Variations in sodium uptake along primary roots of corn seedlings

Entry of Na⁺ into segments of the apical 8-centimeter portion of corn (Zea mays) roots was investigated and analyzed for each centimeter segment separately. Influence of temperature in the 0 C to 30 C range was well described by the Arrhenius equation [U = A exp (-Ea/RT)]. Values of A and Ea differed for each segment, tending to lessen with increasing distance from root apex. Time course of Na⁺ entry was followed up to 70 minutes. Time relations of the process fit well the expression U = m [1 - exp (-nt)]. Calculated maximal uptake capacity (m) diminished with increasing distance from the apex. The data presented indicate that sodium uptake mechanisms vary qualitatively and quantitatively along corn roots. Thus, the use of entire roots for characterization of uptake mechanisms should be reassessed.     

Axial and Radial Hydraulic Resistance to Roots of Maize (Zea mays L.)

A root pressure probe was employed to measure hydraulic properties of primary roots of maize (Zea mays L.). The hydraulic conductivity (Lp_r) of intact root segments was determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. In hydrostatic experiments, Lp_r was constant along the segment except for an apical zone of approximately 20 millimeters in length which was hydraulically isolated due to a high axial resistance. In osmotic experiments, Lp_r decreased toward the base of the roots. Lp_r (osmotic) was significantly smaller than Lp_r (hydrostatic). At various distances from the root tip, the axial hydraulic resistance per unit root length (R_x) was measured either by perfusing excised root segments or was estimated according to Poiseuille's law from cross-sections. The calculated R_x was smaller than the measured R_x by a factor of 2 to 5. Axial resistance varied with the distance from the apex due to the differentiation of early metaxylem vessels. Except for the apical 20 millimeters, radial water movement was limiting water uptake into the root. This is important for the evaluation of Lp_r of roots from root pressure relaxations. Stationary water uptake into the roots was modeled using measured values of axial and radial hydraulic resistances in order to work out profiles of axial water flow and xylem water potentials.     

Resistance to Water Transport in Shoots of Vitis vinifera L. : Relation to Growth at Low Water Potential

Apparent resistances to water transport in the liquid phase were determined from measurements of soil, root, basal shoot internode, shoot apex, and leaf water potentials and water flux in Vitis vinifera (cv White Riesling) during soil drying. Predawn water potential differences (ΔΨ) in the shoots accounted for 20% of the total ΔΨ between the soil and the shoot apex when plants were well-watered but increased to about 90% when shoot growth ceased. The ΔΨ from soil to root was essentially constant during this period. At low water potential, the ΔΨ in the shoot was persistent when transpiration was low (predawn) or completely prevented (plant bagging). The apparent hydraulic resistance between the basal shoot internode and most rapidly expanding leaf (or shoot apex) increased several-fold when water was withheld. Leaf and internode expansion both exhibited high sensitivity to increasing hydraulic resistance. Measurements of pneumatic resistance to air flow through frozen internode segments indicated progressive vapor-filling of vessels as soil drying progressed. From these observations and others in the literature, it was suggested that embolization may be a common occurrence and play an important role in the inhibition of shoot growth at moderate water deficits.     

Root elongation rate is correlated with the length of the bare root apex of maize and lupin roots despite contrasting responses of root growth to compact and dry soils

Background

We investigated interacting effects of matric potential and soil strength on root elongation of maize and lupin, and relations between root elongation rates and the length of bare (hairless) root apex.

Methods

Root elongation rates and the length of bare root apex were determined for maize and lupin seedlings in sandy loam soil of various matric potentials (?0.01 to ?1.6 MPa) and bulk densities (0.9 to 1.5 Mg m^?3).

Results

Root elongation rates slowed with both decreasing matric potential and increasing penetrometer resistance. Root elongation of maize slowed to 10 % of the unimpeded rate when penetrometer resistance increased to 2 MPa, whereas lupin elongated at about 40 % of the unimpeded rate. Maize root elongation rate was more sensitive to changes in matric potential in loosely packed soil (penetrometer resistances <1 MPa) than lupin. Despite these differing responses, root elongation rate of both species was linearly correlated with length of the bare root apex (r² 0.69 to 0.97).

Conclusion

Maize root elongation was more sensitive to changes in matric potential and mechanical impedance than lupin. Robust linear relationships between elongation rate and length of bare apex suggest good potential for estimating root elongation rates for excavated roots.     

