Ascorbate enhancement of H1 histamine receptor sensitivity coincides with ascorbate oxidation inhibition by histamine receptors

Ascorbate has previously been shown to enhance both ₁- and ₂-adrenergic activity. This activity is mediated by ascorbate binding to the extracellular domain of the adrenergic receptor, which also decreases the oxidation rate of ascorbate. H1 histamine receptors have extracellular agonist or ascorbate binding sites with strong similarities to _1- and ₂-adrenergic receptors. Physiological concentrations of ascorbate (50 µM) significantly enhanced histamine contractions of rabbit aorta on the lower half of the histamine dose-response curve, increasing contractions of 0.1, 0.2, and 0.3 µM histamine by two- to threefold. Increases in ascorbate concentration significantly enhanced 0.2 µM histamine (5–500 µM ascorbate) and 0.3 µM histamine (15–500 µM ascorbate) in a dose-dependent manner. Histamine does not measurably oxidize over 20 h in oxygenated PSS at 37°C. Thus the ascorbate enhancement is independent of ascorbate's antioxidant effects. Ascorbate in solution oxidizes rapidly. Transfected histamine receptor membrane suspension with protein concentration at >3.1 µg/ml (56 nM maximum histamine receptor) decreases the oxidation rate of 392 µM ascorbate, and virtually no ascorbate oxidation occurs at >0.0004 mol histamine receptor/mol ascorbate. Histamine receptor membrane had an initial ascorbate oxidation inhibition rate of 0.094 min·µg protein^–1·ml^–1, compared with rates for transfected ANG II membrane (0.055 min·µg protein^–1·ml^–1), untransfected membrane (0.052 min·µg protein^–1·ml^–1), creatine kinase (0.0082 min·µg protein^–1·ml^–1), keyhole limpet hemocyanin (0.00092 min·µg protein^–1·ml^–1), and osmotically lysed aortic rings (0.00057 min·µg wet weight^–1·ml^–1). Ascorbate enhancement of seven-transmembrane-spanning membrane receptor activity occurs in both adrenergic and histaminergic receptors. These receptors may play a significant role in maintaining extracellular ascorbate in a reduced state. molecular complementarity; vitamin C; seven-transmembrane-spanning membrane receptors     

Relationship between Electrical Resistance and Capacitance of Wood in Progressive Stages of Discoloration and Decay

Electrical resistance and capacitance measurements were madeon woody tissues of several tree species. Each tissue was abovethe fibre-saturation point. Values of capacitance were inverselyproportional to values of resistance in woody tissues in progressivestages of discoloration and decay. With the electrode separationused, clear woody tissues had resistances of 110 to 30 k andcapacitances of 0·002 to 0·110 µF. Resistanceand capacitance measure ments of discoloured tissues were 29to 8 k and 0·005 to 0·040 µF, respectively,and those of decayed tissues were 5 to 1 k and 0·60 to0·110 µF. Either electrical measurement could beused to measure the stages of deterioration in woody tissues.Ground woody tissues, cellulose suspensions, starch suspensions,and wood equivalent solutions of potassium chloride had resistanceand capacitance properties similar to those found in intactwoody tissues. Electrical resistance and capacitance propertiesof woody tissues above the fibre-saturation point appear tobe similar to those of dilute solutions of mobile ions.     

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     

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

The Structure and Function of Pine Forest in Central Himalaya. I. Dry Matter Dynamics

The present study deals with structure and function of fourareas of Himalayan chir pine forest. Tree layer was monospecificon all sites with varied density and basal cover in the rangeof 540–1630 individuals per ha and 25·0–47·2m²ha^–1, respectively. Shrubs having low density were sparselydistributed. All allometric equations relating to biomass ofdifferent components, to circumference at breast height (cbh)were significant, with the exception of that for cone biomass.Total vegetation biomass (115–236 t ha^–1) was distributedas 113–283 t ha^–1 in trees. 0·56–0·82t ha^–1 in shrubs and 1·63–2·57 t ha^–1in herbs. Total forest floor biomass including herbaceous litterranged between 9·6 and 13·6 t ha^–1. Of thetotal annual litter fall (4·26–7·38 t ha^–1),60·3–75·1 per cent was distributed in leaflitter and 24·9–39·7 per cent in wood litter.Turnover rate of tree litter varied from 0·45 to 0·53,whereas rates for shrubs and herbs were assumed to 1. Net primaryproduction of total vegetation ranged between 9·91 and21·2 t ha^–1 year^–1, of which the contributionof trees, shrubs and herbs was 76·5– 88·1per cent 0·6–1·8 per cent and 11·3–21·5per cent, respectively. A compartment model of dry matter onthe basis of mean data across sites was developed to show drymatter storage and flow of dry matter within the ecosystem. Pinus roxburghii forest, biomass, litter fall, net primary production, compartmental transfer     

