Manometric Measurement of Turgor Pressures in Laticiferous Phloem Tissues2

A manometric technique for the determination of turgor pressuresin laticiferous phloem tissues has given reproducible resultsin Hevea brasiliensis and a few other tree species. In Hevea,early morning pressures are in the range 7.9–15.0 atmospheres,falling during the day and recovering at night. These diurnalpressure changes are positively correlated with atmosphericrelative humidity and negatively correlated with changes intemperature, evaporation, leaf water deficit, and stomatal opening.They are not displayed by trees devoid of leaves. Thus the lossin turgor is probably the result of withdrawal of water fromthe phloem tissues under transpirational stress. Pressures at the base of the trunk normally exceed those atthe top, the gradient usually approximating to 1 atmosphere/10metres at night and rising up to six times this figure duringthe day. This increase probably reflects the development oftension gradients in the xylem during transpiration. A generalturgor gradient from base to crown does not preclude mass flowin sieve-tubes in the opposite direction provided that ratesof loading and unloading are such that a sufficient osmoticgradient is maintained in them in the required direction. No marked long-term effects of regular tapping on turgor pressurehave been noted in Hevea trees and there is no evidence forseasonal changes in turgor under our conditions.     

Measurements of Electro-osmosis in Nitella translucens

Determinations of the electro-osmotic efficiency (models H₂Oper Faraday) in Nitella translucens have been made under differentconditions. Associated tracer measurements show that the mainpath of current flow ins through the cell, and not in the cellwall external to the membrane, but that the measured influxesof Na and K can account for only a small fraction of the current.The electro-osmotic coefficient increases markedly at very lowcurrents in the light (to as high as 700 H₂O/F), is decreasedin the dark, in the presence of low concentrations of N-dichlorophenyl-N',N' -dimethyl urea (DCMU) or of carbonyl cyanide m-chlorophenylhydrazone(CCCP) and on the addition of I mM NaHCO₃ to the bathing solution;the efficiency is higher at 10.5 °C than at 22 °C. Electro-osmosis,generated by a difference in membrane potential at the two endsof the cell bathed by different solutions, gave values of theelectro-osmotic coefficient similar to those characteristicof low applied currents.The results suggest that the cell membrane,and not only the cell wall, shows electro-osmotic properties.A simplified analysis of the system shows that the electro-osmoticcoefficient measured depends on conditions at each end of thecell and is not simply a property of the cation added at theend of the cell at which positive current enters.     

The Fine Structure of the Sieve Tubes of the Petiole of Nytnphoides peltatum (Gmel.) O. Kunze

A study has been made by electron microscopy of the fine structureof the peti-olar sieve tubes of the water plant Nymphoides peltatum.These are found to have very well-developed nacreous walls.The pores of the sieve plates appear to be filled in functioningsieve tubes with densely staining cytoplasm. The peripheralcytoplasm of the sieve tubes seems to contain an extensive developmentof the endoplasmic reticulum, whose elements become finer nearthe plates and crowd together through the pores. These findingsappear to be compatible with more than one theory of translocation,including the electro-osmotic theory of mass flow.     

Phloem Mobility of Boron is Species Dependent: Evidence for Phloem Mobility in Sorbitol-rich Species

