Osmotic Component Of Salt Stress Research Articles

Impact of salinity on yield-related parameters in two contrasting cultivars of Oryza glaberrima Steud. in Benin

Summary Salinity is one of the major environmental stresses limiting growth and yield of rice. The objective of the present study was to analyze the impact of NaCl on yield-related parameters of Oryza glaberrima. Two contrasted cultivars of Oryza glaberrima previously tested for salt resistance at the vegetative stage [salt-resistant (TOG5307) and salt-sensitive (TOG5949)] were irrigated with a saline solution containing 30 mM NaCl (EC: 3 dS.m−1 NaCl). After 6 months of treatments, mineral nutrient and yield-related parameters were assessed. Proline was quantified in the panicle leaf at the start of the grain filling stage. NaCl treatment affected most yield-related parameters: panicle length, panicle leaf dry weight, number of branches per panicle, panicle leaf length, days to 50% heading, straw fresh weight per plant, grain yield per plant, number of spikelets per panicle, and number of filled grains per panicle. The weight of 1,000 grains decreased in the salt-sensitive cultivar only, suggesting that grain filling processes were compromised. The salt-resistant cultivar TOG5307 was less affected than the salt-sensitive TOG5949 and accumulated lower amounts of Na+ in the grains. For both cultivars, hulls contained higher concentration of Na+ and K+ than grains. TOG5307 also contained more proline in the panicle leaf than TOG5949, suggesting that TOG5307 can cope with the osmotic component of salt stress. The cultivar exhibiting the highest salt resistance at the vegetative stage also exhibited the highest resistance at the reproductive one.

Evaluation of the importance of ionic and osmotic components of salt stress on the photosynthetic efficiency of epiphytic lichens

Salt stress can significantly disrupt the functioning of lichens which are self-sufficient symbiotic organisms inhabiting various severe environments. The aim was to test the effect of salt and sucrose on the photosynthetic efficiency of two selected epiphytic lichens inhabiting the interior of the land. Firstly, we compared the effect of salt and sucrose solutions of different concentrations. Secondly, the effect of salt and sucrose solutions with identical osmotic pressures was compared. The results showed that short-term salt stress leads to a significant reduction of FV/FM, greater changes in chlorophyll fluorescence parameters and OJIP transients compared to the osmotic effects induced by sucrose. This proved that the negative impact of salt stress is associated primarily with ionic effects. The most symptomatic effect of the ionic stress was a significant reduction of the utilisation of trapped energy in electron transport and thereby down-regulation of electron transfer. Since lichens are resistant to a temporary lack of water, ionic stress could have more serious consequences than osmotic stress itself. Hypogymnia physodes was more sensitive to salt stress than Pseudevernia furfuracea, but the reduction of photosynthetic efficiency was not permanent since after 24 h FV/FM returned to the level characteristic for healthy lichens. Nevertheless, repeated exposure to salt may reduce the vitality of lichens growing along communication routes sprinkled with salt in the winter season. Finally, the changes in certain JIP-test parameters were stronger than FV/FM, thus they could be better indicators of salt stress in lichens.

Ability of leaf mesophyll to retain potassium correlates with salinity tolerance in wheat and barley

This work investigated the importance of the ability of leaf mesophyll cells to control K(+) flux across the plasma membrane as a trait conferring tissue tolerance mechanism in plants grown under saline conditions. Four wheat (Triticum aestivum and Triticum turgidum) and four barley (Hordeum vulgare) genotypes contrasting in their salinity tolerance were grown under glasshouse conditions. Seven to 10-day-old leaves were excised, and net K(+) and H(+) fluxes were measured from either epidermal or mesophyll cells upon acute 100 mM treatment (mimicking plant failure to restrict Na(+) delivery to the shoot) using non-invasive microelectrode ion flux estimation (the MIFE) system. To enable net ion flux measurements from leaf epidermal cells, removal of epicuticular waxes was trialed with organic solvents. A series of methodological experiments was conducted to test the efficiency of different methods of wax removal, and the impact of experimental procedures on cell viability, in order to optimize the method. A strong positive correlation was found between plants' ability to retain K(+) in salt-treated leaves and their salinity tolerance, in both wheat and especially barley. The observed effects were related to the ionic but not osmotic component of salt stress. Pharmacological experiments have suggested that voltage-gated K(+) -permeable channels mediate K(+) retention in leaf mesophyll upon elevated NaCl levels in the apoplast. It is concluded that MIFE measurements of NaCl-induced K(+) fluxes from leaf mesophyll may be used as an efficient screening tool for breeding in cereals for salinity tissue tolerance.

