A Reanalysis of Particle Motion in Sieve Tubes of Heracleum

From an analysis of the Brownian motion of particles in sieveelements of Heracleum mantegazzianum and Heracleum sphondylium,Barclay and Johnson have suggested that the in situ viscosityof sieve tube sap is four to six times higher than has previouslybeen assumed. In particular, they obtained a value for the sapviscosity of about 10^–2 Pa s, which compares with a valueof 2 x 10^–3 Pa s for a 20 per cent (w/v) sucrose solution.The present paper describes a reanalysis of their data. It isargued that Barclay & Johnson underestimated the Brownianmotion of sieve element particles and so overestimated the sapviscosity. An exact correction was not possible, but it is concludedthat the in situ viscosity of Heracleum sieve tube sap mustbe less than 3 x 10^–3 Pa s, which corresponds to a sucroseconcentration of less than 29 per cent. Hence it may not beunreasonable to suppose that the viscosity of sieve tube sapis determined primarily by the concentration of sucrose, ashas been assumed in theoretical analyses of the Munch hypothesis.It is also concluded that the sieve tubes studied by Barclayand Johnson were not functional, in the sense that they didnot exhibit an axial bulk flow of sap. Heracleum, sieve tubes, Brownian motion, viscosity, Munch hypothesis     

An Investigation of the Relay Hypothesis of Phloem Transport in Ricinus communis L.

To test for the existence of an apoplastic unloading/reloadingstep in phloem translocation, as envisaged in the relay hypothesisof phloem transport, isotopic trapping experiments were performedon Ricinus comunis L. var. Gibsonii. A CO₂ buffer system wasused to supply ¹⁴CO₂ at constant partial pressure and constantspecific activity to a photosynthesizing leaf. The subtendinginternode was perfused with solutions of sugars or of mannitoland transolation of ¹⁴C past the perfused zone was monitoredby the collection of phloem exudate. Trapping of activity inthe perfusate was enhanced by the presence of sugars as wouldbe expected with an unloading/reloading process. However, therewas no evidence that introduction of the unlabelled sugars tothe apoplast also reduced activity in the phloem exudate. Moreover,the rate of loss of activity to the perfusing solutions representedonly 1% of the rate of appearance in the exudation. It is suggestedthat the trapping results may reflect an unloading of tracerfrom the phloem associated with a subsequent reloading by adjacenttissues rather than by the sieve tubes. To investigate the length of sieve tube continuity in Ricinus,a horizontal incision was made to the bark and the rate of exudationof phloem sap was monitored. Successive circumferential cutswere made above the exuding incision and progressively closerto it. In general, a girdling incision produced a transientdecrease in the rate of change of exudation rate (i.e. the firstderivative became more negative/less positive). The magnitudeof this response rose with exudation rate and fell with thedistance at which a girdling cut was made. Fitting an appropriatemodel yielded an estimate for contributory length of 69 ±6 cm. This was comparable with the distance of the initial tangentialincision from the stem apex, suggesting a continuous sieve tubesystem in Ricinus. A similar investigation on the petiole yieldedan estimate of around 7.0 cm. This lower estimate for contributorylength is believed to reflect a rapid sealing process that limitsthe distance of propagation of turgor-release rather than alimited length of sieve tube continuity. The results of this investigation do not support the relay hypothesisof phloem transport. Rather they suggest a continuous sievetube system which has a distributed capacity to load and unloadsolutes, and which may exhibit a sealing response when injured. Key words: Ricinus communis L, phloem transport, phleom unloading     

Investigation of Bidirectional Movement of Tracers in Sieve Tubes of Salix Viminalis L.