Boron is generally considered to be phloem immobile or to haveonly limited phloem mobility in higher plants. Evidence suggests,however, that B may be mobile in some species within thePyrus,Malus andPrunusgenera. These genera utilize sorbitol as a primarytranslocated photosynthate and it has been clearly demonstratedthat B forms stable complexes with sorbitolin vitro.In the researchpresented here we demonstrate, further, that B is freely phloemmobile inPyrus, MalusandPrunusspecies and suggest that thisis mediated by the formation and transport of B-sorbitol complexes. The pattern of B distribution within shoot organs and the translocationof foliar-applied, isotopically-enriched¹⁰B was studied in sixtree species. Results demonstrate that in species in which sorbitolis a major sugar (sorbitol-rich), B is freely mobile while inspecies that produce little or no sorbitol (sorbitol-poor) Bis largely immobile. The sorbitol-rich species used here werealmond [Prunus amygdalusB. syn.P. dulcis(Mill.)], apple (MalusdomesticaB.) and nectarine (Prunus persicaL. B. var.nectarinaM.),sorbitol-poor species included fig (Ficus caricaL.), pistachio(Pistacia veraL.) and walnut (Juglans regiaL.). In sorbitol-richspecies foliar applied¹⁰B was transported from the treated leavesto adjacent fruit and specifically to the fruit tissues (hull,shell or kernel) that developed during the experimental period.Whereas, foliar-applied¹⁰B was rapidly translocated out of leaves,only a small percentage of the¹¹B present in the leaf at thetime of foliar application was retranslocated. In sorbitol-richspecies, B concentrations differed only slightly between oldand young leaves while fruit tissue had significantly greaterB concentrations than leaves. In contrast, sorbitol-poor specieshad significantly higher B concentrations in older leaves thanyoung leaves while fruit tissue had the lowest B concentration.This occurred irrespective the source of plant B (soil, solutionor foliar-applied). In a subsequent experiment the growth ofapple trees in solutions free of applied B was maintained solelyby foliar applications of B to mature leaves. These resultsindicate that B is mobile in species that produce significantamounts of sorbitol. We propose that the mobility of B in thesespecies is mediated by the formation of B-sorbitol complexes. Almond; Prunus amygdalus ; apple; Malus domestica; nectarine; Prunus persica; fig; Ficus carica; pistachio; Pistacia vera; walnut; Juglans regia; boron; phloem mobility; deficiency; toxicity; inductively coupled plasma-mass; spectrometer     

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.     

Phloem translocation in regenerating in vitro- heterografts of different compatibility

Phloem translocation of [¹⁴C]-sucrose and 5/6-carboxyfluorescein(CF) from scion into the stock was studied in in vitro-heterograftsof Lycopersicon on Solanum (L/S) and Vicia on Helianthus (V/H)at various stages of regeneration. Autografts of all partnersserved as controls. Corresponding with the translocation experimentsnewly formed sieve-tube connections between the graft partnerswere counted. ¹⁴C-translocation experiments with [¹⁴C]-sucrose revealed anage-dependent increase of radioactivity in the stock of allcombinations. In L/S and all autografts the major increase of¹⁴C-label in the stock occurred 5–10 d after grafting.In V/H, however, import of label into the stock remained lowthroughout the regeneration period. In L/S grafts, increasesin the numbers of sieve-tube connections parallel the increasingrate of ¹⁴C-transport, indicating functioning sieve-tube connectionsin the graft union. In contrast, V/H grafts did not show thisstrong correlation between structure and function of wound repairphloem. This suggested the existence of non-transporting sieve-tubesbetween the graft partners. Similar results were obtained withCF-transport, showing that effective phloem translocation acrossthe graft interface occurred in L/S, but not in V/H grafts.The observed differences in phloem translocation are discussedwith regard to compatibility/incompatibility phenomena in heterografts. Key words: Compatibility/incompatibility, in-vitro-heterografts, phloem transport ([¹⁴C]-sucrose, carboxyfluorescein), wound phloem     

Dynamics of Cold Induced Inhibition of Phloem Transport

Inhibition of phloem transport caused by chilling has been studiedby in vivo measurement of temporal tracer profiles observedboth uptstream and downstream of the chilled region. The dynamicsof the profile changes indicate that blockage is immediate butdoes not cause flow along the pathway to cease for some considerabletime (50–100 min). This behaviour is anticipated for apressure driven mass flow system. These findings demonstratethe caution needed in interpretation of sink accumulation rates. Key words: Phloem transport, Chilling     

