Band structure of surface electric potential in growing roots

For growing roots of azuki bean (Phaseolus chrysanthos), an electric potential is measured minutely along the surface of the root, together with the surface pH. It was found that the root begins to display a band-type pattern of potential with a spatial period of about 2 cm in a mature region as soon as it grows to about 10 cm in root length, while the surface potential shows only one convex peak around a position 5-20 mm behind a root tip and a succeeding concave peak around 20-35 mm, providing the length of root is shorter than about 10 cm. Since the surface potential takes a relatively positive value on average at the side of the root base compared with that in an elongation zone near the tip, electric current is supposed to flow into the elongation zone, accompanied by some local current loops in the mature region. The present band-type pattern observed first in multi-cell systems seems to be a kind of dissipative structure appearing far from equilibrium, and hence its relationship to growth is discussed.     

The spatially variable inhibition by water deficit of maize root growth correlates with altered profiles of proton flux and cell wall pH

Growth of elongating primary roots of maize (Zea mays) seedlings was approximately 50% inhibited after 48 h in aerated nutrient solution under water deficit induced by polyethylene glycol 6000 at -0.5 MPa water potential. Proton flux along the root elongation zone was assayed by high resolution analyses of images of acid diffusion around roots contacted for 5 min with pH indicator gel. Profiles of root segmental elongation correlated qualitatively and quantitatively (r(2) = 0.74) with proton flux along the surface of the elongation zone from water-deficit and control treatments. Proton flux and segmental elongation in roots under water deficit were remarkably well maintained in the region 0 to 3 mm behind the root tip and were inhibited from 3 to 10 mm behind the tip. Associated changes in apoplastic pH inside epidermal cell walls were measured in three defined regions along the root elongation zone by confocal laser scanning microscopy using a ratiometric method. Finally, external acidification of roots was shown to specifically induce a partial reversal of growth inhibition by water deficit in the central region of the elongation zone. These new findings, plus evidence in the literature concerning increases induced by acid pH in wall-extensibility parameters, lead us to propose that the apparently adaptive maintenance of growth 0 to 3 mm behind the tip in maize primary roots under water deficit and the associated inhibition of growth further behind the tip are related to spatially variable changes in proton pumping into expanding cell walls.     

Does salinity reduce growth in maize root epidermal cells by inhibiting their capacity for cell wall acidification?

The reduction in growth of maize (Zea mays L.) seedling primary roots induced by salinization of the nutrient medium with 100 millimolar NaCl was accompanied by reductions in the length of the root tip elongation zone, the length of fully elongated epidermal cells, and the apparent rate of cell production: Each was partially restored when calcium levels in the salinized growth medium were increased from 0.5 to 10.0 millimolar. We investigated the possibility that the inhibition of elongation growth by salinity might be associated with an inhibition of cell wall acidification, such as that which occurs when root growth is inhibited by IAA. A qualitative assay of root surface acidification, using bromocresol purple pH indicator in agar, showed that salinized roots, with and without extra calcium, produced a zone of surface acidification which was similar to that produced by control roots. The zone of acidification began 1 to 2 millimeters behind the tip and coincided with the zone of cell elongation. The remainder of the root alkalinized its surface. Kinetics of surface acidification were assayed quantitatively by placing a flat tipped pH electrode in contact with the elongation zone. The pH at the epidermal surfaces of roots grown either with 100 millimolar NaCl (growth inhibitory), or with 10 millimolar calcium ± NaCl (little growth inhibition), declined from 6.0 to 5.1 over 30 minutes. We conclude that NaCl did not inhibit growth by reducing the capacity of epidermal cells to acidify their walls.     

Rapid Changes in the Pattern of Electric Current around the Root Tip of Lepidium sativum L. following Gravistimulation

Using a highly sensitive vibrating electrode, the pattern of naturally occurring electric currents around 1-day-old primary roots of Lepidium sativum L. growing vertically downward and the current pattern following gravistimulation of the root has been examined. A more or less symmetrical pattern of current was found around vertically oriented, downward growing roots. Current entered the root at the root cap, the meristem, and the beginning of the elongation zone and left the root along most of the elongation zone and in the root hair zone. After the root was tilted to a horizontal position, we observed current flowing acropetally at the upper side of the root cap and basipetally at the lower side within about 30 seconds in most cases. After a delay of several minutes, acropetally oriented current was also found flowing along the upper side of the meristematic zone. The apparent density of the acropetal current in the root cap region increased and then decreased with time. Gravitropic curvature was first visible approximately 10 minutes after tilting of the root to the horizontal position. Since the change in the pattern of current in the root cap region precedes bending of the root and is different for the upper and lower side, a close connection is suggested between the current and the transduction of information from the root cap to the elongation zone following graviperception in the cap.     

