A mathematical model for pH patterns in the rhizospheres of growth zones

In the classical model by Nye (1981), the main process for the change in pH across the rhizosphere is assumed to be diffusion. The classical model focuses on the non-growing part of the root and assumes that the distribution of ion fluxes along the root is spatially uniform. We consider the rhizosphere of the growth zone and take into account the root growth rate and spatially varying flux along the root surface. We present both analytical (dimensional analysis) and experimental (computational) evidence of the importance of taking into account the root growth rate. We describe a conceptual and mathematical model to analyse the pH field around the root tip over time. The model is used with published data to show that, for typical growth rates in sandy soil, the pH field becomes steady (independent of time) after 6 h. Dimensional analysis reveals that a version of the Péclet number, related to the quotient of root elongation rate and proton diffusivity, can be used to predict the extent of the rhizosphere and the time required for it to become steady. For Péclet numbers much greater than 1 (soils), the root influences soil pH for distances on the millimetre scale. In contrast, for Péclet numbers much less than one (agar, aqueous solution), the root influences substrate pH for radial distances on the scale of centimetres. We also present some evidence that agar-contact techniques to measure the soil pH may not be appropriate for measuring the millimetre-scale gradients in soil pH.     

Non-destructive stereological method for estimating the length of rigid root systems

A non-destructive method of total vertical projections for estimating the length of rigid root systems, not introduced yet in plant sciences, is described. It is demonstrated on measuring less and more dense root systems of seedlings of Zea mays grown at hypoxic or control conditions. Photographs of six vertical projections (30° apart) of each root system were taken and evaluated. The method being based on proved mathematical formula offers unbiased estimation of the length of a rigid root system, curved in three-dimensional space, by non-destructive means. Thus, it is applicable during ongoing experimentation on plants grown in a solution culture. It was shown that less than 120 intersections between the root projection and test lines in one photograph ensured sufficient precision of the method and that the observer subjectivity could be overcome by presented instructions. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of calcium and pH on growth in primary roots of Zea mays

Hasenstein, K. H. and Evans, M. L. 1988. The influence of calcium and pH on growth in primary roots of Zea mays. - Physiol. Plant. 72: 466–470.
We investigated the interaction of Ca²⁺ and pH on root elongation in Zea mays L. cv. B73 × Missouri 17 and cv. Merit. Seedlings were raised to contain high levels of Ca²⁺ (HC, imbibed and raised in 10 m M CaCl₂) or low levels of Ca²⁺ (LC, imbibed and raised in distilled water). In HC roots, lowering the pH (5 m M MES/Tris) from 6.5 to 4.5 resulted in strong, long-lasting growth promotion. Surprisingly, increasing the pH from 6.5 to 8.5 also resulted in strong growth promotion. In LC roots acidification of the medium (pH 6.5 to 4.5) resulted in transient growth stimulation followed by a gradual decline in the growth rate toward zero. Exposure of LC roots to high pH (pH shift from 6.5 to 8.5) also promoted growth. Addition of EGTA resulted in strong growth promotion in both LC and HC roots. The ability of EGTA to stimulate growth appeared not to be related to H⁺ release from EGTA upon Ca²⁺ chelation since, 1) LC roots showed a strong and prolonged response to EGTA, but only a transient response to acid pH, and 2) promotion of growth by EGTA was observed in strongly buffered solutions. We also examined the pH dependence of the release of ⁴⁵Ca²⁺ from roots of 3-day-old seedlings grown from grains imbibed in ⁴⁵Ca²⁺. Release of ⁴⁵Ca²⁺ from the root into agar blocks placed on the root surface was greater the more acidic the pH of the blocks. The results indicate that Ca²⁺ may be necessary for the acid growth response in roots.     

