Transpiration Induces Radial Turgor Pressure Gradients in Wheat and Maize Roots

Previous studies have shown both the presence and the absence of radial turgor and osmotic pressure gradients across the cortex of roots. In this work, gradients were sought in the roots of wheat (Triticum aestivum) and maize (Zea mays) under conditions in which transpiration flux across the root was varied This was done by altering the relative humidity above the plant, by excising the root, or by using plants in which the leaves were too young to transpire. Roots of different ages (4-65 d) were studied and radial profiles at different distances from the tip (5-30 mm) were measured. In both species, gradients of turgor and osmotic pressure (increasing inward) were found under transpiring conditions but not when transpiration was inhibited. The presence of radial turgor and osmotic pressure gradients, and the behavior of the gradient when transpiration is interrupted, indicate that active membrane transport or radial solvent drag may play an important role in the distribution of solutes across the root cortex in transpiring plants. Contrary to the conventional view, the flow of water and solutes across the symplastic pathway through the plasmodesmata cannot be inwardly directed under transpiring conditions.     

Radial Turgor and Osmotic Pressure Profiles in Intact and Excised Roots of Aster tripolium: Pressure Probe Measurements and Nuclear Magnetic Resonance-Imaging Analysis

High-resolution nuclear magnetic resonance images (using very short spin-echo times of 3.8 milliseconds) of cross-sections of excised roots of the halophyte Aster tripolium showed radial cell strands separated by air-filled spaces. Radial insertion of the pressure probe (along the cell strands) into roots of intact plants revealed a marked increase of the turgor pressure from the outermost to the sixth cortical layer (from about 0.1-0.6 megapascals). Corresponding measurements of intracellular osmotic pressure in individual cortical cells (by means of a nanoliter osmometer) showed an osmotic pressure gradient of equal magnitude to the turgor pressure. Neither gradient changed significantly when the plants were grown in, or exposed for 1 hour to, media of high salinity. Differences were recorded in the ability of salts and nonelectrolytes to penetrate the apoplast in the root. The reflection coefficients of the cortical cells were approximately 1 for all the solutes tested. Excision of the root from the stem resulted in a collapse of the turgor and osmotic pressure gradients. After about 15 to 30 minutes, the turgor pressure throughout the cortex attained an intermediate (quasistationary) level of about 0.3 megapascals. This value agreed well with the osmotic value deduced from plasmolysis experiments on excised root segments. These and other data provided conclusions about the driving forces for water and solute transport in the roots and about the function of the air-filled radial spaces in water transport. They also showed that excised roots may be artifactual systems.     

An Analysis of Turgor and Turgor Pressure

Turgor depends on the excess pressure inside a cell, not upona reaction between the wall and contents. It is independentof the environmental pressure whereas the total reaction isnot. Turgor pressure is not identical with the reaction, or,unless the effects of the protoplasmic membranes be ignored,with the wall pressure. Changes in turgor pressure can be saidto cause changes in volume, but only when both are actuallysimultaneous results of differential diffusion. The effectsof environmental pressures are considered. Some misconceptionsare discussed and some terms more fully defined.     

未分胁迫下不同抗旱性小麦芽鞘膨压和IAA及ABA含量变化（简报）

水分胁迫条件下,小麦细胞膨压下降与芽鞘生长抑制呈正相关。不同品种单位膨压升高引起的伸长生长有差异。水分胁迫0、3、8、20、32、45h后,抗旱性强的品种芽鞘中IAA含量及IAA／ABA比值在接近生长最快的时刻出现高峰;抗旱性差的品种IAA含量及IAA／ABA比值的高峰出现的时间与生长最快的时间不一致。单位膨压升高引起芽鞘长度增加大的品种,腔迫处理后IAA含理及IAA／ABA相对较高或下降程度相对较小;而BA含理升高则相反,抗旱性弱的品种胁迫处理后ABA含量增加相对较多。     

