Root aeration via aerenchymatous phellem: three-dimensional micro-imaging and radial O2 profiles in Melilotus siculus

? Internal root aeration enables waterlogging-tolerant species to grow in anoxic soil. Secondary aerenchyma, in the form of aerenchymatous phellem, is of importance to root aeration in some dicotyledonous species. Little is known about this type of aerenchyma in comparison with primary aerenchyma. ? Micro-computed tomography was employed to visualize, in three dimensions, the microstructure of the aerenchymatous phellem in roots of Melilotus siculus. Tissue porosity and respiration were also measured for phellem and stelar tissues. A multiscale, three-dimensional, diffusion-respiration model compared the predicted O(2) profiles in roots with those measured using O(2) microelectrodes. ? Micro-computed tomography confirmed the measured high porosity of aerenchymatous phellem (44-54%) and the low porosity of stele (2-5%) A network of connected gas spaces existed in the phellem, but not within the stele. O(2) partial pressures were high in the phellem, but fell below the detection limit in the thicker upper part of the stele, consistent with the poorly connected low porosity and high respiratory demand. ? The presented model integrates and validates micro-computed tomography with measured radial O(2) profiles for roots with aerenchymatous phellem, confirming the existence of near-anoxic conditions at the centre of the stele in the basal parts of the root, coupled with only hypoxic conditions towards the apex.     

Experimental and modelling data contradict the idea of respiratory down-regulation in plant tissues at an internal [O2] substantially above the critical oxygen pressure for cytochrome oxidase

? Some recent data on O(2) scavenging by root segments showed a two-phase reduction in respiration rate starting at/above 21 kPa O(2) in the respirometer medium. The initial decline was attributed to a down-regulation of respiration, involving enzymes other than cytochrome oxidase, and interpreted as a means of conserving O(2). As this appeared to contradict earlier findings, we sought to clarify the position by mathematical modelling of the respirometer system. ? The Fortran-based model accommodated the multicylindrical diffusive and respiratory characteristics of roots and the kinetics of the scavenging process. Output included moving images and data files of respiratory activity and [O(2)] from root centre to respirometer medium. ? With respiration at any locus following a mitochondrial cytochrome oxidase O(2) dependence curve (the Michaelis-Menten constant K(m) = 0.0108 kPa; critical O(2) pressure, 1-2 kPa), the declining rate of O(2) consumption proved to be biphasic: an initial, long semi-linear part, reflecting the spread of severe hypoxia within the stele, followed by a short curvilinear fall, reflecting its extension through the pericycle and cortex. ? We conclude that the initial respiratory decline in root respiration recently noted in respirometry studies is attributable to the spread of severe hypoxia from the root centre, rather than a conservation of O(2) by controlled down-regulation of respiration based on O(2) sensors.     

Exploring the Radial and Longitudinal Aeration of Primary Maize Roots by Means of Clark-Type Oxygen Microelectrodes

Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distribution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen demand was met internally by transport from the shoots, but a little oxygen was received by radial inward diffusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally significant. In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low (< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very significant resistance to oxygen transport to the root.     

Metabolic evidence for stelar anoxia in maize roots exposed to low o(2) concentrations

