Turgor Recovery Research Articles

Succulence and aquaporin expression during drought and recovery in the CAM epiphytic bromeliad Acanthostachys strobilacea (Schult. & Schult.f.) Klotzsch

Due to climate change, drought-rewatering cycles might become more intense and frequent, potentially threatening epiphytic bromeliads as they are detached from the soil. Hence, research on drought-rewatering responses is essential to determine the resilience of these species to projected future environmental conditions. The tankless, CAM epiphytic bromeliad Acanthostachys strobilacea shows significant drought tolerance from its early developmental stages. Here, we investigated water storage remobilization and the expression of aquaporin genes in the succulent leaf tissues of juvenile A. strobilacea plants in response to a drought-rewatering cycle and their relation to metabolic status. Under greenhouse conditions, 3-month-old plants were subjected to 14 days without irrigation, followed by 1 day of rewatering. We conducted analyses of water status, leaf anatomy, photosynthetic rates, titratable acidity, metabolic profile, and aquaporin gene expression. Data on water status indicated drought-induced turgor loss, which could be mainly attributed to the collapse of the hydrenchyma (water-storage tissue). The water stored in these cells likely relocated to the photosynthetically active cells of the chlorenchyma, which may have helped maintain metabolic activity. Indeed, titratable acidity, gas exchange, and metabolic data showed intensified CAM activity after drought. Drought also increased proline and antioxidant contents but significantly reduced the expression of AsPIP1;1, AsPIP1;2-like, AsTIP2;2 and AsNIP2;2 genes. After 1 day of rewatering, turgor, CAM activity, and the expression of most aquaporin genes and most metabolite contents were fully restored to control levels. The rapid turgor recovery, even in the absence of leaf water-absorbing trichomes, was due to water storage in hydrenchyma cells upon rehydration. Additionally, the modulation of aquaporin expression likely reduced water loss during drought and aided turgor restoration after rewatering. These results can guide future research on the responses of epiphytic bromeliads to climate change, essential to developing conservation strategies.

Overexpression of an aquaglyceroporin gene from Trichoderma harzianum improves water-use efficiency and drought tolerance in Nicotiana tabacum

Aquaporins (AQPs) and aquaglyceroporins (AQGPs) are integral membrane proteins that mediate the transport of water and solutes, such as glycerol and urea, across membranes. AQP and AQGP genes represent a valuable tool for biotechnological improvement of plant tolerance to environmental stresses. We previously isolated a gene encoding for an aquaglyceroporin (ThAQGP), which was up-regulated in Trichoderma harzianum during interaction with the plant pathogen Fusarium solani. This gene was introduced into Nicotiana tabacum and plants were physiologically characterized. Under favorable growth conditions, transgenic progenies did not had differences in both germination and growth rates when compared to wild type. However, physiological responses under drought stress revealed that transgenic plants presented significantly higher transpiration rate, stomatal conductance, photosynthetic efficiency and faster turgor recovery than wild type. Quantitative RT-PCR analysis demonstrated the presence of ThAQGP transcripts in transgenic lines, showing the cause–effect relationship between the observed phenotype and the expression of the transgene. Our results underscore the high potential of T. harzianum as a source of genes with promising applications in transgenic plants tolerant to drought stress.

Mechanisms underlying turgor regulation in the estuarine alga Vaucheria erythrospora (Xanthophyceae) exposed to hyperosmotic shock.

