Decrease In Turgor Research Articles

Limitations of Photosynthesis in Pinus taeda L. (Loblolly Pine) at Low Soil Temperatures

The relative importance of stomatal and nonstomatal limitations to net photosynthesis (A) and possible signals responsible for stomatal limitations were investigated in unhardened Pinus taeda seedlings at low soil temperatures. After 2 days at soil temperatures between 13 and 7 degrees C, A was reduced by 20 to 50%, respectively. The reduction in A at these moderate root-chilling conditions appeared to be the result of stomatal limitations, based on the decrease in intercellular CO(2) concentrations (c(i)). This conclusion was supported by A versus c(i) analysis and measurements of O(2) evolution at saturating CO(2), which suggested increases in stomatal but not biochemical limitations at these soil temperatures. Nonuniform stomatal apertures, which were demonstrated with abscisic acid, were not apparent 2 days after root chilling, and results of our A versus c(i) analysis appear valid. Bulk shoot water potential (psi) declined as soil temperature dropped below 16 degrees C. When half the root system of seedlings was chilled, shoot psi and gas-exchange rates did not decline. Thus, nonhydraulic root-shoot signals were not implicated in stomatal limitations. The initial decrease in leaf conductance to water vapor after root chilling appeared to precede any detectable decrease in bulk fascicle psi, but may be in response to a decrease in turgor of epidermal cells. These reductions in leaf conductance to water vapor, which occurred within 30 minutes of root chilling, could be delayed and temporarily reversed by reducing the leaf-to-air vapor-pressure deficit, suggesting that hydraulic signals may be involved in initiating stomatal closure. By independently manipulating the leaf-to-air vapor-pressure deficit of individual fascicles, we could induce uptake of water vapor through stomata, suggesting that nonsaturated conditions occur in the intercellular airspaces. There was an anomaly in our results on seedlings maintained for 2 days at soil temperatures below 7 degrees C. Lower A appeared primarily the result of nonstomatal limitations, based on large increases in calculated c(i) and A versus c(i) analysis. In contrast, measurements of O(2) evolution at saturating CO(2) concentrations implied nonstomatal limitations per se did not increase at these temperatures. One explanation for this paradox is that calculations of c(i) are unreliable at very low gas-exchange rates because of inadequate measurement resolution, and limitations of A are predominantly stomatal. An alternative interpretation is that increases in c(i) are real and the results from O(2)-evolution measurements are in error. The high CO(2) concentration used in O(2)-evolution measurements (15%) may have overcome nonstomatal limitations by enzymes that were down-regulated by a feedback mechanism. In this scenario, carbohydrate feedback limitations may be responsible for nonstomatal reductions in A after 2 days at soil temperatures below 7 degrees C.

Gravitropism of cucumber hypocotyls: biophysical mechanism of altered growth.

The biophysical basis for the changes in cell elongation rate during gravitropism was examined in aetiolated cucumber (Cucumis sativus L.) hypocotyls. Bulk osmotic pressures on the two sides of the stem and in the epidermal cells were not altered during the early time course of gravitropism. By the pressure-probe technique, a small increase in turgor (0.3 bar, 30 kPa) was detected on the upper (inhibited) side, whereas there was a negligible decrease in turgor on the lower (stimulated) side. These small changes in turgor and water potential appeared to be indirect, passive consequences of the altered growth and the small resistance for water movement from the xylem, and indicated that the change in growth was principally due to changes in wall properties. The results indicate that the hydraulic conductance of the water-transport pathway was large (.25 h-1 bar-1) and the water potential difference supporting cell expansion was no greater than 0.3 bar (30 kPa). From pressure-block experiments, it appeared that upon gravitropic stimulation (1) the yield threshold of the lower half of the stem did not decrease and (2) the wall on the upper side of the stem was not made more rigid by a cross-linking process. Mechanical measurements of the stress/strain properties of the walls showed that the initial development of gravitropism did not involve an alteration of the mechanical behaviour of the isolated walls. Thus, gravitropism in cucumber hypocotyls occurs principally by an alteration of the wall relaxation process, without a necessary change in wall mechanical properties.

Salinity stress inhibits bean leaf expansion by reducing turgor, not wall extensibility.

