Rhizodermal Cells Research Articles

Radial and axial turgor pressure measurements in individual root cells of Mesembryanthemum crystallinum grown under various saline conditions

Abstract. Radial and axial turgor pressure profiles were measured with the pressure probe in untreated and salt‐treated intact roots of Mesembryanthemum crystallinum. The microcapillary of the pressure probe was inserted step‐wise into the root tissue 5, 25 and 50 mm away from the root cap. For evaluation of the data, only those recordings on a given root were used in which four discontinuous increases in turgor pressure occurred. These four turgor pressure increases could be related to the rhizodermal cells and to the cells in the three cortical layers. The measurements showed that a radial turgor pressure gradient of the same magnitude (directed from the third cortical layer to the external medium) existed along the root axis. The magnitude of this turgor pressure gradient decreased with increasing salinity (up to 400 mol m‐3 NaCl) in the growth medium. Addition of 10 mol m‐3 CaCl2 to the 400 mol m‐3 NaCl medium partly reduced the salt‐induced decrease in turgor pressure, but only in cells 25–50 mm away from the root tip. Combined with this effect, a small axial turgor pressure gradient was generated, therefore, in the cortex layers which was directed to the root tip. Measurements of the volumetric elastic modulus, ɛ, of the wall of the individual cells showed that the presence of salt considerably reduced the magnitude of this parameter and that addition of Ca2+ to the strongly saline medium partially diminished this decrease. This effect was strongest in cells 50 mm away from the root tip. The magnitude of ɛ of rhizodermal and cortical cells increased along the root axis both in untreated and in salt‐treated roots. The ɛ value was significantly smaller for rhizodermal cells compared to the cortical cells, with the exception of cells 50 mm from the tip. In this tissue, rhizodermal and cortical cells exhibited nearly the same values. The decrease of the ɛ‐values with salt and the increase along the root axis under the various growth conditions could be correlated with corresponding changes in cell volume. Diurnal changes in turgor pressure could not be detected in the individual root cells, with the notable exception of the rhizodermal and cortical cells located in the region 50 mm away from the root tip of the control plants. In these cells, an increase in turgor pressure was observed during the morning hours. Determination of the average osmotic pressure in tissue sections along the roots of control and salt‐treated plants revealed that at 400 mol m‐3 NaCl the osmotic pressure gradient between the tissue and the medium is exo‐directed, provided that the water is not (partly) immobilized.

Cryofixation of plant tissues without pretreatment

SUMMARYRoot tips from Sorghum and Dahlia were frozen without cryoprotection by dipping into nitrogen slush, rapid immersion in liquid propane and by the high‐pressure method. Structural preservation of the samples was studied using freeze‐fracture (FF) and freeze‐substitution (FS) techniques for electron microscopy. It was found that most of the organelles were disrupted by freezing in nitrogen slush and that only the boundary beween the cytoplasm and the vacuole remained visible. If the samples were frozen by rapid immersion in liquid propane, small membraneous organelles, such as dictyosomes, were preserved in peripheral regions of the rhizodermal cells up to 10 μm below the surface of the tissue. Specimens frozen by the high‐pressure freezing technique showed good ultrastructural preservation throughout the tissues up to a depth of more than 100 μm.

Ion Concentrations in the Cytoplasm and Vacuoles of Rhizodermis Cells from NaCl treated Sorghum, Spartina and Puccinellia Plants

Summary The graminees Puccinellia peisonis, Spartina townsendit and Sorghum bicolor × S. sudanensis , differing highly in salt tolerance, grew under steady-state conditions on nutrient solutions without or with 40 mol · m −3 NaCl. The concentrations of Na, Mg, P, S, Cl, K and Ca were determined in the cytoplasm and in the vacuoles of rhizodermis cells by quantitated energy dispersive X-ray microanalyses. Vacuolar element concentrations from salt tolerant species as Puccinellia and Spartina are always higher than those of salt sensitive Sorghum species. Cytoplasmic P, K and Cl concentrations decrease under salt stress in Sorghum but not in Puccinellia and Spartina . Cytoplasmic and vacuolar K and Na concentrations in combination with K/Na selectivities (S K,Na ) at plasmalemma and tonoplast point out different strategies of rhizodermis cells for sustaining the applied NaCl stress: The rhizodermis cells of Puccinellia and Spartina behave as salt includers; S K, Na at the tonoplast (from the vacuole versus cytoplasm) and at the plasmalemma (from the growth medium versus cytoplasm) are higher in Puccinellia than in Spartina . The rhizodermis cells of Sorghum are ion excluders maintaining high S K, Na at plasmalemma and tonoplast. The contribution of Na, K and Cl on the osmotic adaptation of rhizodermis cells towards the growth medium is the subject of the discussion.

