A Relay Mechanism for Phloem Translocation

Munch's pressure-flow mechanism is the most popular hypothesisfor phloem translocation; however, difficulties with it arenot resolved. The metabolic pumping hypotheses on the otherhand often run into complications with the high flux-densityrequirement of translocation. A ‘relay’ mechanismis proposed for solute translocation in plants, which is a hybridbetween pressure flow and one or another of the, at present,alternative candidates for the task of driving phloem translocation.Such a compromise may make sense of the confusing experimentaland ultrastructural evidence. phloem transport, Münch pressure flow mechanism     

Phloem Translocation of Storage Nitrogen in Apple

Mobilization of nitrogenous compounds during the spring was studied in ringed isolated shoot sections (bearing one intact bud each) from Golden Delicious apple trees and in intact stem-ringed apple rootstocks M VII. The changes in total, protein and soluble nitrogen and soluble amino acids and amides were followed in the bark of the shoot sections for 3 weeks during leafing-out and in the shoot and stem bark of the rootstocks for 6 weeks starting at bud-break. Ringing prevented nitrogen movement from below the ring both in the shoot sections and in the rootstocks almost completely, thus demonstrating the importance of the phloem as translocation pathway for stored nitrogenous compounds, even over longer distances. Asparagine and arginine were the major soluble amino compounds throughout. The values of the asparagine/arginine quotient in the various tissues suggest that when the distance between points of nitrogen supply and demand is short asparagine is translocated preferentially, but that at increasing distance this preference shifts to arginine.     

The Electroshock-Induced Inhibition of Phloem Translocation

The transient inhibition of phloem translocation which is producedby an electric shock was studied by applying controlled-currentstimuli to short lengths of bean stem. Translocation was monitoredby observing the accumulation of carbon-11 label into the plantapex. The principal findings are: (i) For constant-current electricshocks whose (current) x (time) product was held constant at10 mC, those of higher current (and shorter duration) gave longerlasting inhibitions, (ii) Breaking a 5 s pulse into a trainof 100 ms pulses slightly shortened the duration of the inhibition,even though the same total charge was passed; however, the separationof the pulses within the train did not seem to matter. But,when the inhibition produced by a pair of 50 ms pulses was studied,the duration of inhibition increased with pulse spacing. (iii)Single pulses as short as 10 ms could produce a detectable inhibition.The duration of inhibition grew rapidly with pulse length forpulse lengths between 10 ms and 100 ms. (iv) When a pair of2.5 s pulses was applied, a longer inhibition was obtained ifthe two pulses were of opposite polarity. To explain these phenomena,a qualitative conceptual model is provided in terms of ion channelsin the phloem. In the experiments reported here and in all previous pulse-labellingexperiments from this laboratory, it has been noted that rapidlyincreasing stochastic variability in the count data made itextremely difficult to obtain useful results for a period ofmore than 2 h after loading, even if the number of counts perunit time remained fairly high. A quantitative theory for thislimitation is worked out and shown to agree with experimentaldata. Therefore, since biological variability from plant toplant makes it advisable to compare matched stimuli on the sameplant in order to detect trends reliably, the number of challengeswhich can usefully be applied following a single pulse-labellingwith a short-lived isotope is quite limited. Key words: Phloem translocation, transient inhibition, electric shock     

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     

A Mathematical Treatment of Munch's Pressure-Flow Hypothesis of Phloem Translocation

The steady state solutions of two mathematical models are used to evaluate Münch's pressure-flow hypothesis of phloem translocation. The models assume a continuous active loading and unloading of translocate but differ in the site of loading and unloading and the route of water to the sieve tube. The dimensions of the translocation system taken are the average observed values for sugar beet and are intended to simulate translocation from a mature source leaf to an expanding sink leaf. The volume flow rate of solution along the sieve tube, water flow rate into the sieve tube, hydrostatic pressure, and concentration of sucrose in the sieve tube are obtained from a numerical computer solution of the models. The mass transfer rate, velocity of translocation, and osmotic and hydrostatic pressures are consistent with empirical findings. Owing to the resistance to water flow offered by the lateral membranes, the hydrostatic pressure generated by the osmotic pressure can be considerably less than would be predicted by the solute concentration. These models suggest that translocation at observed rates and velocities can be driven by a water potential difference between the sieve tube and surrounding tissue and are consistent with the pressure-flow hypothesis of translocation.     