Sieve-tube sap was collected as honeydew from the aphid Tuberolachnussalignus (Gmelin) feeding on a young shoot or a mature stemof willow (Salix viminalis L.). Two radioactive tracers (either³H-glucose and ¹⁴C-sucrose, or ¹⁴CO₂, Na₂ ¹⁴CO₃, and ³²P-orthophosphate)were applied to leaves or to bark abrasions at each end of thestem. In some experiments the tracers were applied after thestart of honeydew production, whilst in others the tracers wereapplied before the aphids were sited. In most cases aphids feeding between the tracer applicationpoints produced doubly-labelled honeydew during a 24–48-hperiod from tracer application. In some instances the firstdrops of honeydew contained one tracer, followed several hourslater by the other tracer, whilst some aphids produced doubly-labelledhoneydew from the first drop. Singly-labelled or inactive honeydewwas found in only a few cases. It was demonstrated that when ³H-glucose and ¹⁴C-sucrose wereapplied at opposite ends of a stem that the tracers moved inopposite directions in the form of sucrose. The data suggest that a simultaneous bidirectional movementof two tracers may occur in the same sieve tube. However, thepossibility that the doubly-labelled honeydew could be producedby lateral movement from one sieve tube carrying one tracerto an adjacent punctured sieve tube carrying the other tracercannot be discounted.     

Studies in Sieve-tube Exudation through Aphid Mouth-parts II. The Effects of Pressure Gradients in the Wood and Metabolic Inhibitors

Exudation was studied from the mouth-parts of the willow aphidTuberolachnus salignus (Gmelin) inserted in the phloem of isolated15 -cm. stem segments of Salix viminalis. By supplying waterunder pressure to the cut surface of the wood at one end ofa segment and allowing it to run freely from the other, a pressuregradient could be set up in the xylem. By closing the free end,the pressure in the xylem could be raised uniformly. This lasttreatment caused the rate of exudation to increase. This effectof pressure could be balanced by an equivalent osmoticum introducedinto the xylem. The increase was therefore due to the DPD inthe xylem rather than any intrinsic effect of pressure. The effect of a pressure gradient (4 atm. applied) dependedon the position of the mouth-parts in relation to the gradient.If they were near the low-pressure end, exudation from themincreased. If they were near the high-pressure end, exudationdecreased. In neither case was the total carbohydrate or totalnitrogen concentration of the exudate changed. When longer segments were used (e.g. 41 cm.) the effect of asimilar pressure (4 atm.) was greater than with 15-cm. segmentsalthough the point of application of the pressure was more remotefrom the mouth-parts. When the wood at the midpoint of a segmentwas severed leaving a narrow bridge of bark joining the twohalves, raising the pressure in the xylem of one half led toan increased exudation from mouth-parts situated in the otherhalf. Thus the effect of pressure was transmitted along thephloem since there was no rise in xylem pressure in the halfsegment bearing the mouth-parts. 10^-4 M. dinitrophenol applied to a bark abrasion a few centimetresaway from mouth-parts, reduced exudation. Applied to the cambialsurface, exudation was reduced by 0.1 per cent, phloridzin andstopped completely by 10^-4 M. dinitrophenol and 10^-6 M. potassiumcyanide. Inhibitors had no effect on the concentration of theexudate. No simple interpretation of the effects of pressure gradientsis possible, but it is suggested they give qualified supportto the Munch hypothesis.     

The Relative Contributions of Sugars from Assimilating Leaves and Stem Storage Cells to the Sieve Tube Sap in Willow Cuttings

A method is described for the measurement of the relative contributions of sugars from the assimilating leaves and stem storage cells to the sieve tube sap in leafy cuttings of Salix viminalis L. In these experiments sieve tube sap was collected as honeydew from a small colony of aphids, Tuberolachnus salignus (Gmelin). Using this method it has been shown that in cuttings maintained under continuous illumination prior to experimentation, the leaves contribute about 75 per cent of the total sugars in the sieve tube sap. With cuttings which have been stored by keeping them in darkness, the level of labile carbohydrates in the stem storage cells decreases, in comparison with those growing in light, and the percentage of sugars in the sieve tube sap supplied by the leaves rises to 90 per cent.     

