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1.
Published and additional data for polyethylene glycol 8000 (PEG), formerly PEG 6000, solution water potentials (Ψ) are compared. Actual bars Ψ over the concentration range of 0 to 0.8 gram PEG per gram H 2O and temperature ( T) range of 5 to 40°C are best predicted (probably within ± 5%) by this equation: Ψ = 1.29[PEG] 2T − 140[PEG] 2 − 4.0[PEG]. Although transformable through division by [PEG] to virial equation form, results indicate that the coefficients are not virial. Mannitol (MAN) interacts with PEG to produce Ψ significantly lower than additive. Vapor pressure osmometer (VPO) data for MAN-PEG synergism compared favorably with those from thermocouple hygrometry; and VPO data showing the interactions between PEG and four salts are presented. The synergism of MAN-PEG and of NaCl-PEG are related linearly to the concentration of solute added with PEG. 相似文献
2.
Pressure volume curves for Alternanthera philoxeroides (Mart.) Griseb. (alligator weed) grown in 0 to 400 millimolar NaCl were used to determine water potential (Ψ), osmotic potential (ψ s), turgor potential (ψ p) and the bulk elastic modulus (ε) of shoots at different tissue water contents. Values of ψ s decreased with increasing salinity and tissue Ψ was always lower than rhizosphere Ψ. The relationship between ψ p and tissue water content changed because ε increased with salinity. As a result, salt-stressed plants had larger ranges of positive turgor but smaller ranges of tissue water content over which ψ p was positive. To our knowledge, this is the first report of such a salinity effect on ε in higher plants. These increases in ε with salinity provided a mechanism by which a large difference between plant Ψ and rhizosphere Ψ, the driving force for water uptake, could be produced with relatively little water loss by the plant. A time-course study of response after salinization to 400 millimolar NaCl showed Ψ was constant within 1 day, ψ s and ψ p continued to change for 2 to 4 days, and ε continued to change for 4 to 12 days. Changes in ε modified the capacity of alligator weed to maintain a positive water balance and consideration of such changes in other species of higher plants should improve our understanding of salt stress. 相似文献
3.
Water potential (ψ), the osmotic potential (ψ π), and the pressure potential (ψ p) of detached cotyledons isolated from Cucumis sativus L. cv Marketer seedlings after 0, 1.5, and 3 days growth with and without zeatin were determined. From zero time to 3 days, cotyledons incubated without exogenous zeatin exhibited a slight decrease in ψ (from −0.4 to −1.0 bars), while those grown with zeatin developed even more negative values (about −4 bars). Both groups showed rising ψ π values (decreases in solutes per unit volume), but this rise was more dramatic in those treated with zeatin. These data indicate that the capacity of zeatin-treated cotyledons to take up water more rapidly than controls and thus expand faster must be due to wall loosening, as reflected in ψ p values which declined during 3 days from about +11 bars to about +1.4 bars. It was also found that freshly detached cotyledons or those grown without exogenous zeatin exhibited osmoregulation in polyethylene glycol (PEG) solutions. That is, while cotyledons initially lost H2O into certain PEG solutions, their ψ values decreased over time and they began absorbing water after 1 to 4 hours. After 3 days growth, zeatin-treated cotyledons had lost most of this capacity of osmoregulate. It seems likely that osmoregulation in cotyledons not treated with zeatin is due to wall loosening rather than changes in ψπ. Zeatin-treated cotyledons with already loosened walls may not have this option to deal with water stress and thus simply come to equilibrium with external PEG solutions. 相似文献
4.
Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface ( I), stomatal resistance ( rs), leaf water potential (ψ), osmotic potential (π), and turgor potential ( P) were followed concurrently in crops of maize ( Zea mays L. var. Pa 602A), sorghum ( Sorghum bicolor [L.] Moench var. RS610), and tobacco ( Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the rs, measured with a ventilated diffusion porometer, the ψ, measured with the pressure chamber, the π, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing ψ, but never became zero. Sorghum had a higher P at a ψ of, say −10 bars, than did maize, and maize had a higher P than tobacco at the same ψ. Moreover, at the same ψ the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, rs did not increase as ψ declined to −13, −15, and −10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between rs and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of rs to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher rs than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance ( rl) and maize had the highest rl. Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves. 相似文献
5.
