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.     

The effect of CCCP on ion fluxes in the stele and cortex of maize roots

Summary The accumulation of ⁸⁶Rb labelled potassium by isolated stelar and cortical tissues from 7-day-old roots of Zea mays has been compared with the levels accumulated by these tissues in the intact root. Cortical tissues have similar uptake eapacities in these two conditions whereas stelar tissues only exhibit an uptake capacity in the intact root system. The uncoupler carbonylcyanide m-chlorophenylhydrazone caused a considerable decrease in the uptake of potassium by these tissues. In the intact root system it prevented ions from the bathing medium reaching the stelar tissues. The efflux pattern from preloaded isolated stelar and cortical tissues was considerably altered by the inhibitor, a promotion of the efflux occurring in both of these tissues.It is concluded that stelar tissues only accumulated ions when these are supplied through the root symplasm and that the stelar plasmalemma has only a limited uptake capacity per se. Stelar uptake is thus a reflection of vacuolar accumulation across the tonoplast. There is no evidence in the present study of a carrier-mediated active secretion of ions across the stelar plasmalemma. The fact that the efflux was promoted rather than depressed by the uncoupler supports the postulate that a passive leakage is the final stage in the transport of ions across the plant root.     

Observations on the Exchange of Salt Between the Xylem and Neighbouring Cells in Zea mays Primary Roots

Comparison of two preparations of Zea mays primary roots hasshown that the salt concentration in the xylem fluid is alteredas it passes through a region of the root where no absorptionof salts or water from the external bathing medium can occur.Salts are secreted to the xylem by such a region, presumablyfrom the vacuoles of the cortical cells. It has also been demonstratedthat longitudinal water movement through the root cortex isnegligibly small during root-pressure exudation.     

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     

Vegetable Plant Part Relationships. III. Modification of Carrot (Daucus carota L.) Root and Shoot Weights by Gibberellic Acid and Daminozide

Aqueous foliar sprays of N-dimethylaminosuccinamic acid (daminozide)at 2000 p.p.m. and gibberellic acid (GA) at 100 p.p.m. wereapplied 45, 59, 82 and 100 days after sowing to Chantenay carrotswith population densities of 244, 495 and 883 plants m^–2.The plants were harvested on ten approximately weekly occasions;fresh weights were determined and d. wt estimates were obtainedfor the separated shoots (s) and roots (r). Allometric linearregressions of the logarithm of s on that of r at each harvestseparately, clearly showed that GA always increased shoot: rootratio and reduced root yield (by approximately 35 per cent)but could sometimes also increase whole-plant weight. Daminozideincreased root yield (by approximately 7 per cent from 80 tonnesha^–1) and tended to have effects opposite to those ofGA. Daucus carota L., carrot, root weight, shoot weight, N-dimethylaminosuccinamic acid (daminozide), gibberellic acid     

Infection of sugar cane by the nitrogen-fixing bacterium Acetobacter diazotrophicus

Significant nitrogen fixation has recently been demonstratedin Brazilian sugar cane (Saccharum officinarum) cultivars knownto form associations with a number of diazotrophs, includingAcetobacter diazotrophicus, an acid-tolerant endophytic bacteriumwhich grows best on a sucrose-rich medium. In a series of experiments,aseptically-grown sugar cane plantlets were rooted in a liquidmedium and inoculated with A. diazotrophicus originally isolatedfrom field-grown sugar cane. After 4, 7, 9, and 15 d, plantswere examined under light, scanning and transmission electronmicroscopes and the presence of A. diazotrophicus on and withinplant tissues was confirmed by immunogold labelling. By 15 d,external bacterial colonization was seen on roots and lowerstems, particularly at cavities in lateral root junctions. Theloose cells of the root cap at root tips were a site of entryof the bacteria into root tissues. Both at lateral root junctionsand root tips, bacteria were also seen in enlarged, apparentlyintact, epidermal cells. After 15 d, bacteria were present inxylem vessels at the base of the stem, many connected via mucusto spiral secondary thickening. There was no obvious pathogenicreaction to the bacteria within the xylem. From these observations,it is proposed that, under experimental conditions, A. diazotrophicusfirstly colonized the root and lower stem epidermal surfacesand then used root tips and lateral root junctions to enterthe sugar cane plant where it was distributed around the plantin the transpiration stream. It is further suggested that thexylem vessels in the dense shoots of mature plants are alsoa possible site of N₂-fixation by diazotrophs as they providethe low pO₂ and energy as sucrose necessary for nitrogenaseactivity. Key words: Acetobacter diazotrophicus, endophyte, infection, nitrogen fixation, sugar cane.     