The Relationship between Metabolism and the 'Exchangeability' of Ions in Plant Tissues2

The uptake of the rubidium ion by well-washed disks of carrotparenchyma has been determined at 0·2° C and 25°C., in the presence and absence of 10^–4M. dinitrophenol,from solutions of rubidium chloride containing 0·5 and5·0 meq./l. Readily-exchangeable and non-exchangeablefractions were separately indentified. The lowering of temperature and application of DNP reduced themagnitude of both ion fractions at each of the two concentrationsof rubidium chloride. Despite the fact that the uptake of ionsinto an exchangeable form at 5·0 meq./l. was about 3times greater, the combined effect of low temperature and thepresence of DNP reduced this fraction by a relatively constantabsolute amount. Under the same conditions the uptake of ionsinto a non-exchangeable form from each concentration was reducedby approximately the same percentage. Over a 6-hour period the rate of uptake of rubidium into a non-exchangeableform at 25° C. was relatively constant, whereas at 0·2°C. there was an initial rapid uptake lasting for about 60 minutesfollowed by a slow steady uptake. The Q₁₀ of this latter processmeasured after 360 minutes was 2·3. It is concluded that an appreciable part of the capacity ofthe tissue to hold ions by exchange is dependent on concurrentmetabolism. The significance of measurements of exchangeability in the interpretationof mechanisms of ion uptake is discussed.     

Growth Substances in the Roots of Vicia faba

An investigation has been made into the growth regulators presentin ethanol extracts of the seedling roots of Vicia faba afterseparation on paper partition chromatograms, using segmentsof Avena coleoptiles and mesocotyls and of Pisum sativum.rootsas biological assay material. Acetonitrile purification shows the presence of at least threeauxins running in isobutanol: methanol: water, at Rfs of 0–0·25,0·4–0·6, and 0·65–0·95;the latter may represent two different auxins. A similar, butclearer, picture is shown by the ether-soluble acid fraction.Here an auxin at Rf 0–0·25 also stimulates rootgrowth and could be ‘accelerator ’. A second atRf 0–0·25 is an indole compound which inhibitsroot growth and does not seem to be be IAA. A third at Rf 0·8–1·0is also a root-growth inhibitor and gives no indole reaction.The ‘inhibitor ß’ complex was demonstrated(Rf 0·65–0·85) together with a number ofother inhibitors at lower Rf value. The ether-soluble neutral component also contains auxins orauxin precursors. The water-soluble, ether-insoluble fraction contains four readilyinterconvertible substances with auxin properties. They allappear to inhibit root growth and give no indole reaction.     

Alternative Respiration and Heat Production in Ripening Banana Fruits (Musa paradisiaca van Mysore Kadali)

The involvement of alternative respiration in thermogenesisduring the ripening of banana {Musa paradisiaca var. MysoreKadali) fruits, attached to a bunch, has been examined. Thetemperature of the youngest (unripened) banana fruit increasedfrom 27·0 ± 0·2°C to 30·8±0·1°C and the total respiration (in nmo1 oxygen min¹ g¹ drywt.) increased from 1·39·6 ± 5·5to 167·3 ± 7·0 at the fully ripened stage.Although the capacity for alternative respiration showed littlechange, the actual operation of this pathway increased from38 to 73% (p= 0·38 to 0·73) during ripening. Similarresults were obtained in fruits along the central axis at differentstages of ripening. It is suggested that alternative respirationmay contribute to the temperature rise observed in ripeningbanana fruit. Key words: Alternative respiration, tehrmogenesis, fruit ripening     

Correlations Between Electrolyte Leakage and Degree of Saturation of Polar Lipids from Aged Potato (Solanum tuberosum L) Tuber Tissue

