Role of Calcium on Phenolic Compounds and Enzymes Related to Lignification in Soybean (Glycine max L.) Root Growth

Changes in soluble and cell wall bound peroxidases activities, phenylalanine ammonia-lyase activity and phenolic compounds and lignin contents in roots of calcium-treated soybean (Glycine max (L.) Merr.) seedlings and their relationships with root growth were investigated. Three-day-old soybean seedlings were cultivated in nutrient solution with or without 0.025–5.0 mM calcium for 24 h. In general, length and fresh and dry weights of roots increased, while activities of enzymes (soluble and cell-wall peroxidases and phenylalanine ammonia-lyase) and phenolic compounds and lignin contents decreased against calcium concentrations. In the absence of calcium, phenylalanine ammonia-lyase and peroxidases activities increased by accumulating phenolic compounds and lignin due to restricted growth of roots. Enhanced calcium supply reduced the production of phenolic compounds and lignification due to low phenylalanine ammonia-lyase and peroxidases activities, reinforcing the essential role of calcium to improve the soybean root growth.     

Transpiration integrated model for root ion absorption under salinized condition

Salinization of crop fields is a pressing matter for sustainable agriculture under desertification and is largely attributed to root absorptive functions of the major crops such as maize. The rates of water and ion absorption of intact root system of maize plants were measured under the salinized condition, and the salt absorptive function of maize roots was analyzed by applying different two kinetic models of root ion absorption (i.e. the concentration dependent model and the transpiration integrated model). The absorption rates for salinization ions (Na⁺, Cl^?, Ca²⁺ and Mg²⁺) were found to depend on ion mass flow through roots driven by the transpiration, and therefore the transpiration integrated model represented more accurately rates of root ion absorption. The root absorption of salinization ions was characterized quantitatively by two model parameters of Q′_max and K′_M involved in the transpiration integrated model, which are considered to relate to the potential absorbing power and the ion affinity of transport proteins on root cell membranes, respectively.     

Characterization of S-adenosylmethionine synthetases in soybean under flooding and drought stresses

Soybean is stress-sensitive crop that exhibits markedly reduced growth under flooding and drought conditions. Three S-adenosylmethionine synthetases (SAMs) proteins were identified as flooding and drought responsive proteins in soybean using a proteomic technique. To better understand the role of these SAMs proteins in soybean under flooding and drought stresses, temporal, organ, and stress specificities were examined at mRNA and enzyme activity levels. The activity of SAMs decreased in response to the flooding, however, it was not significantly changed by NaCl, cold, gibberellic acid, and calcium in soybean roots. The activity of SAMs was induced in roots and hypocotyls under drought. The mRNA expression of the S-adenosylmethionine synthetase (SAMs) family was down-regulated in root tips and roots under the flooding and the drought, and SAMs 1 and SAMs 2 were down-regulated in roots under both stresses. A gene 1-aminocyclopropane-1-carboxylate synthase was up-regulated in root tips, roots, and hypocotyls under drought, however, it was not changed in root tips and roots under the flooding. In addition, 1-aminocyclopropane-1-carboxylate oxidase was induced in root tips under flooding and drought. These results suggest that SAMs was involved in the response to the flooding and drought and it might affect ethylene biosynthesis in soybean.     

Subcellular location of delta-pyrroline-5-carboxylate reductase in root/nodule and leaf of soybean

The expression of Δ¹-pyrroline-5-carboxylate reductase (P5CR) gene was found to be higher in soybean root nodules than in leaves and roots, and its expression in roots appeared to be osmoregulated (AJ Delauney, DPS Verma [1990] Mol Gen Genet 221: 299-305). P5CR was purified to homogeneity as a monomeric protein of 29 kilodaltons by overexpression of a soybean P5CR cDNA clone in Escherichia coli. The pH optimum of the purified P5CR was altered by increasing the salt concentration, and maximum enzyme activity was attainable at a lower pH under high salt (0.2-1 molar NaCl). Kinetic studies of the purified enzyme suggested that nicotinamide adenine dinucleotide phosphate⁺ inhibited P5CR activity, whereas nicotinamide adenine dinucleotide⁺ did not. Subcellular fractionation and antibodies raised against purified soybean P5CR were used to investigate location of the enzyme in different parts of soybean as well as in leaves of transgenic tobacco plants synthesizing soybean P5CR. P5CR activity was present in cytoplasm of soybean roots and nodules as well as in leaves, but in leaves, about 15% of the activity was detected in the plastid fraction. The location of P5CR was further confirmed by western blot assay of the proteins from cytosol and plastid fractions of different parts of the plant. Expression of soybean nodule cytosolic P5CR in transgenic tobacco under the control of cauliflower mosaic virus 35S promoter led to the accumulation of this protein exclusively in the cytoplasm, suggesting that the chloroplastic activity may be due to the presence of a plastid form of the enzyme. The different locations of P5CR in root and leaf suggested that proline may be synthesized in different subcellular compartments in root and leaf. Proline concentration was not significantly increased in transgenic plants exhibiting high level P5CR activity, indicating that reduction of P5C is not a rate-limiting step in proline production.     