Salt and drought tolerance of sugarcane under iso-osmotic salt and water stress: growth, osmolytes accumulation, and antioxidant defense

In order to discriminate between the ionic and osmotic components of salt stress, sugarcane (Saccharum officinarum L. cv. Co 86032) plants were treated with salt-NaCl or polyethylene glycol-PEG 8000 solutions (−0.7 MPa) for 15 days. Both the salt and PEG treatments significantly reduced leaf width, number of green leaves, and chlorophyll stability index. Osmotic adjustment (OA) indicated that both the stresses led to significant accumulation of osmolytes and sugars. Salt stressed plants appeared to use salt as an osmoticum while the PEG stressed plants showed an accumulation of sugars. Oxidative damage to membranes was not severe in plants subjected to salt or PEG stress. The salt stressed plants showed an increase in the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX), while PEG stress led to an increase in SOD but not APX activity as compared to the control. Thus, results indicate that the iso-osmotic salt or PEG stress led to differential responses in plants especially with respect to growth, OA, and antioxidant enzyme activities.

Effects of NaCl and iso-osmotic PEG stress on growth, osmolytes accumulation and antioxidant defense in cultured sugarcane cells

In order to discriminate between the ionic and osmotic components of salt stress, sugarcane (Saccharum officinarum L. cv. Co 86032) calli were cultured on media containing NaCl or polyethylene glycol (PEG) 8000 that exerted the same osmotic pressure (−0.7 MPa). PEG stress exposure for 15 days led to significant growth reduction and loss in water content than salt stressed and control tissues. Osmotic adjustment (OA) was observed in callus tissues grown on salt, but was not evident in callus grown on PEG. Oxidative damage to membranes, estimated in terms of accumulation of thiobarbituric acid reactive substances-TBARS and electrolytic leakage was significantly higher in both the stressed calli than the control however, the extent of damage was more in the PEG stressed calli. The stressed callus tissues showed inhibition of ascorbate peroxidase activity, while catalase activity was increased. These results indicate sensitivity of cells to PEG-mediated stress than salt stress and differences in their OA to these two stress conditions. The sensitivity to the osmotic stress indicate that expression of the stress tolerance response requires the coordinated action of different tissues in a plant and hence was not expressed at the cellular level.

A DREB gene from the xero-halophyte Atriplex halimus is induced by osmotic but not ionic stress and shows distinct differences from glycophytic homologues

Studying the regulation of stress inducible genes can lead to understanding of the mechanisms by which halophytes maintain growth and thrive under abiotic stress. Verifying whether ionic or osmotic components of salt stress control the regulation of Dehydration Responsive Element- binding transcription factor (DREB) was investigated in the present study. DREB belongs to AP2/EREB group that is induced under abiotic stress and regulates many stress inducible genes that contain DRE binding sites in their promoters. DREB in Atriplex halimus was regulated by the osmotic component but not by the ionic one of salt stress. It seemed that DREB was not involved in the regulation of sodium manipulating genes like NHX1, SOS1 or H +-PPase. Moreover, DREB could be involved directly of indirectly in CMO regulation because of timing of induction. Also, DREB was the most upregulated gene under salt (fivefold) and drought (twofold) conditions, which reinforced the importance of this gene in A. halimus tolerance to stress. Moreover, its constitutive expression under normal conditions also indicated its involvement in other growth and developmental programs. The tolerance of A. halimus and of halophytes, in general, could be attributed to the constitutive expression of its genes and differences in protein structures between halophytes and glycophytes.

Sensitivity of Trifolium alexandrinum L. to salt stress is related to the lack of long-term stress-induced gene expression

In this study, the physiological and molecular defects leading to growth reduction under salt stress in Egyptian clover ( Trifolium alexandrinum L.) were investigated. Time-course response of growth to salt stress with 200 mM Na + and/or Cl − showed that significant reduction in the growth of the stressed plants was delayed until the sixth day of stress indicating that the plant is more resistant to the osmotic component of salt stress. The rate of photosynthesis started to decrease significantly from the third day of stress although the rate of transpiration decreased to 18.4% of the control after 2 h indicating that reduction in photosynthetic rate may have not been essentially due to CO 2 limitation. The mutual relationships of photosynthesis and growth are discussed and evidence is provided that growth reduction is largely a consequence rather than a cause of inhibition of photosynthesis. Data also shows that inhibition of photosynthesis could have resulted from excessive accumulation of Na + in the leaves along with downregulation of Na +-sequestering genes ( NHX1, H + -PPase and H +-ATPase) that may have led to loss of the efficiency of Na + sequestration into the vacuole and subsequent damage to photosynthesis. The role of HRD gene in regulating transpiration is discussed.