Age-dependent changes in the concn (nmol g d. wt^–1) andrelative proportion of polar lipid fatty acids in potato tubertissue were characterized over a 32-month storage period. Astubers advanced in age from 2 to 14 months, a decrease (approximately22%) in concn of both saturated (16:0, 18:0) and unsaturated(18:2, 18:3) fatty acids was evident. This decrease was followedby an exponential increase through 32 months. Electrolyte leakagestudies with excised tissue revealed age-dependent differencesin membrane permeability. The relationship between maximum electrolyteleakage (% total) and tissue age was described by a cubic polynomial.Leakage declined approximately 4% as tissues aged from 2 to7 months; however, from 7 to 24 months, leakage increased 21%.These trends were followed by partial restoration of membranefunction in 24 to 32-month-old tissues, as indicated by attenuatedleakage. Similarly, the trend in double bond index (DBI) withage was defined by a cubic polynomial; however, the minima andmaxima were the opposite of those for electrolyte leakage vs.age. The regression of DBI upon leakage was inverse and linear(r = –0·97, P < 0.01). Furthermore, the effectof decreasing temperature from 5 to –2 °C on tissuepermeability (after equilibration with the bathing medium) wasalso dependent upon DBI. The higher the DBI, the greater theresistance of the tissue to increased electrolyte leakage inducedby chilling temperatures (r = 0·99, P < 0·05).Although the evidence is largely correlative, it appears thatchanges in the degree of saturation of membrane lipids are relatedto age-dependent fluctuations in membrane integrity and maybe important in dictating chilling sensitivity in potato Potato (Solanum tuberosum L.), tuber age, electrolyte leakage, membrane fatty acids     

Effect of Severity of Defoliation on the Viability of Reproductive and Vegetative Axillary Buds of Trifolium repens L.

This glasshouse experiment was performed to assess the effectsof a range of constant defoliation regimes applied to cuttingsof a single large-leaved genotype ofTrifolium repens L. on theviability of its axillary buds. Plants were established to comprisea single main stolon (axillary branches were removed) and defoliationtreatments were applied by removing the older (basal) leavesuntil leaf complements of 1·0, 1·5, 2·0,2·5, 3·0 or all leaves (control) remained. Basalleaves were subsequently removed as necessary to maintain thetarget leaf complements. Only severe defoliation (leaf complements of 1·0 and1·5) induced a loss of viability in axillary buds. Lossof viability was greatest in reproductive buds present withinthe apical bud when the treatments were first imposed. Althoughthe most severe treatment (leaf complement 1·0) resultedin death of half the plants, in plants surviving that treatment,death of vegetative axillary buds was restricted to 21% of thevegetative buds at the three youngest node positions withinthe apical bud at the time of treatment application. No othertreatment induced any loss of viability of vegetative buds.There was no loss of viability of axillary buds at nodes formedafter the treatments were imposed. The frequency of initiationof inflorescences at nodes formed after treatments were imposeddecreased as defoliation severity increased. Severe defoliation resulted in marked changes in plant morphologyindicative of a sharp decrease in availability of intraplantresources. It was concluded that under severe defoliation: (1)the potential for vegetative growth (as represented by viablevegetative axillary buds) was maintained at the expense of reproductivegrowth; and (2) that the loss of viability of axillary budswas associated with the sudden changes in physiological processesinduced by defoliation as there was no loss of viability inbuds formed after plants had adjusted their phenotype to oneof smaller size. Trifolium repens L.; white clover; defoliation; axillary buds; viability; inflorescences     

Comparison of the Effects of Ammonia and Methylamine on Chloride Transport and Intracellular pH in Chara corallina

Ammonium and methylammonium ions greatly increase the rate ofCl^– transport in Chara corallian. This effect is dependenton the pH of the bathing solution. The amine-stimulated Cl^–influx is small at pH 5·5, increases to a maximum atpH 6·5–7·5, and decreases again as the pHis raised to 8·5. Increased Cl^– influx is accompaniedby an increase in cytoplasmic pH, as calculated from the distributionof DMO. When the external pH lies between 5·5 and 7·3,cytoplasmic pH in the absence of amine is 7·65–7·70,with an increase of 0·15–0·25 in the presenceof amine. As external pH is increased above 7·3, cytoplasmicpH also increases, with progessively less effect of amine. Although the relationship between Cl^– influx and cytoplasmicpH is not simple, the results provide evidence in accord withthe hypothesis that Cl^– transport in Chara involves H⁺—Cl^–symport, or the equivalent OH^–—Cl^– antiport.The possible role of cytoplasmic pH as a factor involved inthe regulation of membrane transport in Chara is discussed.     