Xylem exudation is related to nitrate assimilation pathway in detopped maize seedlings: use of nitrate reductase and glutamine synthetase inhibitors as tools

The xylem exudation of detopped 7-d-old seedlings of Zea maysL. doubled when KCI was present in the root medium comparedto seedlings maintained on water. It was further enhanced whenKCI was replaced by nitrogen compounds such as nitrate, ammoniumand glutamine. The role of the nitrate assimilation pathwayon the enhancement of xylem exudation rate was investigatedusing tungstate, an inhibitor of nitrate reductase (NR) activity,and phosphinothricin or methionine sulphoximine, inhibitorsof glutamine synthetase (GS) activity. The sap levels of NO₃^–,NH₄⁺, glutamine, and asparagine was used to ascertain the invivo inhibition of both enzymes. The tungstate effects werealso checked by measuring leaf in vitro NA activity and NR proteincontent. Xylem exudation rate of detopped seedlings fed withKNO₃ decreased when the nitrate assimilation pathway was blockedeither at the NR or at GS sites. This decrease was preventedwhen urea (acting as NH₄⁺ supply) was given simultaneously withtungstate. KNO₃ does not act directly on exudation, but throughthe involvement of NH₄⁺. The involvement of glutamine was alsoshown since GS inhibition resulted in a cancellation of theenhancing effect of KNO₃ on exudation. As change of exudationrate was not linked to change in sap osmolarity, it is assumedthat the assimilation chain could modify root water conductance.The role of glutamine was discussed. Key words: Exudation, maize, nitrate, conductance, NR, GS     

Chemical Composition of Bleeding Xylem Sap from Kiwifruit Vines

A study of the chemical composition and charge balance was madeof bleeding xylem sap collected from excised one-year-old extensionshoots of healthy, Mn-deficient, Mn-toxic and Zn-deficient kiwifruitvines (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson)immediately prior to leafburst. The exudates were analysed formacronutrient cations and anions, trace elements, amino acids,organic acids and sugars. Major charged species measured wereCa (13.3 mM), K (8.9 mM), Mg (5.6 mM), malate (12.5 mM) andphosphate (5.8 mM). Glutamine (12 mM) was the predominant Ncarrier identified, accounting for 58 per cent of the totalN followed by NO₂^–-N (4.5 per cent), NH₄⁺-N (3.5 per cent)and arginine-N (2.9 per cent). Approximately 22 per cent ofthe N was in a hydrolysable proteinaceous fraction comprisingmainly glutamine and glutamate. Eighteen free proteinaceousamino acids were idetified in sap, the most abundant being glutamine,glutamic acid, valine, isoleucine and phenylalanine. Computersimulation of the chemical composition predicted that in additionto hydrated cations, ion pairs formed between inorganic components(SO₄^2–, HPO₄^2–, H₂PO₄^–) and cations (Ca²⁺,Mg²⁺, Mn²⁺), plus metal-organic ligand complexes (Ca Malate,Zn Malate, FeCit, CuHis, CuGln) are important species involvedin translocation. The solubility product of hydroxyapatite wasexceeded in all exudates although in vitro precipitation wasnot observed. To achieve electroneutrality with the componentsmeasured, however, formation of precipitate precursors (hydroxyapatitenuclei) had to be assumed. Irregularities in Mn nutrition (butnot Zn) were clearly indicated by the elemental compositionof exudate suggesting the use of sap analysis as a possiblepre-season indicator of nutritional status for this species. Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson, kiwifruit, xylem sap composition, trace metals, amino acids, organic acids     

The Movement of Sugars into the Sieve Elements of Bark Strips of Willow: I. METABOLISM DURING TRANSPORT

¹⁴C-labelled sugars (sucrose, glucose, and fructose) were suppliedto the cambial surface of bark strips of willow, and in conjunctionwith the aphid stylet technique (Weatherley, Peel, and Hill,1959), the movement into and distribution of activity in thesieve tube sap was examined. As well as sucrose, free hexosesand sugar phosphates were found to contain the ¹⁴C-label, andrapid interconversions occurred between these compounds somewherealong the entry route. Sucrose entry into the sieve elementswas accompanied by at least a partial breakdown into its componenthexoses, and the involvement of a sugar phosphate pool was alsodemonstrated. Possible transformation sites on the entry routebetween the solution bathing the cambial surface and the sieveelement vacuole are discussed.     