Importance of Ionic and Osmotic Components of Salt Stress on the Germination of Four Quinua (Chenopodium quinoa Willd.) Selections

The influence of the components of saline stress (osmotic and ionic factors) were evaluated during the germination of four quinua (Chenopodium quinoa Willd.) selections in Chile, two collected in the arid northern highland region with saline soils (Amarilla and Roja) and the other two from the rainy southern region with non-saline soils (Hueque and Pucura). The seeds were treated with different saline concentrations (0, 0.2, 0.4, 0.8 and 1.2 M NaCl). Germination kinetics shows that the control treatments of the four selections reach 100% germination. With application of 0.4 M NaCl (LD50max) germination was reduced by 53% in the Amarilla selection and 89.9% in Hueque. The germination rate was lower for the seeds from non-saline areas, which reached germination after 22 h, compared to 10 h for Amarilla. By separating the salinity stress factors (osmotic and ionic), it is evident that they have different degrees of influence on quinua germination, according to each selection. Thus, the germination of Amarilla selection from the arid area was the least influenced by the ionic factor (27%), Pucura and Roja were more affected by the ionic factor (40% and 46% respectively), and Hueque, from the rainy area, was more affected by the osmotic factor, in which caused a 50% loss in germination.

Salt stress-induced cell death in the unicellular green alga Micrasterias denticulata

Programmed cell death (PCD) is a key element in normal plant growth and development which may also be induced by various abiotic and biotic stress factors including salt stress. In the present study, morphological, biochemical, and physiological responses of the theoretically immortal unicellular freshwater green alga Micrasterias denticulata were examined after salt (200 mM NaCl or 200 mM KCl) and osmotic stress induced by iso-osmotic sorbitol. KCl caused morphological changes such as cytoplasmic vacuolization, extreme deformation of mitochondria, and ultrastructural changes of Golgi and ER. However, prolonged salt stress (24 h) led to the degradation of organelles by autophagy, a special form of PCD, both in NaCl- and KCl-treated cells. This was indicated by the enclosure of organelles by ER-derived double membranes. DNA of NaCl- and KCl-stressed cells but not of sorbitol-treated cells showed a ladder-like pattern on agarose gel, which means that the ionic rather than the osmotic component of salt stress leads to the activation of the responsible endonuclease. DNA laddering during salt stress could be abrogated by addition of Zn2+. Neither cytochrome c release from mitochondria nor increase in caspase-3-like activity occurred after salt stress. Reactive oxygen species could be detected within 5 min after the onset of salt and osmotic stress. Respiration, photosynthetic activity, and pigment composition indicated an active metabolism which supports programmed rather than necrotic cell death in Micrasterias after salt stress.

Response To Salinity In Rice: Comparative Effects Of Osmotic And Ionic Stresses

The Effects Of The Osmotic Component Of Salt Stress On Rice Cultivar Ir64 Were Examined. Treatments Were Four Combinations Of Two Levels Of Osmotic Stress At Two Developmental Stages: Medium- And High-Level Stress Applied At The Vegetative And Reproductive Stages Using Salt (Nacl) And Polyethylene Glycol-6000 (Peg) As Sources Of Osmotic Stress. Both Peg And Nacl Reduced The Total Above Ground Biomass And Delayed Flowering And Maturity, With A Longer Delay Observed With The High-Level Stress. The Reduction In Number Of Filled Spikelets, 1,000-Grain Weight, And Hence Grain Yield Was Significantly Greater When They Were Applied During The Reproductive Stage Than During The Vegetative Stage. The Sodium Concentration In Plant Tissues Also Increased In Plants Treated With Nacl, Indicating That Besides Osmotic Stress, Plants Were Also Subjected To Ionic Stress. Treatment With Nacl Decreased The Potassium Concentration In Plant Tissues But Did Not Cause Significant Differences In Phenology, Biomass Accumulation, Yield Or N Uptake Compared With Peg. We Concluded That The Response Of Ir64 To Nacl Was Attributed To The Osmotic Component. These Findings May Be Specific To Ir64, Which Has A Medium Tolerance To Salinity Stress. Further Studies Are Needed With Longer Stress Durations To Achieve A Higher Na+ Concentration In Plant Tissues In Several Varieties With Contrasting Tolerance To Salt Stress To Further Establish The Relative Importance Of Osmotic Versus Ionic Components Of Salt Stress In Rice.