Spatial and Directional Variation of Growth Rates in Arabidopsis Root Apex: A Modelling Study

Growth and cellular organization of the Arabidopsis root apex are investigated in various aspects, but still little is known about spatial and directional variation of growth rates in very apical part of the apex, especially in 3D. The present paper aims to fill this gap with the aid of a computer modelling based on the growth tensor method. The root apex with a typical shape and cellular pattern is considered. Previously, on the basis of two types of empirical data: the published velocity profile along the root axis and dimensions of cell packets formed in the lateral part of the root cap, the displacement velocity field for the root apex was determined. Here this field is adopted to calculate the linear growth rate in different points and directions. The results are interpreted taking principal growth directions into account. The root apex manifests a significant anisotropy of the linear growth rate. The directional preferences depend on a position within the root apex. In the root proper the rate in the periclinal direction predominates everywhere, while in the root cap the predominating direction varies with distance from the quiescent centre. The rhizodermis is distinguished from the neighbouring tissues (cortex, root cap) by relatively high contribution of the growth rate in the anticlinal direction. The degree of growth anisotropy calculated for planes defined by principal growth directions and exemplary cell walls may be as high as 25. The changes in the growth rate variation are modelled.     

Genotypic differences in Al resistance and the role of cell-wall pectin in Al exclusion from the root apex in Fagopyrum tataricum

Background and Aims

Aluminium (Al) toxicity is one of the factors limiting crop production on acid soils. However, genotypic differences exist among plant species or cultivars in response to Al toxicity. This study aims to investigate genotypic differences among eight cultivars of tatary buckwheat (Fagopyrum tataricum) for Al resistance and explore the possible mechanisms of Al resistance.

Methods

Al resistance was evaluated based on relative root elongation (root elongation with Al/root elongation without Al). Root apex Al content, pectin content and exudation of root organic acids were determined and compared.

Key Results

Genotypic differences among the eight cultivars were correlated with exclusion of Al from the root apex. However, there was a lack of correlation between Al exclusion and Al-induced oxalate secretion. Interestingly, cell-wall pectin content of the root apex was generally lower in Al-resistant cultivars than in Al-sensitive cultivars. Although we were unable to establish a significant correlation between Al exclusion and pectin content among the eight cultivars, a strong correlation could be established among six cultivars, in which the pectin content in the most Al-resistant cultivar ‘Chuan’ was significantly lower than that in the most Al-sensitive cultivar ‘Liuku2’. Furthermore, root apex cell-wall pectin methylesterase activity (PME) was similar in ‘Chuan’ and ‘Liuku2’ in the absence of Al, but Al treatment resulted in increased PME activity in ‘Liuku2’ compared with ‘Chuan’. Immunolocalization of pectins also showed that the two cultivars had similar amounts of either low-methyl-ester pectins or high-methyl-ester pectins in the absence of Al, but Al treatment resulted in a more significant increase of low-methyl-ester pectins and decrease of high-methyl-ester pectins in ‘Liuku2’.

Conclusions

Cell-wall pectin content may contribute, at least in part, to differential Al resistance among tatary buckwheat cultivars.     

Tissue and Cellular Distribution of Glutamine Synthetase in Roots of Pea (Pisum sativum) Seedlings

The effect of nitrate application on glutamine synthetase activity in roots of pea (Pisum sativum L.) seedlings (2 weeks old) was studied. Separation of organelles from root fragments by sucrose density-gradient centrifugation revealed that both nitrite reductase and glutamine synthetase activities increased in root plastids as a response to nitrate application and that no such response was induced by ammonium application. Glutamine synthetase activity was also found to increase in plastids with distance from apex in nitrate-treated plants, the highest specific activity being located in the fourth 1-centimeter segment. Separation by SDS-PAGE and characterization by Western blotting showed that cytosolic glutamine synthetase contains one subunit polypeptide (28 kilodaltons) and that plastid glutamine synthetase contains both the 38-kilodalton subunit and a heavier subunit. When nitrate was present in the nutrient solution, the heavier subunit increased in abundance in protein fractions obtained from purified root plastids.     