Osmocytosis and Vacuolar Fragmentation in Guard Cell Protoplasts: Their Relevance to Osmotically-lnduced Volume Changes in Guard Cells

Guard cell protoplasts were prepared from young leaves of peaplants. Under hypertonic conditions they shrink and large numbersof endocytotic (‘osmocytotic’) vacuoles are formedby invagination of the plasma membrane. In thin section theseare indistinguishable from other small vacuoles (‘mini-vacuoles’)which are formed by fragmentation of the large central vacuole.However, the two types of vacuole can be individually recognizedby labelling the central vacuole with neutral red and by performingthe osmotic shrinkage with fluorochromes such as Lucifer Yellow-CHor Cascade Blue present in the extracellular medium. Osmocytoticvacuoles do not fuse with the plasma membrane nor with the mini-vacuolesduring a subsequent swelling phase. After several hours, osmocytosedLucifer Yellow gradually leaks out of the endocytotic vacuoleswhen protoplasts are returned to hypotonic conditions. Thisleakage is not prevented by probenecid at concentrations (20–50mmol m^–3) which do not give rise to pathological changesin protoplast ultrastructure. In order to determine the relevanceof these observations to the situation in planta, intact guardcells in epidermal strips were first allowed to accumulate neutralred in their vacuoles and then subjected to osmotic shrinkagein the presence of external Lucifer Yellow. Osmocytotic vacuoleswere not formed, although the production of mini-vacuoles wasfrequently observed. Key words: Guard cell protoplasts, fluid phase markers, Pisum sativum, probenecid, osmocytosis, shrinkage-swelling cycles     

Studies in the Genus Elaphoglossum Schott: II. The Root and Bud Traces

Further investigations upon some Jamaican species of Elaphoglossumhave shown that, parallel to a series of stelar forms demonstratedearlier, there is another series showing a progressive reductionof the lateral bud trace. The anatomical differences betweenthe root and bud traces are indicated and the development ofthe stele in a young outgrown bud is described.     

Permeation of Ca2+ and monovalent cations through an outwardly rectifying channel in maize root stelar cells

A depolarization-activated outwardly-rectifying channel (OR),most likely involved in the passive release of K⁺ from the rootsymplasm into the stelar apoplast (for subsequent transportto the shoot via the xylem vessels), has been characterizedin the plasma membrane of maize root stelar cells (Roberts andTester, 1995). In the present study, the selectivity of thischannel was further characterized using single channel current-voltagecurves generated using a voltage ramp protocol. This protocolpermitted the accurate and unambiguous measurement of the reversalpotentials of currents resulting from single channel openings.Using the voltage ramp protocol, it was shown that the OR allowsboth K⁺ efflux and Ca²⁺ influx at potentials positive of E_Kand negative of E_Ca. The OR had a P_Ca/P_K of 1.72–0.21decreasing as extracellular Ca²⁺ was increased. The permeabilityof the OR for monovalent cations other than K⁺ was also investigated.In biionic conditions, a relative permeability sequence of was determined (i.e. Eisenman sequenceIV). The physiological implications of the selectivity of theOR are discussed. Key words: Maize roots, K⁺ channel selectivity, Ca²⁺ permeation     

Observations on Gas Movement in the Rhizome of Menyanthes trifoliata L., with Comments on the Role of the Endodermis

Patterns of diffusion of oxygen in the rhizome of the aquaticMenyanthes have been investigated after the interruption ofeither stelar or cortical diffusion path-ways. Forty-eight hoursafter ringing, oxygen levels in cortex, stele, and root weresignificantly depressed, although never to danger point. Interruptionof the stele, involving also partial interruption of the cortex,was only significantly effective when made just above the samplingzone. The addition of M/1000 KCN to depress the metabolic uptakeof oxygen, resulted as expected in higher equilibrium levelsof oxygen in all zones. It was shown (a) that the velocity of diffusion of oxygen acrossthe endodermis essentially conformed to that of a process ofaqucous diffusion and (b) that oxygen diffused through the corticalair-space system at about 1/25 of its diffusion velocity inair. The diminished diffusion rate in the cortex is doubtlessdue to the frictional resistance imposed by the small poreswhich connect adjacent air cavities. An analysis has been made of the pertinent anatomical featuresof this rhizome, and the findings as a whole are discussed inrelation to their functional and ecological significance.     

