Osmotic Constraint Research Articles

Silicon Improves Rice Salinity Resistance by Alleviating Ionic Toxicity and Osmotic Constraint in an Organ-Specific Pattern.

Salinity stress severely inhibits the growth of plant via ionic toxicity and osmotic constraint. Exogenous silicon (Si) can alleviate salinity stress, but the mechanisms behind remain unclear. To investigate the role of Si in alleviating ionic and osmotic components of salinity, rice (Oryza sativa L.) seedlings were grown hydroponically in iso-osmotic stress conditions developed from NaCl or polyethylene glycol (PEG). The effects of Si on the growth of shoot and root of rice under salinity and PEG-derived osmotic stress were evaluated and further compared using principal coordinate analysis (PCoA). We also analyzed the concentrations of Na, K, and compatible osmolytes, tissue sap osmotic potential, antioxidant enzymes activities, and the expression of aquaporin genes. Generally, Si significantly promoted shoot and root growth in rice exposed to both NaCl and PEG. PCoA shows that the Si-induced distance change under NaCl treatment was larger than that under PEG treatment in the shoot, while the Si-induced distance changes under NaCl and PEG treatments were at an equal level in the root. Under salinity, Si decreased Na concentration and Na/K ratio in the shoot but not in the root. However, Si decreased net Na uptake and increased root Na accumulation content. Osmotic potential was increased in the shoot but decreased in the root by Si addition. Si decreased soluble sugar and proline concentrations in the shoot but increased soluble sugar and soluble protein concentrations in the root. Besides, Si promoted shoot transpiration rate and root morphological traits. Although both NaCl and PEG treatments upregulated aquaporin gene expression, Si addition maintained the expression of OsPIPs under NaCl and PEG treatments at same levels as control treatment. Furthermore, Si alleviated oxidative damages under both NaCl and PEG by regulating antioxidant enzyme activities. In summary, our results show that Si improves salt stress tolerance in rice by alleviating ionic toxicity and osmotic constraint in an organ-specific pattern. Si ameliorates ionic toxicity by decreasing Na uptake and increasing root Na reservation. Si alleviates osmotic constraint by regulating root morphological traits and root osmotic potential but not aquaporin gene expression for water uptake, and promoting transpiration force but not osmotic force in shoot for root-to-shoot water transport.

Intracellular osmoprotectant concentrations determine Propionibacterium freudenreichii survival during drying

Propionibacterium freudenreichii is a beneficial bacterium widely used in food as a probiotic and as a cheese-ripening starter. In these different applications, it is produced, dried, and stored before being used. Both freeze-drying and spray-drying were considered for this purpose. Freeze-drying is a discontinuous process that is energy-consuming but that allows high cell survival. Spray-drying is a continuous process that is more energy-efficient but that can lead to massive bacterial death related to heat, osmotic, and oxidative stresses. We have shown that P. freudenreichii cultivated in hyperconcentrated rich media can be spray-dried with limited bacterial death. However, the general stress tolerance conferred by this hyperosmotic constraint remained a black box. In this study, we modulated P. freudenreichii growth conditions and monitored both osmoprotectant accumulation and stress tolerance acquisition. Changing the ratio between the carbohydrates provided and non-protein nitrogen during growth under osmotic constraint modulated osmoprotectant accumulation. This, in turn, was correlated with P. freudenreichii tolerance towards different stresses, on the one hand, and towards freeze-drying and spray-drying, on the other. Surprisingly, trehalose accumulation correlated with spray-drying survival and glycine betaine accumulation with freeze-drying. This first report showing the ability to modulate the trehalose/GB ratio in osmoprotectants accumulated by a probiotic bacterium opens new perspectives for the optimization of probiotics production.