Aquatic organisms are often exposed to dramatic changes in salinity in the environment. Despite decades of research, many questions related to molecular and physiological mechanisms mediating sensing and adaptation to salinity stress remain unanswered. Here, responses of Vaucheria erythrospora, a turgor-regulating xanthophycean alga from an estuarine habitat, have been investigated. The role of ion uptake in turgor regulation was studied using a single cell pressure probe, microelectrode ion flux estimation (MIFE) technique and membrane potential (Em ) measurements. Turgor recovery was inhibited by Gd(3+) , tetraethylammonium chloride (TEA), verapamil and orthovanadate. A NaCl-induced shock rapidly depolarized the plasma membrane while an isotonic sorbitol treatment hyperpolarized it. Turgor recovery was critically dependent on the presence of Na(+) but not K(+) and Cl(-) in the incubation media. Na(+) uptake was strongly decreased by amiloride and changes in net Na(+) and H(+) fluxes were oppositely directed. This suggests active uptake of Na(+) in V. erythrospora mediated by an antiport Na(+) /H(+) system, functioning in the direction opposite to that of the SOS1 exchanger in higher plants. The alga also retains K(+) efficiently when exposed to high NaCl concentrations. Overall, this study provides insights into mechanisms enabling V. erythrospora to regulate turgor via ion movements during hyperosmotic stress.

Exposure of two Eutrema salsugineum (Thellungiella salsuginea) accessions to water deficits reveals different coping strategies in response to drought.

Eutrema salsugineum is an extremophile related to Arabidopsis. Accessions from Yukon, Canada and Shandong, China, were evaluated for their tolerance to water deficits. Plants were exposed to two periods of water deficit separated by an interval of re-watering and recovery. All plants took the same time to wilt during the first drought exposure but Yukon plants took 1 day longer than Shandong plants following the second drought treatment. Following re-watering and turgor recovery, solute potentials of Shandong leaves returned to predrought values while those of Yukon leaves were lower than predrought levels consistent with having undergone osmotic adjustment. Polar metabolites profiled in re-watered plants showed that different metabolites are accumulated by Yukon and Shandong plants recovering from a water deficit with glucose more abundant in Yukon and fructose in Shandong leaves. The drought-responsive expression of dehydrin genes RAB18, ERD1, RD29A and RD22 showed greater changes in transcript abundance in Yukon relative to Shandong leaves during both water deficits and recovery with the greatest difference in expression appearing during the second drought. We propose that the initial exposure of Yukon plants to drought renders them more resilient to water loss during a subsequent water deficit leading to delayed wilting. Yukon plants also established a high leaf water content and increased specific leaf area during the second deficit. Shandong plants undergoing the same treatment regime do not show the same beneficial drought tolerance responses and likely use drought avoidance to cope with water deficits.

An estuarine species of the alga Vaucheria (Xanthophyceae) displays an increased capacity for turgor regulation when compared to a freshwater species.

Turgor regulation is the process by which walled organisms alter their internal osmotic potential to adapt to osmotic changes in the environment. Apart from a few studies on freshwater oomycetes, the ability of stramenopiles to turgor regulate has not been investigated. In this study, turgor regulation and growth were compared in two species of the stramenopile alga Vaucheria, Vaucheria erythrospora isolated from an estuarine habitat, and Vaucheria repens isolated from a freshwater habitat. Species were identified using their rbcL sequences and respective morphologies. Using a single cell pressure probe to directly measure turgor in Vaucheria after hyperosmotic shock, V.erythrospora was found to recover turgor after a larger shock than V.repens. Threshold shock values for this ability were >0.5MPa for V.erythrospora and <0.5MPa for V.repens. Recovery was more rapid in V.erythrospora than V.repens after comparable shocks. Turgor recovery in V.erythrospora was inhibited by Gd(3+) and TEA, suggesting a role for mechanosensitive channels, nonselective cation channels, and K(+) channels in the process. Growth studies showed that V.erythrospora was able to grow over a wider range of NaCl concentrations. These responses may underlie the ability of V.erythrospora to survive in an estuarine habitat and restrict V.repens to freshwater. The fact that both species can turgor regulate may indicate a fundamental difference between members of the Stramenopila, as research to date on oomycetes suggests they are unable to turgor regulate.