Separate de Novo and Salvage Purine Pools Are Involved in the Biosynthesis of Theobromine but Not Caffeine in Leaves of Coffea arabica L

We used a video digitizer system to (a) measure changes in the pattern of longitudinal surface extension in primary roots of maize (Zea mays L.) upon application and withdrawal of auxin and (b) compare these patterns during gravitropism in control roots and roots pretreated with auxin. Special attention was paid to the distal elongation zone (DEZ), arbitrarily defined as the region between the meristem and the point within the elongation zone at which the rate of elongation reaches 0.3 of the peak rate. For roots in aqueous solution, the basal limit of the DEZ is about 2.5 mm behind the tip of the root cap. Auxin suppressed elongation throughout the elongation zone, but, after 1 to 3 h, elongation resumed, primarily as a result of induction of rapid elongation in the DEZ. Withdrawal of auxin during the period of strong inhibition resulted in exceptionally rapid elongation attributable to the initiation of rapid elongation in the DEZ plus recovery in the main elongation zone. Gravistimulation of auxin-inhibited roots induced rapid elongation in the DEZ along the top of the root. This resulted in rapid gravitropism even though the elongation rate of the root was zero before gravistimulation. The results indicate that cells of the DEZ differ from cells in the bulk of the elongation zone with respect to auxin sensitivity and that DEZ cells play an important role in gravitropism.     

Zonal changes in ascorbate and hydrogen peroxide contents,peroxidase, and ascorbate-related enzyme activities in onion roots

Onion (Allium cepa) roots growing hydroponically show differential zonal values for intra- (symplastic) and extra- (apoplastic) cellular ascorbate (ASC) and dehydroascorbate (DHA) contents and for related enzyme activities. In whole roots, ASC and DHA concentrations were higher in root apex and meristem and gradually decreased toward the root base. Guaiacol peroxidase, ASC peroxidase, monodehydroascorbate oxidoreductase, DHA reductase, catalase, and glutathione reductase activities showed differential activity patterns depending on the zone of the root and their apoplastic or symplastic origin. An in vivo staining of peroxidase activity also revealed a specific distribution pattern along the root axis. Using electron microscopy, hydrogen peroxide was found at different locations depending on the root zone but was mainly located in cell walls from epidermal and meristematic cells and in cells undergoing lignification. A balanced control of all of these molecules seems to exist along the root axis and may be directly related to the mechanisms in which the ASC system is involved, as cell division and elongation. The role of ASC on growth and development in relation to its presence at the different zones of the root is discussed.     

Computer-based video digitizer analysis of surface extension in maize roots

We used a video digitizer system to measure surface extension and curvature in gravistimulated primary roots of maize (Zea mays L.). Downward curvature began about 25 +/- 7 min after gravistimulation and resulted from a combination of enhanced growth along the upper surface and reduced growth along the lower surface relative to growth in vertically oriented controls. The roots curved at a rate of 1.4 +/- 0.5 degrees min-1 but the pattern of curvature varied somewhat. In about 35% of the samples the roots curved steadily downward and the rate of curvature slowed as the root neared 90 degrees. A final angle of about 90 degrees was reached 110 +/- 35 min after the start of gravistimulation. In about 65% of the samples there was a period of backward curvature (partial reversal of curvature) during the response. In some cases (about 15% of those showing a period of reverse bending) this period of backward curvature occurred before the root reached 90 degrees. Following transient backward curvature, downward curvature resumed and the root approached a final angle of about 90 degrees. In about 65% of the roots showing a period of reverse curvature, the roots curved steadily past the vertical, reaching maximum curvature about 205 +/- 65 min after gravistimulation. The direction of curvature then reversed back toward the vertical. After one or two oscillations about the vertical the roots obtained a vertical orientation and the distribution of growth within the root tip became the same as that prior to gravistimulation. The period of transient backward curvature coincided with and was evidently caused by enhancement of growth along the concave and inhibition of growth along the convex side of the curve, a pattern opposite to that prevailing in the earlier stages of downward curvature. There were periods during the gravitropic response when the normally unimodal growth-rate distribution within the elongation zone became bimodal with two peaks of rapid elongation separated by a region of reduced elongation rate. This occurred at different times on the convex and concave sides of the graviresponding root. During the period of steady downward curvature the elongation zone along the convex side extended farther toward the tip than in the vertical control. During the period of reduced rate of curvature, the zone of elongation extended farther toward the tip along the concave side of the root. The data show that the gravitropic response pattern varies with time and involves changes in localized elongation rates as well as changes in the length and position of the elongation zone. Models of root gravitropic curvature based on simple unimodal inhibition of growth along the lower side cannot account for these complex growth patterns.     