Seawater stress applied at germination affects mitochondrial function in durum wheat (Triticum durum) early seedlings.

Seawater stress effects on mitochondrial ATP synthesis and membrane potential (ΔΨ) were investigated in germinating durum wheat seedlings under moderate (22% seawater osmolarity, -0.62 MPa) and severe (37% seawater osmolarity, -1.04 MPa) stress. To estimate the osmotic component of salt stress, mannitol solutions (0.25 and 0.42 m) iso-osmotic with the saline ones were used. Moderate stress intensity only delayed mean germination time (MGT), whereas higher seawater osmolarity reduced germination percentage as well. In contrast, Na+ and Cl- accumulation showed a sharp increase under moderate stress and only a small further increase under severe stress, which was more pronounced for Cl-. Only severe stress significantly damaged succinate-dependent oxidative phosphorylation, which may be related to the stress-induced alteration in inner mitochondrial membrane permeability, as indicated by changes in ΔΨ profiles. Proline-dependent oxidative phosphorylation, however, was inhibited under moderate stress. This suggests the occurrence of an adaptation mechanism leading to proline accumulation as an osmoprotectant. Moreover, both the osmotic and the toxic components of seawater stress were detrimental to oxidative phosphorylation. Damage to germination and MGT, in contrast, were mainly caused by osmotic stress. Therefore, mitochondrial function appears to be a more sensitive target of toxic stress than growth. In conclusion, the effects of seawater stress on mitochondrial ATP synthesis vary in relation to the substrate oxidised and stress level, inducing both adaptive responses and damage.

Effect of NaCl and mannitol iso-osmotic stresses on proline and free polyamine levels in embryogenic Fraxinus angustifolia callus

With the aim to differentiate the ionic and osmotic components of salt stress, short and long-term changes in free polyamines and proline induced by iso-osmotic concentrations of NaCl (0.1 mol/L and 0.2 mol/L) and mannitol (0.2 mol/L and 0.4 mol/L) were determined in Fraxinus angustifolia callus. The peculiarities of the short-term responses were: i) a very early (30 min) and temporary increase in Putrescine (Pu) and Spermine (Spm) as a consequence of salt treatment, and ii) a continuous accumulation of Spermidine (Spd) and Spm in response to mannitol. The changes of Proline (Pro) were quite limited both in the short and in the long term, and generally occurred later than Polyamine (PAs) changes took place, suggesting a regulatory mechanism of PAs metabolism on Pro biosynthesis. In the long-term, no drastic accumulations of Pro or PAs in response to NaCl and mannitol were observed, suggesting that their physiological role is unlikely to be that of osmo-compatible solutes in this plant system. The salt induced a higher callus growth inhibition effect than did mannitol and this inhibition was associated with the reduction of endogenous levels of PAs, especially Pu. However, while a diverging time course was observed under lethal salt concentration (0.2 mol/L NaCl), a high parallelism in the endogenous changes of Pro and Pu was observed under all non-lethal conditions (control--0.2 and 0.4 mol/L mannitol--0.1 mol/L NaCl). Therefore the synchronous changes of Pro and Pu can be considered as a physiological trait associated with cell survival. These results indicate a strong metabolic co-ordination between PAs and Pro pathways and suggest that the metabolic fluxes through these pathways start competing only when the stress level is high enough to be lethal for cells.

