Salt tolerance of the annual halophyte <Emphasis Type="Italic">Cakile maritima</Emphasis> as affected by the provenance and the developmental stage

Though halophytes are naturally adapted to salinity, their salt-tolerance limits are greatly influenced by their provenance and developmental stage. In the present study, physio-biochemical responses of two Tunisian ecotypes of the oilseed coastal halophyte Cakile maritima (Brassicaceae) to salinity (0–400 mM NaCl) were monitored during germination and vegetative growth stages. Tabarka and Jerba seeds were collected from humid or arid climatic areas, respectively. Plant response to salinity appeared to depend on the ecotype and salinity levels. Increasing salinity inhibited germination process. Jerba seeds were found to be more salt tolerant than the Tabarka ones. At the autotrophic stage of growth and under salt-free conditions, Jerba was less productive than Tabarka (in terms of dry matter accumulation), but plant biomass production and leaf expansion (area and number) of the former ecotype were progressively improved by 100 mM NaCl, as compared to the control. In contrast, at the same salt concentration, these parameters decreased under increasing salinity in Tabarka (salt sensitive). Leaf chlorophyll content was reduced at severe salinity, but this effect was more conspicuous in the sensitive Tabarka plants. Na⁺ contents in the Jerba and Tabarka leaves collected from the 400 mM NaCl-treated plants were 17- and 12-fold higher than in the respective controls. This effect was accompanied by a significant reduction in the leaf K⁺, Mg²⁺ and Ca²⁺ contents, especially in the salt-treated Tabarka. A significant accumulation of proline and soluble carbohydrates in leaves was found during the period of intensive leaf growth. These organic compounds likely play a role in leaf osmotic adjustment and in protection of membrane stability at severe salinity.     

Kinetin regulation of growth and secondary metabolism in waterlogging and salinity treated Vigna sinensis and Zea mays

Waterlogging mostly increased fresh weight and water content in shoots and roots of Vigna sinensis and Zea mays while salinity seemed to have a decreasing effect. There was a marked induction of proline in shoots and roots of both plants by salinity with lower values in logged plants. In addition, anthocyanin content was increased in Vigna sinensis by both treatments and in Zea mays only by salinity. Meanwhile the treatments significantly accumulated phenolic compounds in plant shoots. Also there were increased activities of phenylalanine ammonia lyase (PAL) and tyrosine ammonia lyase (TAL) in shoots and roots of both plants. Foliar application of kinetin equilibrated, if any, the effects of both treatments on contents of proline, anthocyanin and phenolic compounds as well as activities of PAL and TAL in shoots and roots of treated plants. These findings reveal that kinetin alleviates the stress symptoms and regulates the changes in phenolic metabolism of waterlogged or salinity treated Vigna sinensis and Zea mays.     