Structure and Function of Oak Forests in Central Himalaya. II. Nutrient Dynamics

This paper elucidates nutrient dynamics in oak forests previouslyinvestigated for dry matter dynamics. The nutrient concentrationsin different life forms were of the order: herb > shrub >tree, whereas the standing state of nutrients were of the order:tree > shrub > herb. Soil, litter and vegetation, respectively,accounted for 32·4–98·0 %; 0·3–3·5%, and 10·2–66·6 % of the total nutrientsin the system. Considerable reductions (8·5–41·7%)in concentrations of nutrients in leaves occurred during senescence.The uptake of nutrients by vegetation, and also by differentcomponents with and without adjustment for internal recycling,has been calculated separately. Annual transfer of litter (above+ below ground) to the soil by vegetation was 115·9–187N, 7·5–15·6 P, 122·7–195·1Ca, 36·1–48·8 K and 2·88–5·16Na kg ha^–1 yr^–1. Turnover rate and turnover timefor different nutrients ranged between 0·66–0·84yr^–1 and 1·19–1·56 yr^–1, respectively.Compartment models for nutrient dynamics have been developedto represent the distribution of nutrient contents and net annualfluxes within the system. Quercus leucotrichophora forest, Q.floribunda forest, Q. lanuginosa forest, Nutrient concentration, standing state, uptake, internal cycling, turnover     

The Exodermal Casparian Band of Onion Roots Blocks the Apoplastic Movement of Sulphate Ions

Peterson, C. A. 1987. The exodermal Casparian band of onionroots blocks the apoplastic movement of sulphate ions.—J.exp. Bot. 38: 2068–2081. It has previously been established that the exodermal Casparianbands in the roots of onion and many other angiosperm speciesare impermeable to fluorescent, apoplastic dyes. The permeabilityof the onion root exodermal Casparian band to ions was testedby measuring sulphate free space in the walls using compartmentalanalysis by elution. Results of this analysis revealed fourcompartments for sulphate. The first was a surface film, between15 and 35 µm thick, which had a half-time of elution tooshort to be measured (i.e. less than 15 s). The second was awall compartment with an average half-time of elution of 1·0min. The third and fourth compartments were the cytoplasm andvacuole which had half-times of elution of 12·4 min and4·9 d, respectively. Experiments in which the temperatureand concentration of sulphate in the treatment solution werevaried indicated that the compartment identified as the wallwas a free space. Segments were excised from roots in regionswith a mature exodermal Casparian band. The sulphate free spacein the walls of ‘whole’ segments (i.e. segmentswith their cut ends sealed) was compared with that of ‘bisected’segments (i.e. segments which had been bisected and their stelesremoved). The free space in the walls of the whole segmentswas consistently smaller than the comparable free space in thebisected segments; the ratio whole/bisected ranged from 0·16to 0·26. This ratio compared favourably with predictedvalues obtained by taking the ratio of the wall volumes, i.e.walls external to the exodermal Casparian band/walls internaland external to the exodermal Casparian band. The ratios predictedfrom wall measurements varied from 0·25 to 0·27.Furthermore, the volume of the sulphate free space in the wholesegments was less than the volume of the walls external to theCasparian band. The results of the present study show that theexodermal Casparian band of onion is impermeable to sulphateions, which have a hydrated ionic radius of 0·378 nm. Key words: Allium cepa, exodermis, sulphate     

The Influence of Donor Plant Genotype and Environment on Production of Multicellular Microspores in Cultured Anthers of Brassica napus ssp. oleifera