Characterisation of rice (Oryza sativa) F3 populations selected for saltresistance. 2. Relationships between yield-related parameters andphysiological properties

One somaclonal family (1-23) regenerated from a salt-resistant callus, its initial cultivar I Kong Pao (IKP, salt-sensitive), the breeding line IR31785 (extremely salt-sensitive), the moderately salt-resistant cultivar Aiwu and F3 populations derived from several crosses were exposed until maturity to a low sub-lethal (30�mmol�NaCl/L) dose of salt. The agronomic performance of this material was compared with its physiological behaviour, as recorded during the vegetative growth phase. Although grain yield of 1-23 was similar to the yield of its initial cultivar IKP in both control and salt-stress conditions, a strong increase in salt resistance in terms of grain yield per plant was observed in their F3 population derived from IR31785 × 1-23, which performed better than the corresponding parents and better even than the cultivar Aiwu in the presence of 30 mmol/L NaCl. Such improvement was due to a high number of spikelets per plant and a high spikelet fertility. An improved salt resistance in terms of grain yield was also observed in the F3 population derived from IR31785 × IKP. Among physiological parameters, only K : Na ratios quantified in stress conditions are useful as a predictive criteria for yield potential, and salt resistance to low NaCl does not involve any adaptation to the osmotic component of salt stress. No physiological parameters quantified in the absence of stress were correlated with yield in the presence of NaCl. Results are discussed in relation to the usefulness of somaclonal variation for plant breeding.

Discrimination between the ionic and osmotic components of salt stress in relation to free polyamine level in rice ( Oryzasativa)

In order to precise the relative importance of ionic and osmotic components of salt stress on modification of free polyamine level, seedling of two rice cultivars (I Kong Pao (IKP): salt-sensitive and Pokkali: salt-resistant) were exposed to isoosmotic concentrations of NaCl, KCl (50 and 100 mM) or polyethylene glycol (PEG 6000, 16 and 26%) for a maximal period of 12 h in daylight conditions. Both ion and polyamine concentrations were already modified after 3 h of exposure to ionic stresses. At the shoot level, this response occurred independently of any change in the plant water status. Conversely, exposure to 26% PEG during 12 h induced a strong decrease in shoot osmotic potential and water content but had only a limited impact on endogenous free polyamine level. It was thus suggested that the ionic component by itself might trigger short-term polyamine accumulation. This effect might be attributed to monovalent ions, although K + and Na + accumulation had different effects on polyamine content. Putrescine may assume differential role in non-photosynthetic organs versus photosynthetic ones since it accumulated to high amounts in the roots of the salt-resistant cultivar Pokkali comparatively to salt-sensitive cultivar IKP, while an opposite trend was recorded in the shoots. Tyramine was also present at higher concentrations in the roots of Pokkali; its level clearly increased in response to ionic stresses while cadaverine level increased in water stress conditions only. No clear relationship was observed between the mean level of salinity resistance and the endogenous concentrations of spermidine or spermine.

Effect of salt and osmotic stresses on germination in durum wheat (Triticum durum Desf.)

In order to determine the relative importance of ionic toxicity versus the osmotic component of salt stress on germination in durum wheat (Triticum durum Desf.), seeds of three cultivars differing in their salt and drought resistance (Omrabi-5, drought-resistant; Belikh, salt-resistant and Cando, salt-sensitive) were incubated in various iso-osmotic solutions of NaCl, mannitol and polyethylene-glycol (PEG) (osmotic potential of −0.15 (control solution) −0.58, −1.05 or −1.57 MPa). Moderate stress intensities only delayed germination, whereas the highest concentration of NaCl and PEG reduced final germination percentages. PEG was the most detrimental solute, while mannitol had no effect on final germination percentages. All osmotica reduced endosperm starch and soluble sugars content as well as α-amylase activities recorded after 48 h of treatment while β-amylase activities were, in contrast, slightly stimulated in all cultivars. Deleterious effects of NaCl and PEG were higher on isolated embryos germinated onto an in vitro Linsmaier and Skoog (LS) medium comparatively to whole seeds. All PEG-treated embryos, however, recovered after the stress relief while NaCl-treated embryos exhibited a lower rate of recovery and some extent of abnormal germination after rinsing. It was concluded that stress inhibition of germination could not be attributed to an inhibition of mobilisation of reserves and that the main effect of PEG occurred via an inhibition of water uptake while detrimental effects of NaCl may be linked to long-term effects of accumulated toxic ions. The behaviour of the three cultivars during germination did not fully reflect their mean level of putative stress resistance in field conditions and germination is, therefore, not recommended as a reliable selection criterion for breeding purposes.