Treatment of bean (Phaseolus vulgaris L.) seedlings with low levels of salinity (50 or 100 millimolar NaCl) decreased the rate of light-induced leaf cell expansion in the primary leaves over a 3 day period. This decrease could be due to a reduction in one or both of the primary cellular growth parameters: wall extensibility and cell turgor. Wall extensibility was assessed by the Instron technique. Salinity did not decrease extensibility and caused small increases relative to the controls after 72 hours. On the other hand, 50 millimolar NaCl caused a significant reduction in leaf bulk turgor at 24 hours; adaptive decreases in leaf osmotic potential (osmotic adjustment) were more than compensated by parallel decreases in xylem tension potential and the leaf apoplastic solute potential, resulting in a decreased leaf water potential. It is concluded that in bean seedlings, mild salinity initially affects leaf growth rate by a decrease in turgor rather than by a reduction in wall extensibility. Moreover, long-term salinization (10 days) resulted in an apparent mechanical adjustment, i.e. an increase in wall extensibility, which may help counteract reductions in turgor and maintain leaf growth rates.

Cell enlargement and growth‐induced water potentials

Understanding the origin and role of growth‐induced water potentials helps to explain how cell enlargement is inhibited without a decrease in turgor when water is depleted in the soil or more rapidly lost by transpiration. In many cases, changes in growth‐induced water potentials probably cause the inhibition initially, but eventually the cell walls lose extensibility and specific proteins accumulate so that metabolic changes in the walls probably limit later growth.

Externally Induced Acceleration of Transpiration of Dark-stressed Strawberry Plants

Summary Transpiration of strawberry plants ( Fragaria grandiflora Duch. cv. Red gauntlet), was investigatedwith an infrared gas analyser. It has been found that long darkening of plants (100-500 h) causes considerable inhibition of transpiration recovery after illumination. This effect could be reversed by several hours of illumination or immediately by feeding the leaf with a solution of KCl, NaCl, ATP, HCl, sucrose, mannitol or fusicoccin. Some indirect evidence suggests, that acceleration of transpiration recovery after treatmentwith these substances is not due to decrease of turgor in epidermal cells but rather due to its increase in guard cells. Acceleration of stomatal opening by externally supplied substances can be observed only inthe light. This may suggest that plasmalemma of guard cells disorganized by long darkness is in some way modified by solutes in the apoplast, which in effect ensures intensification of transport of osmoticum into guard cells.

Loss of capacity for acid-induced wall loosening as the principal cause of the cessation of cell enlargement in light-grown bean leaves

Cell enlargement in primary leaves of bean (Phaseolus vulgaris L.) can be induced, free of cell divisions, by exposure of 10-d-old, red-light-grown seedlings to white light. The absolute rate of leaf expansion increases until day 12, then decreases until the leaves reached mature size on day 18. The cause of the reduction in growth rate following day 12 has been investigated. Turgor calculated from measurements of leaf water and osmotic potential fell from 6.5 to 3.5 bar before day 12, but remained constant thereafter. The decline of growth after day 12 is not caused by a decrease in turgor. On the other hand, Instron-measured cell-wall extensibility decreased in parallel with growth rate after day 12. Two parameters influencing extensibility were examined. Light-induced acidification of cell walls, which has been shown to initiate wall extension, remained constant over the growth period (days 10-18). Furthermore, cells of any age could be stimulated to excrete H(+) by fusicoccin. However, older tissue was not able to grow in response to fusicoccin or light. Measurements of acid-induced extension on preparations of isolated cell walls showed that as cells matured, the cell walls became less able to extend when acidified. These data indicate that it is a decline in the capacity for acid-induced wall loosening that reduces wall extensibility and thus cell enlargement in maturing leaves.

Osmotic adjustment and the inhibition of leaf, root, stem and silk growth at low water potentials in maize

The expansion growth of plant organs is inhibited at low water potentials (Ψ w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The Ψ w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the Ψ w in the region of cell expansion decreased in each organ. The Ψ w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these Ψ w and ceased only when Ψ w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the Ψ w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the Ψ w was lower in the expanding regions than in the mature tissues, indicating that there was a Ψ w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a Ψ w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because Ψ w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular Ψ w changed about the same amount as the soil Ψ w. Therefore, the gradient in Ψ w favored water uptake by roots but not leaves at low Ψ w. The data show that expansion growth responds to low Ψ w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of Ψ w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the Ψ w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and Ψ w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.

Determination of Solute Permeability in Chara Internodes by a Turgor Minimum Method

The analysis of Sha'afi et al. (Sha'afi, Rich, Mickulecky, Solomon 1970 J Gen Physiol 55: 427-450) for determining solute permeability in red blood cells has been modified and applied to turgid plant cells. Following the addition of permeating solute to the external medium, a biphasic response of cell turgor can be measured with the pressure probe in isolated internodes of Chara corallina. After an initial decrease in turgor due to water flow (water phase), turgor increases due to the uptake of the solute (solute phase) until the original turgor is reattained. From the pressure/time course in the neighborhood of the minimum turgor, the permeability of the osmotic solute can be determined. The data obtained by the minimum method for rapidly permeating (ethanol, methanol) and slowly permeating (formamide, dimethylformamide) solutes are similar to those calculated from the half-time of pressure changes during the solute phase and to data obtained by other workers using radioactive tracers. The methods employing the pressure probe were applied to examine the effect of high pH (up to pH 11) on the membrane permeability. There appeared to be no effect of high pH on the permeability coefficients, reflection coefficients, and hydraulic conductivity.