Combined effect of boron and salinity on water transport: The role of aquaporins

Boron toxicity is an important disorder that can limit plant growth on soils of arid and semi arid environments throughout the world. Although there are several reports about the combined effect of salinity and boron toxicity on plant growth and yield, there is no consensus about the experimental results. A general antagonistic relationship between boron excess and salinity has been observed, however the mechanisms for this interaction is not clear and several options can be discussed. In addition, there is no information, concerning the interaction between boron toxicity and salinity with respect to water transport and aquaporins function in the plants. We recently documented in the highly boron- and salt-tolerant the ecotype of Zea mays L. amylacea from Lluta valley in Northern Chile that under salt stress, the activity of specific membrane components can be influenced directly by boron, regulating the water uptake and water transport through the functions of certain aquaporin isoforms.Key words: aquaporins, boron, salinity, water relations, Zea maysHigh concentrations of boron are often associated to saline soils in semi arid and/or arid climates and frequently crops are exposed to both stresses simultaneously.¹ As there is no a unique plant response to combination of salinity and boron toxicity, several mechanisms has been proposed to explain the experimental results. Some reports showed no additive effects of boron and salinity on shoot weight of different cultivars suggesting independent of the interaction.^2–5 However, additive effects^6–9 have been also proposed and the interaction of boron and salinity declined the rate of germination and limited growth in maize and sorghum plants.¹⁰ No explanation is currently available for these contradictory observations. Recently, the Abbot method has been applied to characterize the combined effect of boron and salinity at toxic levels in pepper plants, observing mainly an antagonistic relationship regarding growth and yield.¹¹ Antagonism between salinity and boron may be the result of decreased toxicity of boron in the presence of NaCl, reduced toxicity of NaCl in the presence of boron, or both together. Letey et al.,¹² have reported that increased soil salinity may also reduce boron movement to the broccoli plants and hence result in a reduction of boron toxicity symptoms. Reduction of boron accumulation in leaves in the presence of salinity has been also reported for melon,⁵ tomato⁸ jack pine¹³ and grapesvines¹⁴ and could be the result of the reduced rates of transpiration in plants where boron is transported via xylem as consequence of the osmotic effect of the salt. On the other hand, it has been observed that concentration of Na⁺ in leaves decreased with increasing addition of boron to the soil, probably due to the inhibition in root growth and reduction in root density caused by the boron treatment.¹⁵ Grieve and Poss⁷ found in wheat plants that the Cl⁻ content in the leaves was reduced when boron was increased. Similar results were reported in pepper plants suggesting that boron could reduce Cl⁻ toxicity.¹¹ Also, in our recent report although a nutrient imbalance resulted from the effect of salinity or boron alone, a general optimisation was observed when both treatments were applied together.¹⁶Under saline conditions, an optimal water balance is important in order to maintain the plant homeostasis and aquaporins may be one of the mechanisms involved under environmental and developmental changes.^17–19 However, there is no information concerning plant water uptake and transport in response to combined excess boron and salinity.It has been reported that, at high external B concentrations, considerable B transport occurs through the plasma membrane aquaporins, and a specific membrane intrinsic protein (MIP) has been described.²⁰ Thus boron uptake across the plasma membrane, by permeation through the lipid membrane and aquaporins, may be greatly influenced by the plant tolerance to salinity, through the associated changes in root hydraulic conductivity. Wimmer et al.,²¹ showed that salinity could interact with boron toxicity by a combined effect on boron and water uptake. In addition, we reported that the reduction of aquaporin functionality in NaCl-exposed plants could induce the reduction of plant boron concentration, producing a beneficial effect.²²Recently, we showed in a tolerant ecotype of maize a different pattern for PIP1 and PIP2 protein content under the application of excess of boron in combination with salinity, suggesting a differential aquaporin response in this cultivar and pointed out the complexity of the interaction.¹⁶ These results were in consonance with the previous observation that different aquaporin isoforms may represent a response to environmental changes.^18,19,²³ Thus, we concluded that the activity of specific membrane components can be influenced by boron under salt stress regulating the functions of certain aquaporin isoforms as possible components of the salinity tolerance mechanism. However, although a fine water transport control through the aquaporins could be necessary in order to reduce the accumulation of toxic boron levels in the tissues, the contribution of each isoform to water transport through the plasma membrane under boron-salinity combination must be elucidated.     

Effect of Salt Stress on Viability, Germination and Endogenous Levels of Some Metabolites and Ions in Maize (Zea mays L.) Pollen

Maize plants, subjected to 0, 80, 120 and 160 meq l^–1salinity using NaCl, showed adverse effects on viability, germinationand tube growth of pollen, besides enhancing the bursting ofpollen. The endogenous levels of various metabolites in pollenwere also affected. Pollen grains from salinized plants hadmore soluble carbohydrates, free amino acids, especially proline,phenols and DNA and less starch, protein and RNA compared tothe non-saline controls. Salinity also resulted in the accumulationof ions such as Na⁺, K⁺ and Cl^– while it caused a reductionin the boron content of pollen. These metabolic disturbancespossibly lead to decreased viability, germination and tube growthof pollen thereby resulting into a reduction in reproductivecapacity of the plants under salt stress. Zea mays L., maize, pollen, viability, germination, salt stress     

NaCl Uptake in Roots of Zea mays Seedlings: Comparison of Root Pressure Probe and EDX Data

The extent by which salinity affects plant growth depends partlyon the ability of the plant to exclude NaCl. To study the uptakeof NaCl into excised roots of Zea mays L. cv. ‘Tanker’,two different techniques were applied. A root pressure probewas used to record steady state as well as transient valuesof root (xylem) pressure upon exposure of the root to mediacontaining NaCl and KCl as osmotic solutes. In treatments withNaCl, pressure/time responses of the root indicated a significantuptake of NaCl into the xylem. NaCl induced kinetics were completelyreversible when the NaCl solution was replaced by an isosmoticKCl solution. This indicated a passive movement of Na⁺-saltsacross the root cylinder. Root samples were taken at differenttimes of exposure to NaCl and prepared for X-ray microanalysis(EDX analysis). Radial profiles of ion concentrations (Na⁺,K⁺, Cl^–) were measured in cell vacuoles and xylem vesselsalong the root axis. Na⁺ appeared rapidly in mature xylem (earlymetaxylem) and living xylem (late metaxylem) before it was detectablein vacuoles of the root cortex. EDX results confirmed that thekinetics observed by the pressure probe technique correspondedmainly to an influx of Na⁺-salts into early metaxylem. In latemetaxylem, the uptake of Na⁺ was associated with a decline ofK⁺. The Na⁺/K⁺ exchange indicated a mechanism to reduce sodiumfrom the transpiration stream. Ion localization, ion transport, maize, root pressure, salinity, water relations, X-ray microanalysis, Zea mays     

