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. 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     

Transport of Salt and Water in Rabbit and Guinea Pig Gall Bladder

A simple and reproducible method has been developed for following fluid transport by an in vitro preparation of mammalian gall bladder, based upon weighing the organ at 5 minute intervals. Both guinea pig and rabbit gall bladders transport NaCl and water in isotonic proportions from lumen to serosa. In the rabbit bicarbonate stimulates transport, but there is no need for exogenous glucose. The transport rate is not affected by removal of potassium from the bathing solutions. Albumin causes a transient weight loss from the gall bladder wall, apparently by making the serosal smooth muscle fibers contract. Active NaCl transport can carry water against osmotic gradients of up to two atmospheres. Under passive conditions water may also move against its activity gradient in the presence of a permeating solute. The significance of water movement against osmotic gradients during active solute transport is discussed.     

Modeling of ion transport. Analysis of possible mechanisms of intracellular osmoregulation]

It seems likely that the operation of the vacuolar mechanism of active transport of water (VMATW) is based on the following. An exceeding osmotic pressure is created within an originally small vacuole (e. g. due to rapid enzymatis hydrolysis of macromolecules inside the vacuole) and consequently water enters the vacuole, as well as molecules and ions according to the gradients of their chemical potentials. After a while the contents of the swelling vacuole is thrown out to the external medium. An analysis of the efficiency of VMATW system in the stationary case shows that the efficiency of VMATW can be rather high to create the ionic heterogeneity.     

Water Flow Across the Sieve Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. IV. Root Tips and Seed Coats

In the present paper, the theory developed in Part I of thisseries is applied to seed coats of Phaseolus vulgaris and somecombined data on root tips of Hordeum distichum and Hordeumvulgare. Because of the large back-pressures implied, it isconcluded that phloem transport into these primary sinks wouldbe physiologically impossible in the absence of a symplasticpathway for the unloading of water from sieve elements. In thiscase, unloading of water and sucrose will occur predominantlyas a pressure-driven flow of solution through plasmodesmata,although diffusion can contribute significantly to the plasmodesmatalsucrose flux. At least 20% of the plasmodesmata connecting sieveelements and adjacent cells must be unobstructed if large changesin turgor and osmotic pressure are to be avoided. Dependingon the membrane area available for water fluxes, it is possiblethat the difference in water potential across the sieve-tubeplasmalemma can lead to significant errors when axial turgorgradients are estimated from gradients of osmotic pressure andexternal water potential. The magnitude and even the sign ofthese errors is uncertain, but it is possible that sieve-tubeturgor pressures will be significantly underestimated in primarysinks Phloem, turgor, osmotic pressure, plasmodesmata, Munch hypothesis, Phloem unloading     

Controversy regarding the secondary active water transport hypothesis.

Historically, water transport across biological membranes has always been considered a passive process, i.e., the net water transport is proportional to the gradients of hydrostatic and osmotic pressure. More recently, this dogma was challenged by the suggestion that secondary active transporters such as the Na/glucose cotransporter (SGLT1) could perform secondary active water transport with a fixed stoichiometry. In the case of SGLT1, the stoichiometry would consist of one glucose molecule to two Na+ ions to 220-400 water molecules. In the present minireview, we summarize and criticize the evidence supporting and opposing this water cotransport hypothesis. Published and unpublished observations from our own laboratory are also presented in support of the idea that transport-dependent osmotic gradients begin to build up immediately after cotransport commences and are fully responsible for the cell swelling observed.     

Go with the flow: mechanisms driving water transport during vegetative growth and fruiting

