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 (ε), 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 (~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 ε of hydrated leaves was large and there was no evidence of an increased ε 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 ε may have resulted in localized turgor recovery in specific cells in the desiccated leaves.     

Relationship of dehydration rate to drought avoidance,dehydration tolerance and dehydration avoidance of cabbage leaves,and to their acclimation during drought-induced water stress*

Abstract. Drought avoidance due to cuticular control increases with leaf number to a maximum in the intermediate leaves, decreasing to a minimum in the upper leaves. Dehydrated intermediate leaves do not rehydrate detectably when floated on water for several days. Excision of their petioles when submerged, permits full rehydration, presumably via the xylem. Stressing the plant by withholding water for 1–3 weeks fails to increase this already high resistance to water movement through the leaf surface. It does, however, markedly decrease the loss of water from the fully rehydrated (100% RWC) leaves during the first hour of dehydration, presumably due to a more rapid stomatal closure than in the non-stressed leaves. Dehydration tolerance increases with leaf number, without an intermediate maximum. The intermediate and upper leaves were markedly more tolerant of dehydration after drought-induced stress than when non-stressed. Dehydration tolerance in some cases, was inversely proportional to dehydration rate. It was possible, however, to equalize the rates of dehydration of drought-stressed and non-drought-stressed leaves without affecting the greater tolerance of the drought-stressed leaves. Dehydration avoidance by osmotic adjustment was markedly developed in the slowly dehydrated attached leaves of drought-stressed plants, but not in the rapidly dehydrated excised leaves. This is evidence of drought acclimation. It must, therefore, be concluded that the slow dehydration of the drought-stressed plants also leads to the increase in dehydration tolerance by permitting drought-induced acclimation. The overall drought resistance of cabbage leaves depends on the three components: drought avoidance, dehydration avoidance and dehydration tolerance. The latter two increase during acclimation but the cuticular control of drought avoidance does not.     

Cell water potential,osmotic potential,and turgor in the epidermis and mesophyll of transpiring leaves

Water potential, osmotic potential and turgor measurements obtained by using a cell pressure probe together with a nanoliter osmometer were compared with measurements obtained with an isopiestic psychrometer. Both types of measurements were conducted in the mature region of Tradescantia virginiana L. leaves under non-transpiring conditions in the dark, and gave similar values of all potentials. This finding indicates that the pressure probe and the osmometer provide accurate measurements of turgor, osmotic potentials and water potentials. Because the pressure probe does not require long equilibration times and can measure turgor of single cells in intact plants, the pressure probe together with the osmometer was used to determine in-situ cell water potentials, osmotic potentials and turgor of epidermal and mesophyll cells of transpiring leaves as functions of stomatal aperture and xylem water potential. When the xylem water potential was-0.1 MPa, the stomatal aperture was at its maximum, but turgor of both epidermal and mesophyll cells was relatively low. As the xylem water potential decreased, the stomatal aperture became gradually smaller, whereas turgor of both epidermal and mesophyll cells first increased and afterward decreased. Water potentials of the mesophyll cells were always lower than those of the epidermal cells. These findings indicate that evaporation of water is mainly occurring from mesophyll cells and that peristomatal transpiration could be less important than it has been proposed previously, although peristomatal transpiration may be directly related to regulation of turgor in the guard cells.     

Complete turgor maintenance at low water potentials in the elongating region of maize leaves

Leaf elongation rate, water potential, and osmotic potential were measured in the fifth leaf of maize (Zea mays L.) plants growing in soil from which water was withheld for varying times. Elongation occurred in the basal region, which was enclosed by other leaf sheaths. When water was withheld from the soil, leaf elongation decreased and eventually ceased even though enough solutes accumulated in the elongating region to maintain turgor virtually constant. In the exposed blade, however, turgor was lost and wilt symptoms developed. If the night was prolonged, the elongating region lost much of its ability to accumulate solute, which suggests that the accumulating solutes were of recent photosynthetic origin. Under these conditions, leaf elongation was restricted to higher water potentials than under the usual photoperiodic regime.     

Short-term phosphate uptake kinetics in Zostera noltii Hornem: a comparison between excised leaves and sediment-rooted plants

Short-term phosphate uptake by excised leaves of Zostera noltii Hornem. as well as by leaves of sediment-rooted plants were characterized and compared in a kinetic framework. Time courses of phosphate disappearance were measured over a wide range of initial substrate concentrations. Phosphate uptake determined by this perturbation method did not follow Michaelis-Menten kinetics. Both excised leaves and sediment-rooted plants exhibited a biphasic uptake pattern as a function of phosphate concentration. However, rooted plants showed higher uptake rates and accumulated higher amounts of phosphate than excised leaves. The results point out the importance of the structural and functional coupling between shoots and underground parts during the nutrient foliar uptake processes. Our study also indicates that Zostera noltii leaves function as a phosphate sink in the water column.A second objective of this work is to compare the perturbation and the multiple flask methods in determining the uptake kinetic parameters. The obtained results support that both methods provide valuable and complementary information in determining the uptake rates.     