Spatio-temporal dynamics of expansion growth in roots: automatic quantification of diurnal course and temperature response by digital image sequence processing

A newly developed technique based on image sequence analysis allows automatic and precise quantification of the dynamics of the growth velocity of the root tip, the distribution of expansion growth rates along the entire growth zone and the oscillation frequencies of the root tip during growth without the need of artificial landmarks. These three major parameters characterizing expansion growth of primary roots can be analysed over several days with high spatial (20 microm) and temporal resolution (several minutes) as the camera follows the growing root by an image-controlled root tracking device. In combination with a rhizotron set up for hydroponic plant cultivation the impact of rapid changes of environmental factors can be assessed. First applications of this new system proved the absence of diurnal variation of root growth in Zea mays under constant temperature conditions. The distribution profile of relative elemental growth rate (REGR) showed two maxima under constant and varying growth conditions. Lateral oscillatory movements of growing root tips were present even under constant environmental conditions. Dynamic changes in velocity- and REGR-distribution within 1 h could be quantified after a step change in temperature from 21 degrees C to 26 degrees C. Most prominent growth responses were found in the zone of maximal root elongation.     

Growth of aerial roots with an extensive elongation zone by the example of a hemiepiphyte <Emphasis Type="Italic">Monstera deliciosa</Emphasis>

We investigated peculiarities of growth of aerial roots in a hemiepiphyte Monstera deliciosa. Aerial roots show low absolute and relative rates of growth and have an extensive elongation zone. In contrast to common roots, cell elongation in the elongation zone of aerial roots may last for 30 days and sometimes longer. The length of cortex cells increases in direct proportion to the distance from the root tip. This means that there is no drastic change in the relative rate of growth associated with transition to elongation characteristic of common roots. Distribution of growth over the elongation zone of aerial roots is irregular. Within the elongation zone, the cells of rhizodermis can divide, and divisions are distributed nonuniformly. The contact between neighboring growing polycytes (cell complexes) is presumably associated with their sliding against one another (intrusive growth). By the example of aerial roots of Monstera deliciosa, we showed a particular type of growth organization in the root with an extensive elongation zone differing from the growth of common roots and resembling the growth of leaves, stems, and fleshy fruit of dicotyledons.     

Glutamine synthetase: activity and localization in cyanobacteria of the cycadsCycas revoluta andZamia skinneri

Nitrogen-fixing cyanobacteria inhabit the zone between the inner and outer cortex of cycad coralloid roots. In the growing tip of such roots the cyanobacterial heterocyst frequency, nitrogenase activity (C₂H₂-reduction) and glutamine synthetase activity (both transferase and biosynthetic) were comparable to those found in freeliving cyanobacteria. The relative level of glutamine synthetase protein and its pattern of cellular/subcellular localization in heterocysts and vegetative cells were also similar to those of free-living cyanobacteria. However, there was a progressive decline in nitrogenase activity along the coralloid root with maximum reduction occurring in the regions farthest from the growing tip. A similar but less pronounced pattern was observed for glutamine synthetase activity. Distribution of glutamine synthetase protein in cyanobacteria in the first 2–3 mm of the root tip indicated a slight decrease in the heterocysts and vegetative cells. However, the overall level of cyanobacterial glutamine synthetase protein did not change because of a drastic increase in the numbers of heterocysts, which contain a proportionally higher level of glutamine synthetase than the vegetative cells.Abbreviation GS glutamine synthetase     

Growth, Gravitropism, and Endogenous Ion Currents of Cress Roots (Lepidium sativum L.) : Measurements Using a Novel Three-Dimensional Recording Probe