Fungi need water for all stages of life. Notably, mushrooms consist of ∼90% water. Fungi degrade organic matter by secreting enzymes. These enzymes need water to be able to break down the substrate. For instance, when the substrate is too dry, fungi transport water from moist areas to arid areas by hydraulic redistribution. Once nutrients are freed from the substrate, they are taken up by transporters lining the cell membrane. Thereby an intracellular osmotic potential is created which is greater than that of the substrate, and water follows by osmosis. Aquaporins may facilitate water uptake depending on the conditions. Since fungi possess a cell wall, the cell volume will not increase much by water uptake, but the cell membrane will exert higher pressure on the cell wall, thereby building up turgor. Fungi have tightly coordinated osmotic regulatory controls via the HOG pathway. When water is getting scarce, this pathway makes sure that enough osmolytes are synthesized to allow sufficient water uptake for maintaining turgor homeostasis. The fungal network is interconnected and allows water flow when small pressure differences exist. These pressure differences can be the result of growth, differential osmolyte uptake/synthesis or external osmotic conditions. Overall, the water potential of the substrate and of fungal tissues determine whether water will flow, since water flows from an area of high- to a low water potential area, when unobstructed. In this review we aim to give a comprehensive view on how fungi obtain and translocate water needed for their development. We have taken Agaricus bisporus growing on compost and casing soil as a case study, to discuss water relations during fruiting in detail. Using the current state-of-the-art we found that there is a discrepancy between the models describing water transport to mushrooms and the story that water potentials tell us.     

Water-Transporting Proteins

Transport through lipids and aquaporins is osmotic and entirely driven by the difference in osmotic pressure. Water transport in cotransporters and uniporters is different: Water can be cotransported, energized by coupling to the substrate flux by a mechanism closely associated with protein. In the K⁺/Cl⁻ and the Na⁺/K⁺/2Cl⁻ cotransporters, water is entirely cotransported, while water transport in glucose uniporters and Na⁺-coupled transporters of nutrients and neurotransmitters takes place by both osmosis and cotransport. The molecular mechanism behind cotransport of water is not clear. It is associated with the substrate movements in aqueous pathways within the protein; a conventional unstirred layer mechanism can be ruled out, due to high rates of diffusion in the cytoplasm. The physiological roles of the various modes of water transport are reviewed in relation to epithelial transport. Epithelial water transport is energized by the movements of ions, but how the coupling takes place is uncertain. All epithelia can transport water uphill against an osmotic gradient, which is hard to explain by simple osmosis. Furthermore, genetic removal of aquaporins has not given support to osmosis as the exclusive mode of transport. Water cotransport can explain the coupling between ion and water transport, a major fraction of transepithelial water transport and uphill water transport. Aquaporins enhance water transport by utilizing osmotic gradients and cause the osmolarity of the transportate to approach isotonicity.     

The Balance of Fluid and Osmotic Pressures across Active Biological Membranes with Application to the Corneal Endothelium

The movement of fluid and solutes across biological membranes facilitates the transport of nutrients for living organisms and maintains the fluid and osmotic pressures in biological systems. Understanding the pressure balances across membranes is crucial for studying fluid and electrolyte homeostasis in living systems, and is an area of active research. In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments. The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges. The steady-state analysis on active membranes suggests a new pressure mechanism which balances the fluid pressure together with the osmotic pressure. The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport. We apply the proposed theory to study the transendothelial fluid pressure in the in vivo cornea, which is a crucial factor maintaining the hydration and transparency of the tissue. The results show the importance of the proposed pressure mechanism in mediating stromal fluid pressure and provide a new interpretation of the pressure modulation mechanism in the in vivo cornea.     

Turgor pressure changes trigger characteristic changes in the electrical conductance of the tonoplast and the plasmalemma of the marine alga Valonia utricularis

