Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. II. Phloem in the Stem

Confirming a previous analysis by Lang (1974), it is concludedthat in tree trunks, phloem turgor and turgor gradients maybe estimated from osmotic pressure and osmotic-pressure gradients,respectively. The present analysis is an improvement becauseit is based on observed osmotic-pressure gradients rather thansupposed turgor gradients, and allowance is made for sucroseunloading and gradients of external water potential. It is concludedthat the rate of sucrose unloading in tree trunks must be lessthan 50 nmol m^–2 S m^–1. In small plants, higherrates of unloading (100 nmol m m^–2 S m^–1) and steeperconcentration gradients will lead to larger errors, but turgorpressures can still be estimated with acceptable accuracy. Oneshould be more cautious when considering turgor gradients insmall plants, although it seems likely that reasonable estimateswill still be obtained. Assuming plasmodesmatal transport throughan unconstricted cytoplasmic annulus, it is concluded that thesieve elements and their associated cells will sustain verysimilar turgor and osmotic pressures. Convection and diffusioncan both contribute significantly to plasmodesmatal sucroseunloading. Similarly, the plasmodesmatal volume flux will reflecta combination of pressure flow and osmosis. Water fluxes acrossthe sieve element plasmalemma and through the plasmodesmatacan be in opposite directions. It may be possible to assessthe extent of hydraulic coupling between the sieve elementsand their associated cells from studies of phloem water relations Phloem, turgor, osmotic pressure, plasmodesmata, phloem unloading, Munch hypothesis     

Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. III. Phloem in the Leaf

The theory described in Part I of this series is applied hereto the loading of water and sucrose into the sieve-element-companion-cell(se-cc) complexes of minor veins. It is concluded that symplasticphloem loading cannot account for large increases in osmoticpressure from mesophyll to se-cc complex or the dilution ofsieve tube sap which is evident in leaves. By contrast, loadingfrom the free space into a symplastically isolated se-cc complexcan account for both these observations. Within the se-cc complex,sucrose and water will be transported from the companion cellsto the sieve elements by a pressure-driven flow of solutionthrough the cytoplasmic annuli of plasmodesmata. The associatedchanges in turgor and osmotic pressure are small, and so these-cc complex can be regarded as a single compartment with respectto water potential. The assumption that this compartment isat water flux equilibrium will lead to significant overestimatesof turgor gradients. However, if the trans-membrane water potentialdifference and the external water potential are taken into account,the correlation of such gradients with sieve-element dimensionsand / or transport velocities provides one means of testingthe Munch hypothesis of phloem transport Phloem, turgor, osmotic pressure, plasmodesmata, Münch hypothesis, Phloem loading     

Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications For Phloem Loading and Unloading. I. Theory

A mathematical model of water and sucrose transport across thesieve tube boundary is presented, based on conservation of matterand the phenomenological equations for plasmodesmatal transportbetween the sieve elements and their associated cells. Plasmodesmataltransport coefficients are discussed. In parts II–IV,the equations developed here are used to assess: (i) the estimationof phloem turgor gradients from osmotic pressure gradients;(ii) plasmodesmatal transport of water and sucrose between thesieve elements and adjacent cells; and (iii) the plausibilityof symplastic and apoplastic phloem loading and unloading insome primary sources and sinks. A list of symbols is given inAppendix 1 of this paper Phloem, turgor, osmotic pressure, loading, unloading, plasmodesmata, Munch hypothesis     

Turgor Maintenance by Osmoregulation inBrassica napusandB. junceaunder Field Conditions