Effect of salinity and PEG-induced water stress on water status, gas exchange, solute accumulation, and leaf growth in Ipomoea pes-caprae

This study was designed to evaluate the response of I. pes-caprae, a pantropical species growing down to the high tide mark on the beach, under low and high drought and saline stresses and under both stresses applied simultaneously. Leaf water relations, gas exchange parameters, and leaf expansion and production rates were evaluated using NaCl and Polyethylene glycol 6000 to adjust the iso-osmotic potential in the nutrient solution to −0.5 and −1.0 MPa. Additionally, both osmotica were combined in the quantities required to allow that both solutes contribute in the same proportion to the total reduction in the solution water potential. The leaf osmotic potential decreased more rapidly at low stress levels and was always lower in plants growing under salinity. At long term, the osmotic adjustment was reached in all treatments, but under high stress, the osmotic adjustment was delayed and plants experimented eventual turgor loss. The simultaneous addition of NaCl and PEG accelerated turgor recovery after the induction of osmotic stress, compared with both agents applied individually. Na + accumulation was enhanced in plants by salt addition, while proline content per leaf water unit increased significantly only at high stress levels. In all treatments maximum assimilation rate and stomatal conductance decreased the first day after addition of salt and/or PEG, and thereafter progressively increased throughout the treatment. At short term and at high stress level, the leaf expansion rate (LER) decreased compared with control plants. When both stresses were applied simultaneously, the effects on LER and leaf area were less detrimental when compared with each stress separately. The results suggest that I. pes-caprae is more susceptible to high water limitations than to soil salinity, while the water and carbon balance were always enhanced when both stresses were applied simultaneously. Thus, although salinity and drought stress are physiologically related and the tolerance mechanisms overlap, some aspect of plant physiology and metabolism may differ when the plant experiments single saline and water or both stresses simultaneously.

Ptk2 contributes to osmoadaptation in the filamentous fungus Neurospora crassa

Hyphal tip-growing organisms often rely upon an internal hydrostatic pressure (turgor) to drive localized expansion of the cell. Regulation of the turgor in response to osmotic shock is mediated primarily by an osmotic MAP kinase cascade which activates osmolyte synthesis and ion uptake to effect turgor recovery. We characterized a Neurospora crassa homolog (PTK2) of ser/thr kinase regulators of ion transport in yeast to determine its role in turgor regulation in a filamentous fungi. The ptk2 mutant is osmosensitive, and has lower turgor poise than wildtype. The cause appears to be lower activity of the plasma membrane H +-ATPase. Its role in osmoadaptation is unrelated to the activity of the osmotic MAP kinase cascade. Instead, it acts in an alternative pathway that, like the osmotic MAP kinase cascade, also involves ion transport mediated osmoadaptation.

Turgor regulation in the osmosensitive cut mutant of Neurospora crassa

The internal hydrostatic pressure (turgor) of fungal cells is maintained at 400-500 kPa. The turgor is regulated by changes in ion flux and by production of the osmotically active metabolite glycerol. In Neurospora crassa, there are at least two genetically distinct pathways that function in adaptation to hyperosmotic shock. One involves a mitogen-activated protein (MAP) kinase cascade (kinases OS-4, OS-5 and OS-2 downstream of the osmosensing OS-1); the other is less understood, but involves the cut gene, which encodes a putative phosphatase. This study examined turgor regulation, electrical responses, ion fluxes and glycerol accumulation in the cut mutant. Turgor recovery after hyperosmotic treatment was similar to that in the wild-type, for both time-course ( approximately 40 min) and magnitude. Prior to turgor recovery, the hyperosmotic shock caused a rapid transient depolarization of the membrane potential, followed by a sustained hyperpolarization that occurred concomitant with increased H(+) efflux, indicating that the plasma membrane H(+)-ATPase was being activated. These changes also occurred in the wild-type. Net fluxes of Ca(2+) and Cl(-) during turgor recovery were similar to those in the wild-type, but K(+) influx was attenuated in the cut mutant. The similar turgor recovery can be explained by the ion uptake, since glycerol did not accumulate in the cut mutant within the time frame of turgor recovery (but did accumulate in the wild-type). The results suggest that turgor regulation involves multi-faceted coordination of both ion flux and glycerol accumulation. Ion uptake is activated by a MAP kinase cascade, while CUT is required for glycerol accumulation.