Cellular differentiation in root caps of Zea mays that do not secrete mucilage

We quantified the structural changes accompanying cellular differentiation in root caps of Zea mays cv. Ageotropic to determine the developmental basis for the nongraviresponsiveness of their primary roots. Cells of the calyptrogen and columella of primary roots of the ageotropic mutant have structures indistinguishable from those of caps of primary roots of Z. mays cv. Kys the graviresponsive, wild-type parent of Z. mays cv. Ageotropic. However, the relative volumes of dictyosomes, dictyosome-derived vesicles and starch in the outermost peripheral cells of wild-type roots were significantly lower than were those in peripheral cells of mutant roots. This corresponds to a dramatic accumulation of starch and mucilage-filled vesicles in peripheral cells of mutant roots. Cellular differentiation in root caps of graviresponsive seminal roots of the Ageotropic mutant resembled that of primary and seminal roots of the wild-type cultivar, and differed significantly from that of primary roots of the mutant. We conclude that the mutation that blocks secretion of mucilage from peripheral cells of Ageotropic roots: (1) expresses itself late in cellular differentiation in root caps; (2) is expressed only in primary (but not seminal) roots of the Ageotropic mutant; and (3) is consistent with malfunctioning dictyosomes and dictyosome-derived vesicles being the cellular basis for agravitropism of primary roots of this mutant.     

The effect of abscisic acid on cell growth, cell division and DNA synthesis in the maize root meristem

Cis -abscisic acid (ABA), when applied to maize ( Zea mays L. cv. LG 11) roots, decreases the rates of cell growth and cell division in the meristem. It also decreases the rate at which nuclei become labelled with [³H]-thymidine and enter mitosis. Removing the root cap accelerates the entry of nuclei into the DNA synthetic phase of the mitotic cycle and enhances the rate of cell proliferation in the quescent centre. ABA diminishes these effects, but does not suppress them. Thus, ABA cannot wholely substitute for the presence of a cap. One of the primary effects of applied ABA is to retard cell enlargement which may in turn affect the rate of cell division; natural endogenous ABA may act similarly. ABA might in this way assist in maintaining the quiescent centre in intact roots, but cannot be the sole agent involved.     

Development and validation of a model to describe root length density of maize from root counts on soil profiles

Root length density (RLD) is an important determinant of crop water and nutrient acquisition, but is difficult to measure in the field. On a soil profile, in-situ counts of root impacts per unit surface on soil profiles (NI) can be used to calculate RLD if crop-specific parameters for preferential root orientation (anisotropy) are known. An improved method for field determinations of RLD was developed and validated for maize at sites in Côte d'Ivoire and Burkina Faso. Root anisotropy was measured with cubes of undisturbed soil with 0.1 m sidelength, based on NI observed on three planes oriented perpendicularly to each other. RLD was also measured for the enclosed volume. Repetition of such measurements enabled estimation of the robustness across sites of empirical and geometric models for the relationship between RLD and NI:RLD = NI CO, with CO being the coefficient of root orientation, theoretically equals 2 for an isotropic distribution. Root systems were found to be nearly isotropic, except near the root front (0.3 to 0.5 m), where roots had a preferentially orthotropic orientation. Measured RLD was generally about 50% larger than RLD calculated from observed NI and CO, indicating that at least one of the measurement techniques had a systematic error. The ratio between measured and calculated RLD (CE), which ranged from 0.8 to 2, increased with the age of the plants and decreased with soil depth. CE was therefore introduced as an additional coefficient, resulting in RLD = NI CO CE. The empirical value for CO CE was between 2 and 5. The empirical coefficients CO and CE were the same for the sites in Cote d'Ivoire (oxisol with an iron pan at 0.6 to 0.9 m) and Burkina Faso (alfisol with an iron pan at 0.4 to 0.8 m). The model was validated with independent data sets at both sites, and gave satisfactory predictions of RLD on the basis of NI obtained from single soil planes, which can be easily measured in the field.     