Indian mustard (Brassica juncea(L) Czernjacw) maintains higherleaf turgor than canola (B. napusL.) under water deficits andthis is related to the greater yield of mustard under theseconditions. The work reported in this paper was designed tostudy the way mustard maintains this turgor advantage. It wasbased on three field experiments that each used at least twocultivars or lines of each species. The leaf water potentialat which leaves reached zero turgor was consistently lower inmustard than in canola (up to 1.1 MPa lower). This differencearose from a greater rate of decline in leaf osmotic potentialwith declining water potential in mustard rather than from anydifference in the osmotic potential at full turgor. Calculationsof solute accumulation showed that mustard had a greater capacityto osmoregulate than canola, with this capacity being the basisfor its advantage in turgor maintenance. Other differences inplant water relations were consistent with the differences inosmoregulation, with the predicted relative water content ofleaves at an osmotic potential of -2.5 MPa being 0.43 for canolaand 0.61 for mustard. Mustard's greater capacity to accumulatesolutes is concluded to be a major factor in its greater yieldunder water deficits. Brassica napusL.; Brassica juncea(L) Czernjacw; Indian mustard; canola; water deficit; plant water relations; osmoregulation; osmotic adjustment; turgor     

Phloem water relations and translocation

Satisfactory measurements of phloem water potential of trees can be obtained with the Richards and Ogata psychrometer and the vapor equilibration techniques, although corrections for loss of dry weight and for heating by respiration are required for the vapor equilibrium values. The psychrometer technique is the more satisfactory of the 2 because it requires less time for equilibration, less tissue, and less handling of tissue. Phloem water potential of a yellow-poplar tree followed a diurnal pattern quite similar to that of leaves, except that the values were higher (less negative) and changed less than in the leaves.

The psychrometer technique permits a different approach to the study of translocation in trees. Measurements of water potential of phloem discs followed by freezing of samples and determination of osmotic potential allows estimation of turgor pressure in various parts of trees as the difference between osmotic potential and total water potential. This technique was used in evaluating gradients in water potential, osmotic potential, and turgor pressure in red maple trees. The expected gradients in osmotic potential were observed in the phloem, osmotic potential of the cell sap increasing (sap becoming more dilute) down the trunk. However, values of water potential were such that a gradient in turgor pressure apparently did not exist at a time when rate of translocation was expected to be high. These results do not support the mass flow theory of translocation favored by many workers.

     

Evaluation of a Non-Destructive Method for Measuring Turgor Pressure in Helianthus

The portable instrument described by Heathcote, Etherington,and Woodward (1979) for the non-destructive measurement of turgorpressure was evaluated in Helianthus annuus and Helianthus paradoxus.A good correlation was obtained between turgor pressure measuredwith the instrument and turgor pressure estimated by the pressure-volumetechnique for individual leaves allowed to dry after excision;however, variation in both the intercept and slope of the relationshipoccurred between leaves. Consequently, there was no correlationbetween the output of the instrument for individual leaves andthe turgor pressure of the same leaves estimated by conventionalmethods. Moreover, for a given leaf, the instrument had onlya limited ability to detect temporal variation in turgor pressurewhen compared with turgor pressure calculated from measuredvalues of leaf water potential and leaf osmotic potential. Theinstrument's output was influenced by its proximity to majorveins and by leaf thickness. We conclude that variability inleaf thickness and the presence of large veins limits its usefulnessfor measurement of turgor pressure in Helianthus. Key words: Leaf thickness, Turgormeter, Turgor pressure, Helianthus     

Turgor Regulation in a Brackish Charophyte, Lamprothamnium succinctum I. Artificial Modification of Intracellular Osmotic Pressure

Internodal cells of Lamprothamnium succinctum cultured in freshwater and brackish water of different salinities maintainedalmost the same turgor pressure at steady state. When the turgorpressure was increased by decreasing the external osmolality,the cells recovered their original turgor pressure within 2h. However, the recovery from decreased turgor pressure required1 day. When salts of the external medium were replaced with sorbitol,the cells still regulated the turgor pressure, indicating thatthe essential factor for the turgor regulation is not the salinitybut the osmolality. Internodal cells with osmotic pressure andion concentrations artificially modified to higher or lowervalues also regained the original turgor pressure by changingtheir intracellular osmotic pressure, whether the cells werecultured in brackish water or fresh water. These results indicate that turgor regulation is intrinsic toLamprothamnium and is initiated by a deviation of turgor pressurefrom the reference value, which is about 0.35 Osm. (Received November 28, 1983; Accepted March 14, 1984)     