Mathematical models were developed to characterize the physiological bases of the responses of tomato ( Lycopersicon esculentum Mill. cv T5) seed germination to water potential (ψ) and abscisic acid (ABA). Using probit analysis, three parameters were derived that can describe the germination time courses of a seed population at different ψ or ABA levels. For the response of seed germination to reduced ψ, these parameters are the mean base water potential (¯ψ b, MPa), the standard deviation of the base water potential among seeds in the population (σ ψb, MPa), and the “hydrotime constant” (θ H, MPa·h). For the response to ABA, they are the log of the mean base ABA concentration ([unk]ABA b, m), the standard deviation of the base ABA concentration among seeds in the population (σ ABAb, log[ m]), and the “ABA-time constant” (θ ABA, log[ m]·h). The values of ¯ψ b and [unk] ABAb provide quantitative estimates of the mean sensitivity of germination rate to ψ or ABA, whereas σ ψb and σ ABAb account for the variation in sensitivity among seeds in the population. The time constants, θ H and θ ABA, indicate the extent to which germination rate will be affected by a given change in ψ or ABA. Using only these parameters, germination time courses can be predicted with reasonable accuracy at any medium ψ according to the equation probit( g) = [ψ - (θ H/ tg) - ¯ψ b]/σ ψb, or at any ABA concentration according to the equation probit( g) = [log[ ABA] - (θ ABA/ tg) - log[[unk] ABAb]]/σ ABAb, where tg is the time to radicle emergence of percentage g, and ABA is the ABA concentration ( m) in the incubation solution. In the presence of both ABA and reduced ψ, the same parameters can be used to predict seed germination time courses based upon strictly additive effects of ψ and ABA in delaying the time of radicle emergence. Further analysis indicates that ABA and ψ can act both independently and interactively to influence physiological processes preparatory for radicle growth, such as the accumulation of osmotic solutes in the embryo. The models provide quantitative values for the sensitivity of germination to ABA or ψ, allow evaluation of independent and interactive effects of the two factors, and have implications for understanding how ABA and ψ may regulate growth and development. 相似文献
6.
Recent evidence from leaves and stems indicates that gradients in water potential (ψ w) necessary for water movement through growing tissues are larger than previously assumed. Because growth is sensitive to tissue ψ w and the behavior of these gradients has not been investigated in transpiring plants, we examined the water status of all the growing and mature vegetative tissues of maize ( Zea mays L.) during high and low rates of transpiration. The ψ w measured in the mature regions of the plant responded primarily to transpiration, while the ψ w in the growing regions was affected both by transpiration and growth. The transpiration-induced potentials of the mature tissue formed a gradient of decreasing ψ w along the transpiration stream while the growth-induced potentials formed a gradient of decreasing ψ w from the transpiration stream to the expanding cells in the growing tissue. The growth-induced gradient in ψ w within the leaf remained fairly constant as the xylem ψ w decreased during the day and was associated with a decreased osmotic potential (ψ s) of the growing region (osmotic adjustment). The growth-induced gradient in ψ w was not caused by excision of the tissue because intact maize stems exhibited a similar ψ w. These observations support the concept that large gradients in ψ w are required to maintain water flow to expanding cells within all the vegetative tissues and suggest that the maintenance of a favorable gradient in ψ w for cell enlargement may be an important role for osmotic adjustment. 相似文献
7.
The numerous modifications of tRNA play central roles in controlling tRNA structure and translation. Modifications in and around the anticodon loop often have critical roles in decoding mRNA and in maintaining its reading frame. Residues U 38 and U 39 in the anticodon stem–loop are frequently modified to pseudouridine (Ψ) by members of the widely conserved TruA/Pus3 family of pseudouridylases. We investigate here the cause of the temperature sensitivity of pus3Δ mutants of the yeast Saccharomyces cerevisiae and find that, although Ψ 38 or Ψ 39 is found on at least 19 characterized cytoplasmic tRNA species, the temperature sensitivity is primarily due to poor function of tRNA Gln(UUG), which normally has Ψ 38. Further investigation reveals that at elevated temperatures there are substantially reduced levels of the s 2U moiety of mcm 5s 2U 34 of tRNA Gln(UUG) and the other two cytoplasmic species with mcm 5s 2U 34, that the reduced s 2U levels occur in the parent strain BY4741 and in the widely used strain W303, and that reduced levels of the s 2U moiety are detectable in BY4741 at temperatures as low as 33°C. Additional examination of the role of Ψ 38,39 provides evidence that Ψ 38 is important for function of tRNA Gln(UUG) at permissive temperature, and indicates that Ψ 39 is important for the function of tRNA Trp(CCA) in trm10Δ pus3Δ mutants and of tRNA Leu(CAA) as a UAG nonsense suppressor. These results provide evidence for important roles of both Ψ 38 and Ψ 39 in specific tRNAs, and establish that modification of the wobble position is subject to change under relatively mild growth conditions. 相似文献
8.