Global analysis of plasticity in turgor loss point,a key drought tolerance trait

Many species face increasing drought under climate change. Plasticity has been predicted to strongly influence species' drought responses, but broad patterns in plasticity have not been examined for key drought tolerance traits, including turgor loss or ‘wilting’ point (π_tlp). As soil dries, plants shift π_tlp by accumulating solutes (i.e. ‘osmotic adjustment’). We conducted the first global analysis of plasticity in Δπ_tlp and related traits for 283 wild and crop species in ecosystems worldwide. Δπ_tlp was widely prevalent but moderate (?0.44 MPa), accounting for 16% of post‐drought π_tlp. Thus, pre‐drought π_tlp was a considerably stronger predictor of post‐drought π_tlp across species of wild plants. For cultivars of certain crops Δπ_tlp accounted for major differences in post‐drought π_tlp. Climate was correlated with pre‐ and post‐drought π_tlp, but not Δπ_tlp. Thus, despite the wide prevalence of plasticity, π_tlp measured in one season can reliably characterise most species' constitutive drought tolerances and distributions relative to water supply.     

Induction of N–malonyl–D–tryptophan by drought stress. Is D–tryptophan the only D–amino acid appeared in wilted leaves?

D-amino acid were searched in wilted tomato leaves. D-Isomers of free amino acids were not revealed by the treatment with L- and D-amino acid oxidases. The noncationic fraction of the extract contained N-malonyl-D-tryptophan and no other N-acylated amino acids. A special search for endogenous N-malonyl-D-phenylalanine gave negative results. Exogenous¹⁴C-malonate was only incorporated in one Chromatographic zone corresponding to N-malonyl-D-tryptophan. It is concluded that drought stress does not induce the appearance of D-amino acids except for D-tryptophan which is accumulated in the malonylated form.     

Regulation of sulfate uptake by excised barley roots in the presence of selenate

Active transport of SO₄²⁻ and SeO₄²⁻ has been evaluated during 60-hour contact of barley roots with nutrient solutions containing either ³⁵SO₄²⁻ or ⁷⁵SeO₄²⁻, or both ions, at 0.1 milli-equivalent per liter. In the SO₄²⁻ solution the time course of active transport follows a straight line; if SeO₄²⁻ is also present transport is strongly inhibited after 20 to 30 hours for both ions. The S-Se uptake ratio remains 1.4 during the 60 hours; S-Se ratio shifts from 3.0 to 3.3 in proteins and falls to 0.6 in free amino acids. S-Se discrimination is mainly operating at the level of amino acid incorporation into proteins. The presence of Se-amino acids blocks this incorporation and brings about an accumulation of free amino acids; at the same time carrier activity is inhibited. The addition of methionine or Se-methionine causes a 60 to 80% inhibition of the active transport.     

Light and turgor affect the water permeability (aquaporins) of parenchyma cells in the midrib of leaves of Zea mays

In response to light, water relation parameters (turgor, half-time of water exchange, T(1/2), and hydraulic conductivity, Lp; T(1/2) proportional 1/Lp) of individual cells of parenchyma sitting in the midrib of leaves of intact corn (Zea mays L.) plants were investigated using a cell pressure probe. Parenchyma cells were used as model cells for the leaf mesophyll, because they are close to photosynthetically active cells at the abaxial surface, and there are stomata at both the adaxial and abaxial sides. Turgor ranged from 0.2 to 1.0 MPa under laboratory light condition (40 micromol m(-2) s(-1) at the tissue level), and individual cells could be measured for up to 6 h avoiding the variability between cells. In accordance with earlier findings, there was a big variability in T(1/2)s measured ranging from 0.5 s to 100 s, but the action of light on T(1/2)s could nevertheless be worked out for cells having T(1/2)s greater than 2 s. Increasing light intensity ranging from 100 micromol m(-2) s(-1) to 650 micromol m(-2) s(-1) decreased T(1/2) by a factor up to five within 10 min and increased Lp (and aquaporin activity) by the same factor. In the presence of light, turgor decreased due to an increase in transpiration, and this tended to compensate or even overcompensated for the effect of light on T(1/2). For example, during prolonged illumination, cell turgor dropped from 0.2 to 1.0 MPa to -0.03 to 0.4 MPa, and this drop caused an increase of T(1/2) and a reduction of cell Lp, i.e. there was an effect of turgor on cell Lp besides that of light. To separate the two effects, cell turgor (water potential) was kept constant while changing light intensity by applying gas pressure to the roots using a pressure chamber. At a light intensity of 160 micromol m(-2) s(-1), there was a reduction of T(1/2) by a factor of 2.5 after 10-30 min, when turgor was constant within +/-0.05 MPa. Overall, the effects of light on T(1/2) (Lp) were overriding those of turgor only when decreases in turgor were less than about 0.2 MPa. Otherwise, turgor became the dominant factor. The results indicate that the hydraulic conductivity increased with increasing light intensity tending to improve the water status of the shoot. However, when transpiration induced by light tends to cause a low turgidity of the tissue, cell Lp was reduced. It is concluded that, when measuring the overall hydraulic conductivity of leaves, both the effects of light and turgor should be considered. Although the mechanism(s) of how light and turgor influence the cell Lp is still missing, it most likely involves the gating of aquaporins by both parameters.     