Osmoregulation or Turgor Regulation in Chara?

Chara corallina Klein ex Willd. wm. R.D.W. (= C. australis R. Br.), a fresh water alga, maintains a constant internal osmotic pressure when external osmotic pressure is increased. This results in a decrease in turgor pressure. Chara osmoregulates effectively in the presence of high CaCl(2) and raffinose, but is less efficient in response to increased NaCl. Decreasing external pH from 7 to 5 results in a decrease in turgor, but increasing it to values as high as 9 has no effect. Increasing the daily amount of light from 0.5 to 24 hours has no effect on turgor.

The Influence of Water Deficit on the Factors Controlling the Daily Pattern of Growth ofPhaseolusTrifoliates

Phaseolus seedlings were grown in liquid culture under controlled temperature and irradiance and measurements were made of daily variation in growth of the first trifoliate leaves. Leaf growth rate was significantly enhanced within a few hours of the start of the light period. Over a similar time, a small decrease in leaf turgor and an increase in cell wall plasticity were recorded. Slowly declining growth rates as the light period progressed may have been caused by decreases in turgor during this time. When water availability to the leaves was restricted by growing the plants for several days in nutrient solution maintained at a low temperature (12 °C), the daily pattern of growth of the trifoliates was changed quite markedly. Dark-growth rates were slightly enhanced, while light-growth rates were significantly reduced when compared to growth rates of plants well-supplied with water (roots at 20 °C). Relative 'plateau' growth rates of plants well-supplied (ww) with water or suffering a restricted supply (ws) in the light (L) and in the dark (D) were as follows: ww L > ws D > ww D > ws L. In the light, turgors of the two groups of plants were similar, suggesting that the reduced growth rate of the cooled plants resulted from a change in cell wall structure and/or properties. Immediately before the lights were switched on, plants grown with a restricted water supply showed relatively high turgors in the trifoliates and these were presumably responsible for the enhanced growth rates at this time. Restriction of water availability may have slightly increased the plasticity of cell walls and decreased the yield threshold for growth. The control of leaf growth in plants developing water deficit is discussed.

Energy-dependent phases of the circadian clock and the clock-controlled leaf movement in Phaseolus coccineus L.

The energy requirements of the various phases of the circadian clock in the laminar pulvini cells of primary leaves of Phaseolus coccineus L. were investigated using 4-h pulses of NaCN (5 mM) and NaN3 (1 mM). The induced phase shifts were calculated from the timing of the subjective night position during the third cycle after the treatment. Both inhibitors produce advances during phases which are correlated with the upward movement of the leaf (ca. 0-12 h after the maximum of the subjective night position) and during phases which are correlated with the downward movement of the leaf (ca. 20-28 h after the maximum of the subjective night position). Maximal advances are induced during the phase which is correlated with the maximum of the subjective night position (hour 0), whereas during phases which are correlated with the subjective day position (ca. 12-20 h after the maximum of the subjective night position) the inhibitors have no effect or induce only small advances. These results demonstrate that the part of the circadian cycle which, according to Bünning's tension-relaxation model of the circadian clock, is characterized by features of relaxation, represents a sequence of phases with decreasing energy requirement, whereas the tension part of the circadian cycle represents a sequence of phases with increasing energy requirement. The energy requirement for changing and maintaining the leaf positions was investigated by continuously offering NaCN, NaN3, and dinitrophenol (DNP) to leaves with intact and half (flexor cut away) pulvini. The substances inhibit in both pulvini the upward movement or induce a downward movement, depending on the leaf position, when the transfer to the inhibitor solution takes place. These results give evidence that the movement of intact pulvini reflects the turgor (volume) state of the extensor cells and that the increase of turgor (volume) and high turgor (volume) state requires more energy than the decrease of turgor (volume) or low turgor (small volume) state. Therefore, the time course of the energy requirements of the circadian clock and the clock-controlled turgor (volume states or leaf movement) is out of phase during a circadian cycle. Consequently the reaction of the clock-controlled leaf movement to the reduced energy supply can mask the clock behavior in pulse and step experiments. The phase response curves towards CN(-) and N 3 (-) reflect the time course of the CN(-)-induced membrane depolarizations (the energy requirement of the electrogenic pump) in extensor cells of the pulvinus (Freudling et al. (1980), Plant Physiol. 65, 966-968), and both are out of phase with the time course of the energy requirement of the turgor. Consequently it is hypothesized that in Phaseolus advances are due to membrane depolarization and that at least in this organism electric properties of the plasmalemma are essentially involved in the mechanism of the circadian clock.