Water Content-Potential Relationship in Soya Bean: Changes in Component Potentials for Mature and Immature Leaves under Field Conditions

Changes in turgor and osmotic potentials of soya bean leaves(Glycine max.) with changes in water content were measured throughouta season using the pressure-volume technique. Two distinct reponsesto water loss were found. When water was expressed from leavesin the pressure chamber their osmotic behavior was describedby a concentration effect based on the osmotic volume. The osmoticfraction of the total water content averaged 0·72 and0·84 for mature and immature leaves, respectively. Thechanges in turgor pressure in the chamber were described bya volumetric modulus of elasticity which increased linearlywith turgor pressure. The changes in total potential at highturgor pressures were almost exclusively due to changes in turgordue to the high modulus (high tissue rigidity) in that range.Responses were different, however, for leaves drying in thefield. For these, the osmotic changes were always large anddominated by solute adjustment. Diurnal changes in osmotic potentialwere as much as 5 bars (500 kPa), or around 50 per cent, andwere about the same magnitude as the changes in turgor pressurefor both mature and immature leaves. The elastic modulus atthe time of sampling showed the normal turgor dependence forimmature leaves but for mature leaves the initial modulus wasapparently constant at about 180 bars. The different behaviourin the pressure bomb and the field is interpreted in terms ofa rate dependence for turgor and osmotic response to water loss.     

Cell elongation, turgor and osmotic pressure in developing sunflower hypocotyls

The relationship between cell elongation, change in turgor andcell osmotic pressure was investigated in the sub-apical regionof hypocotyls of developing sunflower seedlings (Helianthusannuus L.) that were grown in continuous white light. Cell turgorwas measured with the pressure probe. The same hypocotyl sectionswere used for determination of osmotic pressure of the tissuesap. Acceleration of cell elongation during the early phaseof growth was accompanied by a 25% decrease in both turgor andosmotic pressure. During the linear phase of growth both pressuresremained largely constant. The difference between turgor andosmotic pressure (water potential) was –0.10 to –0.13MPa. Excision of one cotyledon had no effect on growth, turgorand osmotic pressure. However, after removal of both cotyledonscell elongation ceased and a substantial decrease in both pressureswas measured. In addition, we determined the longitudinal tissuepressure in seedlings from which one or both cotyledons hadbeen removed. Tissue pressure and turgor were very similar quantitiesunder all experimental conditions. Our results demonstrate thatturgor and cell osmotic pressure show a parallel change duringdevelopment of the stem. Cessation of cell elongation afterremoval of the cotyledons is attributable to a decrease in turgor(tissue) pressure, which provides the driving force for growthin the hypocotyl of the intact plant. Key words: Cell elongation, Helianthus annuus, osmotic pressure, tissue pressure, turgor     

Aphid stylectomy reveals an osmotic step between sieve tube and cortical cells in barley roots

The aim of the present study was to quantify osmotic pressuresdirectly in the translocation pathway, from leaf to growingroot tip, in order to understand the forces driving solutesfrom a source to a sink. Solutes move through the translocationpathway down an osmotically derived turgor gradient. Accordinglyaphid stylectomy and single cell sampling techniques have beencombined to examine the osmotic pressure of root phloem andgrowing root cells. Sieve tube sap was obtained from shootsand, for the first time, roots of barley seedlings using aphidstylectomy. Vacuolar sap was also obtained from a variety ofcells in leaf and root tissues using single cell sampling methods.Osmotic pressure of sieve tube sap from roots and shoots wasmeasured at high temporal resolution (within min) and over longperiods of time (up to 24 h). Osmotic pressure did not changesignificantly in the minutes immediately following excision,suggesting that confidence can be placed in the assumption thatstylet exudate is representative of sieve tube sap in vivo.There were no differences in the osmotic pressure of sieve tubesap from shoots (1.240.26 MPa, n = 10) or roots (1.420.15MPa, n = 13). However, osmotic pressure of sap from root corticalcells (0.710.09, n = 12) was about 0.7 MPa lower than thatof the sieve elements from roots, this difference may be maintainedby consumption of incoming solutes at the root tip. Resultsare discussed in the context of pressure driven flow in thephloem and symplastic contact between root tip cells and sievetube. It is hoped that the approach described here will provideimportant insights into the nature of the relationship betweenroot cell extension and assimilate supply through the phloem. Key words: Phloem, sieve tube, aphid, root, barley, osmotic pressure, translocation     