This investigation presents metabolic evidence to show that in 4- to 5-day-old roots of maize (Zea mays hybrid GH 5010) exposed to low external O₂ concentrations, the stele receives inadequate O₂ for oxidative phosphorylation, while the cortex continues to respire even when the external solution is at zero O₂ and the roots rely solely on aerenchyma for O₂ transport. Oxygen uptake rates (micromoles per cubic centimeter per hour) declined at higher external O₂ concentrations in excised segments from whole roots than from the isolated cortex; critical O₂ pressures for respiration were greater than 0.26 moles per cubic meter O₂ (aerated solution) for the whole root and only 0.075 moles per cubic meter O₂ for the cortex. For plants with their shoots excised and the cut stem in air, ethanol concentrations (moles per cubic meter) in roots exposed to 0.06 moles per cubic meter O₂ were 3.3 times higher in the stele than in the cortex, whereas this ethanol gradient across the root was not evident in roots exposed to 0 moles per cubic meter O₂. Alanine concentrations (moles per cubic meter) in the stele of roots exposed to 0.13 and 0.09 moles per cubic meter O₂ increased by 26 and 44%, respectively, above the levels found for aerated roots, whereas alanine in the cortex was unchanged; the increase in stelar alanine concentration was not accompanied by changes in the concentration of free amino acids other than alanine. For plants with their shoots intact, alcohol dehydrogenase and pyruvate decarboxylase activities (micromoles per gram protein per minute) in roots exposed to 0.13 moles per cubic meter O₂ increased in the stele by 40 to 50% over the activity in aerated roots, whereas there was no appreciable increase in alcohol dehydrogenase and pyruvate decarboxylase activity in the cortex of these roots. More convincingly, for roots receiving O₂ solely from the shoots via the aerenchyma, pyruvate decarboxylase in the cortex was in an “inactive” state, whereas pyruvate decarboxylase in the stele was in an “active” state. These results suggest that for roots in O₂-free solutions, the aerenchyma provides adequate O₂ for respiration in the cortex but not in the stele, and this was supported by a change in pyruvate decarboxylase in the cortex to an active state when the O₂ supply to the roots via the aerenchyma was blocked.     

Evidence for proteins specific for vascular elements in intact and cultured tissues and cells of maize

A group of antigenically distinct proteins characteristic for the tissue complex of the vascular cylinders was found in maize (Zea mays L.) seedlings using an immunofiltration technique. Specific stelar antigens present in the fully developed stele (vascular cylinder) of the primary root were also found in steles extracted from adventitious roots and from the mesocotyl but were absent, within the limits of sensitivity of the immunodiffusion tests employed, in root cortex and epidermis. Some of the stelar antigens were also evident in the meristem of the primary root and were present in traces in the scutellum, the mesocotyl node, and the primary leaves plus coleoptile. The specific stelar antigens could be traced in 13- and 15-day-old developing embryos and were definitely expressed by the 21 st day after pollination. Several stelar-specific antigens were found in embryo-derived callus tissues and in stem-derived cells maintained in serial suspension culture. Higher resolution of the stelar antigens by a modified technique of crossed immunoelectrophoresis was used to demonstrate several minor stelar antigens that were presumably characteristic exclusively of the completely differentiated stele. This technique along with sequential immunoprecipitation of labelled proteins provided a semiquantitative estimate of the specific stelar antigens in the meristem and the stele of the primary root, and in suspension-cultured cells which were devoid of noticeable signs of vascular differentiation.     

Membrane ATPase Activity of Barley Roots and Potassium Uptake by Isolated Stele and Cortex

Uptake of potassium ions by isolated stelar tissues of barley from 0.5 and 10 mM K⁺ was respectively 13 and 3.6% of that of the cortical tissues. 0.1 mM H₂PO₄, LO mM ATP and 10 mM Ca(NO₃)₂ did not increase the potassium uptake of either stele or cortex during 5 h of uptake period. A time-course incubation for histological demonstration of the ATPase activity of the plasmalemma and tonoplast of the matured sections of the roots demonstrated a greater activity for the cortical than the stelar tissue. In the stelar parenchyma cells, the plasma lemma showed a higher activity than the tonoplast. These results, which support the “leakiness hypothesis” of the stele, are discussed in relation to the proposed mechanisms of radial ion transport in roots.     

Barriers to the radial diffusion of ions in maize roots

Summary The water-extractable and ion-exchangeable fractions of the free space of maize roots for sodium ions has been determined. The free space of whole roots, excised roots and isolated stelar and cortical tissues, has been compared and the results examined for any evidence of a barrier between the cortex and the stele. Similarly the free space of whole roots and excised roots, from which the epidermal and outer cortical cells have been removed by shaving, has been compared and the results examined for any evidence of an epidermal barrier.Whole roots gave a free space value some 20% lower than excised roots. It was calculated that this difference could be accounted for if the cortical tissues only were considered in estimating the whole root value, that is if the stele was considered as participating in the excised root but not in the whole root. Samples in which isolated cortical and stelar material were measured together, or separately and the value calculated, gave similar values to those obtained for excised roots. These results are interpreted as evidence that a barrier to free diffusion exists between the cortical and the stelar tissues at or near the endodermis. Shaving both whole and excised roots increased the free space by about 35%. However, as this value was similar for both, it was concluded that the increase was due to the contribution of damaged cortical cells and does not indicate that the epidermis is an effective barrier to the diffusive entry of sodium ions into the root.     