The Composition of Phloem Exudate and Xylem Sap from Tree Tobacco (Nicotiana glauca Grah.)

The composition of xylem sap and exudate from stem incisionsof Nicotiana glauca Grah. was compared in detail. Exudationfrom stem incisions occurred over a 5 min period in certainplants, enabling collection of 5–30 µl of sap. Therate of exudation showed an exponential decline. Exudate hada high dry matter content (170–196 mg ml^–1) andhigh sugar (sucrose) levels. Xylem sap had a low pH (5.8) andexudate a pH of 7.9. Glutamine dominated the amino compoundsin xylem sap and exudate, and K⁺ was the major cation. Totalamino compounds in stem exudate reached 10.8 mg ml^–1 whereasxylem sap contained much lower levels (0.28 mg ml^–1).All mineral elements and amino compounds with the exceptionof calcium were more concentrated in stem exudate than in xylemsap. Sucrose was labelled heavily in stem exudate following pulsingof an adjacent leaf with ¹⁴CO₂. A concentration gradient ofsugar (2.1 bar m^–1) was recorded for stems. Levels ofsucrose, amino compounds and K⁺ ions in stem exudate showeda diurnal periodicity. Each commodity reached maximum concentrationat or near noon and minimum concentration about dawn. The evidencesuggests that exudate from stem incisions of N. glauca is arepresentative sample of solutes translocated in the phloem. Nicotiana glauca Grah., phloem sap, xylem sap, sucrose, amino compounds, mineral ions     

Phloem sap exudation in Ricinus communis: elastic responses and anatomical implications

Abstract. Ricinus communis plants have an unusually high capacity to exude considerable quantities of phloem sap from bark incisions. We have used Ricinus as an experimental system to study different aspects of sap exudation. Dimensional changes in the bark, monitored by a displacement transducer, showed that pressure release in the sieve tubes was accompanied by elastic shrinkage. The rate of exudation was also controlled by the degree of pressurization and elastic properties of the sieve tubes. A displacement transducer was used to measure the elastic modulus (ɛ) of phloem samples by immersing them in a range of different osmotica. The cells had a low elastic modulus (ɛ= 1.62 ± 0.41 MPa at full turgor). ɛ of phloem tissue in massage pretreated bark, from which exudation was enhanced, was not significantly different from that of unmassaged bark in contrast with the suggestion of Lee (1981). However, anatomical studies showed that massage pretreatment has a stimulating influence on the cambial cell division, which increased the phloem tissue cross-section up to 160%. The newly-formed sieve tubes were 'spliced' into existing ones in the unmassaged zone to re-establish vascular continuity. Plants with a greater capacity to exude phloem sap from a given stem location had a greater cross-sectional area of sieve tubes in the vicinity.
The significance of observations with respect to other sap exudation phenomenon is discussed. The importance of the present work in understanding the technique of palm tapping, on which the palm sugar industry depends, is also considered.     

Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. II. The Effects of Abscisic Acid and Cytokinins

Guttation was used as a non-destructive way to study the flowof water and mineral ions from the roots and compared with parallelmeasurements of root exudation. Guttation of the leaves of barley seedlings depends on age andon the culture solution. Best rates of guttation were obtainedwith the primary leaves of 6- to 7-day-old seedlings grown onfull mineral nutrient solution. The growing leaf tissue becomessaturated with K⁺ below 1.5 mM K⁺ in the medium, whereas K⁺concentration in the guttated fluid still increases furtheras K⁺ concentration in the medium is raised. At 3 mM K⁺ averagevalues of guttation were 1.4–2.4 mm³ h^–1 per plantwith a K⁺ concentration of 10–20 mM; for exuding plantsthe flow was 4.2–7.6 mm³ h^–1 per plant and K⁺ concentration35–55 mM. Abscisic acid (ABA) at 10^–6 to 10^–4 M 0–2h after addition to the root medium increased volume flow ofguttation and exudation and the amount of K⁺ exported. Threeh after addition of ABA both volume and amount of K⁺ were reduced.There was an ABA-dependent increase in water permeability (Lp)of exuding roots shortly after ABA addition. Later Lp was decreasedby 35 per cent and salt export by 60 per cent suggesting aneffect of ABA on salt transport to the xylem apart from itseffect on Lp. Benzyladenine (5 x 10^–8 to 10^–5 M)and kinetin (5 x 10^–6 M) progressively reduced volumeflow and K⁺ export in guttation and exudation and reduced Lp. Guttation showed a qualitatively similar response to phytohormonesas found here and elsewhere using exuding roots. Hordeum vulgare L., barley, guttation, abscisic acid, cytokinins, benzyl adenine, kinetin     

Aphid stylectomy reveals an osmotic step between sieve tube and cortical cells in barley roots

The aim of the present study was to quantify osmotic pressuresdirectly in the translocation pathway, from leaf to growingroot tip, in order to understand the forces driving solutesfrom a source to a sink. Solutes move through the translocationpathway down an osmotically derived turgor gradient. Accordinglyaphid stylectomy and single cell sampling techniques have beencombined to examine the osmotic pressure of root phloem andgrowing root cells. Sieve tube sap was obtained from shootsand, for the first time, roots of barley seedlings using aphidstylectomy. Vacuolar sap was also obtained from a variety ofcells in leaf and root tissues using single cell sampling methods.Osmotic pressure of sieve tube sap from roots and shoots wasmeasured at high temporal resolution (within min) and over longperiods of time (up to 24 h). Osmotic pressure did not changesignificantly in the minutes immediately following excision,suggesting that confidence can be placed in the assumption thatstylet exudate is representative of sieve tube sap in vivo.There were no differences in the osmotic pressure of sieve tubesap from shoots (1.240.26 MPa, n = 10) or roots (1.420.15MPa, n = 13). However, osmotic pressure of sap from root corticalcells (0.710.09, n = 12) was about 0.7 MPa lower than thatof the sieve elements from roots, this difference may be maintainedby consumption of incoming solutes at the root tip. Resultsare discussed in the context of pressure driven flow in thephloem and symplastic contact between root tip cells and sievetube. It is hoped that the approach described here will provideimportant insights into the nature of the relationship betweenroot cell extension and assimilate supply through the phloem. Key words: Phloem, sieve tube, aphid, root, barley, osmotic pressure, translocation     

Cell Length Variation in the Secondary Phloem of Dalbergia spp. with Increasing Age of the Vascular Cambium

Cell length variation of sieve tube elements and phloem fibreswas studied in relation to their relative position within thebark in five species of Dalbergia. It was found that in fourout of the five species studied, the fibre elements occurringin the vicinity of cambium are longer than those occurring atthe periphery of the bark. D. latifolia is found to be an exception,since the shorter elements occur near the cambium while thelonger elements are found in the outer regions of the bark inthis species. In all the species fibre length declines markedlyin the middle region of the bark, and this is followed by asudden but temporary rise, except in D. latifolia in which theshortest elements occur both in the middle as well as near thecambium. The cell length variation of sieve tube cells does not followany recognizable trend in any of the species studied.     