Pressure probe and isopiestic psychrometer measure similar turgor

Turgor measured with a miniature pressure probe was compared to that measured with an isopiestic thermocouple psychrometer in mature regions of soybean (Glycine max [L.] Merr.) stems. The probe measured turgor directly in cells of intact stems whereas the psychrometer measured the water potential and osmotic potential of excised stem segments and turgor was calculated by difference. When care was taken to prevent dehydration when working with the pressure probe, and diffusive resistance and dilution errors with the psychrometer, both methods gave similar values of turgor whether the plants were dehydrating or rehydrating. This finding, together with the previously demonstrated similarity in turgor measured with the isopiestic psychrometer and a pressure chamber, indicates that the pressure probe provides accurate measurements of turgor despite the need to penetrate the cell. On the other hand, it suggests that as long as precautions are taken to obtain accurate values for the water potential and osmotic potential, turgor can be determined by isopiestic psychrometry in tissues not accessible to the pressure probe for physical reasons.     

Influence of Xylem Water Potential on Leaf Elongation and Osmotic Adjustment of Wheat and Lupin

The leaf elongation rate and osmotic pressure at full turgorof wheat (Triticum aestivum L.) and lupin (Lupinus cosentiniiGuss.) were measured in well watered plants, in plants thatwere allowed to dry the soil slowly over 7 d, and in plantsin which the water potential of the leaf xylem was maintainedhigh by applying pressure to the roots during the drying cycle.Maintenance of high xylem water potentials failed to preventa reduction in the rate of leaf elongation as the soil dried,while the osmotic pressure at full turgor and the degree ofosmotic adjustment increased as the soil water content decreased.The rate of leaf elongation was reduced more and the degreeof osmotic adjustment was higher in leaves with high xylem waterpotentials than in those in which leaf xylem potentials wereallowed to decrease as soil water content decreased. Osmoticadjustment was linearly correlated with the reduction in leafelongation rate in both wheat and lupin. Key words: Osmotic adjustment, leaf elongation, turgor regulation     

Relative water content and water potential of tissue 1

Relative water content (RWC) and water potential as measuredwith the pressure chamber were evaluated as indicators of waterstatus of tissue-cultured apple shoots and plantlets (shootswith roots). During the hydration required for RWC measurement,both water content and water potential exhibited the same hydrationkinetics, indicating that 10 h were required for full hydration.Once full hydration was reached, shoot mass remained relativelyconstant. Moisture release characteristics were also constructedand the associated shoot and plantlet water relations parameterswere estimated. Underin vitroconditions, both shoot and plantletwater potential were similar to the water potential of the culturemedium in which they were grown. The moisture release characteristicof shoots and plantlets was consistent with that expected fortypical plant tissues, and gave estimates of maximum modulusof elasticity (6.201.14 MPa), osmotic potential at saturation(–0.85 0.10 MPa), osmotic potential at zero turgor (–1.16 0.14 MPa) and RWC at zero turgor (78 2%) which were similarto values in the literature. Higher values of leaf conductanceand RWC were found in shoots and plantlets placed at 95% RH(21 C) compared to those at 90% RH. Plantlets had higher valuesof both conductance and RWC compared to shoots, suggesting thatinvitroroots are functional in water uptake. Relative water contentwas related to measures of physiological activity such as leafconductance, and it was also easier to measure than water potential.Relative water content is suggested as a sound index of waterstatus in tissue culture plants. Key words: Conductance, microculture, water status, water stress.     