The giant marine alga Valonia utricularis is capable of regulating its turgor pressure in response to changes in the osmotic pressure of the sea water. The turgor pressure response comprises two phases, a fast, exponential phase arising exclusively from water shifting between the vacuole and the external medium (time constant about 10 min) and a second very slow, almost exponential phase adjusting (but not always) the turgor pressure near to the original value by release or uptake of KCl (time constant about 5 h). The changes in the vacuolar membrane potential as well as in the individual conductances of the tonoplast and plasmalemma accompanying turgor pressure regulation were measured by using the vacuolar perfusion assembly (with integrated microelectrodes, pressure transducers and pressure‐regulating valves) as described by Wang et al. (J. Membrane Biology 157, 311–321, 1997). Measurements on pressure‐clamped cells gave strong evidence that the turgor pressure, but not effects related to water flow (i.e. electro‐osmosis or streaming potential) or changes in the internal osmotic pressure and in the osmotic gradients, triggers the cascade of osmotic and electrical events recorded after disturbance of the osmotic equilibrium. The findings definitely exclude the existence of osmosensors as postulated for other plant cells and bacteria. There was also evidence that turgor pressure signals were primarily sensed by ion transporters in the vacuolar membrane because conductance changes were first recorded in the many‐folded tonoplast and then significantly delayed in the plasmalemma independent of the direction of the osmotic challenge. Consistently, turgor pressure up‐regulation (but not down‐regulation) could be inhibited reversibly by external addition of the K⁺ transport inhibitor Ba²⁺ and/or by the Cl^– transport inhibitor 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS). Extensive studies under iso‐, hyper‐ and hypo‐osmotic conditions revealed that K⁺ and Cl^– contribute predominantly to the plasmalemma conductance. Addition of 0.3 mm NaCN showed further that part of the K⁺ and Cl^– transporters depended on ATP. These transporters are apparently up‐regulated upon hyper‐osmotic, but not hypo‐osmotic challenge. These findings explain the strong increase of the K⁺ influx upon lowering turgor pressure and the less pronounced pressure‐dependence of the Cl^– influx of V. utricularis reported in the literature. The data derived from the blockage experiments under hypo‐osmotic conditions were also equally consistent with the experimental findings that the K⁺ efflux is solely passive and progressively increases with increasing turgor pressure due to an increase of the volumetric elastic modulus of the cell wall. However, despite unravelling some of the sequences and other components involved in turgor pressure regulation of V. utricularis the co‐ordination between the ion transporters in the tonoplast and plasmalemma remains unresolved because of the failure to block the tonoplast transporters by addition of Ba²⁺ and DIDS from the vacuolar side.     

A model of stomatal conductance to quantify the relationship between leaf transpiration, microclimate and soil water stress

A model of stomatal conductance was developed to relate plant transpiration rate to photosynthetic active radiation (PAR), vapour pressure deficit and soil water potential. Parameters of the model include sensitivity of osmotic potential of guard cells to photosynthetic active radiation, elastic modulus of guard cell structure, soil‐to‐leaf conductance and osmotic potential of guard cells at zero PAR. The model was applied to field observations on three functional types that include 11 species in subtropical southern China. Non‐linear statistical regression was used to obtain parameters of the model. The result indicated that the model was capable of predicting stomatal conductance of all the 11 species and three functional types under wide ranges of environmental conditions. Major conclusions included that coniferous trees and shrubs were more tolerant for and resistant to soil water stress than broad‐leaf trees due to their lower osmotic potential, lignified guard cell walls, and sunken and suspended guard cell structure under subsidiary epidermal cells. Mid‐day depression in transpiration and photosynthesis of pines may be explained by decreased stomatal conductance under a large vapour pressure deficit. Stomatal conductance of pine trees was more strongly affected by vapour pressure deficit than that of other species because of their small soil‐to‐leaf conductance, which is explainable in terms of xylem tracheids in conifer trees. Tracheids transport water by means of small pit‐pairs in their side walls, and are much less efficient than the end‐perforated vessel members in broad‐leaf xylem systems. These conclusions remain hypothetical until direct measurements of these parameters are available.     

Isotonic Water Transport in Secretory Epithelia ,

The model proposed by Diamond and Bossert [1] for isotonic water transport has received wide acceptance in recent years. It assumes that the local driving force for water transport is a standing osmotic gradient produced in the lateral intercellular spaces of the epithelial cell layer by active solute transport. While this model is based on work done in absorptive epithelia where the closed to open direction of the lateral space and the direction of net transport are the same, it has been proposed that the lateral spaces could also serve as the site of the local osmotic gradients for water transport in secretory epithelia, where the closed to open direction of the lateral space and net transport are opposed, by actively transporting solute out of the space rather than into it. Operation in the backward direction, however, requires a lower than ambient hydrostatic pressure within the lateral space which would seem more likely to cause the space to collapse with loss of function. On the other hand, most secretory epithelia are characterized by transport into a restricted ductal system which is similar to the lateral intercellular space in the absorptive epithelia in that its closed to open direction is the same as that of net transport. In vitro micropuncture studies on the exocrine pancreas of the rabbit indicate the presence of a small but statistically significant increase in juice osmolality, 6 mOsm/kg H₂O, at the site of electrolyte and water secretion in the smallest extralobular ducts with secretin stimulation which suggests that the ductal system in the secretory epithelia rather than the lateral intercellular space is the site of the local osmotic gradients responsible for isotonic water transport.     