Interrelationships between trans-Plasma Membrane Electron/Proton Transfer Stoichiometry, Organic Acid Metabolism, and Nitrate Reduction in Dwarf Bean (Phaseolus vulgaris)

Iron deficiency in dwarf bean (Phaseolus vulgaris L.) induces an increased activity of a system in the rhizodermal cells, which reduces extracellular ferric salts, and an active proton efflux from the roots, which is coupled to accumulation of citrate and malate in the roots and subsequent export of these compounds in the xylem. During reduction of extracellular ferricyanide by Fe-deficient plants, the stoichiometry of electron transport to proton efflux is 2e(-)/1H(+), and citrate and malate levels in the roots are strongly decreased. Reduction of ferricyanide by Fe-sufficient plants has no influence on root and shoot levels of citrate and malate, but in such plants the process is characterized by a e(-)/H(+) efflux stoichiometry close to unity. Apparently, organic acid metabolism and transport are closely associated with the e(-)/H(+) efflux ratio. To assess the significance of organic acid metabolism as one of the direct intracellular components of the induced unbalanced e(-)/H(+) efflux by roots, we studied NO(3) (-) reduction in shoots and roots of Fe-deficient and Fe-sufficient plants. Nitrate reductase activity in the roots was positively correlated with the level of citrate and malate, whereas the enzyme activity in the leaves responded positively to the import of these organic acid anions.

Root hair formation and elongation of primary maize roots

In maize (Zea mays L. cv. LG 11) roots cultured in humid air, the presence of hairs was not related to root growth. However, maximum hair length and length of the hair zone could be correlated to the elongation rate of the primary root. Under the growth conditions used, the emergence of root hairs always took place in the extending zone. In more basal regions, rhizodermal cells could not give rise to root hairs. Results were similar for roots preincubated in a buffer solution.

The roles of cell-wall acidification and proton-pump stimulation in auxin-induced growth: studies using monensin

The carboxylic ionophore, monensin, rapidly induced cell-wall acidification and a decrease in cytosolic pH when added to maize coleoptiles at low external pH and Na(+) concentration. Elongation growth at rates equivalent to those obtained with indole-3-acetic acid was induced for about 1 h. Stimulation of the outwardly directed proton pump apparently occurred, since under the same conditions monensin induced membrane hyperpolarization of maize root rhizodermis cells. When the external pH was high (>8) and Na(+) present, monensin treatment caused only minimal changes in membrane potential and cytosolic pH. Although the ionophore transported protons out of the cell, resulting in cell-wall acidification, no elongation growth occurred. However, under identical conditions, indole-3-acetic acid dit induce growth. The data indicates that stimulation of the outwardly directed electrogenic proton pump rather than the subsequent acidification of the cell wall is vital for the induction of elongation growth.

Evidence that acid solutions induce plant cell elongation by acidifying the cytosol and stimulating the proton pump

Acetic acid (3 mM, pH 4.5) stimulated elongation growth of maizecoleoptiles at a much higher rate than citric acid at the same pH and concentration. The effect of these solutions on cytosolic pH and membrane potential of maize rhizodermis cells was measured with microelectrodes. Citric acid caused a decrease in cytosolic pH and a slow membrane hyperpolarization. Acetic acid induced a larger and more rapid cytosolic acidification and membrane hyperpolarization. Hence, the degree of growth stimulation by the acids was positively correlated with the extent of their cytosolic acidification and stimulation of the proton pump. We suggest the acids induce growth by acidifying the cytosol and stimulating the proton pump rather than via direct acidification of the cell wall. Acid-induced growth Cell wall acidification Cytosolic acidification Indoleacetic acid Membrane potential Zea mays

