to 247–748 µmol g⁻¹ d. wt in the stagnant plus saline treatment. Leaf Cl⁻ also showed only small effects of stagnant plus saline treatment, compared with saline alone. In comparison with H. marinum, wheat was more adversely affected by each stress alone, and particularly when combined; growth reductions were greater, adventitious root porosity was 21 %, it lacked a barrier to ROL, leaf K⁺ declined to lower levels, and leaf Na⁺ and Cl⁻ concentrations were 3·1–9-fold and 2·8–6-fold higher, respectively, in wheat.

Conclusions

Stagnant treatment plus salinity reduced growth more than salinity alone, or stagnant alone, but some accessions of H. marinum were still relatively tolerant of these combined stresses, maintaining Na⁺ and Cl⁻ ‘exclusion’ even in an O₂-deficient, saline rooting medium.Key words: Aerenchyma, combined salinity and waterlogging, leaf Cl⁻, leaf K⁺, leaf Na⁺, radial O₂ loss, salt tolerance, salinity–waterlogging interaction, sea barleygrass, waterlogging tolerance, wheat, wild Triticeae     

A native Glomus intraradices strain from a Mediterranean saline area exhibits salt tolerance and enhanced symbiotic efficiency with maize plants under salt stress conditions

Aims

Arbuscular mycorrhizal (AM) fungi have been shown to occur naturally in saline environments and it has been suggested that differences in fungal behaviour and efficiency can be due to the origin and adaptation of the AM fungus. These findings invite to look out for AM fungal species isolated in saline environments and compare their salt-tolerance mechanisms with those of species living in non-saline areas.

Methods

A fungal strain of G. intraradices (Gi CdG) isolated from a region with serious problems of salinity and affected by desertification, has been compared with a collection strain of the same species, used as a model fungus. An in vitro experiment tested the ability of both AM fungi to grow under increasing salinity and an in vivo experiment compared their symbiotic efficiency with maize plants grown under salt stress conditions.

Results

The isolate Gi CdG developed better under saline conditions and induced considerably the expression of GintBIP, Gint14-3-3 and GintAQP1 genes, while it showed a lower induction of GintSOD1 gene than the collection G. intraradices strain. The isolate Gi CdG also stimulated the growth of maize plants under two levels of salinity more than the collection strain. The higher symbiotic efficiency of Gi CdG was corroborated by the enhanced efficiency of photosystem II and stomatal conductance and the lower electrolyte leakage exhibited by maize plants under the different conditions assayed.

Conclusions

The higher tolerance to salinity and symbiotic efficiency exhibited by strain Gi CdG as compared to the collection G. intraradices strain may be due to a fungal adaptation to saline environments. Such adaptation may be related to the significant up-regulation of genes encoding chaperones or genes encoding aquaporins. The present study remarks that AM fungi isolated from areas affected by salinity can be a powerful tool to enhance the tolerance of crops to saline stress conditions.     

Nostoc, Microcoleus and Leptolyngbya inoculums are detrimental to the growth of wheat (Triticum aestivum L.) under salt stress

Background and aims

This study investigated the effect of cyanobacterial inoculants on salt tolerance in wheat.

Methods

Unicyanobacterial crusts of Nostoc, Leptolyngbya and Microcoleus were established in sand pots. Salt stress was targeted at 6 and 13 dS m^?1, corresponding to the wheat salt tolerance and 50 % yield reduction thresholds, respectively. Germinated wheat seeds were planted and grown for 14 (0 and 6 dS m^?1) and 21 (13 dS m^?1) days by which time seedlings had five emergent leaves. The effects of cyanobacterial inoculation and salinity on wheat growth were quantified using chlorophyll fluorescence, inductively coupled plasma-optical emission spectrometry and biomass measurements.

Results

Chlorophyll fluorescence was negatively affected by soil salinity and no change was observed in inoculated wheat. Effective photochemical efficiency correlated with a large range of plant nutrient concentrations primarily in plant roots. Inoculation negatively affected wheat biomass and nutrient concentrations at all salinities, though the effects were fewer as salinity increased.

Conclusions

The most likely explanation of these results is the sorption of nutrients to cyanobacterial extracellular polymeric substances, making them unavailable for plant uptake. These results suggest that cyanobacterial inoculation may not be appropriate for establishing wheat in saline soils but that cyanobacteria could be very useful for stabilising soils.     

Signalling mechanisms involved in the response of two varieties of Humulus lupulus L. to soil drying: I. changes in xylem sap pH and the concentrations of abscisic acid and anions

Background and aims

Soil drying leads to the generation of chemical signals in plants that regulate water use via control of the stomatal aperture. The aim of our work was to identify the presence and identity of potential chemical signals, their dynamics, and their relationship with transpiration rate during soil drying in hop (Humulus lupulus (L.)) plants.

Methods

We used pressure chamber technique for measurement of shoot water potential and collection of shoot xylem sap. We analyzed concentrations of abscisic acid (ABA), nitrate, phosphate, sulphate and malate in sap and also the rate of whole plant transpiration.

Results

Transpiration rate decreased prior to changes in shoot water potential. The concentration of ABA in xylem sap continuously increased from early to later stages of water stress, whereas in leaves it increased only at later stages. Shoot sap pH increased simultaneously with the decrease of transpiration rate. Xylem sap alkalization was in some cases accompanied by a decrease in nitrate concentration and an increase in malate concentration. Concentration of sulphate increased in xylem sap during drying and sulphate in combination with a higher ABA concentration enhanced stomatal closure.

Conclusions

Several early chemical signals appear in sap of hop plants during soil drying and their impact on transpiration may vary according to the stage of soil drying.     

Response of microbial activity and biomass in rhizosphere and bulk soils to increasing salinity

Background and aims

The impact of salinity on microbes has been studied extensively but little is known about the response of soil microbial activity and biomass to increasing salinity in rhizosphere compared to bulk (non-rhizosphere) soil.

Methods

Barley was grown for 5 weeks in non-saline loamy sand to which salt (NaCl) was added. The electrical conductivity in the saturated extract (ECe) was 1, 13 and 19 dS m^?1 for non-saline and two saline soils. Pots without plants were prepared in the same manner and placed next to those with plants. The water content in all pots was maintained at 75 % of water-holding capacity by weight. After 5 weeks the planted and unplanted pots were harvested to collect rhizosphere and bulk soil, respectively. The collected soil was then used for an incubation experiment. The EC levels in the pot experiment (EC1, EC13 and EC19, referred to as original) were either maintained or increased by adding NaCl to adjust the EC to 13, 19, 31 and 44 dS m^?1. CO₂ release was measured continuously for 20 days, microbial biomass C (MBC) was measured at the start and the end of the incubation experiment.

Results

In general, cumulative respiration and microbial biomass C concentration in rhizosphere and bulk soil decreased to a similar extent with increasing adjusted EC. However, compared to the treatments where the EC was maintained, the percentage decrease in cumulative respiration when the EC was increased to EC44 was smaller in rhizosphere than in bulk soil.

Conclusion

Overall, the reduction of cumulative respiration with increasing salinity did not differ between rhizophere and bulk soil. But microbes in rhizosphere soil were more tolerant to high EC than those in bulk soil which could be due to the greater substrate availability in the rhizosphere even after the soil was removed from the roots.