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum — the role of cell turgor in long-distance water transport: a comment

Summary The calculation of absolute-pressure values on the basis of measurements with differential-gauge pressure sensors, as described by Thürmer et al. (Protoplasma 206: 152–162, 1999), leads to discrepancies with the definition of absolute pressure when negative values are reached. From previous experiments with the xylem pressure probe we can conclude that the recorded pressure signal belongs not only to the xylem pressure, as stated by the authors, but also to the capillary pressure.     

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum — the role of cell turgor in long-distance water transport: a reply

Summary From equilibrium thermodynamics an equation is given to show that in a liquid negative pressures (tensions) are physical reality and may reliably be recorded from any point of the aqueous phase within the xylem conduit by the xylem pressure probe introduced by Balling et al. (Naturwissenschaften 75: 409–411, 1988).     

Osmotic adjustment increases water uptake, remobilization of assimilates and maintains photosynthesis in chickpea under drought

Eight chickpea advanced breeding lines (ABLs) and their parents were evaluated for osmotic adjustment (OA), leaf carbohydrates and gas exchange under dryland field . These (ABLs) were derived from crosses between CTS 60543 x Kaniva and Tyson x Kaniva. Mean leaf water potential (LWP) fell down from -1.00 MPa at pre-stress level to about -2.25 MPa during terminal stress. Relative water content (RWC) showed periodic changes with alternate decrease or increase at certain interval, which also influenced the values of OA (low or high) in number of genotypes e.g. Kaniva, CTS 60543, Tyson and M 75. Significant variation in OA ranging 0.45 to 0.88 MPa was observed at high level of stress at -2.5 MPa. However, none of the genotypes showed stability of OA over the period of stress. Leaf starch declined even at mild stress (LWP, -1.6 MPa) resulting in an increase in hexose sugars and activation state of sucrose-phosphate synthase (SPS) that led to accumulation of sucrose. Both photosynthesis (Pmax) and transpiration decreased concurrently in two chickpea lines M 129 and Tyson with increasing water stress. However, rate of decline in the photosynthesis slowed down even drought was further intensified. The observed periodic changes in OA, RWC and photosynthesis appeared to be associated with drought-induced changes in SPS and carbohydrates which modify water uptake of the leaves.     

Effects of flooding and drought on stomatal activity, transpiration, photosynthesis, water potential and water channel activity in strawberry stolons and leaves

Transpiration, xylem water potential and water channel activity were studied in developing stolons and leaves of strawberry (Fragaria × ananassa Duch.) subjected to drought or flooding, together with morphological studies of their stomata and other surface structures. Stolons had 0.12 stomata mm^–2 and a transpiration rate of 0.6 mmol H₂O m^–2 s^–1, while the leaves had 300 stomata mm^–2 and a transpiration rate of 5.6 mmol H₂O m^–2 s^–1. Midday water potentials of stolons were always less negative than in leaves enabling nutrient ion and water transport via or to the strawberry stolons. Drought stress, but not flooding, decreased stolon and leaf water potential from –0.7 to –1 MPa and from –1 to –2 MPa, respectively, with a concomitant reduction in stomatal conductance from 75 to 30 mmol H₂O m^–2 s^–1. However, leaf water potentials remained unchanged after flooding. Similarly, membrane vesicles derived from stolons of flooded strawberry plants showed no change in water channel activity. In these stolons, turgor may be preserved by maintaining root pressure, an electrochemical and ion gradient and xylem differentiation, assuming water channels remain open. By contrast, water channel activity was reduced in stolons of drought stressed strawberry plants. In every case, the effect of flooding on water relations of strawberry stolons and leaves was less pronounced than that of drought which cannot be explained by increased ABA. Stomatal closure under drought could be attributed to increased delivery of ABA from roots to the leaves. However, stomata closed more rapidly in leaves of flooded strawberry despite ABA delivery from the roots in the xylem to the leaves being strongly depressed. This stomatal closure under flooding may be due to release of stress ethylene. In the relative absence of stomata from the stolons, cellular (apoplastic) water transport in strawberry stolons was primarily driven by water channel activity with a gradient from the tip of the stolon to the base, concomitant with xylem differentiation and decreased water transport potential from the stolon tip to its base. Reduced water potential in the stolons under drought are discussed with respect to reduced putative water channel activity.     

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum: The role of cell turgor in long-distance water transport

Summary Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the cell turgor and the osmotic pressure of leaf cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 1 decrease in the cell turgor pressure of the mesophyll cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the cell turgor pressure (ratio of about 1 1). The cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and cell turgor pressure. From the xylem, cell turgor, and cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero turgor pressure.