A novel, three-dimensional recording, vibrating probe was used for measuring the density and direction of the endogenous ionic current of cress roots (Lepidium sativum L.) bathed in low salt media (artificial pond water, APW). Roots submerged in regular APW and growing vertically show the following current pattern. Current of 0.7 microampere/square centimeter density enters or leaves the root cap; the current changes direction frequently. Current of 1.6 microamperes/square centimeter enters the meristem zone most of the time. Maximum current with a density of 2.2 microamperes/square centimeter enters the apical elongating zone, i.e. between 0.8 and 1.2 millimeters behind the root tip. The current density decreases to 1.4 microamperes/square centimeter at 2 millimeters, i.e. in the central elongating zone, and to 1.0 microampere/square centimeter at 3 millimeters, i.e. in the basal elongating zone. The current direction changes from inward to predominantly outward between 1.2 and 3 millimeters behind the tip. Measurements on opposite flanks of the roots indicate that the current pattern is fairly symmetrical. After placing the roots horizontally, the density of the endogenous current remains stable, but the current direction changes at the root cap and in the meristem zone. The current leaves the root on the upper side and enters on the lower side, causing a highly asymmetrical current pattern at the very tip. The current pattern at the upper and lower side further away from the tip remains the same as in vertical roots. Roots submerged in low Ca²⁺ APW show a very different current pattern, no gravitropism, and no change of the current pattern after horizontal orientation. In these roots current enters the root cap and the basal elongating zone and leaves the apical elongating zone. Three conclusions are drawn from these results: First, plant roots elongate by two different modes of growth that are correlated with different current directions. They grow by cytoplasmic enlargement at sites of inward current and by turgor-driven elongation at sites of outward current. Second, a change in the current pattern at the root cap and in the meristem zone is a clear indicator of later gravitropism. Third, Ca²⁺ ions are involved in the gravistimulated change in the current pattern, probably affecting the activity of plasmalemma H⁺-ATPases.     

Growth of the maize primary root at low water potentials : I. Spatial distribution of expansive growth

Seedlings of maize (Zea mays L. cv WF9 × Mo 17) were grown in vermiculite at various water potentials. The primary root continued slow rates of elongation at water potentials which completely inhibited shoot growth. To gain an increased understanding of the root growth response, we examined the spatial distribution of growth at various water potentials. Time lapse photography of the growth of marked roots revealed that inhibition of root elongation at low water potentials was not explained by a proportional decrease in growth along the length of the growing zone. Instead, longitudinal growth was insensitive to water potentials as low as − 1.6 megapascal close to the root apex, but was inhibited increasingly in more basal locations such that the length of the growing zone decreased progressively as the water potential decreased. Cessation of longitudinal growth occurred in tissue of approximately the same age regardless of spatial location or water status, however. Roots growing at low water potentials were also thinner, and analysis revealed that radial growth rates were decreased throughout the elongation zone, resulting in greatly decreased rates of volume expansion.     

Gravity-Induced Polar Transport of Calcium across Root Tips of Maize

Calcium movement across primary roots of maize (Zea mays, L.) was determined by application of ⁴⁵Ca²⁺ to one side of the root and collection of radioactivity in an agar receiver block on the opposite side. Ca movement across the root tip was found to be at least 20 times greater than movement across the elongation zone. The rapid movement of Ca across the tip was severely inhibited in roots from which the root cap had been removed. Ca movement across the tip was also strongly retarded in roots pretreated with 2,4-dinitrophenol or potassium cyanide. Orientation of roots horizontally had no effect on Ca movement across the elongation zone but caused a strong asymmetry in the pattern of Ca movement across the tip. In gravistimulated roots, the movement of Ca from top to bottom increased while movement from bottom to top decreased. The data indicate that gravistimulation induces polar movement of Ca toward the lower side of the root cap. An earlier report (Lee, Mulkey, Evans 1983 Science 220: 1375-1376) from this laboratory showed that artificial establishment of calcium gradients at the root tip can cause gravitropic-like curvature. Together, the two studies indicate that Ca plays a key role in linking gravistimulation to the gravitropic growth response in roots.     