Scanning electron microscopic aspects of short tuberized roots, with special reference to cell rhizodermis evolution under drought and rehydration

Abstract. Different morphological aspects of short tuberized roots initiated during drought stress in mesophytic species, such as Sinapis alba L., were studied with the scanning electron microscopy technique of cryofixation. Specially adapted for direct and immediate observation of fresh living roots, this method has given precise information about rhizodermal organization and evolution during drought and rehydration.The main difference from a normal lateral root grown in a well‐watered soil appears in the basal enlarged zone of the short root where all the cells show the same round and turgid aspect. In the medium zone of the short root, rhizodermis differentiation into alternating rows of short (trichoblasts) and long (atrichoblasts) cells, which characterizes the typical Sinapis root, remains clearly discernible, though not so regular as in normal lateral roots.The turgid state of rhizodermal cells all over the short tuberized root grown in a drying soil suggests an effective regulatory mechanism for water deficit avoidance.During the first hours of rehydration, immediate absorption of water is noticeable through the rapid swelling of some long cells which appear to protrude considerably among other rhizodermal cells. However, these protrusions will not give rise to hairs, as further observations of short roots after growth has resumed show very distinctly that progressive hair formation occurs in the medium zone of the root, and that their emergence originates from trichoblasts only.These observations may indicate that atrichoblasts, on account of their highly vacuolated condition, are the first cells to absorb water and that they may even be stimulated, in some environmental conditions, to initiate hair formation, although they are not so well adapted to do so as the short cells in this species.

Effect of Fe stress on utilization of Fe chelates by efficient and inefficient plant species

Abstract Using a double labelled Fe chelate (FeEDDHA) it has been possible to show that plants can take up Fe both, as an intact Fe chelate molecule and as ionic Fe after chelate splitting. The rate of chelate splitting in the roots is determined mainly by the Fe nutritional status of the particular plant species but also by the cation exchange capacity of the roots (positive correlation). While Fe stress has no influence on the rate of chelate splitting and the uptake of Fe by Fe‐inefficient plant species (corn), in Fe‐effi‐cient species (sunflower) it brings about a dramatic increase of the rate of Fe uptake following pronounced chelate splitting. This enhanced utilization of chelated Fe by Fe‐efficient plant species under Fe stress is associated with changes in root morphology (thickening of cortical cells, additional division of rhizodermis cells, increased formation of root hairs, presence of transfer cells). Physiological changes in the roots also accompany there morphological developments (increase...

A Special Role of the Rhizodermis in Nutrient Uptake by Plant Roots

Summary Using the possibility of selective killing the rhizodermis by means of osmotic shock further evidence is produced for the hypothesis of a restricted diffusion into the free space of a higher plants root cortex and of a special role of the rhizodermis in uptake processes of plant roots. Osmotic shock causes a strong decrease of phosphate uptake from low-concentrated solutions due to an osmotic rupture of rhizodermal cells. Phosphate uptake from higher concentrated solutions is only slightly influenced. It is concluded that phosphate uptake from low-concentrated solutions takes place at the shock-sensitive tissue surface only whereas higher concentrations permit the diffusion of the solute into the free space and, therefore, allow the shock-insensitive cortical cells to take part in the uptake process. This view is supported by a second line of evidence coming from the investigation of the time course and the temperature dependence of phosphate uptake and from measurements of the trans-membrane potential difference.

Suitability of the Osmotic Shock Procedure for the Analysis of Membrane Transport in Root Tips ofZea maysL.