Osmotic adjustment in leaves of Brassica oilseeds in response to water deficit

The influence of water deficit on water content (WC), total soluble solids (TSS), osmotic potential (OP), sugar content and osmotic adjustment (OA) of expanded and partly expanded leaves of Brassica oilseeds was examined. Nine canola (B. napus) cultivars (Karoo, Monty, Pinnacle, Hyden, Mystic, Rainbow, Surpass 300, Surpass 400, Surpass 501), two doubled haploids, one from Karoo (KDH) and the other from Monty (MDH) and one line of Indian mustard (B. juncea, PI-81792) were grown under glasshouse and net-house conditions. Expanded wilted leaves of Karoo and Monty absorbed excessive amounts of water per dry weight upon in vitro rehydration compared with control non-stressed leaves, resulting in underestimation of OA calculated on the basis of the relative water content (RWC). Hence, estimation of OA based on water weight per leaf dry weight (WC) was preferred. Young expanding leaves maintained visual turgor for 6-7 d after withholding irrigation, while expanded leaves on the same plants ceased to regain turgor overnight. The young expanding leaves exhibited greater accumulation of TSS and, consequently, more negative OPs compared with expanded leaves. Maintenance of OA after irrigation and turgor recovery was evident in both expanded and expanding leaves. Although OA under drought and upon turgor recovery varied within cultivars in different experiments, outstanding OA capacity, in terms of both magnitude and stability, was identified in the cultivar Hyden and in the doubled haploid of Monty, indicating the potential to select for this trait as well as to exploit variability for OA through haploidization. Key words: Brassica oilseeds, drought stress, osmotic adjustment, haploid lines

Role of a Mitogen-Activated Protein Kinase Cascade in Ion Flux-Mediated Turgor Regulation in Fungi

Fungi normally maintain a high internal hydrostatic pressure (turgor) of about 500 kPa. In response to hyperosmotic shock, there are immediate electrical changes: a transient depolarization (1 to 2 min) followed by a sustained hyperpolarization (5 to 10 min) prior to turgor recovery (10 to 60 min). Using ion-selective vibrating probes, we established that the transient depolarization is due to Ca(2+) influx and the sustained hyperpolarization is due to H(+) efflux by activation of the plasma membrane H(+)-ATPase. Protein synthesis is not required for H(+)-ATPase activation. Net K(+) and Cl(-) uptake occurs at the same time as turgor recovery. The magnitude of the ion uptake is more than sufficient to account for the osmotic gradients required for turgor to return to its original level. Two osmotic mutants, os-1 and os-2, homologs of a two-component histidine kinase sensor and the yeast high osmotic glycerol mitogen-activated protein (MAP) kinase, respectively, have lower turgor than the wild type and do not exhibit the sustained hyperpolarization after hyperosmotic treatment. The os-1 mutant does not exhibit all of the wild-type turgor-adaptive ion fluxes (Cl(-) uptake increases, but net K(+) flux barely changes and net H(+) efflux declines) (os-2 was not examined). Both os mutants are able to regulate turgor but at a lower level than the wild type. Our results demonstrate that a MAP kinase cascade regulates ion transport, activation of the H(+)-ATPase, and net K(+) and Cl(-) uptake during turgor regulation. Other pathways regulating turgor must also exist.

Anaerobicity prepares Saccharomyces cerevisiae cells for faster adaptation to osmotic shock.