Changing proton concentrations at the surfaces of gravistimulated Phleum roots

The pH patterns at the surfaces of both vertically growing roots of Phleum pratense L. and roots tilted by 45° were recorded using H ⁺-sensitive microelectrodes. During vertical growth the root cap exhibited lower pH values than the meristematic zone. The highest pH values were found at the border between meristematic and elongation zones. In the apical part of the elongation zone the values strongly decreased basipetally. They reached a minimum value of pH 5.4–5.5 (medium pH of about 6.0) at a distance of 700 m from the root tip. This region of strongest acidification usually coincided with that of the highest relative rates of elongation. The region of the first visible curvature following gravistimulation was positioned at 100–200 m more apically. The pH values increased in the basal elongation zone towards the mature zone. The H⁺-flux pattern around a vertically growing Phleum root was characterized by high influxes in the meristematic zone and smaller effluxes in the elongation zone. Tilting the root by 45° induced changes in the pH values of the upper and lower sides of a Phleum root. At a distance of 300–500 m from the root tip, the upper side was strongly acidified while the pH of the lower side slightly increased compared with the values during vertical orientation. pH differences of up to 0.9 pH units between the two sides of a root were detected. These differences decreased basipetally and could not be measured more distant than 700–800 m from the tip. Compared with a vertically growing root, the H⁺-flux pattern of a Phleum root tilted by 45° exhibited effluxes on the entire upper organ flank while the pattern was scarcely altered on the lower side. The curvature-initiating zone in Phleum roots is positioned within that section of the root in which pH changes occur after tilting. The region of highest pH differences, however, is nearer to the apex of the root than the curvature-initiating zone. The pH changes began 8.2 min after a root had been tilted. The bending process started after 17.2 min, i.e. after double the time needed for differential acidification. After reorienting a root, which had just begun to bend, to its previous vertical position the inversion of the pH gradient could be measured within the same mean time of about 8 min. This is again significantly earlier than the beginning of the rebending process. The results indicate that, during the graviresponse, ionic movements occur much earlier than the changes in hormonal activities reported in the literature.Abbreviation CIZ curvature-initiating zone A preliminary report was presented at the 29th Plenary Meeting of the Committee on Space Research (COSPAR) in Washington D.C., USA, 28 Aug – 5 Sept 1992 (Zieschang and Sievers 1993)This work was supported by the Deutsche Forschungsgemeinschaft. We thank Professor H. Felle (Botanisches Institut, Universität Gießen, Gießen, FRG) for practical instructions concerning the method of H⁺-sensitive microelectrodes and Professor W. Simonis (Botanisches Institut, Universität Würzburg, Würzburg, FRG) for allowing to use the microelectrode amplifier.     

Colonization of Tomato Root Seedling by Pseudomonas fluorescens 92rkG5: Spatio–temporal Dynamics, Localization, Organization, Viability, and Culturability

The localization, viability, and culturability of Pseudomonas fluorescens 92rkG5 were analyzed on three morphological root zones (root tip + elongation, root hair, and collar) of 3-, 5-, and 7-day-old tomato plants. Qualitative information about the localization and viability was collected by confocal laser scanning microscopy. Quantitative data concerning the distribution, viability, and culturability were obtained through combined dilution plating and flow cytometry. Colonization by P. fluorescens affected root development in a complex way, causing a general increase in the length of the collar and early stimulation of the primary root growth (3rd day), followed by a reduction in length (7th day). The three root zones showed different distribution, organization, and viability of the bacterial cells, but the distribution pattern within each zone did not change with time. Root tips were always devoid of bacteria, whereas with increasing distance from the apex, microcolonies or strings of cells became more and more prominent. Viability was high in the elongation zone, but it declined in the older parts of the roots. The so-called viable but not culturable cells were observed on the root, and their proportion in the distal (root tip + elongation) zone dramatically increased with time. These results suggest the existence of a specific temporal and spatial pattern of root colonization, related to cell viability and culturability, expressed by the plant-beneficial strain P. fluorescens 92rkG5.     

Wavelike distributions of infections by an introduced and naturally occurring root pathogen along wheat roots

Previously, we showed that copiotrophic and oligotrophic bacteria fluctuate as moving waves along roots. These waves probably originate as a result of growth and death cycles at any location where a moving nutrient source passed. In this study, we placed sclerotia of Rhizoctonia solani AG8 along growing roots of wheat and showed that the proportions of root sections from which R. solani was isolated fluctuated with distance from the root tip. Similarly, proportions of root sections from which naturally occurring Pythium spp. were isolated fluctuated with distance from the root tip. Fourier analysis showed that these fluctuations constituted significant waves. Cross-correlation analyses demonstrated that there were negative correlations between R. solani infections and colony forming units of copiotrophic bacteria at the time of inoculation at the same locations on the root (lag = 0 cm), indicating that infection by R. solani could have been inhibited by these bacteria. There was a positive correlation between Pythium infections and copiotrophic bacteria at a lag of 6 cm along the roots. It therefore appears that Pythium infection took place shortly after the initial peak in copiotrophic bacteria following the passage of the root tip.     