Impact of Molecular Architecture on Dynamics of Miktoarm Star Copolymers

Broadband dielectric spectroscopy (BDS) is used to probe the chain and segmental dynamics of A2B2 and AB2 miktoarm star copolymers based on polystyrene (PS, A block) and polyisoprene (PI, B block) that display lamellar morphologies as determined using small-angle X-ray scattering (SAXS). While no changes in the distribution of PI segmental relaxation times are observed with variation of the molecular architecture, an unexpected increase in the normalized PI chain relaxation intensity is realized for AB2 miktoarm star copolymers as well as a change in the distribution of chain relaxation rates as compared to A2B2 and AB diblock copolymer systems. This result is attributed to asymmetry in the molecular architecture near the junction point, which affects the osmotic constraint of the tethered PI chains within the interfacial region of the lamellae. The results highlight the importance of macromolecular design on fundamental chain dynamics in phase-separated thermoplastic elastomers.

Glycine betaine transport conditions of Lactobacillus delbrueckii subsp. bulgaricus in salt induced hyperosmotic stress

Glycine betaine (GB) is a naturally occurring osmolyte that helps cells adapt to osmotic stress. In this study, the osmoprotective role of GB in Lactobacillus delbrueckii subsp. bulgaricus 34.5 under salt stress was investigated. A concentration of 0.2 m NaCl provided sufficient osmotic constraint while allowing the cells to survive. Under conditions of salt stress, Lactobacillus bulgaricus 34.5 continued to proliferate in the presence of GB, and the optimum growth was seen at 1.2 mm GB. The optimum culture conditions for L. bulgaricus 34.5 were 37 °C and pH 6.0 for 24 h. In addition, intracellular GB content increased in response to salt stress, and the extent of GB transport in L. bulgaricus 34.5 under high salt concentrations depended on the culture conditions. In conclusion, GB played a protective role in alleviating the detrimental effect of salt on L. bulgaricus 34.5.

Effect of salinity on locomotor performance and thermal extremes of metamorphic Andean Toads (Rhinella spinulosa) from Monte Desert, Argentina

Rhinella spinulosa is distributed from Peru to Argentina (from 1200 to 5000 m elevation), inhabiting arid mountain valleys of the Andes, characterized by salty soils. The variations in soil salinity, caused by high evapotranspiration of water, can create an osmotic constraint and high thermal oscillations for metamorphsed Andean toad (R. spinulosa), affecting their thermoregulation and extreme thermal tolerances. We investigated the changes in thermal tolerance parameters (critical thermal maximum and crystallization temperature) of a population of metamorphosed R. spinulosa from the Monte Desert of San Juan, Argentina, under different substrate salinity conditions. Our results suggest that the locomotor performance of metamorphs of R. spinulosa is affected by increasing salinity concentrations in the environment where they develop. On the other hand, the thermal extremes of metamorphs of R. spinulosa also showed changes associated with different salinity conditions. According to other studies on different organisms, the increase of the osmolarity of the internal medium may increase the thermal tolerance of this species. More studies are needed to understand the thermo-osmolar adjustments of the metamorphs of toads to environmental variability.

Reproductive traits of the “ocellated killifish” Floridichthys polyommus Hubbs, 1936 (Pisces: Cyprinodontidae) inhabiting estuary of the Champotón River (Campeche, Mexico)

This study explores the reproductive biology and the influence of environmental factors on the reproductive traits of Floridichthys polyommus inhabiting the estuary of the Champoton River. The analysis included population structure, gonadal developmental stages, reproductive period, size at first sexual maturity, fecundity, sex ratio, and somatic indexes. A spatio-temporal pattern in the distribution of the seven size classes was detected. The species showed a positive allometric growth. Sex ratio was 1.21:1 (females:males). Size at first maturity was 42.23 mm (females) and 47.8 mm (males). A higher absolute and relative fecundity was detected in Puente Champotón estuary (PCH) with lower salinities than those detected in the Delta (DE). The adaptive significance of this trait could be in response to the heavy osmotic constraint imposed by extreme salinities in the DE. Floridichthys polyommus prefers PCH for spawning, and as a result a higher number of ripe individuals was observed in this site, characterized by the highest nutrient levels, which can boost productivity and food availability for fish. Hepatosomatic index was negatively correlated with gonadosomatic index, suggesting energy transfer from the liver towards gamete production. Floridichthys polyommus exhibits an opportunistic strategy (early maturity, small oocytes, small clutches) suggesting that energy is allocated towards reproduction to compensate for the unpredictable hydrological conditions imposed by the estuary (particularly during the hurricane season). Floridichthys polyommus shows a reproductive strategy exhibited by fish living in unstable systems. A seasonal divergence was observed in the reproductive traits of F. polyommus, while differences between study sites were for fecundity.