Studies of the effects of genotype and pre-flowering environmentalconditions on the production of multicellular microspores wereundertaken th four highly inbred lines of Brassica napus sap.oleifera. These lines were first grown in shaded and unshadedenvironments at 20/15°C arid unshaded at 30/25°C ina daylight phytotron. Buds were harvested from half the plantswhen first visible in the rosette and later from the remainingplants at the time when the first flower opened. The frequencyof microspores at a specific stage of development varied widelywithin a relatively narrow range of bud lengths. Uninucleatemicrospores were not detected in anthers from buds less than1·5 m or greater than 3·0mm long, but were generallypresent in frequencies of greater than 50 per cent in anthersfrom buds which were between 2·0 and 2·5 mm inlength. However, the bud length at which the highest frequencyof uninucleate microspores was detected varied significantlybetween genotypes and between the environments in which theywere grown. Examination of the remaining anthers from each budafter a period in culture revealed that the proportion of microsporesdeveloping into multicellular units varied greatly with budlength, an increase in frequency of multicellular microsporesbeing associated with an increase in the frequency of uninucleatemicrospores in the uncultured anther. Genotypes differed, however,in respect of the relationship between uninucleate microsporefrequency and production of multicellular units. Although thefrequency of multicellular units was as high as 57 percent,further development was limited and the number of embryoldsformed was low in all cases (<10 per cent). The frequency of multicellular units in pollen samples frombuds of a length in which uninucleate microspore frequency washigh varied significantly with genotype, temperature and lightconditions under which donor plants were grown, and the stageof inflorescence development at which buds were removed. Underconditions most conducive to multicellular unit formation (20/15°C,unshaded), the maximum frequency of multicellular units foreach genotype in buds from young inflorescences ranged from11·5 to 56·5 per cent. Shading or exposure tothe higher temperature was associated with a marked reductionin production of multicellular units. Higher frequencies ofmulticellular units were generally detected in microspore samplesfrom younger inflorescences irrespective of genotype or environment. Two of the four inbred lines were selected for a second experimentin which responses to vernalization and photoperiod durationwere monitored. There was a significant reduction in the numberof leaf nodes formed prior to floral initiation in both genotypesfollowing exposure to vernalization and/or a longer photoperiod,the response to photoperiod being more pronounced. Exposureto 4 weeks vernalization was accompanied by a significant increasein the frequency of multicellular units in both genotypes, thefrequency being double that in unvernalized plants under thelonger photoperiod. By contrast, genotypes differed sharplyin their response to photoperiod. In TB 20, the frequency ofmulticellular units was unaffected by an increase in day lengthirrespective of whether seed had been vernalized or not. Onthe other hand, in TB 42 the frequency of multicellular unitswas substantially greater in the 24 h day than in the 12 h day,being 27·3 per cent vs 13·0 per cent in the caseof unvernalized plants and 66·7 per cent vs 18·2per cent in the case of vernalized plants. Brassica napus, anther culture, pollen embryogenesis, genotype-environment interaction     

A Sensitive Auxanometer for Field Use

The essential characteristics of an auxanometer designed tomeasure the extension rates of grass leaves in the field areoutlined. The construction, using a linear-variable differentialtransformer (LVDT) to measure displacement, is described. Laboratorytests showed a linear relation between output and armature displacement,which was independent of changes in temperature between 1 and24 °C. Field tests on winter wheat leaves showed good agreementbetween auxanometer and rule measurements of leaf extensionmade over time periods of 24, 12, and 2 h. The tests and ananalysis of the errors, including the thermal expansion of theauxanometer stand, show that this auxanometer can reliably andaccurately measure leaf extension rates of between 0·1and 10 mm h^–1 in wind speeds up to 4·5 m s^–1.     

Partitioning of Dry Matter in Tanier (Xanthosoma spp.) Irrigated with Fractions of Evapotranspiration

The accumulation and partitioning of dry matter was determinedin tanier plants irrigated with fractions of the water lostthrough evapotranspiration (WLET) in an effort to establishgrowth analysis data from which a tanier growth (simulationmodel could be developed. The irrigation regimes were basedon Class A pan factors ranging from 0·33 to 1·32with increments of 0·33. Tanier plants were planted inthe field and harvested for biomass production about every 6weeks during the growing season. At each harvest, plants wereseparated into various plant parts, and their dry matter contentwas determined. The first 90 d after planting (DAP) were characterizedby low rates of dry matter accumulation, with only leaves andpetioles showing substantial growth. A grand growth period followedin which leaves, petioles, and roots rapidly accumulated drymatter until 278 DAP. During this period, plants that received0·99 and 1·32 WLET exhibited similar total drymatter content, and this was significantly greater than in plantssupplied with 0·33 and 0·66 WLET. Cormel dry mattercontent peaked at 29% of total plant dry matter by 322 DAP inplants replenished with 1·32 WLET. Partitioning of drymatter to cormels in other treatments was significantly reduced.Partitioning of dry matter to corms increased linearly throughoutthe growing season in all treatments. Dry matter partitioningto suckers and the number of suckers formed from plants replenishedwith 0·33 and 0·66 WLET was greater than in themore irrigated treatments.Copyright 1994, 1999 Academic Press Tanier, Xanthosoma spp., growth, dry matter partition, irrigation, evapotranspiration     

Structure and Function of High Altitude Forests of Central Himalaya II. Nutrient Dynamics