A critical re-examination of pressure-volume analysis of conifer shoots: comparison of three procedures for generating PV curves on shoots of Pinus resinosa Ait. seedlings

Prediction of water relations attributes for red pine (Pinusresinosa Ait.) derived from pressure-volume (PV) curves varieddepending on which of three methods was used. The sap expressionmethod entailed the enclosure of a shoot in a pressure chamberand expression of xylem sap by applying a constant selectedpressure until sap flow ceased, at which point xylem water potentialand shoot weight were measured. A sap expression PV curve wasformed by aggregating pairs of water potential-weight measurements,each pair supplied by one of 25 shoots. The repeat pressurizationmethod involved repeatedly measuring xylem water potential andshoot weight on a single shoot drying on a laboratory bench.Repeat pressurization PV curves were constructed from data providedby a single shoot. The composite method utilized single measurementsof xylem water potential and shoot weight on 25-30 differentshoots ranging in relative water content from about 1.0 to 0.5achieved by bench drying. Composite PV curves were constructedfrom aggregate data supplied by a population of shoots. Therewas close agreement in all PV attributes generated using repeatpressurization and sap expression methods. In contrast, withthe composite PV method, there was a fundamental differencein the slope of the linear region of the PV curves, causingosmotic potentials at full turgor and turgor loss to be morenegative, and relative water content at turgor loss to be lowerand symplast fraction to be higher. Comparison of compositeand repeat pressurization PV curves over the same ranges inwater content did not eliminate differences in derived waterrelations attributes. Differences in water potential isothermsrelated to the PV procedures used suggest that prolongedor repeatedexposure to gas at high pressure may introduce errors in theestimation of water relations attributes. Key words: Pinus resinosa, pressure chamber, pressure volume, tissue water relations     

Quantitative Analysis of Photosynthate Unloading in Developing Seeds of Phaseolus vulgaris L. : II. Pathway and Turgor Sensitivity

Phloem import and unloading in perfused bean (Phaseolus vulgaris L.) seed coats were investigated using steady-state labeling. Though photosynthate import and unloading were significantly reduced by perfusion, measurements of photosynthate fluxes in perfused seed coats proved useful for the study of unloading mechanisms in vivo. Phloem import was stimulated by lowered seed coat cell turgor, as demonstrated by an increase in tracer and sucrose import to seed coats perfused with high concentrations of an osmoticum. The partitioning of photosynthates between retention in the seed coat and release to the perfusion solution also was turgor sensitive; increases in seed coat cell turgor stimulated photosynthate release to the apoplast at the expense of photosynthate retention within the seed coat. There was no evidence of a turgor-sensitive sucrose uptake mechanism in perfused seed coats. Thus, the turgor sensitivity of photosynthate partitioning within perfused seed coats was consistent with a turgor-sensitive efflux control mechanism. Measurements of tracer equilibration and sugar partitioning in perfused seed coats provided strong evidence for symplastic phloem unloading in seed coats.     

Lamprothamnium, a Euryhaline Charophyte: I. OSMOTIC RELATIONS AND MEMBRANE POTENTIAL AT STEADY STATE

The charophyte Lamprothamnium papulosum (Wallr.) J. Gr. is foundat salinities varying from nearly fresh water to twice thatof sea water. It can maintain its turgor constant at 302 mosmolkg^–1 (0.73 MPa) when exposed to external osmotic pressuresof 550 to 1350 mosmol kg^–1 (1.3–3.3 MPa). Turgorshows a tendency to rise slightly at lower osmotic pressure(388 mosmol kg^–1 of turgor at 150 mosmol kg^–1 externalosmolality). K⁺ and Cl^– are the main solutes in the vacuole,and are most important in controlling internal osmotic pressure.Mg²⁺, Ca²⁺, and SO^2–₄ are present in significant amountsbut their concentrations do not change with changes in externalsalinity. Na⁺ is present in lower concentration than K⁺, andplays a minor role in regulating turgor. Sucrose is presentin significant concentrations, but changes little with changesin salinity. Two enzymes involved in sucrose metabolism, sucrosephosphate synthetase (EC 2.4.1.14 [EC] ), and sucrose synthetase (EC2.4.1.13 [EC] ) are active in whole cell extracts of Lamprothamnium.As in the fresh water charophytes, Lamprothamnium membrane potentialmay be depolarized (close to E_K) or hyperpolarized, and presumablyof electrogenic origin. Both types of potential are found atall salinities tested.     