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre‐dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.     

Effect of the medium composition on the resting membrane potential in the earthworm longitudinal somatic muscle fibers

Norepinephrine or increased extracellular K+ hyperpolarize the membrane of the earthworm somatic muscle fibre, whereas removal of Cl- from external solution or a hypotonic solution depolarize the membrane. The dependence of the membrane resting potential on the extracellular K+ is quite characteristic against the background of ouabain action. A preliminary membrane depolarisation by ouabain eliminates the above effects on the membrane resting potential. The data obtained suggest that the ouabain-sensitive active ion pump directly contributes to the membrane resting potential value. This hypothesis is discussed with respect to existence of active Cl- transport combined with Na+, K(+)-pump which presumably takes part in the intracellular osmotic pressure regulation in the earthworm somatic muscle.     

Solute transport process in intestinal epithelial cells

In rat small intestine, the active transport of organic solutes results in significant depolarization of the membrane potential measured in an epithelial cell with respect to a grounded mucosal solution and in an increase in the transepithelial potential difference. According to the analysis with an equivalent circuit model for the epithelium, the changes in emf's of mucosal and serosal membranes induced by active solute transport were calculated using the measured conductive parameters. The result indicates that the mucosal cell membrane depolarizes while the serosal cell membrane remarkably hyperpolarizes on the active solute transport. Corresponding results are derived from the calculations of emf's in a variety of intestines, using the data that have hitherto been reported. The hyperpolarization of serosal membrane induced by the active solute transport might be ascribed to activation of the serosal electrogenic sodium pump. In an attempt to determine the causative factors in mucosal membrane depolarization during active solute transport, cell water contents and ion concentrations were measured. The cell water content remarkably increased and, at the same time, intracellular monovalent ion concentrations significantly decreased with glucose transport. Net gain of glucose within the cell was estimated from the restraint of osmotic balance between intracellular and extracellular fluids. In contrast to the apparent decreases in intracellular Na+ and K+ concentrations, significant gains of Na+ and K+ occurred with glucose transport. The quantitative relationships among net gains of Na+, K+ and glucose during active glucose transport suggest that the coupling ratio between glucose and Na+ entry by the carrier mechanism on the mucosal membrane is approximately 1:1 and the coupling ratio between Na+-efflux and K+-influx of the serosal electrogenic sodium pump is approximately 4:3 in rat small intestine. In addition to the electrogenic ternary complex inflow across the mucosal cell membrane, the decreases in intracellular monovalent ion concentrations, the temporary formation of an osmotic pressure gradient across the cell membrane and the streaming potential induced by water inflow through negatively charged pores of the cell membrane in the course of an active solute transport in intestinal epithelial cells are apparently all possible causes of mucosal membrane depolarization.     

Effects of cyclic AMP on fluid absorption and ion transport across frog retinal pigment epithelium. Measurements in the open-circuit state

A modified version of a capacitance probe technique has been used to measure fluid transport across the isolated retinal pigment epithelium (RPE)-choroid of the bullfrog. The accuracy of this measurement is 0.5-1.0 nl/min. Experiments carried out in the absence of external osmotic or hydrostatic gradients show that the RPE-choroid transports fluid from the retinal to the choroid side of the tissue at a rate of approximately 10 nl/min (4-6 microliters/cm2 X h). Net fluid absorption (Jv) was abolished within 10 min by the mitochondrial uncoupler 2,4-dinitrophenol. It was also inhibited (70%) by the removal of bicarbonate from the bulk solutions bathing the tissue. Ouabain caused a slow decrease in Jv (no effect at 10 min, 70% at 3 h), which indicates that RPE fluid transport is not directly coupled to the activity of the Na-K pump located at the apical membrane of this epithelium. In contrast to ouabain, cyclic AMP (cAMP) produced a quick decrease in Jv (84% within 5 min). Radioisotope experiments in the open circuit show that cAMP stimulated secretory fluxes of Na and Cl, which accounted for the observed cAMP-induced decrease in Jv. The direction of net fluid absorption, the magnitudes of the net ionic fluxes in the open circuit, and the dependence of Jv on external bicarbonate concentration strongly suggest that fluid absorption is generated primarily by the active absorption of bicarbonate.     