Immunological evidence for the presence of plant homologues of the actin- related protein Arp3 in tobacco and maize: subcellular localization to actin-enriched pit fields and emerging root hairs

Summary. The actin-nucleating and -organizing Arp2/3 protein complex is well known to be conserved throughout the eukaryotic kingdom. For higher plants, however, only limited evidence is available for the presence of the Arp2/3 complex so far. Using heterologous antibodies against the Dictyostelium discoideum and Schizosaccharomyces pombe proteins and a bovine peptide, we found immunological evidence for the presence of Arp3 homologues in plants. First, proteins with a molecular mass of about 47–50 kDa were clearly recognized in extracts of both a dicotyledonous plant (tobacco) and a monocotyledonous plant (maize) in immunoblots with the anti-Arp3 antibodies. Second, immunolocalization with these Arp3 antibodies was performed on different plant cells, selected for their diverse actin organizations and functions. On isolated plasma membrane ghosts derived from tobacco leaf protoplasts, a putative Arp3 was localized along cortical actin filaments. In the inner cortex of maize roots, Arp3 was localized to actin-rich plasmodesmata and pit fields and to multivesicular bodies in the cytoplasm. During root hair formation, distinct site-specific localization was found at the protruding apical plasma membrane portions of these tip-growing cells.Correspondence and reprints: Department of Biology, Universitaire Instelling Antwerpen, Universiteitsplein 1, 2610 Wilrijk, Belgium.Received January 3, 2003; accepted February 7, 2003; published online August 26, 2003     

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.     

Differential sensitivities of water diffusion in maize root cortex and stele to HgCl<Subscript>2</Subscript>-induced blockade of aquaporins

Spin-echo NMR comparative study of water diffusion in the cortex and stele of maize (Zea mays L.) roots was made with the aim to determine predominant pathways of radial water movement in the root. The root parts examined differed in terms of water diffusion coefficients and sensitivity to HgCl₂, the aquaporin blocker. These differences are discussed from the viewpoint of unequal contributions of separate transport pathways (apoplastic, symplastic, and transmembrane) to the overall water flow. Characteristics of water diffusion in roots with the endodermis damaged suggest an inconsiderable contribution of the endodermis into resistance to water movement.     

Detection of root mucilage using an anti-fucose antibody

Plant root mucilage is known to enhance soil quality by contributing towards the soil carbon pool, soil aggregation, detoxification of heavy metal ions and interactions with rhizospheric microflora. Mucilage consists of many monosaccharide units, including fucose which can be used as an indicator for plant root based polysaccharides. This is the first report of an immunological technique developed to use anti-fucose antibodies as markers for probing and localizing fucosyl residues in mucilage polysaccharide and, in turn, for localization of plant root mucilage. Fucose was complexed with bovine serum albumin to raise antibodies against fucose. A fucose-directed antibody was shown to cross-react with root cap mucilages from grasses. This antibody was used to localize root mucilage polysaccharide in maize and wheat root caps using immunogold electron microscopy. Abundant labelling could be localized on the cell wall, and in the intercellular matrix and vesicles of the peripheral root cap cells. Labelling was less intense in cells towards the centre of the root cap tissue. Control experiments confirmed that immunogold localization of fucose was specific and reliable.     

Hardening of root cell walls: a growth inhibitory response to salinity stress

Primary roots of intact maize plants (Zea mays L.) grown for several days in nutrient solutions containing 100 mol m⁻³ NaCl and additional calcium, had relatively inhibited rates of elongation. Possible physical restraints underlying this salt induced inhibition were investigated. The inhibition did not involve reductions in osmotic potential gradients and turgor in the tip tissues responsible for root elongation growth. The apparent yield threshold pressure, which is related to capacity of cell walls to undergo loosening by stress relaxation, was estimated psychrometrically in excised root tips. Salinity increased yield threshold values. Comparative root extensibility values were obtained for intact plants by determining the initial (1 min) increase in root elongation rate induced by an 0.1 MPa osmotic jump. Comparative extensibility was significantly reduced in the salinized root tips. Salinity did not reduce capacities for water efflux and associated elastic contraction in root tip tissues of intact plants exposed to hypertonic mannitol. We conclude that cell wall hardening in the elongating root tips is an important component of root growth inhibition induced by long-term salinization.     