Seed germination ecology of a medicinal halophyte Zygophyllum propinquum: responses to abiotic factors

Information about the seed biology of the medicinally important halophyte Zygophyllum propinquum is limited. We therefore studied seed germination and recovery responses to different abiotic factors. Seed germination and recovery responses to different thermoperiods (10/20, 15/25, 20/30 and 25/35°C), photoperiods (12h light/12h dark and 24h dark), salts (chloride and sulphate salts of Na, K, Ca and Mg), and mannitol were studied. Seeds were non-dormant and germinated maximally (≈95%) in distilled water. Relatively high seed germination was observed under moderate (15/25 and 20/30°C) temperature regimes and in 12h photoperiod. Seed germination decreased with increases in salinity and only few seeds (<30%) germinated at 200mM NaCl. However, most un-germinated seeds from high salinity showed recovery of germination when transferred to water. Among chloride (Cl−) salts inhibitory effects at −1.2MPa under light were in following order of magnitude: KCl>CaCl2> NaCl=MgCl2, while for sulphate (SO4−−) salts inhibitory effects were in following order: K2SO4>MgSO4>Na2SO4. Germination inhibition under various osmotic treatments of mannitol was generally lesser than the iso-osmotic NaCl treatments. These results indicate that seeds Z. propinquum were moderately salt tolerant and preferred to germinate under moderate temperatures and in the presence of light. Recovery data following different salt treatments hints at greater role of osmotic constraint than the specific ion effects.

Response of Osmotic Adjustment of Lactobacillus bulgaricus to NaCl Stress

Growth and osmotic response of Lactobacillus bulgaricus ATCC 11842 under hyperosmotic constraint were investigated in a chemically defined medium (CDM) and MRS medium. NaCl could inhibit the growth of L. bulgaricus which decreased with increasing NaCl concentration. In the MRS, NaCl of 1.0 mol · L−1 was the biggest salt stress concentration; in the CDM, 0.8 mol · L−1 was the biggest inhibition concentration. In contrast to what was observed in other lactic acid bacteria, proline, glycine betaine and related molecules were unable to relieve inhibition of growth of L. bulgaricus under osmotic constraint. This was correlated to the absence of sequences homologous to the genes coding for glycine-betaine and/or proline transporters described in Lactococcus lactis and Bacillus subtilis. The amino acid aspartate and alanine were proved to be osmoprotective under NaCl stress. Addition of peptone (0.25% w/v) in the presence of salt led to a stimulation of the growth, as the decrease of the lag time and generation time, and the final biomass increased from 0.31 to 0.64.

Molecular basis of the adaption of the anchovy isolate Lactobacillus sakei CRL1756 to salted environments through a proteomic approach