This paper deals with nutrient dynamics in horse chestnut, silverfir and kharsu oak forests in a high altitude region of CentralHimalaya. In general, the nutrient concentrations in differentlife forms were of the order: herb > seedling > shrub> sapling > tree, whereas the standing state of nutrientswere of the order: tree > herb > shrub > sapling >seedling. Of the total nutrients in the system, soil litterand vegetation, respectively accounted 66·5, 0·6and 32·9% in horse chestnut, 61·4, 0·8and 37·8% in silver fir, 58·1, 0·8 and41·1% in kharsu oak forests. Considerable reductionsin concentrations of nutrients in leaves occurred during senescence.Annual transfer of litter (above-ground+below-ground) to thesoil by vegetation of all forests ranged from 68-163 for N,4-7 for P, 26-48 for K, 62-150 for Ca and 2-4 kg ha^-1 year^-1for Na. Turnover time for different nutrients ranged between1·41 and 1·75 years for horse chestnut, 1·33and 2·13 years for silver fir, and 1·32 and 1·75years for kharsu oak forests. The distribution of nutrient contentsand net annual fluxes within the system have been developedto represent nutrient dynamics in compartment models.Copyright1995, 1999 Academic Press Standing state, turnover, retranslocation, nutrient concentration, internal cycling, uptake     

Unidirectional Ca2+ Fluxes in Roots of Rye (Secale cereale L). A Comparison of Excised Roots with Roots of Intact Plants

The unidirectional Ca²⁺ fluxes across the plasma membrane andtonoplast were determined in both excised roots and roots ofintact seedlings of rye (Secale cereale L. cv. Rheidol). Theunidirectional Ca²⁺ fluxes across the plasma membrane and tonoplastmeasured in excised roots were of a similar order of magnitudeto those determined in roots of intact plants. Influx and effluxof Ca²⁺ across the root plasma membrane were similar (estimatedto be between 0·7 and 3·4 µmol g     

Electrical Impedance Analysis in Plant Tissues8

Electrical impedance spectra (100 Hz–800 kHz) were measuredin leaves of Peperomia obtusifolia L. (a succulent) and Brassicaoleracea L. (cabbage). By measuring impedances at three or moreinter-electrode distances in a single leaf, electrode impedanceand specific tissue impedance were separated. Analysis of impedance data from B. oleracea leaves in relationto an equivalent circuit model showed that leaf developmentwas accompanied by increases in extracellular resistance, cytoplasmicresistance and vacuole interior resistance, together with decreasesin plasma membrane capacitance and tonoplast capacitance. AfterB. oleracea leaves were subjected to a –6 °C freeze-thawstress, extracellular resistance, cytoplasmic resistance andvacuole interior resistance decreased, but plasma membrane capacitanceand tonoplast capacitance did not change. These results indicatethat useful measurements of leaf parameters can be obtainedby this technique. Examination of the electrode impedance spectrum showed thatelectrode insertion produced a damaged collar, 0·4–0·5mm wide, around the electrode. This was confirmed by visualobservation of the damage in P. obtusifolia leaf. Key words: Peperomia obtusifolia L., Brassica oleracea L. (cabbage), electrical impedance, equivalent circuit, electrode polarization     

Imaging the Vascular Continuity of Prunus avium Petioles During Leaf Senescence Using Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) micro-imaging was used to studythe distribution of mobile protons in the petiole and stem ofPrunus avium throughout the period of senescence of an attachedleaf. The base of the NMR micro-imaging probe was modified toallow access to the coil from below, enabling a branch of theintact tree to be inserted into the probe which, in turn, wasinserted into the base of the NMR cryomagnet. A multi-sliceimage was used to determine the final orientation of the petiolein the probe. NMR spin echo imaging parameters were investigatedby using variable relaxation delays (D1) ranging from 0·1to 4·0 s. Transverse slices of the petiole with a D1ranging between 1·0 to 4·0 s produced bright imagescorresponding to the cortex and leaving the xylem vessels asa dark horseshoe-shaped area. Reducing D1 to 0·25 s or0·1 s produced images with more detail, differentiatingthe phloem and xylem vessels from the surrounding cortex. A longitudinal slice, 0·5 mm thick, through the stemand subtending petiole was imaged repeatedly over 6 d, duringwhich the leaf senesced and abscinded. The images showed detailsof the vascular strands in the stem with a section continuouswith the petiole. During leaf senescence the development ofthe abscission zone in the petiole was observed. The petioleremained hydrated throughout leaf senescence, demonstratingthe usefulness of NMR imaging to probe the vascular integrityof the intact plant. Key words: Spin echo pulsed gradient, cherry, petiole, vascular integrity