Studies on fungi in the root region

Summary Fungal isolations were made from roots ofPhaseolus vulgaris after washing in sterile water, at monthly intervals throughout the life of the plant, and from other roots after dissection and after surface sterilization at certain plant ages only. A table is provided showing the relative importance of the most common species isolated in each of four clearly distinct microhabitats — the root surface, the cortex, the outer stele and the inner stele.Fusarium oxysporum andCylindrocarpon radicicola were the most frequently isolated fungi from the roots.Fusarium oxysporum was most abundant on young roots and seemed to be associated, particularly, with the root surface and cortical tissues.Cylindrocarpon radicicola, although common on young roots, was more abundant on older roots and was an important initial colonist of the stelar tissues. Sterile mycelia were isolated mainly from older roots and seemed to be responsible, withC. radicicola, for the initial colonization of the stele. Microscopic examination of roots showed the cortical tissues to be increasingly penetrated by fungal hyphae with plant age but extensive fungal penetration of the endodermis and stelar tissues did not occur until the plants were at least five months old.     

Oxygen uptake by roots of Rumex species at different temperatures: the relative importance of diffusive resistance and enzyme kinetics

Abstract. Oxygen uptake characteristics of the roots of three Rumex species were compared, and related to kinetics of the respiratory system and to root anatomy. The observed differences could not be explained by differences in fundamental characteristics of the oxygen uptake system: with all three species, cytochrome-mediated respiration contributed 70% and cyanide-insensitive (alternative) respiration 30% of the total respiration rate, and apparent K_m values of cytochrome oxidase were lower than those obtained for the alternative oxidase in all cases. However, differences in critical oxygen pressure for respiration (COPR) and in apparent K_m for oxygen, were strongly correlated with differences in root porosity and root diameter. K_m(O₂) values at high and low temperatures were determined, and from Arrhenius plots of oxygen uptake rates between 11 and 32°C, the role of diffusional impedance could be estimated. Root respiration of Rumex maritimus and R. crispus , both with high root porosity, but differing in root diameter, had a low K_m for oxygen (3–7 mmol m⁻³). In contrast with this were the responses of R. thvrsiflorus , which has thin roots but low root porosity: a high K_m (10-20 mmol m⁻³) was found at all temperatures. The role of diffusional impedance as a function of temperature in oxygen uptake rate by the three species is discussed and related to the differential resistance of the species towards flooding.     

落叶松和水曲柳不同根序细根形态结构、组织氮浓度与根呼吸的关系

根系具有高度的形态和生理功能异质性, 在森林生态系统碳和养分循环中起重要作用。根系分枝的顺序构成根序,是根系最基本的构型特征, 根序代表根系不同的发育阶段。然而, 目前直接测定不同根序细根生理功能的研究很少。以落叶松(Larix gmelinii)和水曲柳(Fraxinus mandshurica)的细根为研究对象, 使用气相氧电极测定不同根序细根的呼吸速率, 探讨根系呼吸速率与其形态、结构和组织氮浓度的关系。结果表明: 落叶松和水曲柳细根的直径、根长和维管束直径均随着根序的增加(1–5级)而增加, 而比根长、组织氮浓度和呼吸速率随着根序的增加而降低, 各根序之间差异显著(P < 0.05); 1级根比根长最大、皮层组织发达、组织氮浓度最高且呼吸速率也最高, 其呼吸速率分别为17.57 nmolO₂·g^–1·s^–1(落叶松)和18.80 nmolO₂·g^–1·s^–1(水曲柳), 比5级根分别高148%(落叶松)和124%(水曲柳); 并且, 落叶松根的呼吸速率几乎有96%与根系组织氮浓度相关, 而水曲柳根的呼吸速率则有89%与根系组织氮浓度相关。上述结果说明, 细根的形态和生理功能异质性是紧密相连的, 低级根的形态、结构决定其功能是吸收养分和水, 而高级根的形态、结构决定其功能是运输和贮存养分。     