Osmotic stress triggered by high salt concentrations results in the injury of starters during food processing. The adaption of the anchovy isolate Lactobacillus sakei CRL1756 to salted environments in the presence of osmoprotectants was analyzed by using a proteomic approach. Glycine–betaine (GB) pre-treated cells exposed to 1.7M NaCl+3mMGB showed improved adaption achieving growth after a long lag phase while non-proliferative cells in the salted medium without GB was observed. Comparative proteomic analyses of the Lactobacillus cells grown in MRS+GB exposed or not to 10% salt revealed eighteen significantly regulated proteins. Under hypertonic stress, decreased synthesis of four glycolytic enzymes (Fba, Pgk, Gpm5, Tpi) and induction of other two (MleS, Pox2) related to an alternative energetic pathway, were registered. Proteins related to general stress response and nucleotide metabolism were up-regulated. Noticeably, the induction of DyP-type peroxidase, involved in iron transport, detoxification and oxidative stress, was observed for the first time in lactic acid bacteria under osmotic constraint. Our results demonstrate that GB played a significant role in protecting L. sakei CRL1756 against salt stress. Indeed, GB commonly found in marine fish may be used by the starter culture cells ensuring their robustness in salted-anchovy based products.

Dielectric Behavior of Guestcis-Polyisoprene Confined in Spherical Microdomain of Triblock Copolymer.

Dielectric behavior was examined for ternary blends of guestcis-polyisoprene (PI) chains, coguest PS chains, and host PS–PI–PS triblock copolymers. The guest PI chains had the molecular weight Mg-PI well below Mb-PI of the PI blocks and were mixed in the spherical PI domains formed in the copolymers. PI has the type-A dipoles parallel along the chain backbone and its end-to-end vector fluctuation activates dielectric relaxation. The dielectric test was conducted at temperatures well below the glass transition temperature of the PS blocks, so that the PI blocks had their ends fixed on the glassy PS matrix and exhibited no end-to-end vector fluctuation. Thus, the slow dielectric response of the systems exclusively detected the global motion of the guest PI chains confined in the spherical PI domain. The dielectric mode distribution of the guest PI chains thus observed was broader than that of the same chains in bulk systems, reflecting the osmotic and spatial constraints on the chain dynamics in the spherical domains. The guest PI chains of different Mg-PI exhibited nearly the same extent of mode broadening, despite their average coil size approached the domain size with increasing Mg-PI. This result suggested that the osmotic constraint was the primary factor affecting (broadening and retarding) the guest chain dynamics. The osmotic constraint appeared to be stronger in the spherical domain than in the lamellar domain, the latter being examined in literature.

Comparative Analysis of Reproductive Traits in Black-Chinned Tilapia Females from Various Coastal Marine, Estuarine and Freshwater Ecosystems

The black-chinned tilapia Sarotherodon melanotheron is a marine teleost characterised by an extreme euryhalinity. However, beyond a certain threshold at very high salinity, the species exhibits impaired growth and precocious reproduction. In this study, the relationships between reproductive parameters, environmental salinity and condition factor were investigated in wild populations of this species that were sampled in two consecutive years (2003 and 2004) from three locations in Senegal with different salinities: Guiers lake (freshwater, 0 psu), Hann bay (seawater, 37 psu) and Saloum estuary (hypersaline water, 66–127 psu). The highest absolute fecundity and spawning weight were recorded in seawater by comparison to either freshwater or hypersaline water whereas the poorest condition factors were observed in the most saline sampling site. These results reflect higher resource allocation to the reproduction due to the lowest costs of adaptation to salinity in seawater (the natural environment of this species) rather than differences in food resources at sites and/or efficiency at foraging and prey availability. Fecundities, oocyte size as well as spawning weight were consistent from year to year. However, the relative fecundity in the Saloum estuary varied significantly between the dry and rainy raisons with higher values in the wet season, which seems to reflect seasonal variations in environmental salinity. Such a reproductive tactic of producing large amounts of eggs in the rainy season when the salinity in the estuary was lower, would give the fry a better chance at survival and therefore assures a high larval recruitment. An inverse correlation was found between relative fecundity and oocyte size at the two extreme salinity locations, indicating that S. melanotheron has different reproductive strategies in these ecosystems. The adaptive significance of these two reproductive modes is discussed in regard to the heavy osmotic constraint imposed by extreme salinities and high inter-specific competition.