Differential effects of salinity on leaf elongation kinetics of three grass species

This study focuses on the inhibitory effect of salinity on the leaf extension of three different grass species: Hordeum jubatum L., Hordeum vulgare L. and Zea mays L. Leaf elongation rates (LER) were measured on the third leaf of the plants. NaCl was added to the hydroponic solution (0, 40, 80 and 120 mM) and changes in LER were measured over time with a displacement transducer. Salinity inhibited LER immediately in all three species, and a new, but lower steady-state LER was reached within 5 h. The decrease in LER was proportional to the salinity level. Differences in salt tolerance (% of control LER) were evident between genotypes within 5 h after salinization, but the relative salt tolerance of the plant at this stage was not necessarily indicative of the long-term salt tolerance of the species. In general, H. jubatum was more tolerant than maize, which was more tolerant than barley to these short-term salinity stresses. In contrast, barley is more salt tolerant than maize over the long term. The mechanisms of inhibition of LER by salinity, as tested by the applied-tension technique, varied with the species examined, affecting either the apparent yield threshold, the hydraulic conductance of the whole plant or both. The cell wall extensibility was not significantly affected by salinity in the three species tested in this study.     

The effects of salinity on ion concentrations within the root cells of Zea mays L.

Zea mays is a salt-sensitive crop species which in saline (100 mol m^-3 NaCl) conditions suffers considerable growth reduction correlated with elevated Na⁺ and Cl^- concentration within the leaves. To increase understanding of the regulation of ion uptake and transport by the roots in saline conditions, ion concentrations within individual root cortical cells were determined by X-ray microanalysis. There was variation in Na⁺, K⁺ and Cl^- distributions among individual cells, which could not be correlated with their spatial position in the roots. Generally, however, in response to saline growth conditions (100 mol m³ NaCl) Na⁺ and Cl^- were mostly localized in the vacuoles, although their concentrations were also sometimes increased in the cytoplasm and cell walls. The concentration of K⁺ in the cytoplasm was usually maintained at a level (mean 79 mol m^-3) compatible with the biochemical functions ascribed to this ion.Abbreviation (T)AEM (Transmission) analytical electron microscopy     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

Agronomical and physiological characterization of salinity tolerance in a commercial tomato hybrid

The salt tolerance of the commercial F1 tomato hybrid (Lycopersicon esculentum Mill) Radja (GC-793) has been agronomically and physiologically evaluated under greenhouse conditions, using a control (nutrient solution), a moderate (70 mM NaCl added to the nutrient solution) and a high salt level (140 mM NaCl), applied for 130 days. The results show that Radja is a Na⁺-excluder genotype, tolerant to moderate salinity. Fruit yield was reduced by 16% and 60% and the shoot biomass by 30% and more than 75% under moderate and high salinities, respectively. At 90 days of salt treatment (DST), the mature leaves feeding the 4th truss at fruiting accumulated little Na⁺ (178 mmol kg^-1 DW). At this time, the sucrose concentration in these leaves even increased with moderate salinity and the amino acid proline was not accumulated under salt conditions as compared to control. At 130 DST, Na⁺ was accumulated mainly by the roots in proportion to the salt level applied, while in leaves appreciable amounts were found only at high salinity (452 mmol kg^-1 DW). In the leaves, Cl^- was always accumulated in proportion to the salt level and in a very much greater amounts than Na⁺ (until 1640 mmol kg^-1 DW). The sucrose content was reduced in all plants by salinity, and was distributed preferentially toward the distal stem and peduncle of a truss at fruiting under moderate salinity, and toward the basal stem and root at high salinity. Moreover, proline was accumulated in different organs of the plant only at high salinity, coinciding with Na⁺ accumulation in leaves. Attempts are made to find a clear relationship between physiological behaviour triggered by stress and the agronomical behaviour, in order to assess the validity of physiological traits used for salt-tolerance selection and breeding in tomato.     