Intensity of hydrostimulation for the induction of root hydrotropism and its sensing by the root cap

Roots of Pisum sativum L. and Zea mays L. were exposed to different moisture gradients established by placing both wet cheesecloth (hydrostimulant) and saturated aqueous solutions of various salts in a closed chamber. Atmospheric conditions with different relative humidity (RH) in a range between 98 and 86% RH were obtained at root level, 2 to 3mm from the water-saturated hydrostimulant. Roots of Silver Queen corn placed vertically with the tips down curved sideways toward the hydrostimulant in response to approximately 94% RH but did not respond positively to RH higher than approximately 95%. The positive hydrotropic response increased linearly as RH was lowered from 95 to 90%. A maximum response was observed at RH between 90 and 86%. However, RH required for the induction of hydrotropism as well as the responsiveness differed among plant species used; gravitropically sensitive roots appeared to require a somewhat greater moisture gradient for the induction of hydrotropism. Decapped roots of corn failed to curve hydrotropically, suggesting the root cap as a major site of hydrosensing.     

Radial diffusion transport of water in various zones of maize root and its sensitivity to mercury chloride

NMR-spin echo method was used for comparative study of radial water diffusion in various zones of maize (Zea mays L., cv. Donskaya 1) primary root. Coefficients of water diffusion varied strongly along the root length; the pattern of variations depended on the period during which the diffusion of water molecules was traced. Water diffusion transport in various root zones was unevently sensitive to mercury chloride, an aquaporin inhibitor. The discovered variations in the mobility of water molecules were assigned to morphological and functional features of cells and tissues in the root zones examined; they were interpreted in terms of variable contribution and redistribution of water flows along several transport pathways. The decrease in diffusional water flows could be caused by cell wall modifications (deposition of suberin) that emerge in the endoderm regions distant from the root apex and diminish the contribution of apoplastic transport.     

Evidence for the Active and Passive Chemotropisms in Roots

Changes in the elongation of root cells during the negative (away from the salt) or positive (towards the salt) chemotropic bending of roots induced by the unilateral application of agar blocks (1 mm³) with 10^–3and 10^–2M Cd(NO₃)₂solutions to the meristem zone of the root were studied. The root bending was not accompanied by differential changes in the number of cells that elongated during the 3-h period of chemical stimulation. The bending was only due to differential changes in the cell elongation rates. In most chemically stimulated roots, both concentrations of Cd(NO₃)₂solutions inhibited cell elongation at the stimulated and nonstimulated sides. Cell elongation was inhibited by 10^–2M Cd(NO₃)₂mainly on the stimulated side of the roots, hence, the roots bent towards the salt. On the contrary, 10^–3M Cd(NO₃)₂inhibited cell elongation mainly at the nonstimulated side of the roots. As a result, the roots bent away from the salt, i.e., in the direction opposite to that expected in the case of the direct inhibition of cell growth by Cd(NO₃)₂. It is concluded that the root chemotropisms induced by the above two Cd(NO₃)₂concentrations are, correspondingly, of a passive or active nature.     

Effects of low temperature on lateral diffusion in plasma membranes on maize (Zea mays L.) root cortex protoplasts: relevance to chilling sensitivity

The effects of temperature on the lateral diffusion of fluorescent phospholipids, sterols and proteins in the plasma membranes of maize root cortex protoplasts were monitored using fluorescence photobleaching recovery (FPR). Diffusion parameters were measured in two cultivars of maize having different chilling tolerance. Hydrodynamic theory predicts that the diffusion coefficient, D, should increase with increasing temperature. In the more chilling-tolerant cultivar, however, D for all three probes was nearly insensitive to temperature. In the more chilling-sensitive cultivar, D was also insensitive to temperature over the range from 12 to 21°C, but D for the lipid probes tended to be higher and more variable at lower temperatures. The proportion of probe molecules free to diffuse in the membrane was less than 1 for all probes, and increased significantly with increasing temperature for the protein probe. These results, taken together, support the concept that the plasma membrane contains domains having differing diffusional characteristics. Temperature effects on membrane diffusion are moderated by the existence of these domains to limit significant changes. The observed tendency for higher diffusion coefficients at low temperatures in the chilling-sensitive cultivar may correlate to morphological changes observed with protoplasts of that cultivar at low temperatures.     