The initiation of radicle growth during seed germination may be driven by solute accumulation and increased turgor pressure, by cell wall relaxation, or by weakening of tissues surrounding the embryo. To investigate these possibilities, imbibition kinetics, water contents, and water (Ψ) and solute (ψ s) potentials of intact muskmelon ( Cucumis melo L.) seeds, decoated seeds (testa removed, but a thin perisperm/endosperm envelope remains around the embryo), and isolated cotyledons and embryonic axes were measured. Cotyledons and embryonic axes excised and imbibed as isolated tissues attained water contents 25 and 50% greater, respectively, than the same tissues hydrated within intact seeds. The effect of the testa and perisperm on embryo water content was due to mechanical restriction of embryo swelling and not to impermeability to water. The Ψ and ψ s of embryo tissues were measured by psychrometry after excision from imbibed intact seeds. For intact or decoated seeds and excised cotyledons, Ψ values were >−0.2 MPa just prior to radicle emergence. The Ψ of excised embryonic axes, however, averaged only −0.6 MPa over the same period. The embryonic axis apparently is mechanically constrained within the testa/perisperm, increasing its total pressure potential until axis Ψ is in equilibrium with cotyledon Ψ, but reducing its water content and resulting in a low Ψ when the constraint is removed. There was no evidence of decreasing ψ s or increasing turgor pressure (Ψ-ψ s) prior to radicle growth for either intact seeds or excised tissues. Given the low relative water content of the axes within intact seeds, cell wall relaxation would be ineffective in creating a Ψ gradient for water uptake. Rather, axis growth may be initiated by weakening of the perisperm, thus releasing the external pressure and creating a Ψ gradient for water uptake into the axis. The perisperm envelope contains a cap of small, thin-walled endosperm cells adjacent to the radicle tip. We hypothesize that weakening or separation of cells in this region could initiate radicle expansion. 相似文献
9.
We grew sunflower ( Helianthus annuus L.) plants in nutrient solutions having nutritionally adequate but low or high Mg 2+ concentrations and determined whether photosynthesis was effected as leaf water potentials (ψ w) decreased. Leaf Mg contents were 3- to 4-fold higher in the plants grown in high Mg 2+ concentrations (10 millimolar) than in those grown in low concentrations (0.25 millimolar). These contents were sufficient to support maximum growth, plant dry weight, and photosynthesis, and the plants appeared normal. As low ψ w developed, photosynthesis was inhibited but moreso in high Mg leaves than in low Mg leaves. The effect was particularly apparent under conditions of light- and CO 2-saturation, indicating that the chloroplast capacity to fix CO 2 was altered. The differential inhibition observed in leaves of differing Mg contents was not observed in leaves having differing K contents, suggesting that the effect may have been specific for Mg. Because Mg 2+ inhibits photophosphorylation and coupling factor activities at concentrations likely to occur as leaves dehydrate, Mg may play a role in the inhibition of chloroplast reactions at low ψ w, especially in leaves such as sunflower that markedly decrease in water content as ψ w decreases. 相似文献
10.
It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO 2 and H 2O was determined in two growth-chamber-grown wheat cultivars ( Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO 2-saturated A, and stomatal conductance (g s) components. The limitations on A measured at ambient CO 2 concentration (A 350) were estimated. No cultivar difference was observed when A 350 was plotted versus leaf water potential (Ψ w). Light- and CO 2-saturated A, c.e., and g s decreased with decreasing leaf Ψ w, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g s increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A 350. WUE measured as A 350/g s first increased with stomatal closure down to a g s of about 0.25 mol H 2O m −2 s −1 (Ψ w = −1.6 MPa). However, it was predicted that A 350/g s would decrease with more severe stress due to inhibition of c.e. 相似文献
11.