Localization and identification of auxin in roots of Zea mays

Summary Roots of 3.5-day-old seedlings of Zea mays cv. Giant White Horsetooth contain an extractable auxin which has chromatographic properties and reactions to chromogenic sprays identical with those of indole-3-acetic acid (IAA). By separating stele from cortex (and root tips) before extraction it was shown that the auxin is localized predominantly in the stele, with little being found in the cortex. Whole roots, isolated cortices and isolated steles accumulate and metabolize exogenously applied IAA-1-¹⁴C. The stelar tissue is distinguished from whole roots and cortical tissue in having a different pattern of IAA metabolism.     

Ecophysiology of Wetland Plant Roots: A Modelling Comparison of Aeration in Relation to Species Distribution

This study examined the potential for inter-specific differencesin root aeration to determine wetland plant distribution innature. We compared aeration in species that differ in the typeof sediment and depth of water they colonize. Differences inroot anatomy, structure and physiology were applied to aerationmodels that predicted the maximum possible aerobic lengths anddevelopment of anoxic zones in primary adventitious roots. Differencesin anatomy and metabolism that provided higher axial fluxesof oxygen allowed deeper root growth in species that favourmore reducing sediments and deeper water. Modelling identifiedfactors that affected growth in anoxic soils through their effectson aeration. These included lateral root formation, which occurredat the expense of extension of the primary root because of theadditional respiratory demand they imposed, reducing oxygenfluxes to the tip and stele, and the development of stelar anoxia.However, changes in sediment oxygen demand had little detectableeffect on aeration in the primary roots due to their low wallpermeability and high surface impedance, but appeared to reduceinternal oxygen availability by accelerating loss from laterals.The development of pressurized convective gas flow in shootsand rhizomes was also found to be important in assisting rootaeration, as it maintained higher basal oxygen concentrationsat the rhizome–root junctions in species growing intodeep water. Copyright 2000 Annals of Botany Company Aeration, diffusion, ecophysiology, flooding, model, oxygen, respiration, root, wetland     

An Early Cytochemical Marker of Commitment to Stelar Differentiation in Meristems from Dicotyledonous Plants

A cytochemical study of root apices from Vicia faba and Pisumsativum showed esterase activity to be present in the stele,root cap and rhizodermis, but almost completely absent fromthe developing cortex and quiescent centres. The meristem cellsgiving rise to the cortex were almost negative whilst thosegiving rise to the stele were positive for esterase activity.Cambia from roots, shoots and petioles of a number of dicotyledonousspecies were all positive for esterase activity. It is proposedthat esterase activity may be used as an early marker of commitmentto differentiation into stele in roots of dicotyledonous plants,and that the cambia are fully committed meristems. Pisum sativum L., Vicia fabaL., garden pea, broad bean, meristems, stelar differentiation, esterase activity, xylem differentiation, cytochemistry, cambium     

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.     

Sequential depolarization of root cortical and stelar cells induced by an acute salt shock - implications for Na(+) and K(+) transport into xylem vessels

Early events in NaCl-induced root ion and water transport were investigated in maize (Zea mays L) roots using a range of microelectrode and imaging techniques. Addition of 100 mm NaCl to the bath resulted in an exponential drop in root xylem pressure, rapid depolarization of trans-root potential and a transient drop in xylem K(+) activity (A(K+) ) within ～1 min after stress onset. At this time, no detectable amounts of Na(+) were released into the xylem vessels. The observed drop in A(K+) was unexpected, given the fact that application of the physiologically relevant concentrations of Na(+) to isolated stele has caused rapid plasma membrane depolarization and a subsequent K(+) efflux from the stelar tissues. This controversy was explained by the difference in kinetics of NaCl-induced depolarization between cortical and stelar cells. As root cortical cells are first to be depolarized and lose K(+) to the environment, this is associated with some K(+) shift from the stelar symplast to the cortex, resulting in K(+) being transiently removed from the xylem. Once Na(+) is loaded into the xylem (between 1 and 5 min of root exposure to NaCl), stelar cells become more depolarized, and a gradual recovery in A(K+) occurs.     