Yeast cells adapt to hyperosmotic shock by accumulating glycerol and altering expression of hundreds of genes. This transcriptional response of Saccharomyces cerevisiae to osmotic shock encompasses genes whose products are implicated in protection from oxidative damage. We addressed the question of whether osmotic shock caused oxidative stress. Osmotic shock did not result in the generation of detectable levels of reactive oxygen species (ROS). To preclude any generation of ROS, osmotic shock treatments were performed in anaerobic cultures. Global gene expression response profiles were compared by employing a novel two-dimensional cluster analysis. The transcriptional profiles following osmotic shock under anaerobic and aerobic conditions were qualitatively very similar. In particular, it appeared that expression of the oxidative stress genes was stimulated upon osmotic shock even if there was no apparent need for their function. Interestingly, cells adapted to osmotic shock much more rapidly under anaerobiosis, and the signaling as well as the transcriptional response was clearly attenuated under these conditions. This more rapid adaptation is due to an enhanced glycerol production capacity in anaerobic cells, which is caused by the need for glycerol production in redox balancing. Artificially enhanced glycerol production led to an attenuated response even under aerobic conditions. These observations demonstrate the crucial role of glycerol accumulation and turgor recovery in determining the period of osmotic shock-induced signaling and the profile of cellular adaptation to osmotic shock.

Turgor regulation in hyphal organisms

Turgor regulation in two saprophytic hyphal organisms was examined directly with the pressure probe technique. The ascomycete Neurospora crassa, a terrestrial fungi, regulates turgor after hyperosmotic treatments when growing in a minimal medium containing K +, Mg 2+, Ca 2+, Cl −, and sucrose. Turgor recovery by N. crassa after hyperosmotic treatment is concurrent with changes in ion transport: hyperpolarization of the plasma membrane potential and a decline in transmembrane ion conductance. In contrast the oomycete Achlya bisexualis, a freshwater hyphal organism, does not regulate turgor after hyperosmotic treatment, although small transient increases in turgor were occasionally observed. We also monitored turgor in both organisms during hypoosmotic treatment and did not observe a turgor increase, possibly due to turgor regulation. Both hyphal organisms grow with similar morphologies, cellular expansion rates and turgor (0.4–0.7 MPa), yet respond differently to osmotic stress. The results do not support the assumption of a universal mechanism of tip growth driven by cell turgor.

Osmotic effects on the electrical properties of Arabidopsis root hair vacuoles in situ.

To assess the role of the vacuole in responses to hyperosmotic and hypo-osmotic stress, the electrical properties of the vacuole were measured in situ. A double-barrel micropipette was inserted into the vacuole for voltage clamping. A second double-barrel micropipette was inserted into the cytoplasm to provide a virtual ground that separated the electrical properties of the vacuole from those of the plasma membrane. Osmotic stress causes immediate electrical responses at the plasma membrane (Lew RR [1996] Plant Physiol 97: 2002-2005) and ion flux changes and turgor recovery (Shabala SN, Lew RR [2002] 129: 290-299) in Arabidopsis root cells. In situ, the vacuole also responds rapidly to changes in extracellular osmotic potential. Hyperosmotic treatment caused a very large increase in the ionic conductance of the vacuole. Hypo-osmotic treatment did not affect the vacuolar conductance. In either case, the vacuolar electrical potential was unchanged. Taken in concert with previous studies of changes at the plasma membrane, these results demonstrate a highly coordinated system in which the vacuole and plasma membrane are primed to respond immediately to hyperosmotic stress before changes in gene expression.

Turgor regulation in osmotically stressed Arabidopsis epidermal root cells. Direct support for the role of inorganic ion uptake as revealed by concurrent flux and cell turgor measurements.