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

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

负压处理对农杆菌介导小麦成熟胚转化效率的影响

目的:提高小麦成熟胚遗传转化效率,为建立快速高效的小麦成熟胚遗传转化体系奠定基础.方法:以普通小麦HB341、SN2618、TS021和栽培二粒小麦成熟种子为试材,采用整粒切胚诱愈的方法,通过农杆菌GV3101/pBI121::gus介导,研究负压处理对小麦成熟胚遗传转化的影响.结果:接种侵染过程中的负压处理,对小麦成熟胚愈伤组织的诱导没有显著影响,但对小麦成熟胚的遗传转化却有显著性影响.负压处理10、20、30次,转化率分别较对照组提高了9.37倍、10.90倍和10.03倍,最高转化率可达12.50%.同时负压处理能够提高小麦成熟胚转化效率,具有普遍意义.结论:该研究为快速高效小麦成熟胚转化体系的建立提供了依据.     

Turgor Pressure and Phototropism in Sinapis alba L. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropismin Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls.Phototropism was induced by adding 0.27 µmol m^–2s^–1 unilateral blue light to a background of low pressuresodium (SOX) lamp light. Curvatures of some 6° from thevertical were reached by 60 min, the curvature rate between20 min and 60 min being 0.14° min^–1. From the axialgrowth rate and tissue geometry the local growth rates of illuminatedand shaded sides of the hypocotyl were calculated to be 1.5and 4.5 µmin^–1 respectively. Turgor pressures ofexpanding cells in control plants and in the shaded and illuminatedsides of the blue light illuminated hypocotyls were measuredto be 0.40-0.55 MPa with a pressure probe. No changes in turgorpressure were observed on initiation of curvature. The decayof pressure in the cells of non-transpiring plants followingexcision indicated that the yield stress threshold of the tissuemay be as low as 0.1 MPa. These results indicate that the phototropicgrowth response in this tissue is not mediated by changes inturgor pressure. Key words: Sinapis alba L., phototropism, turgor pressure     

Localization of DNA in Mature and Young Wheat Chloroplasts Using the Fluorescent Probe 4'-6-Diamidino-2-phenylindole

The spatial organization of chloroplast DNA in developing and dividing wheat chloroplasts was studied in the light microscope using the fluorescent probe 4′-6-diamidino-2-phenylindole, which binds specifically to DNA.     

Water Relations of Growing Maize Coleoptiles : Comparison between Mannitol and Polyethylene Glycol 6000 as External Osmotica for Adjusting Turgor Pressure

Water relations of growing segments of maize (Zea mays L.) coleoptiles were investigated with osmotic methods using either mannitol (MAN) or polyethylene glycol 6000 (PEG) as external osmotica. Segments were incubated in MAN or PEG solutions at 0 to - 15 bar water potential (Ψ_o) and the effects were compared on elongation growth, osmotic shrinkage, cell sap osmolality (OC), and osmotic pressure (π_i). The nonpenetrating osmoticum PEG affects π_i in agreement with Boyle-Mariotte's law, i.e. the segments behave in principle as ideal osmometers. There is no osmotic adjustment in the Ψ_o range permitting growth (0 to −5 bar) nor in the Ψ_o range inducing osmotic shrinkage (−5 to −10 bar). Promoting growth by auxin (IAA) has no effect on the osmotic behavior of the tissue toward PEG. In contrast to PEG, MAN produces an apparent increase in π_i accompanied by anomalous effects on segment elongation and shrinkage leading to a lower value for Ψ_o which establishes a growth rate of zero and to an apparent recovery from osmotic shrinkage after 2 hours of incubation. These effects can be quantitatively attributed to uptake of MAN into the tissue. MAN is taken up into the apoplastic space and the symplast as revealed by a large temperature-dependent component of MAN uptake. It is concluded that MAN, in contrast to PEG, is unsuitable as an extemal osmoticum for the quantitative determination of water relations of growing maize coleoptiles.     

The Development of Cells in the Growing Zones of the Root

1. Data are presented which show the change in volume, dry weight,respiration, and protein content of the cell during developmentfrom the meristematic to the mature fully extended state.
2. Thedata have been obtained by making serial sections of knownthicknessalong the root, making the relevant observations onthe sections,and relating the values obtained to the totalnumber of cellsin the sections. An apparatus is described forobtaining serialsections from a group of roots.
3. It is shown that the weightof the wall, the protein content,and respiration all increaseduring extension, and the increasesduring active growth withrespect to respiration and proteincontent occur in two phases.In the first the increases arelarge per unit increment in volume;in the second they are relativelysmaller. When extension ceasesa third phase in the developmentof the cell sets in in whichrespiration and protein contentdecrease slightly.
4. The datashow that the protein content and the respiration inthe meristematiccell are considerably lower than they are infully vacuolatedcell.