Characterization of Cd translocation and identification of the Cd form in xylem sap of the Cd-hyperaccumulator Arabidopsis halleri

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanismsinvolved in the root to shoot translocation of Cd are not wellunderstood. In this study, we characterized Cd transfer fromthe root medium to xylem in this species. Arabidopsis halleriaccumulated 1,500 mg kg^–1 Cd in the shoot without growthinhibition. A time-course experiment showed that the releaseof Cd into the xylem was very rapid; by 2 h exposure to Cd,Cd concentration in the xylem sap was 5-fold higher than thatin the external solution. The concentration of Cd in the xylemsap increased linearly with increasing Cd concentration in theexternal solution. Cd transfer to the xylem was completely inhibitedby the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone(CCCP). Cd concentration in the xylem sap was decreased by increasingthe concentration of external Zn, but enhanced by Fe deficiencytreatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR)showed that the chemical shift of ¹¹³Cd in the xylem sap wasthe same as that of Cd(NO₃)₂. Metal speciation with Geochem-PCalso showed that Cd occurred mainly in the free ionic form inthe xylem sap. These results suggest that Cd transfer from theroot medium to the xylem in A. halleri is an energy-dependentprocess that is partly shared with Zn and/or Fe transport. Furthermore,Cd is translocated from roots to shoots in inorganic forms.     

Distribution of Lateral Root Primordia in Root Tips of Musa

The distribution of lateral root primordia in the root tipsof four Musa landraces (Grande Naine, Pisang Berlin, Ngok Egomeand Yangambi Km5) grown in the field has been investigated toevaluate the range of genetic variation of lateral root initiation.In banana (Musa sp.), lateral roots are initiated in the roottip, 0.6–4 mm behind the root/cap junction and arise inseveral protoxylem-based longitudinal rows or ‘ranks’.Significant differences were observed among landraces for theposition of the most distal primordium, however the longitudinalspacing between successive primordia along the ranks was similarfor all landraces. All ranks were involved in lateral root initiation.The number of ranks also showed significant variations amonglandraces and was proportional to the stelar diameter. Hencethe density of lateral roots (roots cm^-1) was affected by stelardiameter variations. Finally, root elongation in the root tipwas landrace-specific and not necessarily exponential, unlikesuggested in previous studies. It is concluded that lateralroot initiation in Musa is not involved in the genetic variationsof root architecture in the field. A dissection of root architectureinto components which may account for these variations is proposedin relation to the improvement of root system architecture.Copyright 1999 Annals of Botany Company Lateral root initiation, root architecture, Musa, banana.     

A Biochemical and Cytochemical Study of Peroxidase Activity in Roots of Pisum sativum: II. DISTRIBUTION OF ENZYMES IN RELATION TO ROOT DEVELOPMENT

Changes in the specific activity and multiple forms of peroxidasealong the differentiating root of Pisum sativum have been examined.The specific activity of the total homogenate increased steadilyalong the root, although that of the low speed supernatant fractionshowed a fall in the first 20 mm. This finding supports previouscytochemical studies which showed low activity in the meristemand increased cell wall activity in the older regions. Acidphosphatase also increased gradually along the root, althoughphenylalanine ammonia lyase (PAL) showed a peak in the 20–40mm region which corresponded with the first positive stainingfor lignin. When stelar and cortical tissues were compared,peroxidase specific activity was highest in a cortical supernatantfraction while PAL was highest in the stelar tissues. The separationof peroxidase by gel electrophoresis showed the presence ofa number of bands'which varied both along the root and betweenstele and cortex. The results are discussed in relation to thepossible functions of peroxidase and, in particular, to itsproposed role in lignification.     

