Cell Expansion during the Elongation of Lateral Roots of Vicia faba L.

Cell width, breadth, length, cross-sectional area, volume andthe ratio of the volume of the nucleus to that of the cell havebeen determined in the epidermis, cortex and stele over theapical mm of lateral roots of Vicia faba as they elongated fromjust-emerged to 4 cm in length. Cell volume increased basallyalong the root in each tissue, but this was not a result ofcell expansion taking place in all dimensions in the epidermis,cortex and stele. Thus, while increase in cell length, was themajor factor involved in cell volume increase basally alongthe root in the stele, the corresponding dimensions in the epidermisand cortex were cell width and cell breadth respectively. Cellvolume was greater in the cortex than in the other tissues andusually greater in the epidermis than in the stele. Cell lengthwas greater in the stele than in the other tissues, while cellbreadth was maximal in the cortex and cell width in the epidermis.Changes took place in the various measurements made as the rootselongated. These are discussed with respect to the onset oflateral root growth and to changes in the rate of growth ofthese roots as they elongate.     

Lipid and Sterol Composition of Plasma Membranes Isolated from Stele and Cortex of Maize Roots

The phospholipid and sterol contents of plasma membranes isolatedfrom stele and cortex of maize roots were compared. The majorsterol present in both tissues was stigmasterol which was foundin significantly higher quantities in the cortex (27·4µg mg^–1 membrane protein) compared to the stele(17·4 µg mg^–1). Other sterols detected includedsitosterol, campesterol and small quantities of cholesterol.The major phospholipids found were phosphatidylethanolamine(PE) and phosphatidylcholine (PC), with PC more abundant inthe stele. The fatty acid composition indicated that the majorfatty acid in both stele and cortex was 16:0 (palmitic), withothers found in lesser amounts. Key words: Zea mays, cortex, phospholipids, plasma membrane, stele, sterols     

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     

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.

     

The Difference Between Open and Closed Meristems

An open and a closed root meristem have been compared by investigatingthe cell kinetics of small regions of the apices of Helianthusand Zea. The cells of the stelar pole are quiescent in both and thereis no exchange of cells between stele and cortex or stele andcap. The immediately distal cells in the closed meristem (Zea)are also quiescent and the few divisions that do occur can betransverse or longitudinal. In the open meristem (Helianthus)these cells are not quiescent, but they go out of cycle transiently,prolonging the potential cell-doubling time. Their divisionsare transverse. It is a consequence of these differences thatclosed meristems form root caps discrete from the cortex whereasopen meristems force instability in the boundary between theperipheral part of the cap and the cortex. Another consequencein roots with open meristems is a succession of columella complexestransversely displaced from each other by the state of fluxin the meristem during the non-cycling phase of the proximaltier of cells, those immediately distal to the stelar pole. The results are discussed in relation to the ontogenetic onsetof quiescence and the evidence for switches between open andclosed operation of meristems. meristem, root apex, Helianthus annuus, Zea mays L.     

Root Cell Ultrastructure in Developing Aerenchyma Tissue of Three Wetland Species

Patterns of root cortex cell development and ultrastructurewere analysed in Sagittaria lancifolia L., Thalia geniculataL. and Pontederia cordata L. using scanning and transmissionelectron microscopy (SEM, TEM). In all three species, cortexcells were arranged in radial columns extending from the endodermisto the hypodermis/epidermis. During gas space formation, thecortex cells elongated parallel to the root radius and shrankin the plane perpendicular to the radius leaving long and thinrows of cortex cells extending from the endodermis to the epidermis.Although the cortex cells appeared collapsed in tissue withwell-developed gas spaces, TEM revealed that the cortical cellsas well as the epidermal cells maintained intact membranes andmany normal organelles. Formation of root cortex tissue withwell-developed gas spaces does not require cell death in thesespecies. Living cortex cells in root tissue with mature gasspaces could provide a symplastic pathway for transport betweenthe root stele and the living epidermal cells. Copyright 2000Annals of Botany Company Sagittaria lancifolia, Thalia geniculata, Pontederia cordata, aerenchyma, root, wetland, development     

A Comparison Between the ATPase and Proton Pumping Activities of Plasma Membranes Isolated from the Stele and Cortex of Zea mays Roots