The contribution of different regions of the seminal roots of wheat to uptake of nitrate from soil

A technique was developed to determine the physiological activity of defined sections of seminal roots of wheat grown in sand. Wheat plants were grown for 2 weeks in narrow columns of N-deficient sand to which all other nutrients had been added. The columns were split longitudinally and ¹⁵N-labelled nitrate, in an agar medium, supplied to 2 cm sections of root. Shoots and roots were analysed after 24 h to determine the uptake of ¹⁵N. Three sections were examined on either the secondary or tertiary seminal root: 1 cm from the seed (basal segment), 35 cm from the seed (middle segment) and 4 cm from the root apex (apical segment). Total uptake was greatest from the basal and middle segments, declining by 50% from the apical segment. However, uptake per unit root length, including exposed sections of lateral roots, was not significantly different along the root.     

Multiple Aluminum-Resistance Mechanisms in Wheat (Roles of Root Apical Phosphate and Malate Exudation)

Although it is well known that aluminum (Al) resistance in wheat (Triticum aestivum) is multigenic, physiological evidence for multiple mechanisms of Al resistance has not yet been documented. The role of root apical phosphate and malate exudation in Al resistance was investigated in two wheat cultivars (Al-resistant Atlas and Al-sensitive Scout) and two near-isogenic lines (Al-resistant ET3 and Al-sensitive ES3). In Atlas Al resistance is multigenic, whereas in ET3 resistance is conditioned by the single Alt1 locus. Based on root- growth experiments, Atlas was found to be 3-fold more resistant in 20 [mu]M Al than ET3. Root-exudation experiments were conducted under sterile conditions; a large malate efflux localized to the root apex was observed only in Atlas and in ET3 and only in the presence of Al (5 and 20 [mu]M). Furthermore, the more Al-resistant Atlas exhibited a constitutive phosphate release localized to the root apex. As predicted from the formation constants for the Al-malate and Al-phosphate complexes, the addition of either ligand to the root bathing solution alleviated Al inhibition of root growth in Al-sensitive Scout. These results provide physiological evidence that Al resistance in Atlas is conditioned by at least two genes. In addition to the alt locus that controls Al-induced malate release from the root apex, other genetic loci appear to control constitutive phosphate release from the apex. We suggest that both exudation processes act in concert to enhance Al exclusion and Al resistance in Atlas.     

Aluminum Interactions with Voltage-Dependent Calcium Transport in Plasma Membrane Vesicles Isolated from Roots of Aluminum-Sensitive and -Resistant Wheat Cultivars

The role of Al interactions with root-cell plasma membrane (PM) Ca2+ channels in Al toxicity and resistance was studied. The experimental approach involved the imposition of a transmembrane electrical potential (via K+ diffusion) in right-side-out PM vesicles derived from roots of two wheat (Triticum aestivum L.) cultivars (Al-sensitive Scout 66 and Al-resistant Atlas 66). We previously used this technique to characterize a voltage-dependent Ca2+ channel in the wheat root PM (J.W. Huang, D.L. Grunes, L.V. Kochian [1994] Proc Natl Acad Sci USA 91: 3473-3477). We found that Al3+ effectively blocked this PM Ca2+ channel; however, Al3+ blocked this Ca2+ channel equally well in both the Al-sensitive and -resistant cultivars. It was found that the differential genotypic sensitivity of this Ca2+ transport system to Al in intact roots versus isolated PM vesicles was due to Al-induced malate exudation localized to the root apex in Al-resistant Atlas but not in Al-sensitive Scout. Because malate can effectively chelate Al3+ in the rhizosphere and exclude it from the root apex, the differential sensitivity of Ca2+ influx to Al in intact roots of Al-resistant versus Al-sensitive wheat cultivars is probably due to the maintenance of lower Al3+ activities in the root apical rhizosphere of the resistant cultivar.     

Toxicities of soluble Al, Cu, and La include ruptures to rhizodermal and root cortical cells of cowpea