Junction Complexes between Sieve Tubes in the Secondary Phloem of Myrtaceae

Junction complexes of unusual structure form between neighbouringsieve tubes in the secondary phloem of Eucalyptus species. Thick-walledribs support thin-walled ‘sieve areas’. In longitudinalsections the structures have a ‘concertina’- likeappearance. They are relatively large, up to 0.2 mm in length.Electron micrographs confirmed that the structures consistedof thin-walled areas perforated with pores, supported by muchthicker ribs. The structures provide a vast surface area fortransfer of metabolites between sieve tubes compared with thatof lateral wall sieve areas of other plants. Hydrolysis of parenchymacell walls occurs during the development of the junction complexes.The structures are only found when sieve tubes are in closeproximity and it is the redifferentiation and partitioning ofintervening parenchyma cells which result in junction complexformation. A survey for the presence of the structures in thephloem of other genera in the family Myrtaceae was made andthey were found in Tristania and Angophora but were not observedin Acmena and Metrosideros. Eucalyptus, sieve tubes, lateral walls, ultrastructure     

Tension in the Phloem?

The hydrostatic pressure in sieve tubes may probably be estimatedreasonably accurately from experimental measurements of theosmolarity of the sieve tube sap and of the tension in the xylem.The possibility of the existence of phloem tension is advanced.     

Apoplastic Phloem Unloading in the Stem of Bean

Sucrose has been found in the apoplast of bean stems at a concentrationof 25–60 mM with an axial concentration gradient in theappropriate direction for Munch translocation. Removal of theepidermis from a 50 mm length of stem enabled the washout oflabelled photosynthate from the apoplast. The rate of labelwashout was strongly dependent on temperature, and the rateincreased on blockage of phloem pathways to the main sink forthat assimilate. Washout did not reduce when the bathed tissuewas plasmolyzed. We propose that sucrose is unloaded from thephloem into the apoplast, and a sucrose concentration is maintainedthere by a balance of sucrose uptake into sink tissue or reloadinginto the phloem. It is proposed that the apoplastic pool ofphotosynthate can act to buffer sudden changes in phloem contentswhen there are rapid changes in source-sink configuration. Key words: Sucrose, Phaseolus vulgaris, Apoplast, Phloem unloading     

Effects of Wounding on Transport in the Phloem

The problem dealt with was the nature of the mechanisms of therecovery of the phloem of Phaseolus vulgaris from the effectsof wounds. The influence of root removal on the transport of[¹⁴C]sucrose and of fluorescein was followed both in the directionof the root and in the opposite direction, towards the intactshoot. The removal of the root was found to cause a sharp buttemporary decrease in the transport of sucrose towards the shoot;this was interpreted as the direct influence of the wound onthe transport system. Similar results were observed concerningfluorescein transport, which also showed that the resumptionof transport was in the entire phloem and not only in newlydifferentiated elements.     

The Control of Wound Callose Formation in Willow Phloem

Callose formation in phloem tissue was promoted by 0.5% eosinin 1 cm segments taken from first year Willow (Salix viminalisL.) shoots and by localized heat treatments in intact secondyear shoots. The eosin did not cause any change in the incorporationof ¹⁴C from labelled sucrose into an insoluble fraction butless of the label: from UDP-glucose labelled in the glucosemoiety was incorporated when eosin was present. Localized heattreatments of intact second-year tissue which was translocating[¹⁴C]sucrose failed to cause an increase in the amount of labelin an insoluble fraction in the zone where callose formationwas stimulated. The results indicate that the callose formedrapidly in response to stress and wounding is not derived directlyfrom translocated sucrose. An alternative possible origin, fromsugar nucleotides released from ruptured plastids, is discussed.     