Root tips of Zea mays L. previously subjected to osmotic shock were studied as to uptake and efflux of glucose and uptake of phosphate, as well as release of protein and other Lowry-positive substances. The osmotic shock procedure applied was similar to that frequently used in order to release periplasmic proteins from bacteria. After treatment with hypertonic solutions of sorbitol (>0·3 M) the uptake of phosphate and glucose is reduced and protein is released into the medium. All osmotic treatments leading to a significant reduction of either glucose or phosphate uptake are accompanied by a strongly increased release of glucose and Lowry-positive substances. Damage of plasma membrane barrier function can be directly observed by the uptake of Evans Blue (Gaff and Okong'o-Ogola, 1971) into the cells involving mainly older vacuolated rhizodermal cells. The occurrence and extent of the alterations of membrane semipermeability are strongly dependent on the duration of exposure to and the concentration of the sorbitol solution used. The velocity of the osmotic transitions (from hypotonic to hypertonic and vice versa) is of minor importance. Sensibility of cells to osmotic stress increases with differentiation and vacuolation. Conditions for a selective reduction of phosphate uptake seem to occur when only meristem and near tip regions subjected to a mild osmotic shock are used.

Mitochondrial inner membrane particles seen in sections of in situ large amplitude swollen mitochondria in rhizodermal cells of cress (Lepidium sativum L.)

Addition of 5–40 mM sodium acetate to root explants of cress (Lepidium sativum L.) growing in a nutrient medium causes large amplitude swelling of mitochondria in rhizodermal cells. On the average, 10 mM sodium acetate causes a fourfold increase in mitochondrial volume, with 40 mM sodium acetate producing an up to tenfold increase in mitochondrial volume. During swelling, however, the mitochondrial membranes remain predominantly intact, and only the outer membrane occasionally appears to be broken. Two types of swelling can be observed: an overall swelling of mitochondrial matrices and a less frequent local swelling which leads to clearly different matrical regions. These regions may sometimes be separated by a septum formed from the inner mitochondrial membrane. After large amplitude swelling in 10 or 40 mM sodium acetate, the visibility of lollipop-like particles on the matrix side of inner mitochondrial membranes is strongly enhanced. These particles are suggested to be identical with mitochondrial inner membrane particles as visualized by negative staining. The distribution of these particles is described. Possible mechanisms which may cause enhanced visibility are discussed.

Influence of humus substances on the toxic effect of 2-methyl-4-chlorphenoxyacetic acid

Effect of humus fractions on the toxicity of 2-methyl-4-chlorphenoxyacetic acid (MCPA) was examined in water cultures. Humus fractions and different MCPA quantities were added to Richter's nutrient solution used for maize (Zea mays L., cv. Kocovska rana) cultivation. The results show that MCPA and humus substances in the applied concentrations intensively influence the maize root system, especially the rhizodermis and apical meristem cells. MCPA reduces ion absorption, induces phosphorus excretion from root, reduces tissue hydratation and relatively quickly kills these cells, at the end stage and/or in the highest concentration. Humus fractions have an unambiguously positive effect on these processes, they increase the cell vitality. When humus substances are applied, the MCPA effect does not appear in potassium and nitrogen absorption, the inhibitory effect is maintained in phosphorus absorption and in tissue hydratation even with the most effective fraction—with fulvo acids. But all fractions increased the cell vitality of sorption tissue and meristems in the presence of MCPA.

A quantitative laboratory method for inoculating cabbage seedlings with Plasmodiophora brassicae Woron

SUMMARYIn studies on root‐hair and rhizodermal cell infection of cabbage seedlings by Plasmodiophora brassicae Woron. seeds were germinated in an incubator at 25°C. on nylon mesh circles placed on glass dishes containing nutrient solution. After 3 days the original nutrient solution was replaced by a similar one containing a known quantity of resting spores of the fungus. After a further 5–7 days in the incubator the roots were removed and stained. The infected plants were counted and the extent of infection on each plant was assessed in grades 0–4.Optimal infection was obtained by using spores from decomposing clubbed roots, by thoroughly washing the resting spores, and by using thirty plants per dish grown from seed of 1·5‐2·0 mm. diam. There was a rapid increase in infection from the 3rd to the 6th day after introduction of the spores.Infection of large numbers of seedlings was easily obtained by this technique, which is now being used for assessing the effects of chemicals in controlling clubroot.