Effects of fusaric acid on respiration in maize root mitochondria

The effects of fusaric acid, a phytotoxin produced byFusarium pathogens, on the metabolism of isolated maize root mitochondria and on maize seed germination and seedling growth were investigated. The phytotoxin inhibited basal and coupled respiration when succinate and α-ketoglutarate were the substrates. Coupled respiration dependent on NADH was inhibited, but basal respiration was not. Consistently, succinate cytochromec oxidoreductase activity was decreased whereas NADH cytochromec oxidoreductase was not affected. The ATPase activities of carbonyl cyanide p-trifluoro-methoxyphenyl hydrazone stimulated mitochondria and of freeze-thawing disrupted mitochondria were inhibited. These results indicate that the phytotoxin impairs the respiratory activity of maize mitochondria by at least three mechanisms: (1) it inhibits the flow of electrons between succinate dehydrogenase and coenzyme Q, (2) it inhibits ATPase/ATP-synthase activity and (3) it possibly inhibits α-ketoglutarate dehydrogenase. Seed germination and seedling growth were also affected by fusaric acid with the most pronounced effect on root development. These effects can possibly contribute to the diseases ofFusarium- infected plants     

The effect of a day at low irradiance of a maize crop. II. Photosynthesis, transpiration and respiration

The kinetics of daily photosynthesis, transpiration and dark respiration of shoots of maize plants ( Zea mays L.) were studied in a growth chamber. After one or two days of reduced light intensity, the photosynthesis and the transpiration returned immediately to a level higher than expected. Respiration, which was reduced by at least 50%, returned slowly for over 2 days towards the initial level. These results suggest the existence of reserves of assimilation products which are used up while maintaining the growth of the aerial part in spite of the lack of assimilation. The prolonged reduction of the respiration shows that it is not strongly correlated to photosyntheis or to growth but rather to the amount of available reserves of assimilate.     

Stimulation of proton extrusion by K+ and divalent cations (Ni2+, Co2+ Zn2+) in maize root segments

Entry of the divalent cations Ni²⁺, Co²⁺ and Zn²⁺ into cells of maize ( Zea mays L. cv. Dekalb XL 85) root tissue is accompanied by an acidification of the incubation medium, a decrease in both the pH of the cell sap and the level of malate in the cells, and by an inhibition of dark fixation of CO₂. K⁺, on the contrary, induces only a very low acidification of the incubation medium, does not change either the pH of the cell sap or the malate level in the cells, and induces an increase in CO₂ dark fixation. Different mechanisms are postulated for the stimulation of proton extrusion by divalent cations and K⁺.     

Characterization of a Ca2+-translocating ATPase from corn root microsomes

Brauer, D., Schubert C. and Tu, S,-I. 1990. Characterization of a Ca²⁺-translocating ATPase from corn root microsomes. - Physiol. Plant. 78: 335-344.
The existence of a Ca²⁺-translocating ATPase in microsomes from maize ( Zea mays L. cv, WF9 × Mo17) roots was evaluated using assays to follow Ca²⁺-stimulation of ATP hydrolysis and Ca²⁺ transport by changes in the fluorescence of chlorotetracycline, ATP hydrolysis by microsomes was stimulated by the addition of Ca²⁺ and further enhanced by the Ca ionophore A23187 and bovine brain calmodulin only in the presence of Ca²⁺, Stimulation by these agents was additive and sensitive to vanadate. These results were consistent with the presence of a Ca²⁺-translocating ATPase in microsomal membranes. The fluorescence of chlorotetracycline in the presence of microsomes and Ca²⁺ increased upon the addition of ATP, indicating the transport of Ca²⁺, The initial rate and extent of change in fluorescence were stimulated by calmodulin and quenched by the addition of either A23187 or EGTA, but not by protonophores. Changes in chlorotetracycline fluorescence were prevented by vanadate. Therefore, results using chlorotetracycline also indicated the presence of a Ca²⁺-translocating ATPase, Localization experiments indicated that the majority of the Ca²⁺-translocating ATPase was on the endoplasmic reticulum.