The Phloem Translocation of Enantiomeric Pairs of Weak Acids in Ricinus communis

A possible role for specific carrier mechanisms in the phloemtranslocation of xenobiotics in Ricinus communis L. var. Gibsonii(Nichols) has been investigated by comparing the phloem transportof enantiomers of three acidic compounds. No differences intranslocation were found between the R– and S-enantiomersof phenylalanine, 2-methoxy-2-phenylpropanoic acid or 5-methyl-5-phenyloxazolidine-2,4-dione.These similarities between transport of enantiomers indicatethat stereospecific carriers do not occur, even for the endogenousamino acid phenylalanine of which only one enantiomer occursnaturally in phloem sap. Whilst transport of the enantiomerswas similar, there were differences between the mobility ofthe three compounds, and these could largely be explained interms of their physicochemical properties, without the needto invoke the existence of specific carrier mechanisms. Key words: Phloem transport, enantiomers, Ricinus communis, weak acids     

Elasticity of Phloem Tissues1

Linear displacement transducers and a pressure transducer wereused to simultaneously monitor pressure reduction and shrinkageassociated with phloem exudation following incision of the barkof Fraxinus americana L. The shrinkage associated with the pressurereduction indicated that the sieve tubes had elastic walls.Mathematical models of a mass flow system of phloem transporthave assumed rigid walls for the sieve tubes, but results demonstratedthat the dimensional changes associated with pressure potentialchanges would produce significant differences in volume flowrates if the pressure gradient were maintained at a constantvalue, and thus indicate a requirement for modification of themodels to accommodate elastic cell walls.     

Analysis of Tracer Profiles with Applications to Phloem Transport

Tracer experiments are quantitatively described in a way whichis independent of the tracer profile, and hence the loadingprocess. This enables properties of the transport system tobe determined which are independent of tracer profile shapeand hence of the loading process. Comparison is made betweentransit times obtained by this approach and by measurement directlyfrom tracer fronts. The direct method is shown to give resultswhich are dependent on the shape of the tracer profile. Thismethod also enables in vivo measurement of leakage from thetransport pathway.     

The Inhibition of Phloem Translocation by Ammonia

Bean plants (Phaseolus vulgaris L. cv. Fardenlosa Shiny) werelabelled with carbon-11 via their first trifoliate leaves when3-weeks-old and the transient inhibitions of translocation causedby the application of ammonium chloride solutions (10 mol m^–3)to a peeled region of stem were studied. At pH 6·5 theammonium was without effect. At pH 11·0 even a briefapplication inhibited translocation for many minutes, whilelonger applications inhibited translocation for considerablylonger. Solutions of 10 mol m^–3 sodium chloride were withouteffect at either pH. At pH 6·5 ammonium chloride solution contains predominantlyammonium ions (NH₄⁺) and at pH 11·0 predominantly dissolvedammonia gas (NH₃). Hence we conclude that phloem transport withinbean stems is inhibited by dissolved ammonia gas but not ammoniumions. Key words: Phloem translocation, transient inhibition, ammonia, ammonium ion     

Unusual Network of Internal Phloem in the Pod Mesocarp of Cowpea [Vigna unguiculata (L.) Walp. (Fabaceae)]

A mycelium-like network of internal phloem was observed in theinner mesocarp of the lateral pod walls of the fruit of certaingenotypes of cowpea [Vigna unguiculata (L.) Walp.] In the cultivarVita 3, the network consists of single, or rarely double, strandsof sieve elements and associated phloem parenchyma, orientedmainly parallel with the fibres of the adjacent endocarp, andstretching marginally beyond the sheets of fibres to connectabove and below with the outermost phloem of the longitudinalstrands of the dorsal and ventral sutures of the fruit. Theinternal phloem network does not relate conformationally to,or interconnect with the conventional (xylem+phloem) vasculatureof the mid mesocarp of the pod wall. In Vita 3, sieve elementsdifferentiate in the internal phloem after those in the majorveins of the pod, but before the presumptive endocarp fibrescommence wall thickening. The pod walls of twenty-one otherspecies of legumes proved negative for internal phloem, whileof nine varied genotypes of cowpea examined, six proved positive,three negative for the trait. Presence of internal phloem incowpea is not always associated with presence of endocarp fibresor necessarily with large fruits with large seeds. Possiblefunctions suggested for the phloem network are to provide assimilatesfor fibre wall thickening or to transport solutes to or fromsites of temporary storage in the fleshy inner layers of thepod wall. Internal phloem, legume fruit, translocation, mesocarp, pod wall, Vigna unguiculata, cowpea     