Aluminum inhibits the H(+)-ATPase activity by permanently altering the plasma membrane surface potentials in squash roots

Although aluminum (AL) toxicity has been widely studied in monocotyledonous crop plants, the mechanism of Al impact on economically important dicotyledonous plants is poorly understood. Here, we report the spatial pattern of Al-induced root growth inhibition, which is closely associated with inhibition of H(+)-ATPase activity coupled with decreased surface negativity of plasma membrane (PM) vesicles isolated from apical 5-mm root segments of squash (Cucurbita pepo L. cv Tetsukabuto) plants. High-sensitivity growth measurements indicated that the central elongation zone, located 2 to 4 mm from the tip, was preferentially inhibited where high Al accumulation was found. The highest positive shifts (depolarization) in zeta potential of the isolated PM vesicles from 0- to 5-mm regions of Al-treated roots were corresponded to pronounced inhibition of H(+)-ATPase activity. The depolarization of PM vesicles isolated from Al-treated roots in response to added Al in vitro was less than that of control roots, suggesting, particularly in the first 5-mm root apex, a tight Al binding to PM target sites or irreversible alteration of PM properties upon Al treatment to intact plants. In line with these data, immunolocalization of H(+)-ATPase revealed decreases in tissue-specific H(+)-ATPase in the epidermal and cortex cells (2--3 mm from tip) following Al treatments. Our report provides the first circumstantial evidence for a zone-specific depolarization of PM surface potential coupled with inhibition of H(+)-ATPase activity. These effects may indicate a direct Al interaction with H(+)-ATPase from the cytoplasmic side of the PM.     

Effect of heavy metals on root growth and peroxidase activity in barley root tip

In the present work, we investigated the alteration of oxidative and peroxidative activities of peroxidases (PODs) along the longitudinal root axis of barley seedlings during heavy metal (HM; e.g., Cd, Cu, Hg, Ni, Pb) treatment. Analysis of the individual root segments revealed that all of the analyzed HMs caused an increase of guaiacol-POD activity, however to a different extent and spatial distribution. Cd-induced ferulic acid POD activity was observed along the whole root tip (RT), while Cu and Hg caused its increase in the meristematic zone and Ni mainly at the end of the differentiation zone of RT. The activation of coniferyl alcohol POD by HMs was detected along the whole RT. HM-induced hydrogen peroxide-generating POD activity was localized mainly to the elongation zone of RT. Elevated chlorogenic acid POD activity was observed in the meristematic zone and at the end of the differentiation zone of RTs. The activation of several PODs is probably associated with enhanced H₂O₂ production and lignification as a defense response of roots to several HM, to prevent their uncontrolled flux. On the other hand, this defense response is accompanied by root growth inhibition, due to the enhanced rigidification of cell wall and accelerated differentiation of RTs.     

The Correlation of Profiles of Surface pH and Elongation Growth in Maize Roots

High-resolution profiles of surface pH and growth along vertically growing maize (Zea mays) primary root tips were determined simultaneously by pH-sensitive microelectrodes and marking experiments. Methodological tests were carried out that proved the reliability of our kinematic growth analysis, while questioning the validity of an alternative technique employed previously. A distal acidic zone around the meristematic region and a proximal one around the elongation zone proper were detected. This pattern as such persisted irrespective of the bulk pH value. The proximal acidic region coincided with maximum relative elemental growth rates (REGR), and both characters reacted in a correlated manner to auxin and cyanide. The distal acidic band was unrelated to growth, but was abolished by cyanide treatment. We conclude that: (a) the pattern of surface pH as such is a regulated feature of growing root tips; (b) the correlation of extracellular pH and growth rate suggests a functional relationship only along proximal portions of the growing root tip; and (c) the distal acidic band is not caused by pH buffering by root cap mucilage, as suggested previously, but rather is controlled by cellular activity.