Hyperosmotic stress is known to significantly enhance net uptake of inorganic ions into plant cells. Direct evidence for cell turgor recovery via such a mechanism, however, is still lacking. In the present study, we performed concurrent measurements of net ion fluxes (with the noninvasive microelectrode ion flux estimation technique) and cell turgor changes (with the pressure-probe technique) to provide direct evidence that inorganic ion uptake regulates turgor in osmotically stressed Arabidopsis epidermal root cells. Immediately after onset of hyperosmotic stress (100/100 mM mannitol/sorbitol treatment), the cell turgor dropped from 0.65 to about 0.25 MPa. Turgor recovery started within 2 to 10 min after the treatment and was accompanied by a significant (30-80 nmol m-2 s-1) increase in uptake of K+, Cl-, and Na+ by root cells. In most cells, almost complete (>90% of initial values) recovery of the cell turgor was observed within 40 to 50 min after stress onset. In another set of experiments, we combined the voltage-clamp and the microelectrode ion flux estimation techniques to show that this process is, in part, mediated by voltage-gated K+ transporters at the cell plasma membrane. The possible physiological significance of these findings is discussed.

Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport.

Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (psi) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re-establishment of psi gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield-threshold turgor. These adjustments permit roots to resume growth under low psi. In contrast, in leaves under reductions in psi of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low psi in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone psi is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that psi of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of psi in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade-off between high xylem conductance and strong osmotic adjustment.

Greenhouse Tomato Photosynthetic Acclimation to Water Deficit and Response to Salt Accumulation in the Substrate

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in a peat-moss based substrate (70% sphag peat and 30% perlite, v/v) and treated with a salinity stress (4.5 mS cm-1 of electrical conductivity, EC) and a low (55 ± 8% on gra-vimetric basis) substrate water content (SWC) to examine the effects of salt accumulation and a prolonged substrate water deficit on photosynthesis and plant water relations. Net photosynthetic rate (Pn) decreased by 24% compared to the control one day after SWC was depleted to 55%. However, as SWC was maintained at the same level for several days, the effect of water stress diminished, with the decreasing extent of 14%, 15%, and 14% compared to the control on the 11th, 16th, and 28th days, respectively, from the beginning of treatments. This demonstrated that tomato plants acclimated to substrate water deficit. One day after SWC reached 55%, leaf turgor potential (ΨP) decreased sub-stantially as leaf water potential (ΨW) declined. However, as SWC was maintained con-stant over a period, ΨP recovered to a large extent even at the same Ψw level. This tur-gor recovery was based on osmotic adjustment shown by the decrease in osmotic poten-tial (Ψπ) at fully hydrated status. The effect of salinity on Pn was not observed under both high and low SWC one day after the beginning of treatments, but that effect became larger and larger as the treatment was prolonged. Although ΨW and Ψp declined steadi-ly in salinity stressed plants during the experiment, osmotic adjustment also occurred, re-sulting in a partial turgor maintenance. The combined treatment of salinity and water de-ficit imposed an additive effect on Pn, ΨW, and Ψp, which did not allow Pn to recover despite the osmotic adjustment.

Rapid Response of the Yield Threshold and Turgor Regulation during Adjustment of Root Growth to Water Stress in Zea mays.

Responses of cortical cell turgor (P) following rapid changes in osmotic pressure ([pi]m) were measured throughout the elongation zone of maize (Zea mays L.) roots using a cell pressure probe and compared with simultaneously measured root elongation to evaluate: yield threshold (Y) (minimum P for growth), wall extensibility, growth-zone radial hydraulic conductivity (K), and turgor recovery rate. Small increases in [pi]m (0.1 MPa) temporarily decreased P and growth, which recovered fully in 5 to 10 min. Under stronger [pi]m (up to 0.6 MPa), elongation stopped for up to 30 min and then resumed at lower rates. Recoveries in P through solute accumulation and lowering of Y enabled growth under water stress. P recovery was as much as 0.3 MPa at [pi]m = 0.6 MPa, but recovery rate declined as water stress increased, suggesting turgor-sensitive solute transport into the growth zone. Under strong [pi]m, P did not recover in the basal part of the growth zone, in conjunction with a 30% shortening of the growth zone. Time courses showed Y beginning to decrease within several minutes after stress imposition, from about 0.65 MPa to a minimum of about 0.3 MPa in about 15 min. The data concerning Y were not confounded significantly by elastic shrinkage. K was high (1.3 x 10-10 m2 s-1 MPa-1), suggesting very small growth-induced water potential gradients.