Effect of different sugars on stomatal behaviour inMerremia aegyptia (L.) Urban andM. dissecta Hallier F.

The effect of mannitol, glucose and sucrose on the stomatal behaviour of two desert species,Merremia aegyptia andM. dissecta has been studied. Stomatal opening did not uniformly depend on the decrease in turgor of the epidermal and subsidiary cells caused by the different osmotic potential of the sugars. Sucrose caused plasmolysis of the subsidiary cells only but this was not accompanied by the opening of the stomatal pore. InM. aegyptia, no plasmolysis was seen either in epidermal or subsidiary cells, even the stomata opened; inM. dissecta, on the other hand, plasmolysis occurred in these cells without any stomatal opening, after incubation in glucose or mannitol. Mannitol is least absorbed, glucose slightly more and sucrose is absorbed to a very large extent in the guard cells when the materials were inoubated in the respective sugar solutions. However, the absorption of these three sugars was almost always larger in isolated epidermal strips than in discs; in detached intact leaves it was still more reduced.

Turgor-dependent Changes in Avena Coleoptile Cell Wall Composition

The effects of reduced turgor pressure on growth, as measured by cell elongation, and on auxin-mediated changes in cell walls, as measured by analyses of wall composition, were examined using Avena coleoptile segments. Although moderate (1-4 bar) decreases in turgor resulted in a progressive decline in growth proportional to the decrease in turgor, the major auxin-induced change in wall composition, a decrease in noncellulosic wall glucose, was unaffected. Severe (5-8 bar) decreases, however, did inhibit this auxin effect on the wall, and with turgor decreases of 9 bars or more this auxin effect was no longer apparent. The results show that turgor pressure is required for this auxin-mediated wall modification and also that this modification of wall glucose occurs at turgor pressures less than those required for wall extension. Changes in other wall components were generally unaffected by altering turgor pressure.

Chloroplast Response to Low Leaf Water Potentials

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.

Mechanism of the Seismonastic Reaction in Mimosa pudica

The efflux of K(+) from the pulvinar cells of Mimosa pudica was shown to increase substantially during the seismonastic reaction. This result is shown to indicate a decrease in sigma (reflection coefficient) of pulvinar cell membrane for potassium salts which could account for the pulvinar cell turgor decrease during the seismonastic reaction.Membrane potentials and concentrations of Ca(2+), K(+), Cl(-), S, and P were measured in the top and bottom halves of pulvini. Pulvinar cells showed a large negative membrane potential, cell vacuole relative to external solution, but no significant difference in membrane potential could be detected between upper and lower pulvinar cells. A large difference in K(+) and Cl(-) concentration between top and bottom pulvinar halves was evident in reactive pulvini but not in unreactive pulvini. The effect of K(+) concentration on plant growth and leaf reactivity was also investigated.

Studies of the water relations of crop plants grown under natural rainfall in northern Australia.

Studies of the water relations of cotton, peanuts, and grain sorghum were made at Katherine, N.T., on crops grown under natural rainfall during the 1952-53 rainfall season. In the early part of this season, rainfall was more or less normal and little evidence of stress was seen in the plants. The latter part, however, was abnormally dry and resulted in the appearance of severe water stress symptoms in all crops. The water balance of the plants through the season was followed using Weatherley's (1950) "relative turgidity" technique of leaf turgor measurement. In each crop the relative turgidity level was maintained at a fairly high level until the onset of dry conditions, when a progressive decline commenced. This decline, although continuous, appeared to be in two stages. It was felt that the lag of absorption behind transpiration which resulted in loss of turgor was initially due to the rapid rise in transpiration, which followed the increase in atmospheric aridity with the onset of the dry period. As atmospheric conditions became more static, the continued decline in turgor was attributed primarily to the influence of soil moisture stress, in limiting absorption. Of the three crops, grain sorghum appeared to have the best-developed root system and also the most effective internal control over transpiration. Cotton appeared to be least well equipped in these respects. These features were reflected in generally higher turgor levels in grain sorghum than in other crops, and in a slower rate of decrease in turgor with the onset of dry weather. This decrease was particularly rapid in cotton. This relative resistance to turgor loss was in turn reflected in growth rate reductions in cotton as soon as soil moisture stress appeared, but not in grain sorghum until severe soil moisture stress was evident. The peanut responses throughout appeared intermediate between those of the other two crops.