Previous studies observed a higher ratio of divergences at nonsynonymous and synonymous sites (ω = dN/ dS) in species with a small population size compared to that estimated for those with a large population size. Here we examined the theoretical relationship between ω, effective population size ( Ne), and selection coefficient ( s). Our analysis revealed that when purifying selection is high, ω of species with small Ne is much higher than that of species with large Ne. However the difference between the two ω reduces with the decline in selection pressure ( s → 0). We examined this relationship using primate and rodent genes and found that the ω estimated for highly constrained genes of primates was up to 2.9 times higher than that obtained for their orthologous rodent genes. Conversely, for genes under weak purifying selection the ω of primates was only 17% higher than that of rodents. When tissue specificity was used as a proxy for selection pressure we found that the ω of broadly expressed genes of primates was up to 2.1-fold higher than that of their rodent counterparts and this difference was only 27% for tissue specific genes. Since most of the nonsynonymous mutations in constrained or broadly expressed genes are deleterious, fixation of these mutations is influenced by Ne. This results in a higher ω of these genes in primates compared to those from rodents. Conversely, the majority of nonsynonymous mutations in less-constrained or tissue-specific genes are neutral or nearly neutral and therefore fixation of them is largely independent of Ne, which leads to the similarity of ω in primates and rodents. 相似文献
12.
The unidirectional uptake of sodium across the outer surface of the isolated frog skin ( J12Na) was measured in the presence of transepithelial potential difference (Δ ψ) ranging from +100 to -100 mV. With a sodium concentration of 115 mM in the bathing solutions J12Na increases significantly when the spontaneous Δ ψ is reduced to zero by short-circuiting the skin. With an Na concentration of 6 mM a progressive increase J12Na can be observed when Δ ψ is decreased in several steps from +100 to -100 mV (serosal side positive and negative, respectively). The observed change J12Na amounts to a fraction only of that predicted from the shift in Δ ψ. The results suggest that under open circuit conditions the potential step across the outside surface is at most one half of Δ ψ and that the resistance across the outside and inside barrier of the skin is ohmic. This is in agreement with measurements of intracellular potentials in the frog skin and with resistance measurements carried out in the toad skin. The data strongly support the view that the saturating component of Jψ proceeds via a charged carrier system. Exposure to negative values of Δ ψ of 50 mV or more for times of 24 min or more result in a marked reduction of J12Na which shows only partial or no reversibility. 相似文献
13.
A double pressure probe technique was used to measure simultaneously water flows and hydraulic parameters of individual cells and of excised roots of young seedlings of maize ( Zea mays L.) in osmotic experiments. By following initial flows of water at the cell and root level and by estimating the profiles of driving forces (water potentials) across the root, the hydraulic conductivity of individual cell layers was evaluated. Since the hydraulic conductivity of the cell-to-cell path was determined separately, the hydraulic conductivity of the cell wall material could be evaluated as well ( Lpcw = 0.3 to 6.10 −9 per meter per second per megapascal). Although, for radial water flow across the cortex and rhizodermis, the apoplasmic path was predominant, the contribution of the hydraulic conductance of the cell-to-cell path to the overall conductance increased significantly from the first layer of the cortex toward the inner layers from 2% to 23%. This change was mainly due to an increase of the hydraulic conductivity of the cell membranes which was Lp = 1.9.10 −7 per meter per second per megapascal in the first layer and Lp = 14 to 9.10 −7 per meter per second per megapascal in the inner layers of the cortex. The hydraulic conductivity of entire roots depended on whether hydrostatic or osmotic forces were used to induce water flows. Hydrostatic Lpr was 1.2 to 2.3.10 −7 per meter per second per megapascal and osmotic Lpr = 1.6 to 2.8.10 −8 per meter per second per megapascal. The apparent reflection coefficients of root cells (σ s) of nonpermeating solutes (KCI, PEG 6000) decreased from values close to unity in the rhizodermis to about 0.7 to 0.8 in the cortex. In all cases, however, σ s was significantly larger than the reflection coefficient of entire roots (σ sr). For KCI and PEG 6000, σ sr was 0.53 and 0.64, respectively. The results are discussed in terms of a composite membrane model of the root. 相似文献
14.