Root respiration associated with nitrogenase activity (c(2)h(2)) of soybean, and a comparison of estimates

Root respiration associated with symbiotic fixation in soybean (Glycine max [L.] Merr.) was estimated by four methods.

Averaged over the life of the plant, the root respires 5.8 milligrams C per milligram N accumulated from fixation. When nitrogenase (C₂H₂) activity and root respiration were decreased by treating roots briefly with 1.0 atmosphere O₂, the respiration associated with nitrogenase was estimated as 2.10 micromoles CO₂ per micromole C₂H₄.

When nitrogenase activity and respiration were decreased by addition of nitrate, the respiration associated with fixation was calculated as 2.90 micromoles CO₂ per micromole C₂H₄. Removing nodules from roots decreased fixation and root respiration, and the ratio was 4.08 micromoles CO₂ per micromole C₂H₄. When soybean plants were kept in prolonged darkness, then returned to light, the associated drop and recovery of respiration and nitrogenase activity had a ratio of 4.36 micromoles CO₂ per micromole C₂H₂.

     

Expression of a Soybean Hydroxyproline-Rich Glycoprotein Gene Is Correlated with Maturation of Roots

A novel extensin gene has been identified in soybean (Glycine max L.) that encodes a hydroxyproline-rich glycoprotein (SbHRGP3) with two different domains. In this study expression of SbHRGP3 was investigated during soybean root development. SbHRGP was expressed in roots of mature plants, as well as seedlings, and showed a distinct pattern of expression during root development. The expression of SbHRGP3 increased gradually during root development of seedlings and reached a maximum while the secondary roots were maturing. The maximum expression level was contributed mainly by the secondary roots rather than by the primary root. Furthermore, expression of SbHRGP3 was preferentially detected in the regions undergoing maturation of the primary and secondary roots. These results imply that the expression of SbHRGP3 is regulated in an organ- and development-specific manner and that in soybean SbHRGP3 expression may be required for root maturation, presumably for the cessation of root elongation. Wounding and sucrose in combination enhanced expression of SbHRGP3 in roots, whereas both wounding and sucrose were required for the expression of SbHRGP3 in leaves.     

Effects of Manganese Toxicity on the Growth and Gene Expression at the Seedling Stage of Soybean

In order to investigate the effects of Manganese (Mn) toxicity stress on the growth and gene expression at the seedling stage of soybean, soybean seedlings were treated with normal Mn concentration (5 μmol/L MnSO₄) and excess Mn concentration (100 μmol/L MnSO₄) by the method of hydroponic culture in this study. When soybean was subjected to Mn toxicity stress, excessive Mn could affect seedling growth, root development, the number of Mn oxide spots in leaves, and the Mn accumulation content in different parts of soybean. With the increase of exogenous Mn concentration and the prolongation of culture time, the shoot and root biomasses of soybean decreased significantly, but the plant height of soybean had no obvious effects. The total root length, root surface area and root volume of soybean decreased significantly, but the taproot length, taproot tip length and root diameter did not change significantly. The number of Mn spots in the primary leaves (first leaf) was significantly more than that in the old leaves (second leaf) and the youngest leaves (fourth leaf). The Mn concentration in leaves was significantly higher than that in roots, and the Mn concentration in the old leaves was significantly higher than that in youngest leaves with the method of inductively coupled plasma atomic emission spectrometry (ICP-AES). Moreover, the results in the present study suggested that the 10 selected genes were significantly up-regulated or down-regulated by Mn toxicity in the old and young leaves by quantitative real-time PCR (qRT-PCR) analysis. This indicates that these genes might be important in the process of regulation in old and young leaves of the physiological responses and ion transporting to Mn toxicity stress.     