A Quantitative Investigation of a Peristaltic Model for Phloem Translocation

An analysis is made of a peristaltic model of phloem translocationIt is postulated that the periodic action of contractile orbending organelles drive a longitudinal flow of solution withintubules which connect sieve pores in successive sieve plates Plausible values are assumed for the velocity of propagationof the contraction wave, the frequency, the amplitude, and theviscosity of the solution and its concentration. Using relationswhich describe peristaltic flow, predictions are made for thevalues of parameters such as the velocity of solution, the drivingpressure, and rate of energy dissipation. These predicted valuesare seen to be reasonable when compared with the known propertiesof other biological contractile systems. Thus the model is quantitativelyacceptable.     

The Effect of Petiolar Temperature upon the Translocation Rate of 137Cs in the Phloem of Nymphoides peltata

A fixed length of the petiole of Nymphoides peltata was subjectedto a variety of temperature treatments. A Q₁₀ is calculatedfor the velocity of flow, from the mass transport rate of ¹³⁷Csin the phloem at temperatures between 0 and 50?C. Associatedwith lowered temperatures there was a temporary interruptionof transport overriding the longer term temperature dependence.The characteristics of the interruption were dependent uponthe size of the temperature drop and the magnitude of the ‘new’temperature.     

The Influence of Low Temperature on the Rate of Translocation in the Phloem of Salix viminalis L.

The rate of phloem translocation in willow stems was found tobe little reduced on cooling considerable lengths of the transportpathway. Even when 65 cm lengths of stem were cooled from about23 to 0 ?C the reduction in rate was judged to be less than20 per cent. It is argued that the velocity of translocationis well maintained in cooled stems and that there is no goodcase for supposing that the concentration of the translocatehad altered. The rate of radial transfer of materials from thephloem to the xylem did show a decrease in cooled zones. Theimplications of the results on current theories of phloem transportare discussed.     

Phloem Pressure Differences and C-Assimilate Translocation in Ecballium elaterium

The role of phloem turgor pressure in ¹⁴C-assimilate translocation in Ecballium elaterium A. Rich was studied. The direction of translocation was manipulated by two methods: darkening, or defoliation, of the upper or lower halves of the shoots. After 24 hours of labeled assimilate movement, sieve tube turgor levels were measured with the phloem needle technique. Distribution of label, determined by autoradiography and counting, revealed a direct correlation between the direction of assimilate transport and the pressure difference. Phloem turgor levels always decreased in the stem of darkened shoots; this resulted in greater pressure differences in the stem between the source leaf receiving ¹⁴CO₂ and treated regions.     

The Effect of 2,4-Dinitrophenol on Translocation in the Phloem

The effect of 2,4-dinitrophenol (DNP) on sucrose-¹⁴C transport in Soya seedlings has been analysed. The aim was to distinguish between an effect of the inhibitor on sugar movement within the phloem sieve tubes themselves, and on the prior steps of uptake and secretion of sugar into the conducting cells. DNP drastically inhibited sucrose-¹⁴C transport if it was applied to the ¹⁴C-treated leaf immediately before, or during, ¹⁴C supply. Transport was also strongly inhibited if DNP was applied along the translocation path while the ¹⁴C-treated leaflet was still in position on the plant. When, bowever, DNP was applied through the cut petioles of the primary leaves after removal of the ¹⁴C-treated terminal leaflet of the first trifoliate leaf, no inbibition was observed. On the contrary, transport appeared to have been promoted: significantly more ¹⁴C disappeared from the upper regions of DNP-treated plants as compared with controls, while in the lower plant parts more ¹⁴C accumulated. Different rates of synthesis of sucrose-¹⁴C into non-alcohol-soluble compounds could not account for this result. A similar stimulatory effect was observed when DNP was applied to the cut petiole of a primary leaf opposite that treated with ¹⁴C. Several indications were obtained that ¹⁴C which has reached the lower parts of the plant may circulate upwards again through the phloem within about 15 minutes. When sucrose-¹⁴C was introduced into the roots via the xylem, both DNP treatment and prior steam girdling resulted in the apparent accumulation of ¹⁴C in the lower plant parts. the results would be compatible with DNP inhibition of upwards movement in the phloem. DNP might also have affected sugar uptake processes in cells neighbouring the translocation path. It is concluded that the inhibitory effect of DNP on downwards phloem transport reported by earlier workers was probably due to an effect on uptake and/or secretion into the sieve tubes, not to an effect on the conducting cells themselves. Modern theories for phloem transport are discussed in the light of these findings.     