Plasma membrane vesicles of high purity, determined by markerenzyme assays, were obtained by phase partitioning microsomalfractions from stelar and cortical tissues of Zea mays (cv.LG11) roots. ATP hydrolytic activities in both of the plasmamembrane fractions were inhibited by vanadate, SW26 and erythrosinB, but were insensitive to nitrate. Activity in both fractionsexhibited a marked pH optimum of 6·5 and displayed typicalMichaelis-Menten kinetics. A high substrate specificity wasapparent in both the stele and cortex plasma membrane fractions,while the lower fractions, after phase partitioning, showedlower specificity for nucleotide substrates. Specific activitiesof the stele (67·8 µmol Pi mg^–1 h^–1)and cortex (78·4 µmol Pi mg^–1 h–¹)plasma membrane H⁺ -ATPases were very similar. Proton pumping activities in microsomal membrane fractions fromstele and cortex were inhibited by nitrate and insensitive tovanadate. Homogenization of stele and cortex tissue in the presenceof 250 mol m^–3 KI resulted in microsomal fractions exhibitingvanadate-sensitive, nitrate-insensitive proton pumping activity,suggesting a plasma membrane origin for this activity. SW26was also an effective inhibitor of proton pumping activity,although results indicated an interaction between SW26 and thefluorescent probes quinacrine and acridine orange. The results are discussed in relation to models for the transportof ions into the stele and are consistent with a role for theH⁺ -ATPase activity in this process. Key words: ATPase, cortex, plasma membrane, stele, Zea mays     

Ion Distribution Across the Cortex of Soybean Nodules: Possible Involvement in Control of Oxygen Diffusion

X-ray microanalysis was used to study ion distribution in nodulesof soybean [Glycine max (L.)] cv. Clarke formed with two strainsof Bradyrhizobium japonicum ; RCR3407 and RCR3442. Previousstudies have shown that the oxygen diffusion barrier in nodulesof the RCR3442 symbiosis has slower responses to acetylene andincreased external oxygen than those of the RCR3407 symbiosisand this variation is correlated with differences in glycoproteincontent. X-ray maps and cross-cortical cell counts of nodulesformed by either strain show very similar zonal distributionsof Mg, K, S and Ca across the cortex. Levels of K appear tobe similar but levels of Mg, S and Ca seem to be lower in nodulesof the RCR3442 symbiosis. These results suggest that the contentand distribution of Mg, S and Ca reflect an involvement in theoperation of the cortical diffusion barrier.Copyright 1994,1999 Academic Press Glycine max, X-ray microanalysis, nodules, oxygen diffusion, ion distribution, Mg, Ca, K, S     

A New Procedure for the Calculation of Oxygen Diffusion Resistance in Legume Nodules from Flow-through Gas Analysis Data

Plants of white lupin (Lupinus albus L cv Multolupa) and soyabean(Glycine max L cv Clarke) were grown in controlled-environmentcabinets, subjected to various stresses and their nodular nitrogenaseactivity and total root respiration measured When these measurementswere used to calculate nodular oxygen diffusion resistance,using a simplified equation for Fick's first law of diffusion,it was found that the apparent resistance of stressed nodulesincreased anomalously with decreases in external oxygen concentrationA new analysis procedure is proposed to alleviate this anomalyThis procedure also uses the simplified Fick's law equationbut includes a respiratory contribution to the total oxygenflux across the diffusion barrier which is not coupled to nitrogenaseactivity Also, resistance is modelled as an exponential functionof external oxygen concentration Use of this analysis procedureproduces realistic values for total resistance and providesa characterisation of this resistance into a minimum value andan adjustment factor for changes in external oxygen It is postulatedthat the additional respiration component represents the activityof nodule cortex cells involved in the diffusion barrier, particularlythat of vascular bundles Oxygen diffusion resistance, nodule, nitrogen fixation, respiration     

Radial transport of ions in roots

Measurements were made of the relative amounts of ⁸⁶Rb, ³⁶Cl, and ³²P accumulated in the cortex and stele of intact roots of corn (Zea mays), either detached or attached to their shoots. Both 4- and 7-day-old roots accumulated as much or more ⁸⁶Rb in the stele as in the cortex. In experiments with ³⁶Cl, cortex and stele accumulated the same amount, except for 4-day-old and 7-day-old attached roots, in which the cortex contained more ³⁶Cl than the stele after 23 hr. An additional study of ³²P uptake showed greater accumulation in the cortex than the stele for a short period of time, but as much in the stele as in the cortex after 8 to 24 hr. Transport of ⁸⁶Rb, ³⁶Cl, and ³²P into the xylem exudate increased with increasing accumulation of these ions in stele and cortex of the root. These experiments show no consistent difference between cortex and stele of intact corn roots with respect to their ability to accumulate several kinds of ions.     

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.     