Ion uptake and respiration of dry bean roots subjected to localized anoxia

Summary Ion uptake by dry bean root systems was examined during a three day treatment period. Three aeration treatments were applied to split root systems where both halves were aerated, both halves were nonaerated and one half aerated and the remaining half nonaerated (localized anoxia). Ion absorption was similar for the aerated control and localized anoxia treatments. The nonaerated control absorbed 2, 40, and 60 percent of the aerated control for K⁺, Ca⁺⁺, and NO₃ ⁻, respectively. Ion absorption by stressed plants appeared to increase directly with root growth in the aerated portions of the localized anoxia treatments. Localized anoxia resulted in greater potassium ion uptake per unit root weight and in greater root respiration rates of the aerated half of the Pinto III cultivar root system. Transpiration rates of Seafarer subjected to localized anoxia were 135% of the aerated control. The additional water use may have contributed to greater ion uptake, by mass flow, in the nonaerated portion of the localized anoxia treatment. Nutrient solutions of the nonaerated controls became more alkaline during stress than did the nonaerated portions of the localized anoxia treatments, indicating a possible direct or indirect effect of the aerated portions of the localized anoxia treatments on the corresponding nonaerated half. Compensation in ion uptake by dry bean roots subjected to localized anoxia appeared to be the result of increased root growth, greater respiration rates, greater transpiration rates and, for Pinto III, an increase in the ion uptake rate per unit root weight. This compensatory uptake of water and nutrients by the root system may be one mechanism by which roots overcome localized stress within a soil profile.     

Proteomic and Biochemical Analyses of the Cotyledon and Root of Flooding-Stressed Soybean Plants

Background

Flooding significantly reduces the growth and grain yield of soybean plants. Proteomic and biochemical techniques were used to determine whether the function of cotyledon and root is altered in soybean under flooding stress.

Results

Two-day-old soybean plants were flooded for 2 days, after which the proteins from root and cotyledon were extracted for proteomic analysis. In response to flooding stress, the abundance of 73 and 28 proteins was significantly altered in the root and cotyledon, respectively. The accumulation of only one protein, 70 kDa heat shock protein (HSP70) (Glyma17g08020.1), increased in both organs following flooding. The ratio of protein abundance of HSP70 and biophoton emission in the cotyledon was higher than those detected in the root under flooding stress. Computed tomography and elemental analyses revealed that flooding stress decreases the number of calcium oxalate crystal the cotyledon, indicating calcium ion was elevated in the cotyledon under flooding stress.

Conclusion

These results suggest that calcium might play one role through HSP70 in the cotyledon under flooding stress.     

Transport of copper and manganese to the xylem exudate of sunflower

Abstract Absorption of copper and manganese by sunflower roots from solution cultures of varying composition was followed by measuring the concentrations of the metals appearing in whole roots, root cell sap and xylem exudate. Total copper in the fibrous roots was linearly related to the concentration of copper in the external solution but the concentration of copper released to the xylem exudate was buffered somewhat against the changes made externally. No such buffering was observed for managenese. A copper-sensitive electrode, responsive only to free cupric ions was used in conjunction with total copper analysis by atomic absorption spectrophotometry to show that little of the copper (usually < 1%) existed as a free ion in any phase of the system. Copper in the xylem exudate may be strongly complexed. An electron paramagnetic resonance spectrum of the xylem exudate indicated that manganese probably was a free divalent ion. Calculation of the electrochemical potential gradient for free cupric ions showed that no special metabolically-linked mechanism need be postulated to account for absorption of copper (or manganese) other than that necessary to maintain the transmembrane potential.     