Tissue-6

In order to study a possible involvement of cdc-like proteinkinases in cell development and tissue differentiation, a polyclonalantibody raised against the evolutionary conserved PSTAIR-regionof p34^cdc2-homologue protein kinases (PSTAIR-proteins) was appliedto sections of the maize root apices. PSTAIR-proteins were localizedin the nuclei and the cytoplasm of cells in the root meristem,including the quiescent centre (QC), and of all dividing cellsthat form the lateral root primordia. In most root tissues,the amount of cytoplasmic PSTAIR-proteins progressively declinedwith increasing distance from the root cap junction, becomingrestricted to the nucleus after the cessation of cell divisions.This occurred much nearer to the root cap junction in cellsof the stele, especially in metaxylem cells, than in cells ofthe root cortex. Interesting exceptions were cells of the pericycle,endodermis and the outermost cell rows of stelar parenchyma,which exhibited relatively high levels of the cytoplasmic PSTAIR-proteinsthroughout all developmental zones. After root wounding, rapid cytoplasmic accumulation of PSTAIR-proteinsin cells adjacent to the wound was observed in all tissues ofthe meristem and of the elongation zone. This wound response,which was usually followed by newly-induced cell divisions,was delayed with increasing distance from the root cap junctionin a tissue-specific manner. Since PSTAIR-proteins were foundin the cell nuclei throughout all developmental zones, theyseem to have some nuclear functions which continue even aftercell division has stopped. Key words: Cell cycle, maize roots, cyclin-dependent protein kinases, wounding     

植物根系呼吸代谢及影响根系呼吸的环境因子研究进展

根系呼吸是植物通过活根向环境释放CO2的过程。根系的呼吸作用集物质代谢与能量代谢为一体,构成了地下部代谢的中心。根系呼吸进行顺利与否是衡量植物根系功能和逆境胁迫的重要指标之一,相关代谢研究已成为目前植物生理、生化和生态学等领域的热点。该文对植物根系呼吸途径、呼吸代谢关键酶和中间产物、影响根系呼吸代谢的根域环境因子以及研究进展进行了综述,并对其研究前景进行展望。     

Lateral root elongation inVicia faba L.

Summary Mitotic index (MI) was determined in the cap, epidermis, cortex and stele at successive intervals along the apical mm of lateral roots of various specific lengths. This parameter was found to decrease basally along roots of each length examined and to decline as the lateral root elongated. Moreover, MI was different in the various tissues investigated. These results have been discussed with respect to changes in the other parameters of cell proliferation and to root growth by elongation. It is suggested on the basis of the data obtained, and other results in the literature, that epidermal initial cells could be differentiated from those of the cortex and stele before the quiescent centre appeared in these roots, but the initial cells of the latter two tissues were indistinguishable until after the quiescent centre began to form.     

Radial salt transport in corn roots

Primary roots of solution-grown, 5-day-old or 6-day-old seedlings of corn (Zea mays L.) 10 to 14 cm in length were used to study radial salt transport. Measurements were made of the volume of root pressure exudation, salt concentration of the exudate, and rate of salt movement into the xylem exudate. The ³²P uptake, O₂ consumption, and dehydrogenase activity of the root cortex and stele also were studied.

These roots produced copious root pressure exudate containing 4 to 10 times the concentration of ³²P in the external solution. Freshly separated stele from 5-day-old roots accumulated ³²P as rapidly as the cortex from which it was separated and the stele of intact roots also accumulated ³²P. Separated stele has a higher oxygen uptake than cortex. It also shows strong dehydrogenase activity with the tetrazolium test. The high oxygen consumption, ³²P uptake and strong dehydrogenase activity indicate that the cells of the stele probably play a direct role in salt transport.

These data raise doubts concerning theories of radial salt transport into the xylem based on the assumption that the stele is unable to accumulate salt vigorously.