Seasonal Production of Secondary Phloem in the Twigs of Certain Tropical Timber Trees

Secondary phloem production in four deciduous (Albizzia lebbeck,Dalbergia sissoo, Tectona grandis and Terminalia crenulata)and three evergreen plants (Calophyllum inophyllum, Mangiferaindica and Morinda tinctoria) is briefly described. The totalduration of phloem production for each year was worked out forall these plants. In three of the four deciduous trees therewere two instalments of phloem production in correspondencewith the presence of two flushes of cambial activity while inTectona grandis and in all the three evergreen trees there wasonly one instalment. The time of initiation and cessation ofphloem tissue production was found to be variable in the differentplants studied. Periodicity in the production of different componentsof phloem tissue as well as the difference in the dimensionsof the different phloic elements produced during each flushof cambial activity resulted in detectable growth increments(or ‘rings’) within the phloem. There was a distinctshortening of the different phloem elements during the approachof dormancy/least activity. Conspicuous changes were found inthe ergastic contents of phloem parenchyma and ray cells adjacentto the cambial zone during the initiation of cambial activityand during the approach of dormancy/least activity. Seasonal growth, secondary phloem, deciduous and evergreen trees, cambial activity     

Phloem Unloading within the Seed coat of Pisum sativum Observed using Surgically Modified Seeds

Phloem unloading in pea seed coats was observed by removingthe embryos from developing seeds and washing the attached coatswith a weakly buffered solution. The quantity of labelled photosynthateappearing in the washing solution varied immediately when thesolute concentration was changed, and is shown to be an osmoticresponse. This response is predicted by the Münch theoryof phloem transport with concentration dependent unloading.Respiratory inhibitors and the sulphydryl modifying reagentPCMBS had a slow effect upon the washout of tracer, which arrivedwithin the seed coat prior to inhibitor application, but completelystopped any washout of tracer arriving after its application.This time-course suggests that the inhibitors were not directlyinhibiting unloading, but preventing further tracer from enteringthe region of unloading within the seed coat. Phloem unloadingwithin the seed coats of Pisum appears to be passive and notdependent upon a PCMBS-sensitive carrier. Key words: Pisum sativum, seeds, phloem unloading     

Electron Microscopic Observations of the Structure of Sieve-connexions in the Phloem of Angiosperms and Gymnosperms

The sieve elements of Pinus silvestris L., Sorbus aucupariaL., Vitis vinifera L., and Cucurbita pepo L. have been examinedelectron microscopically in ultra-thin section, and the structuresof the corresponding sieve areas or sieve plates have been describedand compared. In Pinus the sieve areas contain groups of connectingstrands which enter the wall from the lumen side as individualsbut coalesce within it in the median cavity. This cavity hasdeveloped by wall breakdown in the middle lamella and primarywall region and corresponds to the median nodule visible undera light microscope. Neither in this nor in the other speciesobserved is there any visible closing membrane. Structural differences between the four species are shown tosuggest that the major evolutionary trend in the evolution ofspecialized conducting strands has been the enlargement of theconnecting strands from groups of small separate strands toa smaller number of larger strands as the median cavity becomesenlarged to form a canal through the wall. The connecting strands appear invariably to be dense, highlyosmiophilic and continuous with the cytoplasmic surface of thecell. No signs of micropores or of other tubular structure inthe strands have been found. The structures thus revealed aremore nearly compatible with active transport of materials acrossthe sieve plate than they are with any purely physical mechanism.It is suggested that they are incompatible with any mass flowhypothesis.