Epithelial water transport in a balanced gradient system.

The relationship between epithelial fluid transport, standing osmotic gradients, and standing hydrostatic pressure gradients has been investigated using a perturbation expansion of the governing equations. The assumptions used in the expansion are: (a) the volume of lateral intercellular space per unit volume of epithelium is small; (b) the membrane osmotic permeability is much larger than the solute permeability. We find that the rate of fluid reabsorption is set by the rate of active solute transport across lateral membranes. The fluid that crosses the lateral membranes and enters the intercellular cleft is driven longitudinally by small gradients in hydrostatic pressure. The small hydrostatic pressure in the intercellular space is capable of causing significant transmembrane fluid movement, however, the transmembrane effect is countered by the presence of a small standing osmotic gradient. Longitudinal hydrostatic and osmotic gradients balance such that their combined effect on transmembrane fluid flow is zero, whereas longitudinal flow is driven by the hydrostatic gradient. Because of this balance, standing gradients within intercellular clefts are effectively uncoupled from the rate of fluid reabsorption, which is driven by small, localized osmotic gradients within the cells. Water enters the cells across apical membranes and leaves across the lateral intercellular membranes. Fluid that enters the intercellular clefts can, in principle, exit either the basal end or be secreted from the apical end through tight junctions. Fluid flow through tight junctions is shown to depend on a dimensionless parameter, which scales the resistance to solute flow of the entire cleft relative to that of the junction. Estimates of the value of this parameter suggest that an electrically leaky epithelium may be effectively a tight epithelium in regard to fluid flow.     

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.

     

Corneal endothelium transports fluid in the absence of net solute transport

The corneal endothelium transports fluid from the corneal stroma to the aqueous humor, thus maintaining stromal transparency by keeping it relatively dehydrated. This fluid transport mechanism is thought to be driven by the transcellular transports of HCO(3)(-) and Cl(-) in the same direction, from stroma to aqueous. In parallel to these anion movements, for electroneutrality, there are paracellular Na(+) and transcellular K(+) transports in the same direction. The resulting net flow of solute might generate local osmotic gradients that drive fluid transport. However, there are reports that some 50% residual fluid transport remains in nominally HCO(3)(-) free solutions. We have examined the driving force for this residual fluid transport. We confirm that in nominally HCO(3)(-) free solutions, 48% of control fluid transport remains. When in addition Cl(-) channels are inhibited, 30% of control fluid movement still remains. Addition of a carbonic anhydrase inhibitor has no further effect. These manipulations combined inhibit the transcellular transport of all anions, without which there cannot be any net transport of solute and consequently no local osmotic gradients, yet there is residual fluid movement. Only the further addition of benzamil, an inhibitor of epithelial Na(+) channels, abolishes fluid transport completely. Our data are inconsistent with transcellular local osmosis and instead support the paradigm of paracellular fluid transport driven by electro-osmotic coupling.     

Proline accumulation and the adaptation of cultured plant cells to water stress

The transfer of cultured tomato cells (Lycopersicon esculentum cv VFNT-Cherry) to a low water potential environment resulted in an increased dry weight to fresh weight ratio accompanied by a rapid accumulation of proline. Proline content continued to increase as osmotic adjustment and growth occurred. The initial increase in proline concentration was accompanied by a drop in turgor. However, proline levels continued to increase with a gain in turgor during osmotic adjustment. Thus, the accumulation of proline depended not only on cell water potential, or on the initial loss of turgor but more closely on cell osmotic potential. The ultimate level of proline depended on the level of adaptation. Proline levels remained high after more than 100 cell generations in low water potential media, but declined rapidly after transfer to media with a less negative water potential. Addition of exogenous proline to the medium during water stress and during osmotic downshock alleviated the normally resulting inhibition of growth. The results suggest a positive role for proline accumulation in adaptation of cells to changing external water potentials.