Transient Responses of Cell Turgor and Growth of Maize Roots as Affected by Changes in Water Potential.

Transient responses of cell turgor (P) and root elongation to changes in water potential were measured in maize (Zea mays L.) to evaluate mechanisms of adaptation to water stress. Changes of water potential were induced by exposing roots to solutions of KCl and mannitol (osmotic pressure about 0.3 MPa). Prior to a treatment, root elongation was about 1.2 mm h-1 and P was about 0.67 MPa across the cortex of the expansion zone (3-10 mm behind the root tip). Upon addition of an osmoticum, P decreased rapidly and growth stopped completely at pressure below approximately 0.6 MPa, which indicated that the yield threshold (Ytrans,1) was just below the initial turgor. Turgor recovered partly within the next 30 min and reached a new steady value at about 0.53 MPa. The root continued to elongate as soon as P rose above a new threshold (Ytrans,2) of about 0.45 MPa. The time between Ytrans,1 and Ytrans,2 was about 10 min. During this transition turgor gradients of as much as 0.15 MPa were measured across the cortex. They resulted from a faster rate of turgor recovery of cells deeper inside the tissue compared with cells near the root periphery. Presumably, the phloem was the source of the compounds for the osmotic adjustment. Turgor recovery was restricted to the expansion zone, as was confirmed by measurements of pressure kinetics in mature root tissue. Withdrawal of the osmoticum caused an enormous transient increase of elongation, which was related to only a small initial increase of P. Throughout the experiment, the relationship between root elongation rate and turgor was nonlinear. Consequently, when Y were calculated from steady-state conditions of P and root elongation before and after the osmotic treatment, Yss was only 0.21 MPa and significantly smaller compared with the values obtained from direct measurements (0.42-0.64 MPa). Thus, we strongly emphasize the need for measurements of short-term responses of elongation and turgor to determine cell wall mechanics appropriately. Our results indicate that the rate of solute flow into the growth zone could become rate-limiting for cell expansion under conditions of mild water stress.

Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3 degrees C, 0.2-0.4 kPa vapor pressure deficit, 100 mumol.m(-2).s(-1) photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3 degrees C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3 degrees C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3 degrees C compared with 20 degrees C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.

Water Relations of Turgor Recovery and Restiffening of Wilted Cabbage Leaves in the Absence of Water Uptake

A novel phenomenon in which wilted cabbage leaves appeared to regain positive turgor pressures without additional water uptake has been previously reported (J Levitt [1986] Plant Physiol 82: 147-153). These experiments were replicated and the biophysical nature of turgor recovery characterized. Leaf water potential and its components were assayed in hydrated, wilted, and desiccated leaves which appeared to regain turgor after wilting. The hypotheses that turgor recovery was due to an increased volumetric elastic modulus (epsilon), or alternatively the result of solute redistribution were tested. Quantitative evidence that turgor recovery occurs in excised leaves was found. Leaf turgor pressure in hydrated leaves ( approximately 0.6 megapascal) decreased to zero upon wilting. After continued desiccation, turgor pressure returned to approximately 0.3 megapascal even though leaf relative water content declined. The epsilon of hydrated leaves was large and there was no evidence of an increased epsilon in the turgor-recovered leaves. Solute mobilization occurred during desiccation. The apoplastic osmotic potential decreased from -0.15 to -0.44 megapascal in hydrated and turgor-recovered leaves, respectively, and solutes were transported from the lamina to the midrib tissue. Solute redistribution coupled with the high epsilon may have resulted in localized turgor recovery in specific cells in the desiccated leaves.