Response of Tunisian autochthonous pearl millet (Pennisetum glaucum (L.) R. Br.) to drought stress induced by polyethylene glycol (PEG) 6000

Seeds of pearl millet (Pennisetum glaucum (L) R. Br.) from six provenances of Tunisia were subjected to germination and shoot and root length tests on filter paper treated with polyethylene glycol 6000 (PEG 6000) solutions made up to provide osmotic potentials of 0, -1 and -2 MPa. Mean germination percent for all provenances decreased about 73% in -2 MPa compared to control (0 MPa) treatment. Osmotic potential of -1.0 MPa improves the rate of germination but not significantly compared to the control. There were significant differences among the relative germination percent of the provenances in different treatments. Decreases in the external osmotic potential induced decreased shoot growth while a slight increase in root length associate with the -1 MPa treatments was observed for some ecotypes. This reflects an adaptive response involving an increase in root length to reach deeper water in the soil. It was also notified that the elongation of the radicle is more sensitive to the osmotic constraint than the coleoptile.

Tolerance to high osmolality of the lactic acid bacterium Oenococcus oeni and identification of potential osmoprotectants

Growth of the lactic acid bacterium Oenococcus oeni under hyperosmotic constraint was investigated in a chemically defined medium. The bacterium could grow on media with an elevated osmolality, preferably below 1.5 Osm kg − 1 H 2O. At osmolalities comprised between 0.6 and 1.5 Osm kg − 1 H 2O, the growth deficit elicited by the sugars glucose and fructose was slightly more severe than with salts (NaCl or KCl). In contrast to what was observed in other lactic acid bacteria, proline, glycine betaine and related molecules were unable to relieve inhibition of growth of O. oeni under osmotic constraint. This was correlated to the absence of sequences homologous to the genes coding for glycine betaine and/or proline transporters described in Lactococcus lactis and Lactobacillus plantarum. The amino acid aspartate proved to be osmoprotective under electrolyte and non-electrolyte stress. Examination of the role of peptides during osmoregulation showed that proline- and glutamate-containing peptides were protective under salt-induced stress, and not under sugar-induced stress. Under high salt, PepQ a cytoplasmic prolidase that specifically liberated proline from di-peptides increased activity, while PepX (X-prolyl-dipeptidyl aminopeptidase) and PepI (iminopeptidase) activities were unaffected. Our data suggest that proline- and glutamate-containing peptides may contribute to the adaptation of O. oeni to high salt through their intracellular hydrolysis and/or direct accumulation.

Response of Tunisian autochthonous pearl millet (Pennisetum glaucum (L.) R. Br.) to drought stress induced by polyethylene glycol (PEG) 6000

Seeds of pearl millet (Pennisetum glaucum (L) R. Br.) from six provenances of Tunisia were subjected to germination and shoot and root length tests on filter paper treated with polyethylene glycol 6000 (PEG 6000) solutions made up to provide osmotic potentials of 0, -1 and -2 MPa. Mean germination percent for all provenances decreased about 73% in -2 MPa compared to control (0 MPa) treatment. Osmotic potential of -1.0 MPa improves the rate of germination but not significantly compared to the control. There were significant differences among the relative germination percent of the provenances in different treatments. Decreases in the external osmotic potential induced decreased shoot growth while a slight increase in root length associate with the -1 MPa treatments was observed for some ecotypes. This reflects an adaptive response involving an increase in root length to reach deeper water in the soil. It was also notified that the elongation of the radicle is more sensitive to the osmotic constraint than the coleoptile.