Chlorolemma-A Continuous Membrane Layer between the Plasmalemma and the Streaming Cytoplasm in Nitellopsis obtusa and Nitella translucens, which takes part in the Generation of the Resting Potential, in its Light-Induced Changes and in the Generation of the Action Potential

At slow stepwise insertion of the glass micro-electrode intothe cell of Nitellopsis obtusa andNitella translucens four potentialjumps were recorded which corresponded to the penetration ofthe micro-electrode across the cell wall, the plasmalemma, thesupposed chlorolemma and the tonoplast, respectively. The photo-electricdepolarization appeared after the third jump only, which wassupposed to be the penetration of a membrane layer between theplasmalemma and the streaming part of the cytoplasm. The vibrationof this chlorolemma, caused probably by the protoplasmic streaming,resulted in large (up to 100 mV) hyperpolarizing and depolarizingpotential fluctuations when the micro-electrode tip was nearto this membrane. During cell excitation the potential difference(p.d.) had changed also across the chlorolemma- It was shownthat the potential changes across the chlorolemma, caused bylocal illumination on the isolated part of the cell, were measurablealso at a considerable distance (more than 1.4 cm) from theilluminated and isolated sequence. The light- and electron microphotographsobtained showed that the chloroplasts might really form a verycompact layer with tight connections between the neighbouringchloroplasts and that there existed at least one continuousmembrane layer which separated the immobile chloroplasts fromthe streaming cytoplasm. It was concluded that the p.d. measuredbetween the streaming cytoplasm and the external medium wasthe sum of the p.d.s across the cell wall, the plasmalemma andthe chlorolemma, respectively. Key words: Resting potential, Photo-electric responses, Characean algae, Chlorolemma     

Nutrient-Limited Growth Rates: Roles of Nutrient-Use Efficiency and of Adaptations to Increase Uptake Rate

When stressed by low nutrient availability, young sunflowerplants (Helianthus annuus) showed responses seen in many otherspecies: increases in root uptake capacity (V^max, l/K_m), root:shoot ratio, and putative nutrient-use efficiency, nUE=l/(tissuenutrient content). A straightforward mechanistic model is derivedfor relative growth rate (RGR) in solution culture in termsof these factors. A linear regression based on the model indicatesa negative role for nUE, which violates a premise of the model.A revised model proposes that primary adaptations are only inuptake rate and growth or nutrient allocations, and these actthrough the photosynthetic utility of nutrient. The tissue nutrientcontent and associated nUE become dependent quantities. Thepredictions for RGR, as tested by linear regression, are improved.The model predicts that nUE can increase as external solutionconcentration decreases, but decreases with increased uptakeadaptations in one given environment. The decrease in nUE compromisespotential gains in RGR from uptake adaptations, and makes increasesin root: shoot ratio a nearly insignificant contributor to earlyRGR. The model and associated regression analyses are generalizedfor additional adaptations such as increased root fineness andfor different quantitative ways that a nutrient may limit photosynthesis.The model and analyses are also generalized to plant growthin soil and growth without functional balance between root andshoot. Key words: Relative growth rate, Helianthus annuus, nutrient stress, nutrient use efficiency, functional balance     

An Electrophysiological Study of the Stomatal Complex of Tradescantia virginiana

Measurements of electrical potential difference (PD) and potassiumactivity were carried out on the intact leaf of Tradescantiavirginiana. PD gradients across the stomatal complex were observedwith both open and closed stomata. The guard cell PD appearedto be linearly related to stomatal aperture. With the stomataopen a gradient of potassium activity across the stomatal complexwas observed which became reversed on stomatal closure. Calculationof the driving forces on potassium suggested that it was distributedpassively between the vacuoles of the cells of the stomatalcomplex. The electrophysiological data obtained from this investigationenabled potassium activity in the apoplast to be calculated.The results showed that on stomatal closure there was a massiveincrease in the potassium activity in the guard cell wall. Key words: Stomata, Ionic gradients, Electrical potentials     

An electrophysiological study of the membrane in aplanospore-like cell of Boergesenia forbesii