Elevated levels of many metals are toxic to plant roots, but their modes of action are not well understood. We investigated the toxicities of aluminium (Al), copper (Cu), and lanthanum (La) in solution on the growth and external morphology of 3-d-old cowpea (Vigna unguiculata L.) roots for periods of up to 48 h. Root elongation rate decreased by 50% at ca. 30 μM Al, 0.3 μM Cu, or 2.0 μM La, accompanied by a decrease in the distance from the root tip to the proximal lateral root. Kinks developed in some roots 2.0 ± 0.4 mm from the root apex on exposure to Al or La (but not Cu). Light and scanning electron microscopy showed that soluble Al, Cu, or La caused similar transverse ruptures to develop > 1 mm from the root apex through the breaking and separation of the rhizodermis and outer cortex from inner-layers. The metals differed, however, in the range in concentration at which they had this effect; developing in solutions containing 54 to‑600 μM Al, but only from 0.85 to 1.8 μM Cu or 2.0 to 5.5 μM La. These findings suggest that Al, Cu, and La bind to the walls of cells, causing increased cell wall rigidity and eventual cell rupturing of the rhizodermis and outer cortex in the elongating zone. We propose that this is a major toxic effect of Al, and that Cu and La also have additional toxic effects.     

Aluminium distribution in soybean root tip for a short time Al treatment

Using the lumogallion staining method which we developed (Kataoka et al. 1997a), Al movement in soybean (Glycine max. (L.)Merr. cv. Tsurunoko) root tips treated for a short time was studied. We have indicated that the majority of Al accumulated in the root was found between 0 and 2 mm from the root apex within 2 h (Kataoka et al. 1997a, b). In the study presented here two-day seedlings of the soybean were treated with 50 μmol/L AlCl₃ (pH 4.4), including 0.2 mmol/L CaCl₂, for 1 h, and Al accumulation in the root sections at both 1 and 2 mm apart from root apex was analyzed by a confocal laser microscopy. Although the early indicators, callose induction and the decrease of growth recovery, were not observed in the root when treated for 15 min, a trace amount of Al was already incorporated into the nucleus of cells and the middle tissue of the root. The non-toxic level of Al was more rapidly absorbed than previously thought. The initial increase of callose accumulation and the reduction of the growth recovery were found after 30 min. Therefore, the difference between Al accumulation profiles of 15 and 30 min was analyzed to find out what triggered a toxic Al effect. Increase of Al accumulation in whole root tissue was observed in the root sections, at both 1 and 2 mm from the root apex, and the greatest amount of Al was found in the cytoplasm of the outer cortex, 1 mm away from the root apex. These results are consistent with the fact that Al exclusion from root tip cells is an important mechanism of Al tolerance.     

The longevity and activity of the primary root of maize

The longevity of the main root cylinder and the laterals of the primary root of maize plants was determined under controlled greenhouse conditions by means of nuclear staining with acridine orange. The cortex of the main root was found to be alive for the whole life-span of the plant, whereas the life-span of the root hairs was only 2 to 3 days as evidenced by electronmicroscopical examination of cell integrity. The onset of senescence of laterals was observed at the older part of the main root at the 6-leaf stage of the plant. Senescence of 1st and 2nd order laterals commenced near the root tip a few days after their protrusion and advanced towards the basal region of the root. In any root segment death of the cortex cells preceeded that of the stele. At the late grain filling stage all laterals along the main root exhibited advanced senescence, but stainable nuclei were seen in the root tissues of the basal part of 1^st order laterals (both cortex and stele) as well as of the 2^nd order laterals which emerged from that root segment. The pattern of the dying of the root tissue is discussed with regard to the P-nutrition of the shoot system by the primary root.     

Highly sensitive analytical method for aluminum movement in soybean root through lumogallion staining

We present highly sensitive aluminum detection method in root using fluorescent lumogallion. Roots treated with 100 μM AlCl₃ including 0.2 mM CaCl₂ (pH 4.5) were stained for 60 min with 10 μM lumogallion fluorescence solution and fluorescence from aluminum complex in root was observed under confocal laser microscope. There was a good correlation between the intensity of fluorescence and aluminum content. When the amount of aluminum lost during each step in staining process was measured, it was found that about 10% of aluminum was lost only at staining stage. Through lumogallion staining method, aluminum accumulation especially at an early stage of aluminum treatment in root was shown. At the beginning (2 hr), aluminum began to be accumulated in root cap. After 4 hr treatment, the aluminum distribution was spread to about 3 mm from root apex in the root cap and outer cortex. When aluminum was found in the outer cortex in 3–5 mm from the root apex, the viability was tended to be decreased in the same area (6 hr). At the same time, aluminum amount in meristem was increased. However the comparison of lumogallion staining method with that of morin, which has been widely used to detect aluminum in root, the sensitivity of lumogallion method was found to be much higher.     