Further Investigations into the Physiological Relationship Between an Epiphyllous Liverwort and its Host Leaves

Experiments were carried out to investigate the physiologicalrelationship between an epiphyllous liverwort, Radula flaccidaLbg. et G., and its host leaves. The osmotic potential of theepiphylla cell sap (-30 to -35 bar) was found to be much lowerthan that of the host cells (-10 to -12 bar). There is thereforea good physiological basis for the movement of water from thehost leaves to the epiphylla. The amount of light incident onthe host leaf which is intercepted by even the heaviest epiphyllacolony investigated was less than 2 per cent and this was foundto produce no measurable difference between the chlorophyllcontents of epiphylla-colonized and uncolonized parts of thehost leaf. ¹⁴CO₂ light fixation products were found not to movebetween the host leaf and the epiphylla in either direction.It is concluded that the dependence (parasitism) of R.flaccidaon its host leaves is partial and does not include the derivationof organic food substances.     

A combinatory approach for analysis of protein sets in barley sieve-tube samples using EDTA-facilitated exudation and aphid stylectomy

This study investigated advantages and drawbacks of two sieve-tube sap sampling methods for comparison of phloem proteins in powdery mildew-infested vs. non-infested Hordeum vulgare plants. In one approach, sieve tube sap was collected by stylectomy. Aphid stylets were cut and immediately covered with silicon oil to prevent any contamination or modification of exudates. In this way, a maximum of 1muL pure phloem sap could be obtained per hour. Interestingly, after pathogen infection exudation from microcauterized stylets was reduced to less than 40% of control plants, suggesting that powdery mildew induced sieve tube-occlusion mechanisms. In contrast to the laborious stylectomy, facilitated exudation using EDTA to prevent calcium-mediated callose formation is quick and easy with a large volume yield. After two-dimensional (2D) electrophoresis, a digital overlay of the protein sets extracted from EDTA solutions and stylet exudates showed that some major spots were the same with both sampling techniques. However, EDTA exudates also contained large amounts of contaminative proteins of unknown origin. A combinatory approach may be most favourable for studies in which the protein composition of phloem sap is compared between control and pathogen-infected plants. Facilitated exudation may be applied for subtractive identification of differentially expressed proteins by 2D/mass spectrometry, which requires large amounts of protein. A reference gel loaded with pure phloem sap from stylectomy may be useful for confirmation of phloem origin of candidate spots by digital overlay. The method provides a novel opportunity to study differential expression of phloem proteins in monocotyledonous plant species.     

Investigations of Carbon Transport in Plants: I. THE USE OF CARBON-11 TO ESTIMATE VARIOUS PARAMETERS OF THE TRANSLOCATION PROCESS

A technique is described which enables several components ofthe translocation system to be measured routinely, in vivo andsimultaneously. Plants are exposed to a 1-min pulse of ¹¹CO₂and the movement of the ¹¹C pulse through the plant followedusing an array of scintillation counters. It is possible toestimate the time taken for the ¹¹C to enter the sieve tubes.In addition, the speed of translocation can be obtained frommeasurements of the arrival of the ¹¹C pulse at different positionsalong the translocation pathway. The increasing half-width ofthe pulse as it enters and moves through the sieve tubes givessome indication of the extent of the delays in the differentparts of the translocation system. The short half-life of the¹¹C allows the measurements to be repeated on the same plantseveral times per day. A technique is discussed whereby dataon the movement of ¹¹C could be combined with simultaneous measurementsof the rate of uptake of carbon dioxide to provide an estimateof the rate of movement of carbon.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.     

Flavanoids in Pollen and Stigma of Brassica oleracea and Their Effects on Pollen Germination In Vitro

Brassica oleracea pollen was applied to a basic medium of 1.5per cent agar and 15 per cent sucrose to which flavanoids wereadded at three concentrations. Two types of agar were used;with agar 1, quercetin at a concentration of 0.5 x 10^–3per cent gave an increase in percentage germinating grains.With agar 2, an increase in germination occurred with kaempferoland naringin at concentrations of 0.5 x 10^–3 and 0.5 x10^–1 per cent respectively. Increase in pollen tube lengthoccurred with agar 2 and quercetin at a concentration of 0.5x 10^–3 per cent. The stigma tissue of B. oleracea contains at least three andthe pollen at least one glycoside of quercetin. The sugars inthe glycosides were not identified. Pollen germination and pollentube extension were not stimulated exclusively by the flavanoidspresent in the stigma. The flavanoid composition of the stigmadid not vary amongst five different S-allele genotypes, indicatingthat flavanoids are probably not directly involved in the incompatibilityreaction of B. oleracea.     