     

Leaf Diffusive Conductance and Tap Root Cell Turgor Pressure of Sugarbeet

Abstract. The interrelationships of leaf diffusive conductance, tap root cell turgor pressure and the diameter of the tap root of sugarbeet were studied. The study was conducted on well-watered plants growing in pots under artificial light in the glasshouse. In a typical experiment, on illumination (400 μmol m⁻² s⁻¹) leaf conductance increased from 0.6 to 7.4 mm s⁻¹. Cell turgor pressure in the tap root decreased from 0.8 MPa to 0.45 MPa and the root diameter (9.0 cm) contracted by 145μm. Removal of light resulted in the reversal of each of the above parameters to their previous values. Quantitively similar results were obtained when sugar beet plants were uprooted and the response of each of the parameters was measured. The sequence of events however was different. On stimulation by light, changes in leaf diffusive conductance preceded the turgor and root diameter changes (which were simultaneous) by some 15–20min. In contrast, on uprooting the simultaneous changes in root turgor pressure and diameter preceded the changes in leaf conductance. The lag times between changes in diffusive conductance and turgor pressure in the root were between 20 and 30 min.
Tap root turgor pressure and diameter correlated strongly and permitted the calculation of an apparent whole root volumetric elastic modules (55–63 MPa). The small changes in tissue volume relative to the transpiration rate suggest that the tap root is not a significant source of transpirational water during the day.     

Expression of the Kdp ATPase Is Consistent with Regulation by Turgor Pressure

The kdpFABC operon of Escherichia coli encodes the four protein subunits of the Kdp K⁺ transport system. Kdp is expressed when growth is limited by the availability of K⁺. Expression of Kdp is dependent on the products of the adjacent kdpDE operon, which encodes a pair of two-component regulators. Studies with kdp-lac fusions led to the suggestion that change in turgor pressure acts as the signal to express Kdp (L. A. Laimins, D. B. Rhoads, and W. Epstein, Proc. Natl. Acad. Sci. USA 78:464–468, 1981). More recently, effects of compatible solutes, among others, have been interpreted as inconsistent with the turgor model (H. Asha and J. Gowrishankar, J. Bacteriol. 175:4528–4537, 1993). We re-examined the effects of compatible solutes and of medium pH on expression of Kdp in studies in which growth rate was also measured. In all cases, Kdp expression correlated with the K⁺ concentration when growth began to slow. Making the reasonable but currently untestable assumptions that the reduction in growth rate by K⁺ limitation is due to a reduction in turgor and that addition of betaine does not increase turgor, we concluded that all of the data on Kdp expression are consistent with control by turgor pressure.     

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

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

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

Shoot Turgor Does Not Limit Shoot Growth of NaCl-Affected Wheat and Barley

The aim of this work was to test the hypothesis that the reduced growth rate of wheat and barley that results when the roots are exposed to NaCl is due to inadequate turgor in the expanding cells of the leaves. The hypothesis was tested by exposing plants to 100 millimolar NaCl (which reduced their growth rates by about 20%), growing them for 7 to 10 days with their roots in pressure chambers, and applying sufficient pneumatic pressure in the chambers to offset the osmotic pressure of the NaCl, namely, 0.48 megapascals. The results showed that applying the pressure had no sustained effect (relative to unpressurized controls) on growth rates, transpiration rates, or osmotic pressures of the cell sap, in either the fully expanded or currently expanding leaf tissue, of both wheat and barley. The results indicate that the applied pressure correspondingly increased turgor in the shoot although this was not directly measured. We conclude that shoot turgor alone was not regulating the growth of these NaCl-affected plants, and, after discussing other possible influences, argue that a message arising in the roots may be regulating the growth of the shoot.