Water relations of growing segments of maize ( Zea mays L.) coleoptiles were investigated with osmotic methods using either mannitol (MAN) or polyethylene glycol 6000 (PEG) as external osmotica. Segments were incubated in MAN or PEG solutions at 0 to - 15 bar water potential (Ψ o) and the effects were compared on elongation growth, osmotic shrinkage, cell sap osmolality (OC), and osmotic pressure (π i). The nonpenetrating osmoticum PEG affects π i in agreement with Boyle-Mariotte's law, i.e. the segments behave in principle as ideal osmometers. There is no osmotic adjustment in the Ψ o range permitting growth (0 to −5 bar) nor in the Ψ o range inducing osmotic shrinkage (−5 to −10 bar). Promoting growth by auxin (IAA) has no effect on the osmotic behavior of the tissue toward PEG. In contrast to PEG, MAN produces an apparent increase in π i accompanied by anomalous effects on segment elongation and shrinkage leading to a lower value for Ψ o which establishes a growth rate of zero and to an apparent recovery from osmotic shrinkage after 2 hours of incubation. These effects can be quantitatively attributed to uptake of MAN into the tissue. MAN is taken up into the apoplastic space and the symplast as revealed by a large temperature-dependent component of MAN uptake. It is concluded that MAN, in contrast to PEG, is unsuitable as an extemal osmoticum for the quantitative determination of water relations of growing maize coleoptiles. 相似文献
15.
At low water potential (ψ w), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor'), and one that lacks this capacity (`Capelle Desprez'). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψ s) declined to a greater degree in Condor plants. Measuring ψ s at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast ( i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψ w. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψ w for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψ w in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψ w in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψ w in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO 2 uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO 2] and 21.8% higher at elevated external [CO 2]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψ w and low ψ w inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψ w, thus reducing low ψ w inhibition of chloroplast photosynthetic potential. 相似文献
16.
The transport parameters of a cellulose acetate membrane prepared from a mixture of cellulose acetate, formamide, and acetone, 25:25:50 by weight, were studied. The membrane consists of a thin, porous layer, the skin, in series with a thick, highly porous layer, the coarse support. In the skin the diffusional permeability coefficient, ω, of a number of small amides and alcohols depends critically upon the partition coefficient, Ks, the size of the molecule, and the apparent hydrogen-bonding ability, Ns, of the solute. These observations are in general agreement with our earlier conclusions on the properties of nonporous membranes. On the other hand, the corrected reflection coefficient, σ', is not a very sensitive function of either Ns or Ks taken separately. The correlation between σ' and molecular diameter is reasonably good; however, it is much improved when both Ns and Ks are taken into consideration. Isotope interaction was also studied in the present preparation and was found to provide only a small (5–8%) contribution to the diffusional permeability coefficient of ethylene glycol. The contribution of solute-water friction was found to be less than 24% of the total solute friction. 相似文献
17.
The relative magnitude of adjustment in osmotic potential (ψ s) of water-stressed cotton ( Gossypium hirsutum L.) leaves and roots was studied using plants raised in pots of sand and grown in a growth chamber. One and three water-stress preconditioning cycles were imposed by withholding water, and the subsequent adjustment in solute potential upon relief of the stress and complete rehydration was monitored with thermocouple psychrometers. Both leaves and roots exhibited a substantial adjustment in ψ s in response to water stress with the former exhibiting the larger absolute adjustment. The osmotic adjustment of leaves was 0.41 megapascal compared to 0.19 megapascal in the roots. The roots, however, exhibited much larger percentage osmotic adjustments of 46 and 63% in the one and three stress cycles, respectively, compared to 22 and 40% in the leaves in similar stress cycles. The osmotically adjusted condition of leaves and roots decreased after relief of the single cycle stress to about half the initial value within 3 days, and to the well-watered control level within 6 days. In contrast, increasing the number of water-stress preconditioning cycles resulted in significant percentage osmotic adjustment still being present after 6 days in roots but not in the leaves. The decrease in ψ s of leaves persisted longer in field-grown cotton plants compared to plants of the same age grown in the growth chamber. The advantage of decreased ψ s in leaves and roots of water-stressed cotton plants was associated with the maintenance of turgor during periods of decreasing water potentials. 相似文献
18.