The Occurrence of Nitrate Reduction in the Leaves of Woody Plants

Nitrate reductase activities greater than 02 µmol h^–1g^–1 f. wt, measured by an in vivo assay, occurred in 41per cent of a large sample (555 species) of woody plants. Ifseveral taxonomic groups (Gymnosperms, Ericaceae and Proteaceae)with consistently low activities were discounted activitiesgreater than 02 µmol h^–1 g^–1 f. wt occurredin 73 per cent of the species. This compares with 93 per centin herbaceous species, suggesting that leaf nitrate reductionis of common occurrence in woody plants. In a small sample ofspecies leaf nitrate reductase activity correlated with nitrateconcentration in the xylem sap. Low activities occurred consistentlyin the Gymnosperms, Ericaceae and Proteaceae. Feeding cut shootsof representatives of these groups with nitrate caused inductionof leaf nitrate reductase activity in the Gymnosperms and Proteaceae,but only limited induction in the Ericaceae. The Ericaceae,with the exception of two species, had low activities and lownitrate reductase inducibility. Root assimilation may predominatein the Gymnosperms and Proteaceae. It is suggested that nitratereduction generally occurs in the leaves of trees from a varietyof plant communities and that this may be related to the lowerenergy cost of leaf, as opposed to root, nitrate assimilation. Nitrate reductase, trees and shrubs, leaves, nitrate assimilation, nitrate translocation, nitrate reductase induction, energy cost, plant ecology     

The Cellular Pathway of Short-distance Transfer of Photosynthates and Potassium in the Elongating Stem ofPhaseolus vulgarisL. A Physiological Assessment

Plasmolytic disruption of plasmodesmata interconnecting metaphloemsieve element-companion cell complexes with small and largephloem parenchyma cells in the elongating region of internode2 ofPhaseolus vulgarisL. seedlings did not affect accumulationof phloem-imported¹⁴C-photosynthates and⁸⁶rubidium. The membrane-impermeantdye, 5(6) carboxyfluorescein, loaded into leaf phloem as themembrane-permeant diacetate ester, was found not to move radiallyout of the importing sieve elements in the internode elongationregion. In contrast, the apoplasmic tracer, Calcuofluor White,rapidly moved laterally throughout all tissues of the elongationzone. Hexoses, sucrose and potassium were identified as themain osmotica in internode apoplasmic sap. Label asymmetry in[¹⁴C](fructosyl)sucrose was retained on accumulation by excisedstem segments. Uptake of [¹⁴C]sucrose and⁸⁶rubidium by stemsegments exhibited saturation kinetics. Sucrose uptake was inhibitedby the slowly penetrating sulphydryl reagent, para-chloromercuribenzenesulphonicacid.In vitrorates of sucrose uptake, at apoplasmic concentrations,corresponded to its predictedin vivorate of delivery to thestem ground tissues from mature sieve elements when respiratorylosses were assumed to be confined to the stem phloem. For potassium,the total delivery rate could be accounted for by itsin vitrorateof uptake. Overall, it was concluded that radial transport,in the elongation zone of internode 2 ofPhaseolus vulgarisL.seedlings, follows an apoplasmic route from mature sieve elementsto stem ground tissues.Copyright 1998 Annals of Botany Company PhaseoluLes vulgaris, apoplasm, elongating stem, French bean, photosynthates, potassium, radial transfer, symplasm.