Osmotic stress at seedling stage impairs reproductive development in buckwheat (Fagopyrum esculentum)

Common buckwheat (Fagopyrum esculentum Moench) is highly sensitive to drought. Many studies have investigated the impact of water stresses, applied late during plant development, on seed production by this non‐Poaceae cereal but information concerning the effects of stresses occurring early during plant’s life is essentially lacking. The aim of the present work was to define which yield‐related parameters are more affected by a 4‐day water constraint, applied at three different developmental stages, by transferring buckwheat plants of the cultivar ‘La Harpe’ grown in a hydroponic system to a medium supplemented with polyethylene glycol. Stresses during the vegetative phase, when the shoot apical meristem (SAM) was still initiating leaves, or later, when its activity was stopped after production of approximately 10 inflorescences, were less detrimental than an osmotic constraint applied to plants recently engaged in the reproductive phase, with SAM having initiated six to seven inflorescences. The number of cymes and flowers per inflorescence and the number of pollen grains per anther were decreased. The production of seeds in the first inflorescence was also reduced but this reduction paralleled the decrease in cyme number, suggesting that seed set, per se, was not affected. Pollen viability, female fertility and weight of seeds reaching maturity were not altered by the water deficit. The present work clearly identified two processes that, in buckwheat, are particularly sensitive to an osmotic stress: (1) the activity of reproductive meristems involved in the production of cymes and flowers and (2) male sporogenesis.

Lack of an osmotic constraint on intake rate of the eastern curlew Numenius madagascariensis

Rates of food intake in animals consuming abundant prey can be constrained by the rates of digestion or excretion of ingested substances, such as salt, particularly so in the animals that regularly migrate between freshwater and saltwater environments. We tested this hypothesis in a long‐distance migrant shorebird, the eastern curlew Numenius madagascariensis (suborder Charadrii), foraging on intertidal decapods in eastern Australia. We predicted that if food intake rates are constrained osmotically, individuals with access to freshwater and less saline prey (FW group) would have higher rates of food and water intake than individuals with seawater‐only access (SW group). Food intake rates did not differ between the FW and SW groups (0.14 g ash‐free dry mass min−1), nor did the water influx rates (0.75 g min−1). Salt intake rates were lower at FW sites (19.3 versus 23.3 mg NaCl min−1) and overall they were similar to those of marine birds. Food intake rate in the eastern curlew appeared limited by digestive rather than by osmoregulatory capacity.

The Essential Light Chain N-terminal Extension Alters Force and Fiber Kinetics in Mouse Cardiac Muscle

The functional significance of the actin-binding region at the N terminus of the cardiac myosin essential light chain (ELC) remains elusive. In a previous experiment, the endogenous ventricular ELC was replaced with a protein containing a 10-amino acid deletion at positions 5-14 (ELC1vDelta5-14, referred to as 1vDelta5-14), a region that interacts with actin. 1vDelta5-14 mice showed no discernable mutant phenotype in skinned ventricular strips. However, because the myofilament lattice swells upon skinning, the mutant phenotype may have been concealed by the inability of the ELC to reach the actin-binding site. Using the same mouse model, we repeated earlier measurements and performed additional experiments on skinned strips osmotically compressed to the intact lattice spacing as determined by x-ray diffraction. 1vDelta5-14 mice exhibited decreased maximum isometric tension without a change in calcium sensitivity. The decreased force was most evident in 5-6-month-old mice compared with 13-15-month-old mice and may account for the greater ventricular wall thickness in young 1vDelta5-14 mice compared with age-matched controls. No differences were observed in unloaded shortening velocity at maximum calcium activation. However, 1vDelta5-14 mice exhibited a significant difference in the frequency at which minimum complex modulus amplitude occurred, indicating a change in cross-bridge kinetics. We hypothesize that the ELC N-terminal extension interaction with actin inhibits the reversal of the power stroke, thereby increasing isometric force. Our results strongly suggest that an interaction between residues 5-14 of the ELC N terminus and the C-terminal residues of actin enhances cardiac performance.