The supply of nutrient ions by diffusion to plant roots in soil

Summary Measurements were made of the diffusion of P³²-labelled phosphate to single roots of onion, leek and rye-grass growing in an Upper Greensand sandy loam (UGS) and a Coral Rag Clay (CRC) to which different amounts of phosphate had been added. Concentration-dependent diffusion coefficients for phosphate ions in the soils were calculated from phosphate desorption isotherms in calcium chloride. The experimental uptake by roots of known dimensions was compared with supply expected by diffusion to a cylindrical model root of the same dimensions. Allowance was made for absorption by the root hairs on rye-grass roots. Phosphate absorption by a cm length of intact root was found to continue for at least 16 days for onion, 10 days for leek and 5 days for rye-grass. Over a wide range of conditions (phosphate concentrations, soils, plant species), experimental uptake was close to the maximum calculated to be possible for the diffusion model except on one soil at a high level of phosphate. Although the concentration of phosphate in the soil solution at the root boundary appeared to be reduced to a small fraction of the initial concentration, because of the extreme non-linear form of the desorption isotherm less than 1/2 of the P³² exchangeable pool of P was considered to contribute to diffusion. Phosphate uptake by rye grass could only be accounted for if the root hairs were active. Although only a small fraction of the uptake is derived from inside the root hair cylinder, this increases the efficiency of the central root 2.3 fold by providing a zone close to the central root through which phosphate moves very readily.     

Association of the Plant-beneficial Fungus Verticillium lecanii with Soybean Roots and Rhizosphere

Colonization of soybean roots by the biocontrol fungus Verticillium lecanii was studied in vitro and in situ. For in vitro experiments, V. lecanii was applied to soybean root tip explant cultures. Four weeks after inoculation, the fungus grew externally on at least half of the roots (all treatments combined), colonizing 31% to 71% of root length (treatment means). However, when a potato dextrose agar plug was available as a nutrient source for the fungus, root tips inoculated soon after transfer were not colonized by V. lecanii unless Heterodera glycines was present. Scanning electron microscopy of colonized roots from in vitro cultures revealed a close fungus-root association, including fungal penetration of root cells in some specimens. In the greenhouse, soybeans in sandy soil and in loamy sand soil were treated with V. lecanii applied in alginate prills. The fungus was detected at greater depths from the sandy soil than from the loamy sand soil treatment, but fungus population numbers were small and variable in both soils. Root box studies coupled with image processing analysis of the spatial distribution of V. lecanii in sandy soil supported these findings. Verticillium lecanii was detected randomly in the rhizosphere and soil of root boxes, and was rarely extensively distributed. These in vitro and in situ experiments indicate that V. lecanii can grow in association with soybean roots but is a poor colonizer of soybean rhizosphere in the soil environment.     

Genetic variability for root hair traits as related to phosphorus status in soybean

Plant root hairs are believed to be very important for phosphorus (P) uptake from the soil by expanding the absorptive surface area of the root and increasing the soil volume explored by the roots, but genetic information about root hair traits in soybean is relatively scarce. In the present study, two contrasting genotypes of soybean (Glycine max and Glycine soja), CN4 and XM6, and their 88 F₉-derived recombinant inbred lines (RILs) were grown in a field with moderately low P availability. Some important root hair traits, including root hair density (RHD), average root hair length (ARHL), and root hair length per unit root (RHLUR) were investigated and quantified with an automatic image analysis system and the genetic variability for these root hair traits was estimated with the RIL population. The results indicated that the two parental genotypes differed significantly in the three root hair traits measured, with XM6 generally having larger RHD and RHLUR (but smaller ARHL) than CN4, which may in part explain the difference in biomass and P status between the two parents. All the three root hair traits were continually segregated in the progenial RIL population with a normal distribution of the phenotypic values, indicating that these traits are possibly controlled by quantitative trait loci (QTLs). Analysis of variance for the RIL population showed that RHD had a low heritability (h² _b = 27.32, 31.04, 33.97% for basal roots, tap roots, total roots, respectively), while ARHL had a relatively higher genetic variance and hence a higher heritability (h² _b = 53.85, 59.18, 60.98% for basal roots, tap roots, total roots, respectively), suggesting that RHD is influenced more by environmental factors than ARHL. Both RHD and ARHL were positively correlated with RHLUR, indicating that the former two traits may be the attributes to the latter. On the other hand, RHD and ARHL were negatively correlated with each other, implying a possible complementary relationship between the two traits. Both RHD and RHLUR (but not ARHL) were positively correlated with P concentration in the plant, suggesting an important role of root hairs in P status. The basal roots had denser and higher total root hair length than the tap roots, and this is in accordance with previous observations with other plants that basal roots are more effective for P uptake than tap roots in cultivated soils.     