X-Ray Micro-Analysis of Cells and Cell Compartments of Atripiex spongiosa: II. ROOTS

Atripiex spongiosa was grown in hydroponic culture with additionof 0, 50, 200, 400 and 600 mM NaCl. Frozen, hydrated cells ofthe roots were analysed by X-ray micro-analysis. Comparisonswere made of meristematic cells at the root apex and vacuolatedcells 5.0 mm from the apex. High selectivitiy for K relativeto Na was found for the cytoplasm of meristematic cells andthere was little effect of increasing salinity on the ratiosNa/K, Cl/K, Na/P and CI/P. In the cell vacuoles of the cortex,selective uptake of K relative to Na also occurred, but to alesser extent than in the meristematic cells. Gradients werefound of decreasing ratio of Na/K from the epidermis to thestele. Measurements of chemical content of the roots and shoots ofthese plants showed that the ratio of Na/K was higher in theshoot than in the cortical cell vacuoles and higher again thanin the stele or meristematic cytoplasm. It is suggested thattransport of ions to the shoot of Atripiex spongiosa involvesselective exclusion of Na from the xylem parenchyma into thexylem, and that this may be general to other halophytes. Key words: Micro-analysis, X-ray, Cells, Atripiex spongiosa     

Oxygen Diffusion Pathways and Nitrogen Fixation in Legume Root Nodules

The theoretical analysis presented in this paper suggests thatthe nature of the diffusion pathway from the surface of a noduleto the infected zone depends on the morphology of the nodule;in particular the cross-sectional area of the intercellularspaces in the inner cortex. If the diffusion barrier containscontinuous pathways there will be no pressure difference betweenthe atmosphere and the infected zone. The conditions under whichthe intercellular spaces of the inner cortex could be air-filled,water-filled, or a combination of both are explored. An experimentto resolve this issue is suggested. Information obtained usingcryo-scanning electron microscopy and oxygen electrode datahave been used to illustrate various points. Expressions arederived for the diffusion resistance of the nodule and its componentparts. To provoke further discussion a simple mechanism forthe control of diffusion is described in general terms. Oxygen, diffusion resistance, nodule, nitrogen fixation, soybean, Glycine max. (L.) Merr. cv. Fiskeby V, Trifolium repens L. cv. Blanca, lucerne, Medicago saliva L. cv. Europe     

Observations on the Gaseous Exchanges which take place between Menyanthes trifoliata L. and its Environment: I. THE COMPOSITION OF THE INTERNAL GAS OF THE PLANT

1. As part of a general investigation into the exchange of gasesbetween the bogbean, Menyanthes trifoliata, and its environment,determinations have been made of the composition of the internalatmosphere of the plant at five different levels, viz. leafyshoot, three successive portions of stem, and roots.
2. Procedureis described for the analysis of small gas samplesusing a modificationof the Bonnier and Mangin apparatus.
3. In darkness with thelower part of the plant surrounded by anatmosphere of nitrogen,it has not been possible to show thepresence of a regular downwardsgradient of oxygen, such aswould be expected on a basis ofnormal gaseous diffusion.
4. Under these Air Top/Nitrogen Bottomconditions the roots areshown to contain between 12?5 and 17?5percent, of oxygen intheir intercellular gas system. The portionof stem immediatelyadjacent to the roots showed an oxygen levelof from 14?0 to18?0 per cent.
5. In plants kept under NitrogenTop/Air Bottom conditions in thedark for 48 hours, the concentrationof oxygen in the rootsfell to a level of 3?1 per cent, or belowin three plants andto 6?o per cent, in a fourth. The oxygenconcentration in theadjacent lowermost part of the stem wasas high as 18?0 percent., depending on the degree of aerationof the solution bathingthe stem. It is suggested that thisdistribution of oxygen canbe related to the relative impermeabilityof roots and of stemendodermis to gaseous diffusion from without,and it is thoughtthat the major part of the oxygen supply tothe roots is transportedto them through the stelar air passages.
6. Some evidence is presented to show that oxygen passes fromthelower parts of the plant into the medium surrounding it.

     

Localization of Thermo-Osmotically Active Partitions in Young Leaves of Nuphar lutea

The submerged roots and rhizomes of the aquatic vascular macrophyteNuphar lutea (L.) Sm. are aerated, at least in part, by pressurizedventilation. Depending on temperature differences of up to 5K between the inside of young, just-emerged leaves and the surroundingair, pressure differences of 79 to 100 Pa higher than atmosphericare detectable inside the lacunuous spongy parenchyma of theleaf blades. The pressurization is a consequence of structuralfeatures of leaf tissues separating the air filled spaces ofthe spongy parenchyma from the atmosphere. These tissues areacting as thermo-osmotic partitions. Whereas the dimensionsof the stomatal openings (about 5·6 x 2·4 µm)and of the intercellular spaces of the palisade parenchyma (diametersabout 15 µm) are too large, those of the monolayers ofcells separating the palisade and the spongy parenchyma (diameters:0·7–1·2 µm) are small enough to impedefree gaseous diffusion. This inner non-homogeneous partitioninggives rise to the so-called Knudsen diffusion, a physical phenomenonleading to pressurization of the warmer air inside the spongyparenchyma. The rising pressure difference is strong enoughto establish an air flow through the aerenchyma of the wholeplant and out of the most porous older leaves in which a temperatureinduced pressurization is never detectable. These thermo-osmoticallyactive leaves enhance the influx of air to the rhizome and thediffusion path for oxygen to the roots is shortened to the distancebetween rhizome and root tips. Therefore, pressurized ventilationin Nuphar is seen to be of considerable ecological importancefor plant life in anaerobic environments. Key words: Aeration, leaf anatomy, thermo-osmosis of gases, Nuphar lutea     