Membrane potential and resistance, each of which was the sumof those of the plasmalemma and tonoplast, measured in the coenocyticthallus of Boergesenia forbesii were 6.7 mv inside positiveand 2.8 k.cm², respectively. Protoplasm squeezed from the thallus into artificial sea water(ASW) formed numerous spherical bodies, which are termed aplanospore-likecells (simply "spores"). The following electrical propertiesof the "spores" 20–40 hr after squeezing were obtained:potential difference (p.d.) across plasmalemma (E_co) was –66mv (– means inside negative), plasmalemma resistance 665cm², p.d. across the tonoplast (E_vc) +73 mv, and tonoplast resistance2.6 k.cm². Tenfold increase in external [K⁺] caused +45 mv changein E_co and +17 mv in E_vc. The plasmalemma was entirely depolarizedin Ca⁺⁺-free ASW or ASW containing Triton X-100. When the "spore" was immersed in potassium-rich (277 mil) ASW,E_co was almost zero and the tonoplast showed two states (I andII, E_ve about +70 mv and +20 mv, respectively). E_vc went backand forth between the two states spontaneously or when a smallcurrent was applied. In most cases oscillatory changes in E^vcoccurred after the lapse of a long time in the K+-rich sea water.Membrane resistances in states I and II were 5 and 9 k.cm²,respectively. (Received July 11, 1977; )     

AtHKT1;1 mediates nernstian sodium channel transport properties in Arabidopsis root stelar cells

The Arabidopsis AtHKT1;1 protein was identified as a sodium (Na⁺) transporter by heterologous expression in Xenopus laevis oocytes and Saccharomyces cerevisiae. However, direct comparative in vivo electrophysiological analyses of a plant HKT transporter in wild-type and hkt loss-of-function mutants has not yet been reported and it has been recently argued that heterologous expression systems may alter properties of plant transporters, including HKT transporters. In this report, we analyze several key functions of AtHKT1;1-mediated ion currents in their native root stelar cells, including Na⁺ and K⁺ conductances, AtHKT1;1-mediated outward currents, and shifts in reversal potentials in the presence of defined intracellular and extracellular salt concentrations. Enhancer trap Arabidopsis plants with GFP-labeled root stelar cells were used to investigate AtHKT1;1-dependent ion transport properties using patch clamp electrophysiology in wild-type and athkt1;1 mutant plants. AtHKT1;1-dependent currents were carried by sodium ions and these currents were not observed in athkt1;1 mutant stelar cells. However, K⁺ currents in wild-type and athkt1;1 root stelar cell protoplasts were indistinguishable correlating with the Na⁺ over K⁺ selectivity of AtHKT1;1-mediated transport. Moreover, AtHKT1;1-mediated currents did not show a strong voltage dependence in vivo. Unexpectedly, removal of extracellular Na⁺ caused a reduction in AtHKT1;1-mediated outward currents in Columbia root stelar cells and Xenopus oocytes, indicating a role for external Na⁺ in regulation of AtHKT1;1 activity. Shifting the NaCl gradient in root stelar cells showed a Nernstian shift in the reversal potential providing biophysical evidence for the model that AtHKT1;1 mediates passive Na⁺ channel transport properties.     

Uptake of Neutral Red by the Vacuoles of a Green Alga, Micrasterias pinnatifida

Neutral red (NR) in the culture medium entered the vacuolesof a green alga, Micrasterias pinnatifida, at a higher rateat pH 8 than at pH 5. NR remained soluble in vacuoles of cellscultured at pH 5, while it precipitated and formed granulesin cells cultured at pH 8. The vacuoles of cells cultured atpH 8 contained fibrils, but those of cells cultured at pH 5did not. The amount of NR that entered the cells was markedlyreduced by the addition to the medium of nigericin at 10^-5M,monensin at 10^-5M, bafilo-mycin A₁ at 10^-5M, or ammonium chlorideat 50 mM. The formation of NR granules in vacuoles were stronglyinhibited and the disorganization of NR granules were acceleratedby the addition of nigericin at 10^-5M, or bafilomycin A₁ at10^-5M to the culture medium. The possibility is discussed thatNR which enters vacuoles might become positively charged (NRH⁺)by protons brought into vacuoles by proton pumps and that NRH⁺might combine with some negatively charged macromolecules toform aggregates or granules. (Received April 18, 1996; Accepted May 27, 1996)