Spatial characteristics of aluminum uptake and translocation in roots of buckwheat (Fagopyrum esculentum)

The detoxification of aluminum (Al) in root tips of the Al accumulator buckwheat by exudation of oxalate leading to reduced Al uptake (Al resistance) is difficult to reconcile with the Al accumulation (Al tolerance). The objective of this study was to analyze resistance and tolerance mechanisms at the same time evaluating particularly possible stratification of Al uptake, Al transport and oxalate exudation along the root apex. The use of a minirhizotron made it possible to differentiate between spatial responses to Al along the root apex with regard to Al uptake and organic acid anion exudation, but also to measure at the same time Al and organic acid transport in the xylem. Al accumulates particularly in the 3‐mm root apex. The study showed that Al taken up by the 10‐mm root apex is rapidly transferred to the xylem which differentiates in the 10 to 15‐mm root zone as revealed by a microscopic study. Al induces the release of oxalate from the root apex but particularly from the subapical 6–20 mm root zone even when Al was applied only to the 5‐mm root apex suggesting a basipetal signal transduction. Citrate proved to be the most likely ligand for Al in the xylem because Al and citrate transport rates were positively correlated. In conclusion, the data presented show that the Al‐induced release of oxalate, and Al uptake as well as Al accumulation are spatially not separated in the root apex.     

Passive Electrical Properties of Toad Urinary Bladder Epithelium : Intercellular Electrical Coupling and Transepithelial Cellular and Shunt Conductances

The electrical resistances of the transcellular and paracellular pathways across the toad urinary bladder epithelium (a typical "tight" sodium-transporting epithelium) were determined by two independent sets of electrophysiological measurements: (a) the measurement of the total transepithelial resistance, the ratio of resistance of the apical to the basal cell membrane, and cable analysis of the voltage spread into the epithelium; (b) the measurement of the total transepithelial resistance and the ratio of resistances of both cell membranes before and after replacing all mucosal sodium with potassium (thus, increasing selectively the resistance of the apical membrane). The results obtained with both methods indicate the presence of a finite transepithelial shunt pathway, whose resistance is about 1.8 times the resistance of the transcellular pathway. Appropriate calculations show that the resistance of the shunt pathway is almost exclusively determined by the zonula occludens section of the limiting junctions. The mean resistance of the apical cell membrane is 1.7 times that of the basal cell membrane. The use of nonconducting materials on the mucosal side allowed us to demonstrate that apparently all epithelial cells are electrically coupled, with a mean space constant of 460 µm, and a voltage spread consistent with a thin sheet model.     

Assessing the applicability of the earth impedance method for in situ studies of tree root systems

Several electrical methods have been introduced as non-invasive techniques to overcome the limited accessibility to root systems. Among them, the earth impedance method (EIM) represents the most recent development. Applying an electrical field between a cormus and the rooted soil, the EIM measures the absorptive root surface area (ARSA) from grounding resistance patterns. Allometric relationships suggested that this method was a valuable tool. Crucial assumptions for the applicability of the EIM, however, have not been tested experimentally. Focusing on tree root systems, the present study assesses the applicability of the EIM. Six hypotheses, deduced from the EIM approach, were tested in several experiments and the results were compared with conventional methods. None of the hypotheses could be verified and the results allow two major conclusions. First, in terms of an analogue electrical circuit, a tree-root-soil continuum appears as a serial circuit with xylem and soil resistance being the dominant components. Allometric variation in contact resistance, with the latter being the proxy for root surface area, are thus overruled by the spatial and seasonal variation of soil and xylem resistances. Second, in a tree-root-soil continuum, distal roots conduct only a negligible portion of the electric charge. Most of charge carriers leave the root system in the proximal parts of the root-soil interface.     