Characterization of the Resistance to Oxygen Diffusion in Legume Nodules

A method for characterizing the resistance to oxygen diffusionin legume nodules has been developed. This is based on the assumptionsthat diffusion can be described using a simple resistance analogueand that the respiratory response of the bacteriod-containingcells to external oxygen concentrations can be analysed as adiffusion-limited process. Applying this analysis to experimentaldata from infact white clover plants allowed the total diffusionresistance to be separated into (a) a minimum resistance and(b) the extent to which this resistance can be increased. Whenthe carbohydrate status of the nodules was reduced by dark treatments,the minimum diffusion resistance increased, and after 24–28h darkness equalled the maximum resistance. At the same timethe ability to control this resistance was lost. White clover, nitrogen fixation, oxygen diffusion, nodule respiration     

Effects of Acetylene and External Oxygen Concentration on the Respiratory Quotient (RQ) of Nodulated Roots of Soyabean and White Clover

Carbon dioxide output and oxygen uptake by intact nodulatedroots of soyabean and white clover were determined over a rangeof external oxygen concentrations (10 to 60%) in the presenceand absence of 10% acetylene. The responses of both sets ofroot systems were in agreement with the known characteristicsof the variable oxygen diffusion resistance of the nodules.These measurements were used to calculate the respiratory quotient(RQ, CO2/O2) for both total root respiration and nitrogenase-linkedrespiration values for each external oxygen concentration. Inthe absence of acetylene, the RQ values of soyabean total rootrespiration showed no significant response to external oxygen,with a mean of 1.06, but differences in values for white cloverroots were significant at P=0.05, with a mean of 1.09. The sametrend was shown for RQ values of nitrogenase-linked respiration,with mean values of 1.10 and 1.12 for soyabean and clover, respectively.The addition of acetylene caused a small but significant initialreduction in RQ of total root respiration with both soyabeanand white clover, as did the subsequent reduction of externaloxygen to 10%. For white clover, increases in external oxygenconcentration to 50% and 60% also produced significant increasesin the RQ of total root respiration compared to that in air.These variations were more pronounced for RQ values of nitrogenaselinkedrespiration. Results are discussed in relation to the use ofRQ values of unity in calculations of nodular oxygen diffusionresistance and the involvement of physical and biochemical processesin the regulation of oxygen supply to bacteroids. Key words: Respiratory quotient, nodules, oxygen, diffusion resistance     

Observations on the Gaseous Exchanges which take place between Menyanthes Trifotiata L. and its Environment

The exchange of gases between whole plants of Menyanthes andtheir environment has been investigated. When a 45 cm. rhizomeis surrounded by an almost oxygen-free environment, leavingthe leafy shoot exposed to air, some oxygen passes right throughthe rhizome, and the Q_O2 of the plant is significantly reduced. Analyses of gases from the air-space system show that oxygenconcentrations fall and CO concentrations rise when plants aretransferred to, and maintained under conditions like these.Oxygen concentrations show a linear gradient from top to bottomof the plant, whilst the reverse is true for CO₂. The main barrier to gas movement is shown to lie within theair-space system of the rhizome, and the number of leaves presenton the plant has little effect upon the oxygen content of theinternal atmosphere. It is shown that as the concentration of CO₂ increases in theair space system of a rhizome, so the consumption of oxygenfalls off, thus allowing more oxygen to pass through the rhizometo more distal regions. Variation in the seasonal activity of the plant is discussedin relation to these find ings and to reported studies on therespiration of rhizome-tissue slices. Deter minations of respirationon tissues such as these are held to be less meaningful if carriedout according to the ‘direct’ method. The biological significance of these features is discussed.     

Gas Transport in the Marsh Grass, Spartina alterniflora

Spartina plants have continuous gas spaces from the leaves tothe tips of the roots. Oxygen values in the roots are as lowas 3 per cent and increase toward the stem. Carbon dioxide valuesare highest in the rhizome and decrease up the stem and towardthe root tips. Oxygen and carbon dioxide moved through the plantsat equal rates for equal gradients, and these rates agree withmeasurements made on plants in the marsh. Calculated oxygenand carbon dioxide fluxes for the observed gradients in theobserved gas spaces agreed with measured fluxes. We concludethat gases move in and out of Spartina roots by diffusion throughuninterrupted gas spaces within the plant.