Sites, pathways, and mechanism of absorption of Cu-EDDS complex in primary roots of maize (Zea Mays L.): anatomical, chemical and histochemical analysis

To study the mechanism of chelant-metal complexes to be absorbed into plant roots in the presence of different concentration chelating agents, the sites, pathways, and mechanism of absorption of Cu-EDDS complex ([S, S’]-ethylene diamine disuccinic acid) in maize (Zea mays L.) primary roots were systematically studied. The results showed that, at low concentrations of the Cu-EDDS complex (<200 μmol L^?1) in hydroponic culture, the complex was passively absorbed mainly from the apoplastic spaces where lateral roots penetrate the endodermis and the cortical region into the root xylem, the lateral root zone were the main absorption sites. At higher concentrations (<3,000 μmol L^?1), under hydroponic culture and soil culture conditions, the passage cells, which form a physiological barrier controlling ion absorption, were either injured or killed, and the complex could enter the root xylem. Injury to the physiological barrier was a key factor in the complex being absorbed by roots in substantially larger quantities. In addition, the histochemical analysis of rubeanic acid can also be used for other researches involving Cu, and the negative–pressure measuring device provides a new research tool for studying the apoplastic absorption of other metal–chelating complexes, molecules, and ions.     

Ontogenetic variation of four cytokinins in soybean root pressure exudate

Cytokinins exported from the root may be involved in the correlative control of plant development. To test this hypothesis in soybean ((Glycine max [L.] Merr. cv. McCall, cv Chippewa 64, and cv Hodgson 78), cytokinins were intercepted en route from the root to the shoot by collecting root pressure exudate from detopped roots. The quantities of four cytokinins in the exudate were studied throughout the development of plants grown in the field and in controlled environment chambers. Zeatin, zeatin riboside, and their dihydro derivatives, dihydrozeatin and dihydrozeatin riboside, were isolated and quantitated using high-performance liquid chromatography.

Cytokinin fluxes (pmoles per plant per hour) were independent of exudate flux (grams per plant per hour). All fluxes are averages for a 6- or 8-h collection period. The ribosides accounted for the majority of the observed cytokinin transport. The fluxes of zeatin riboside and dihydrozeatin riboside increased from low levels during vegetative growth to maxima during late flowering or early pod formation. Before the seeds began rapid dry matter accumulation, zeatin riboside and dihydrozeatin riboside fluxes decreased and remained at low levels through maturation. The fluxes of zeatin and dihydrozeatin were low throughout development.

No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).

The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.

     

The Absorption of Ions by Excised Root Systems: I. APPARATUS AND PRELIMINARY EXPERIMENTS

1. An apparatus is described by means of which the absorptionofions from a complete nutrient solution of constant compositionby excised root systems of plants, grown under known nutrientdeficiencies, may be measured in standard conditions of aerationand temperature. Results of some prelimi nary experiments aredescribed.
2. It was found that the roots readily absorbed theelement inwhich they were deficient, but tended to lose thoseelementswhich were already present in normal amounts.
3. Therewas almost invariably a loss in fresh weight of the rootsafterthe absorption period and also a loss in dry weight. Thislossappears to be complex and is partly attributable to lossofrespiratory material.
4. The addition of 2 per cent. sucroseto the solution from whichthe root systems of phosphorus-deficientbarley plants wereabsorbing increased the nitrogen and phosphoruscontents ofthe roots and maintained the potassium content,while in absenceof sucrose only the phosphorus content increased,but this increasewas significantly less than in the presenceof sucrose.
5. It was shown that roots excised from plants growingin soilwere capable of absorbing phosphorus or nitrogen—elementsin which they were apparently deficient.
6. The interpretationof data obtained from excised roots is discussed,and it isconcluded that excised roots from plants grown incomplete nutrientare not likely to behave in the same way,as regards absorption,as corresponding roots of intact plants,but that roots grownunder conditions of deficiency will behaverather similarlywhether excised or intact. This fact providesa potential methodfor diagnosing and evaluating nutrient deficiencies.
7. The low-saltcondition of roots postulated by Hoagland and Broyeris notnecessarily the primary requisite for rapid absorptionof aparticular ion. It is rather that the roots should be deficientin that ion. The roots could be high in other salts.

     

Effect of Root Temperature on Cytokinin Activity in Root Exudate of Vitis vinifera L

Root exudates of plants of Vitis vinifera L. cv. Thompson Seedless, grown in nutrient cultures with root temperatures maintained at either 20° or 30° and with shoots at a common air temperature, were assayed for cytokinin activity. After chromatography of freeze-dried sap on paper with n-butanol/acetic acid/water (4:1:1). activity was detected with a soybean callus assay. For both root temperatures, major activity appeared between R_F 0.6 and R_F 0.8, at about the same concentration in each case. The major difference between the 2 samples was the presence of activity at R_F 0.1 to 0.2 in the 20° sample and its absence in the 30° sample.