Sucrose Hydrolysis in Relation to Phloem Translocation in Beta vulgaris

Asymmetrically labeled sucrose, ¹⁴C(fructosyl)sucrose, was used to determine whether sucrose undergoes extracellular hydrolysis during phloem translocation in the sugar beet, Beta vulgaris. In addition, the metabolism of various sugars accumulated and translocated was determined in various regious of the plant. These processes were studied in detached regions as well as in the intact, translocating plant in the source leaf, along the translocation path, and in a rapidly growing sink leaf and storage beet. The data show that, unlike sucrose accumulation into the sink tissue of sugarcane, sucrose is neither hydrolzyed prior to phloem loading or during transit, nor is it extracellularly hydrolyzed during accumulation into sink leaves or the storage beet.     

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     

Phloem Translocation and Heat-induced Callose Formation in Field-grown Gossypium hirsutum L

Phloem translocation rates in field-grown cotton (Gossypium hirsutum L.) dropped from morning to afternoon and continued to decline toward evening, except that recovery occurred following the hottest afternoon when the maximum temperature was 44 C. Water deficits increased from morning to evening, and severity of deficits generally were proportional to daytime heating. Water stress contributed toward reducing translocation but was not always the governing factor. Callose breakdown appeared to be slower than heat-induced synthesis, and in the evening callose still reflected the influence of high afternoon temperatures. Translocation was considerably reduced when about 50% or more of the hypocotyl sieve plates had large amounts of callose. While heat-induced callose may have reduced translocation because of sieve plate pore constriction, temperatures of 39 to 44 C appeared to inhibit an additional component of translocation as well, possibly in the leaf blade.     

Calculation of Translocation Coefficients from Phloem Anatomy for use in Crop Models

Translocation coefficients, computed for unit ground area, arerequired in crop models as part of the simulation of the partitioningof assimilates. An equation for translocation is derived byconsidering pressure-driven flow and the physical dimensionsof the pathway in the phloem. Numerical estimates of the translocationcoefficients for grasses and trees are calculated using anatomicaldata. The values found are compatible with published rates oftranslocation in the phloem.Copyright 1995, 1999 Academic Press Model, Münch, phloem, sieve tube, translocation     

Respiration and Phloem Translocation in the Roots of Chickpea (Cicer arietinum)

Root growth in chickpea (Cicer arietinum) has been studied fromthe early vegetative phase to the reproductive stage in orderto elucidate its growth and maintenance respiration and to quantifythe translocation of assimilates from shoot to root. A carbonbalance has been drawn for this purpose using the growth andrespiration data. The increase in the sieve tube cross-sectionalarea was also followed simultaneously. Plants growing in a nutrient culture medium were studied todetermine the relative growth rate (RGR) 5–60 d aftergermination. RGR declined from 113 to 41 mg d^–1 g^–1during the measurement period. Simultaneous with the RGR analysis,respiration rate was also measured using an oxygen electrode.The respiration rate declined as the plants aged and a drasticreduction was recorded following anthesis. The relationshipbetween RGR and respiration rate was used to extrapolate themaintenance respiration (m) and growth respiration (1/Y_EG).The respiration quotient (r.q.) of the roots was 1.2 and theQ₁₀ in the range 20–25 °C was 2·2. A carbon balance for the roots was constructed by subtractingthe carbon lost during respiration from that gained during growth.The roots were found to respire no less than 80% of the carbontranslocated. The increase in the cross-sectional area composed of sieve tubeswas measured near the root-shoot junction as the plants grew.Chickpea has storied sieve plates which simplifies these measurements.Their cross-sectional area increased during growth mainly becauseof an increase in sieve tube number. The diameter of individualsieve tubes remained constant. Specific mass transfer (SMT) values for seive tubes into theroots have been computed during various stages of growth. SMTvalues were relatively constant before anthesis (approx. 6·5g h^–1 cm^–2), but decreased following anthesis. Wedid not evaluate possible retranslocation from roots: any suchretranslocation would have the effect of increasing our SMTvalues. Chickpea, Cicer arietinum, legume, root, respiration, phloem, translocation, carbon balance, specific mass transfer, sieve-tube dimensions     