Polyethylene glycol (PEG), which is often used to impose low water potentials (ψ w) in solution culture, decreases O 2 movement by increasing solution viscosity. We investigated whether this property causes O 2 deficiency that affects the elongation or metabolism of maize ( Zea mays L.) primary roots. Seedlings grown in vigorously aerated PEG solutions at ambient solution O 2 partial pressure ( pO 2) had decreased steady-state root elongation rates, increased root-tip alanine concentrations, and decreased root-tip proline concentrations relative to seedlings grown in PEG solutions of above-ambient pO 2 (alanine and proline accumulation are responses to hypoxia and low ψ w, respectively). Measurements of root pO 2 were made using an O 2 microsensor to ensure that increased solution pO 2 did not increase root pO 2 above physiological levels. In oxygenated PEG solutions that gave maximal root elongation rates, root pO 2 was similar to or less than (depending on depth in the tissue) pO 2 of roots growing in vermiculite at the same ψ w. Even without PEG, high solution pO 2 was necessary to raise root pO 2 to the levels found in vermiculite-grown roots. Vermiculite was used for comparison because it has large air spaces that allow free movement of O 2 to the root surface. The results show that supplemental oxygenation is required to avoid hypoxia in PEG solutions. Also, the data suggest that the O 2 demand of the root elongation zone may be greater at low relative to high ψ w, compounding the effect of PEG on O 2 supply. Under O 2-sufficient conditions root elongation was substantially less sensitive to the low ψ w imposed by PEG than that imposed by dry vermiculite. 相似文献
19.
The effect of transepithelial potential difference (ψ) on Na and Cl flux across toad bladder was assessed by measuring isotopic flux between identical media at various values of ψ. The contribution of edge damage to ionic permeability was eliminated, resulting in relatively high spontaneous ψ (-97 ±4 mv) and low electrical conductance g. Bidirectional Na fluxes were measured simultaneously. Unidirectional Cl fluxes were measured in paired hemibladders at ψ = 0 mv or -97 mv. Net Na flux JNa, at ψ = 0 mv, was slightly less than short-circuit current (SCC). At ψ = -97 mv, JNa averaged 17% of SCC, and was sometimes zero. Δ JNa/Δψ (= g+) averaged 60% of g between -97 mv and +75 mv; at -150 mv, g+ fell, indicating rectification. Analysis of unidirectional Na fluxes indicates low passive conductance (1.5 μmho/mg wet weight), a bidirectional, electrically neutral flux of approximately 0.13 μa/mg, and relatively large conductance of the active transport path at ψ ≥ -97 mv. The absence of appreciable transstimulation of serosal (S)-to-mucosal (M) Na flux (in response to increasing mucosal Na concentration) indicates that the electrically neutral flux is not exchange diffusion in the usual sense. Analysis of Cl fluxes indicates similar values for passive conductance and neutral flux, suggesting linked neutral flux of Na and Cl. Either the electromotive force of the Na pump E, its conductance ga, or both are strong functions of ψ. The product of these two quantities, Ega, is a measure of the “transport capacity” at any given value of ψ, independent of the direct effect of ψ on JNa through the pump path. Ega varies with ψ. Hence estimation of the net Na flux or current at any one value of ψ, including ψ = 0, fails to reveal the maximal transport capacity of the pump, its resting electromotive force (when JNa = 0 through the pump), or the dependence of transport capacity on potential. 相似文献
20.
The elastic behavior of the cell wall as a function of the temperature has been studied with particular attention being given to the swelling of egg cells of Strongylocentrotus purpuratus and Crassostrea virginica in different sea water concentrations at different temperatures. It was found that the modulus of elasticity is a nonlinear function of temperature. At about 12-13°C the modulus of elasticity ( E) is constant, independent of the stress (σ) and strain (ε ν) which exist at the cell wall; the membranous material follows Hooke's law, and E ≈ 3 × 10 7 dyn/cm 2 for S. purpuratus and C. virginica. When the temperature is higher or lower than 12-13°C, the modulus of elasticity increases, and the membranous material does not follow Hooke's law, but is almost directly proportional to the stresses existing at the cell wall. On increasing the stress, the function Eσ = E(σ) approaches saturation. The corresponding stress-strain diagrams, σ = σ(ε ν), and the graphs, Eσ = E(σ) and Eσ = E( t) are given. The cyto-elastic phenomena at the membrane are discussed. 相似文献
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