Physiological and growth changes in micropropagated Citrus macrophylla explants due to salinity

Salinity is one of the major abiotic stresses affecting arable crops worldwide, and is the most stringent factor limiting plant distribution and productivity. In the present study, the possible use of in vitro culture to evaluate the growth and physiological responses to salt-induced stress in cultivated explants of Citrus macrophylla was analyzed. For this purpose, micropropagated adult explants were grown in proliferation and rooting media supplemented with different concentrations of NaCl. All growth parameters were decreased significantly by these NaCl treatments; this was accompanied by visible symptoms of salt injury in the proliferated shoots from 60 mM NaCl and in the rooted shoots from 40 mM NaCl. Malondialdehyde (MDA) increased with increasing salinity in proliferated shoots, indicating a rising degree of membrane damage. The concentration of total chlorophyll significantly decreased in the presence of NaCl, and this effect was more pronounced in the rooted explants. The Na⁺ and Cl⁻ concentrations in the explants increased significantly with the salinity level, but Cl⁻ levels were higher in the proliferated explants than in the rooted explants. For osmotic adjustment, high concentrations of compatible solutes (proline and quaternary ammonium compounds—QAC) accumulated in salt-stressed plants in proliferation, but differences were not observed in rooted explants. In proliferation, proline and QAC were highly correlated with the sodium and chloride concentrations in the explants, indicating a possible role of these compounds in osmotic adjustment. The plant concentrations of NO₃⁻, K⁺, Mg²⁺, Ca⁺ and Fe were also affected by the NaCl concentration of the medium. We suggest that the important deleterious effects in the in vitro explants of Citrus macrophylla grown at increasing NaCl concentrations were due mainly to toxic effects of saline ions, particularly Cl⁻, at the cellular level.     

Thermal Sensitivity of the Pool Size of Electrons Available to P700+ Reduction in Intact Maize Leaves

The relative size of the pool of electrons accumulated in stroma reductants during actinic irradiation, which can be donated to P700⁺ via the intersystem chain, was estimated after short-term exposure of intact Zea mays leaves to elevated temperatures. When the temperature increased from 25 to 50 °C by 5 °C steps, the relative size of the stroma electron pool went through a maximum at around 30 °C, and decreased gradually thereafter.     

Effect of potassium on growth and nitrate reductase during water stress and recovery in maize

Abstract The effect of potassium (0,50, 100 and 200 mg/pot) was studied on growth characteristics and nitrate reductase activity in maize (Zea mays) seedlings during water stress and subsequent recovery. In irrigated plants K⁺ increased the rate of leaf area expansion, leading to increased leaf area per plant. Increased leaf area was associated with decreased chlorophyll content. Water stress (–15 bars) enhanced the stomatal resistance of leaves which was further accentuated by K⁺ application. Nitrate reductase activity rose in irrigated plants 24 h after K⁺ application. Subsequently, as water stress developed, K⁺ helped to maintain higher NR activity for the first two days. However, K⁺ had no effect on half life of NR in light or darkness. During recovery from stress K⁺ aided to maintain the higher leaf expansion rate, the chlorophyll content and the stomatal resistance. The results above are discussed in relation to the ability of K⁺ to maintain better growth under water stress.     

Influence of washing on elemental analysis of leaves of fieldgrown crop plants

Summary Information is limited on soil contamination of leaves from field-grown row crops, especially with respect to aluminum (Al) analyses. The objective of this study was to determine the influence of washing leaf samples with either deionized water or detergent solution on elemental analyses for several agronomic crop plants. The crop plants sampled were corn (Zea mays L.), soybean (Glycine max L. Merr.), grain sorghum (Sorghum bicolor L. Moench), and wheat (Triticum aestivum L.). The crops were grown on a range of soil types, soil pH values, and tillage practices. Samples of upper leaves and lower leaves were collected separately. The samples were either not washed, washed with deionized water, or washed with detergent solution. After drying, grinding, and digesting, the samples were analyzed for Al, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu). For all crop plants and conditions studied, there was no effect on measured N, P, K, Ca, Mg, Mn, Zn, or Cu concentrations, but measured Al and Fe concentrations were influenced by washing. In general, washing had a greater effect on Al analyses than on Fe analyses. Soybean samples were most affected by washing, while wheat samples seemed to be least affected. The results reflected greater contamination of lower leaves than upper leaves. Decontamination procedures appear necessary prior to Al and Fe analyses of field-grown crop plants.     

Estimation of growth parameters in salt-stressed maize: comparison of the pressure-block and applied-tension techniques

A custom-built pressure block was used to estimate the effective turgor (turgor pressure minus the yield threshold) and the cell wall extensibility of the growing zone of the third leaves of 8-d-old maize (Zea mays L.) seedlings. In response to cell wall loosening, pressure in the chamber increased rapidly and reached a maximum after approximately 60 min. Plants treated with 80 mol m^?3 NaCl for 4 h were compared to control plants. Pressure-block analysis revealed that salinity reduced effective turgor, but had no effect on cell wall extensibility. These results are qualitatively and quantitatively similar to those obtained with an applied-tension technique used previously in our laboratory. This study indicates that the pressure-block and applied-tension techniques, which use very different methodologies, estimate similar growth parameters.     