Ionic and osmotic components of salt stress specifically modulate net ion fluxes from bean leaf mesophyll

ABSTRACTIonic mechanisms of salt stress perception were investigated by non‐invasive measurements of net H+, K+, Ca2+, Na+, and Cl− fluxes from leaf mesophyll of broad bean (Vicia faba L.) plants using vibrating ion‐selective microelectrodes (the MIFE technique). Treatment with 90 m M NaCl led to a significant increase in the net K+ efflux and enhanced activity of the plasma membrane H+‐pump. Both these events were effectively prevented by high (10 m M) Ca2+ concentrations in the bath. At the same time, no significant difference in the net Na+ flux has been found between low‐ and high‐calcium treatments. It is likely that plasma membrane K+ and H+ transporters, but not the VIC channels, play the key role in the amelioration of negative salt effects by Ca2+ in the bean mesophyll. Experiments with isotonic mannitol application showed that cell ionic responses to hyperosmotic treatment are highly stress‐specific. The most striking difference in response was shown by K+ fluxes, which varied from an increased net K+ efflux (NaCl treatment) to a net K+ influx (mannitol treatment). It is concluded that different ionic mechanisms are involved in the perception of the ‘ionic’ and ‘osmotic’ components of salt stress.

Salinity stress inhibits calcium loading into the xylem of excised barley (Hordeum vulgare) roots

summarySalinity stress inhibits Ca translocation to the shoot, leading to Ca deficiency. The objective of this study was to determine whether salt stress inhibits Ca translocation through effects on younger root regions, where radial Ca transport is largely apoplastic, or through effects on mature regions of the roots, where radial Ca transport is largely symplastic. Roots were excised from 4‐d‐old dark‐grown barley seedlings. Calcium translocation was studied by application of 45Ca 3 or 6 cm from the root tip and measurement of its appearance in the root exudate. Calcium uptake along the axis of excised roots was studied with a vibrating Ca+2 microelectrode. Salt stress (60 mM NaCl) inhibited Ca translocation from the 6 cm region more than from the 3 cm region. Sodium sulphate treatments (30 and 36 mM) were more deleterious to Ca translocation than was NaCl in the 3 cm region, and 110 mM mannitol and 60 mM KC1 affected Ca translocation similarly to Na2SO4. The salt and mannitol treatments were more deleterious to Ca translocation in the 6 cm region than in the 3 cm region, and mannitol and KC1 inhibited Ca translocation more than the sodium treatments. Supplemental Ca could overcome the inhibition of Ca translocation by NaCl stress. Calcium uptake into the root at the 3 and 6 cm regions was not reduced by NaCl, but was retarded 0–05 cm from the apex. We conclude that symplastic Ca transport is more inhibited than apoplastic transport in salinized roots, and that the osmotic component of salt stress causes most of the inhibition. Since symplastic transport is inhibited, the possibility exists that research into the effects of salinity on the transport functions of endodermal cells will yield information that can be used to improve Ca translocation in salt‐affected plants.

Salt regulation of transcript levels for the c subunit of a leaf vacuolar H+‐ATPase in the halophyte Mesembryanthemum crystallinum

The halophyte Mesembryanthemum crystallinum is an inducible crassulacean acid metabolism (CAM) plant native to seasonally arid coastal environments that has been widely used to study plant responses to environmental stress. On exposure of plants to salt, the activities of both the tonoplast (vacuolar) H(+)-ATPase (V-ATPase) and Na+/H+ antiporter increase in leaf cells, thereby energizing vacuolar salt accumulation. To investigate the molecular basis of this response, a cDNA (Vmac1) encoding the H(+)-conducting c subunit (16.6 kDa) of an M. crystallinum V-ATPase has been cloned. Northern analysis of RNA from leaves of plants treated with NaCl or with isoosmotic mannitol solutions demonstrated (i) that NaCl increased steady-state transcript levels for the V-ATPase c subunit, and (ii) that this effect was caused by the ionic rather than the osmotic component of salt stress. Southern analysis of genomic DNA suggested the probable existence of more than one gene for this subunit of the V-ATPase in M. crystallinum. Expression studies using the 3'-untranslated region of the Vmac1 cDNa as a probe showed that the corresponding salt-inducible transcript was preferentially expressed in leaves. Induction by salt was also observed in juvenile plants in addition to adult ones. These findings, as well as the inability of mannitol to upregulate mRNA levels for this gene, clearly differentiate between the induction of transcript for the V-ATPase c subunit and for genes involved in the CAM pathway in M. crystallinum. Further, the plant growth regulator abscisic acid (ABA) was able to mimic the effect of salt on transcript levels for the V-ATPase c subunit, suggesting the possible involvement of ABA in a distinct signal-transduction pathway linked to vacuolar salt accumulation in this highly salt-tolerant species.