The higher root temperature resulted in increased shoot and root elongation, increased dry matter accumulation by both shoots and roots, and also altered the morphological appearance of the roots.

     

A Gradient of Endogenous Calcium Forms in Mucilage of Graviresponding Roots of Zea mays

Agar blocks that contacted the upper sides of tips of horizontally-orientedroots of Zea mays contain significantly less calcium (Ca) thanblocks that contacted the lower sides of such roots. This gravity-inducedgradient of Ca forms prior to the onset of gravicurvature, anddoes not form across tips of vertically-oriented roots or rootsof agravitropic mutants. These results indicate that (1) Cacan be collected from mucilage of graviresponding roots, (2)gravity induces a downward movement of endogenous Ca in mucilageoverlying the root tip, (3) this gravity-induced gradient ofCa does not form across tips of agravitropic roots, and (4)formation of a Ca gradient is not a consequence of gravicurvature.These results are consistent with gravity-induced movement ofCa being a trigger for subsequent redistribution of growth effectors(e.g. auxin) that induce differential growth and gravicurvature. Atomic absorption, calcium, corn, gravitropism (root), Zea mays     

The Interactions Between Monovalent Cations and Calcium During Their Adsorption on Isolated Cell Walls and Absorption by Intact Barley Roots

The adsorption of sodium, potassium, rubidium and calcium ionsat different concentrations was measured on cell walls frombarley roots (Hordeum vulgare L. cv. Union), prepared by detergenttreatment. It was found that sodium and calcium interacted verystrongly during their simultaneous adsorption, whereas potassiumdid not interfere with calcium. This has led us to concludethat calcium and sodium are adsorbed on identical sites in thecell wall, whereas potassium is adsorbed at another site. Rubidiumseems to be less specific for both sites and interferes onlymoderately with calcium. The adsorption on cell walls of thesecations was compared with their adsorption on intact roots at2 °C, where beside the cell wall, sites may be availableat the outer surface of the membrane, and further measurementswere made of absorption at 25 °C. The fact that sodium interactswith calcium and potassium alters the ratio of K to Na in thecell wall compared to their concentrations in the medium. Thepreferential shift towards potassium when calcium is presentcould be very important for the rates of initial uptake in lowsalt barley roots, since the membrane is in contact with a differentproportion of K to Na in the cell wall from the one suppliedin the medium. Hordeum vulgare L., barley, absorption of cations, adsorption, calcium, sodium, potassium rubidium     

Effects of Cations on Hormone Transport in Primary Roots of Zea mays

We examined the influence of aluminum and calcium (and certain other cations) on hormone transport in corn roots. When aluminum was applied unilaterally to the caps of 15 mm apical root sections the roots curved strongly away from the aluminum. When aluminum was applied unilaterally to the cap and ³H-indole-3-acetic acid was applied to the basal cut surface twice as much radioactivity (assumed to be IAA) accumulated on the concave side of the curved root as on the convex side. Auxin transport in the apical region of intact roots was preferentially basipetal, with a polarity (basipetal transport divided by acropetal transport) of 6.3. In decapped 5 mm apical root segments, auxin transport was acropetally polar (polarity = 0.63). Application of aluminum to the root cap strongly promoted acropetal transport of auxin reducing polarity from 6.3 to 2.1. Application of calcium to the root cap enhanced basipetal movement of auxin, increasing polarity from 6.3 to 7.6. Application of the calcium chelator, ethylene-glycol-bis-(β-aminoethylether)-N,N,N′, N′-tetraacetic acid, greatly decreased basipetal auxin movement, reducing polarity from 6.3 to 3.7. Transport of label after application of tritiated abscisic acid showed no polarity and was not affected by calcium or aluminum. The results indicate that the root cap is particularly important in maintaining basipetal polarity of auxin transport in primary roots of corn. The induction of root curvature by unilateral application of aluminum or calcium to root caps is likely to result from localized effects of these ions on auxin transport. The findings are discussed relative to the possible role of calcium redistribution in the gravitropic curvature of roots and the possibility of calmodulin involvement in the action of calcium and aluminum on auxin transport.