Oxygen distribution and movement,respiration and nutrient loading in banana roots (Musa spp. L.) subjected to aerated and oxygen-depleted environments

Excessive soil wetness is a common feature where bananas (Musa spp.) evolved. Under O₂ deficiency, a property of wet soils, root growth and functions will be influenced by the respiratory demand for O₂ in root tissues, the transport of O₂ from the shoot to root and the supply of O₂ from the medium. In laboratory experiments with nodal roots of banana, we examined how these features influenced the longitudinal and radial distributions of O₂ within roots, radial O₂ loss, solute accumulation in the xylem, root hydraulic conductivity, root elongation and root tip survival. In aerated roots, the stele respired about 6 times faster than the cortex on a volume basis. Respiratory O₂ consumption decreased substantially with distance from the root apex and at 300–500 mm it was 80% lower than at the apex. Respiration of lateral roots constituted a sink for O₂ supplied via aerenchyma, and reduced O₂ flow towards the tip of the supporting root. Stelar anoxia could be induced either by lowering the O₂ partial pressure in the bathing medium from 21 to 4 kPa (excised roots) or, in the case of intact roots, by reducing the O₂ concentration around the shoot. The root hair zone sometimes extended to 1.0 mm from the root surface and contributed up to a 60% drop in O₂ concentration from a free-flowing aerated solution to the root surface. There was a steep decline in O₂ concentration across the epidermal-hypodermal cylinder and some evidence of a decline in the O₂ permeability of the epidermal-hypodermal cylinder with increasing distance from the root apex. The differences in O₂ concentration between cortex and stele were smaller than reported for maize and possibly indicated a substantial transfer rate of dissolved O₂ from cortex to stele in banana, mediated by a convective water flow component. An O₂ partial pressure of 4 kPa in the medium reduced net nutrient transfer into the vascular tissue in the stele within 1 or 2 h. Hypoxia also caused a temporary decrease in radial root hydraulic conductivity by an order of magnitude. In O₂ deficient environments, the stele would be among the first tissues to suffer anoxia and O₂ consumption within the root hair zone might be a major contributor to root anoxia/hypoxia in banana growing in temporarily flooded soils.     

Discerning Apical and Basolateral Properties of HT-29/B6 and IPEC-J2 Cell Layers by Impedance Spectroscopy,Mathematical Modeling and Machine Learning

Quantifying changes in partial resistances of epithelial barriers in vitro is a challenging and time-consuming task in physiology and pathophysiology. Here, we demonstrate that electrical properties of epithelial barriers can be estimated reliably by combining impedance spectroscopy measurements, mathematical modeling and machine learning algorithms. Conventional impedance spectroscopy is often used to estimate epithelial capacitance as well as epithelial and subepithelial resistance. Based on this, the more refined two-path impedance spectroscopy makes it possible to further distinguish transcellular and paracellular resistances. In a next step, transcellular properties may be further divided into their apical and basolateral components. The accuracy of these derived values, however, strongly depends on the accuracy of the initial estimates. To obtain adequate accuracy in estimating subepithelial and epithelial resistance, artificial neural networks were trained to estimate these parameters from model impedance spectra. Spectra that reflect behavior of either HT-29/B6 or IPEC-J2 cells as well as the data scatter intrinsic to the used experimental setup were created computationally. To prove the proposed approach, reliability of the estimations was assessed with both modeled and measured impedance spectra. Transcellular and paracellular resistances obtained by such neural network-enhanced two-path impedance spectroscopy are shown to be sufficiently reliable to derive the underlying apical and basolateral resistances and capacitances. As an exemplary perturbation of pathophysiological importance, the effect of forskolin on the apical resistance of HT-29/B6 cells was quantified.     

Auxin Transport in Roots: VI. MOVEMENT OF IAA THROUGH DIFFERENT ZONES OF ZEA ROOTS

The movement of IAA through 6-mm segments excised 1 mm, 7 mm,and 13 mm behind the apex of the primary root of Zea mays seedlingshas been investigated at temperatures between 10 and 25°C. In all segments, and at all temperatures, the movement of IAAwas polarized acropetally, more IAA being found in apical receiverblocks than in basal ones after transport periods of up to 24h. The amounts of IAA which moved acropetally through a segmentdecreased as the segment was taken at an increasing distancebehind the root apex. Similarly, at least after transport periodsof 8 h, more IAA moved basipetally through the apical segmentthan through the basal ones. At 10°C the velocity of acropetal movement was similar inall three segments, but the acropetbut the acropetal flux wasgreatest in the apical segment and smallest in the most basalone. The same situation appears to exist at the other temperatures. The flux and velocity of the acropetal movement of IAA througha 6-mm segment taken 7 mm behind the apex of the root were similarto those previously reported for the acropetal movement througha 12-mm segment excised 1 mm behind the apex. The smaller amountsof IAA which move acropetally through longer root segments aretherefore attributable to a limitation of the flux in the mostbasal regions of the segment.