Salicylic acid-induced salinity tolerance in corn grown under NaCl stress

Effects of salicylic acid on some physiological and biochemical characteristics of maize ( Zea mays L.) seedlings under NaCl stress were studied. Pre-soaking treatments of NaCl (0, 50, 100 and 200 mM) were given to maize seeds in the presence as well as in the absence of 0.5 mM salicylic acid. Two-week-old maize seedlings exhibited significant decrease in dry weight, root length, shoot length and leaf area on 6 h exposure of 100 and 200 mM NaCl stress. Photosynthetic pigments and NR activity in leaves decreased sharply with increasing stress levels. Both proline content and lipid peroxidation (measured in terms of MDA) levels increased significantly under saline conditions. However, seedlings pretreated with 0.5 mM salicylic acid along with the salinity levels showed enhancement in growth parameters, photosynthetic pigments, NR activity while, free proline and MDA levels decreased. The results showed that salt-induced deleterious effects in maize seedlings were significantly encountered by the pretreatment of salicylic acid. It is concluded that 0.5 mM salicylic acid improves the adaptabilities of maize plants to NaCl stress.     

Effects of NaCl salinity on growth, morphology, photosynthesis and proline accumulation of Salvinia natans

The effects of NaCl salinity on growth, morphology and photosynthesis of Salvinia natans (L.) All. were investigated by growing plants in a growth chamber at NaCl concentrations of 0, 50, 100 and 150 mM. The relative growth rates were high (ca. 0.3 d⁻¹) at salinities up to 50 mM and decreased to less than 0.2 d⁻¹ at higher salinities, but plants produced smaller and thicker leaves and had shorter stems and roots, probably imposed by the osmotic stress and lowered turgor pressure restricting cell expansion. Na⁺ concentrations in the plant tissue only increased three-fold, but uptake of K⁺ was reduced, resulting in very high Na⁺/K⁺ ratios at high salinities, indicating that S. natans lacks mechanisms to maintain ionic homeostasis in the cells. The contents of proline in the plant tissue increased at high salinity, but concentrations were very low (<0.1 μmol g⁻¹ FW), indicating a limited capacity of S. natans to synthesize proline as a compatible compound. The potential photochemical efficiency of PSII (F_v/F_m) of S. natans remained unchanged at 50 mM NaCl but was reduced at higher salinities, and the photosynthetic capacity (ETR_max) was significantly reduced at 50 mM NaCl and higher. It is concluded that S. natans is a salt-sensitive species lacking physiological measures to cope with exposure to high NaCl salinity. At low salinities salts are taken up and accumulate in old leaves, and high growth rates are maintained because new leaves are produced at a higher rate than for plants not exposed to salt.     

Comparison of salt tolerance and osmotic adjustment of low-sodium and high-sodium subspecies of the C4 halophyte, Atriplex canescens

The relationship between Na⁺ accumulation and salt tolerance was tested by comparing subspecies of the halophyte, Atriplex canescens (fourwing saltbush), that differed markedly in Na⁺ content and Na:K ratios. Above ground tissues of one low-sodium and two high-sodium subspecies were compared with respect to cation accumulation, osmotic adjustment and growth along a salinity gradient in greenhouse trials. Plants of each subspecies were grown for 80 d on 2.2, 180, 540 and 720 mol m^?3 NaCl. At harvest, A. canescens ssp. canescens had significantly lower Na⁺ levels, higher K⁺ levels and lower Na:K ratios in leaf and stem tissues than A. canescens ssp. macropoda and linearis over the salinity range (P < 0.05 or 0.01). Na:K ratios in leaves of the latter two, high-sodium, subspecies were approximately 2 on the lowest salinity treatment and ranged from 5 to 10 on the more saline solutions. By contrast, Na:K ratios in leaves of the low-sodium subspecies canescens, were only 0.4 on the lowest salinity and ranged narrowly from 1.7 to 2.3 at higher salinities. However, despite different patterns of Na⁺ and K⁺ accumulation, all three subspecies exhibited equally high salt tolerance and had similar osmotic pressures in their leaves or stems over the salinity range. Contrary to expectations, high salt tolerance was not necessarily dependent on high levels of Na⁺ accumulation in this species.