Potential osmoprotectants for the lactic acid bacteria Pediococcus pentosaceus and Tetragenococcus halophila

The physiological responses of the lactic acid bacteria Pediococcus pentosaceus and Tetragenococcus halophila (formely known as P. halophila), subjected to osmotic stress in the presence of molecules known to act as osmoprotectants for other bacteria were studied. In a defined medium, glycine betaine, dimethylsulfonioacetate, choline, proline and l-carnitine were able to relieve inhibition of growth at 0.8 M NaCl. The five compounds were shown to efficiently compete with glycine betaine transport, suggesting the existence of common transporter(s) for these molecules. T. halophila, the most tolerant strain, exhibited a larger spectrum of compatible solutes including dimethylsulfonioacetate, dimethylsulfoniopropionate and ectoine. Preliminary data suggest that restoration of growth by ectoine under osmotic constraint seems specific to the genus Tetragenococcus.

Test, rejection, and reformulation of a chemical reactor-based model of gut function in a fruit-eating bird.

We explored modulation of retention time in cedar waxwings (Bombycilla cedrorum) by feeding them diets varying in hexose concentration. Our goals were to (1) test three predictions of a chemical reactor-based model of how guts might respond optimally to diet shifts; (2) determine whether modulation of retention time can occur quickly, thereby facilitating rapid changes in diet; (3) tease apart the relative influence of ingestion rate and nutrient concentration on retention time; and (4) examine the degree of axial mixing in the intestine and its relationship with retention time. The model's predictions were rejected: mean retention time did not decrease, ingestion rate did not increase, and glucose assimilation efficiency did not decrease with increased hexose concentration of the diet. Instead, birds displayed maximal intake rate at intermediate sugar concentration, and mouth to cloaca mean retention times increased with hexose concentration. Significant modulation of retention time occurred quickly, within 3 h of exposure to a different diet. Birds did equally well in terms of total energy assimilated on diets differing 3.3-fold in hexose concentration (from 500 mmol/L to 1660 mmol/L) but showed reduced intake when fed food with low hexose concentration (110 mmol/L). Far more variation in retention time was explained by direct effects of ingestion rate than by direct effects of hexose concentration. Finally, a gut dispersion index that measured degree of axial mixing was positively correlated with mean retention time, indicating that higher retention times are accompanied by increased axial mixing. We propose a modification of the assumptions of the original model. The resulting "osmotic constraint" model better captures the interaction between feeding rate and digestive function in fruit-eating birds.

Rheological and Dielectric Behavior of a Styrene−Isoprene−Styrene Triblock Copolymer in Selective Solvents. 2. Contribution of Loop-Type Middle Blocks to Elasticity and Plasticity

Rheological and dielectric behavior was examined for concentrated solutions of a styrene−isoprene−styrene (SIS) triblock copolymer in monomeric and polymeric I-selective solvents, n-tetradecane (C14) and a low-M homopolyisoprene (I-1; M = 1.4K). The I blocks had symmetrically once-inverted dipoles along the block contour, and their midpoint motion was dielectrically detected. The SIS solutions exhibited rubbery, plastic, and viscous behavior at low, intermediate, and high temperatures (T). Dielectric and viscoelastic data strongly suggested that the S and I blocks were more or less homogeneously mixed in the viscous regime. In the rubbery and plastic regimes, the S blocks were segregated to form spherical domains, and the I blocks took either the loop or bridge conformation. In these regimes, the inverted dipoles of the I blocks enabled us to dielectrically estimate the loop fraction, φ1 ≅ 60% in C14 and I-1. These loops, having osmotically constrained conformations, strongly affected the rheological properties of the SIS solutions. A strong osmotic constraint in C14 resulted in almost equal contributions of the loops and bridges to the equilibrium modulus. The loop contribution became less significant in I-1 that (partly) screened this constraint. Similarly, the yield stress σy in C14 was essentially determined by dangling (noninterdigitated) loops at relatively high T where the S/I mixing barrier was rather small, while the bridges and interdigitated loops had a large contribution when this barrier was enhanced, i.e., at lower T and/or in I-1 (a poorer solvent for the S blocks than C14).