Some Observations on the Water-Relations of the Hyphae of Neurospora crassa

The morphology of the colony and of individual cells of thehyphae of Neurospora crassa are described. The behaviour ofthe hyphal apices in solutions of differing molarity is describedand this behaviour is related to the behaviour of Fusarium oxysporum.Measurements of the water potential of the apical cells aremade and it is shown that this corresponds to the balancing-pointin this fungus and in F. oxysporum. Measurements are then madeof the water potential of cells at various points along thelength of the hyphae, and of the osmotic potential of thesecells, and turgor pressure is calculated. The lowest turgorpressure of 12.4 atmospheres is shown by the apical cells andthe highest of 17.5 atmospheres by the basal cells. The validity of the methods used is discussed.     

Water Potential, Translocation and Assimilate Partitioning

Lang, A. and Thorpe, M. R. 1986. Water potential, translocationand assimilate partitioning.—J. exp. Bot. 37: 495–503. The effect of water status upon translocation and assimilatepartitioning is examined both from theory and in an experimentwith young Phaseolus plants. Theory predicts that translocationis unlikely to be directly affected by water status. However,water potential differences within plants should influence translocationflow, with regions at lower potentials attracting disproportionatelylarge shares of assimilate. This prediction is supported in the experiment with Phaseolusin which the pattern of partitioning in the root changed rapidlyin response to bathing portions of it in solutions of differentosmolarity. The relevance of these findings to the growth of plants undernatural conditions is considered and evidence is presented thatwater potential gradients may be an Important factor in thecontrol of partitioning Key words: Phloem translocation, xylem transport, partitioning, water potential, control, osmotic potential     

Responses of a CAM Plant to Drought and Rainfall: Capacitance and Osmotic Pressure Influences on Water Movement

Daily and seasonal patterns in water flow and water potentialwere investigated for the Crassulacean acid metabolism succulentAgave deserti during an extended summer drought and for a periodfollowing rainfall. Field measurements of transpiration andof osmotic pressure changes over selected 24 h periods wereused as input variables for a computer model of water flow thatwas based on an electrical circuit analog of the whole plant.Parameters such as root resistance and tissue capacitance werealso varied to reflect the effects of changing plant or soilwater status. The model predicted internal water flow and waterpotential during the drought cycle and was used to assess therole of tissue osmotic properties in water uptake from the soiland in internal water redistribution. For plants under wet soil conditions, 55% of the night-timetranspiration was derived from water storage, this storage beingrecharged during the day. As drought progressed, transpirationand the nocturnal increase in osmotic pressure declined, althoughthe osmotic pressure itself increased. The difference in osmoticpressure between the water storage tissue and the chlorenchymacaused a net flow of water into the chlorenchyma after 3 weeksof drought, thereby increasing chlorenchyma turgor pressure.Simulations also indicated that a large increase in root resistancemust occur to prevent substantial water loss from the plantto the dry soil. After rainfall, recharge of plant water storagewas complete within one week, although full recovery in theamplitude of daily osmotic pressure variations took longer. Key words: Agave deserti, transpiration, water potential, water storage     

Chloroplast response to low leaf water potentials: I. Role of turgor

The effect of decreases in turgor on chloroplast activity was studied by measuring the photochemical activity of intact sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves having low water potentials. Leaf turgor, calculated from leaf water potential and osmotic potential, was found to be affected by the dilution of cell contents by water in the cell walls, when osmotic potentials were measured with a thermocouple psychrometer. After the correction of measurements of leaf osmotic potential, both the thermocouple psychrometer and a pressure chamber indicated that turgor became zero in sunflower leaves at leaf water potentials of −10 bars. Since most of the loss in photochemical activity occurred at water potentials below −10 bars, it was concluded that turgor had little effect on the photochemical activity of the leaves.