The Effect of Cooling on Translocation in Salix viminalis

Radioactive phosphate was applied externally to a small areaof the bark of the main stem below the point of insertion ofa leafy branch. When photosynthesis and hence translocationwas proceeding, phosphate was carried down the stem and wasmonitored externally by two Geiger counters spaced 10 cm apartfurther down the stem. Sudden cooling to 0 °C of the leaves,or the stem between the leaves and the point of applicationof the isotope, caused a rapid and seemingly simultaneous responseby the two Geiger counters. It is concluded that cooling atone point causes an instantaneous reduction of velocity alongthe whole pathway. If cooling is continued for several hoursthe rate of translocation of phosphate recovers. The evidencesuggests that this recovery is due to a compensatory increasein concentration of radioactive phosphate in the translocationstream rather than a recovery of velocity.     

A Pod Leakage Technique for Phloem Translocation Studies in Soybean (Glycine max [L.] Merr.)

Radioactive photosynthetic assimilates, translocated to a soybean (Glycine max [L.] Merr. `Fiskeby V') pod can be measured directly by excising the stylar tip of the pod under 20 mm ethylenediaminetetraacetate solution (pH 7.0) and allowing the material to leak into the solution. Pods at the source node received approximately 50% of the ¹⁴C exported from the source leaf to the pod and leaked approximately 1 to 3% of this into the solution. More than 90% of the ¹⁴C that leaked from the pods was found in the neutral fraction and, of this, about 93% was in sucrose. Fifteen amino acids were identified in the leakage including: alanine, arginine, asparagine, γ-aminobutyric acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, phenylalanine, serine, threonine, tyrosine, and valine. The majority of the ¹⁴C in the basic fraction was found in serine (30%) and asparagine (23%). The inorganic ions K, Ca, P, Mg, Zn, and Fe were found in the leakage component. Nitrate was not detectable in the collected leakage solution. The absence of NO₃⁻ and the large proportion of the label in sucrose suggest a possible phloem origin for most of the material. The technique provides an uncomplicated, reproducible means of analyzing the material translocated into and through the soybean pod, as well as following the time course of label arrival at the pod.     

Glutamine Transfer from Xylem to Phloem and Translocation to Developing Leaves of Populus deltoides

The distribution of ¹⁴C from xylem-borne [¹⁴C]glutamine, the major nitrogen compound moving in xylem sap of cottonwood (Populus deltoides Bartr. ex Marsh), was followed in rapidly growing shoots with a combination of autoradiographic, microautoradiographic, and radioassay techniques. Autoradiography and ¹⁴C analyses of tissues showed that xylem-borne glutamine did not move with the transpiration stream into mature leaves. Instead, most of it was transferred from xylem to phloem in the upper stem and then translocated to young developing tissues. Microautoradiography showed that metaxylem parenchyma, secondary xylem parenchyma, and rays were the major areas of uptake from xylem vessels in the stem. Accumulation in phloem (high ¹⁴C concentrations in sieve tubes) took place in internodes subtending recently mature leaves. Little ¹⁴C from xylem-borne glutamine was found in phloem of mature leaves, which indicates restricted retransport of glutamine that did enter the leaf. In the primary tissues of the upper stem, most ¹⁴C was found in the phloem. Cottonwood stems have an efficient uptake and transfer system that enhances glutamine movement to developing tissues of the upper stem.     

Cold-Inhibited Phloem Translocation in Sugar Beet: IV. ANALYSIS OF THE COOLING-INDUCED REPARTITIONING HYPOTHESIS

Systems identification techniques were used to calculate photo-assimilatetransport characteristics in Beta vulgaris and Pisum sativum,before and after the application of localized perturbationsto the transport path. Changes in photo-assimilate partitioningto various monitored sinks were found to be promoted by slowcool (25 ?C to 1 ?C in 40 min), quick cool (25 ?C to 1 ?C in2 min)/quick warm, and apoplastic osmotic treatments of a localizedregion of the source leaf petiole in Beta. Photo-assimilatepartitioning into both intact and surgically modified (embryo-less)ovules of Pisum was also observed to change following quickcool/quick warm treatments applied to a 2.0 cm region of thepeduncle leading to the monitored pod. No changes in transportproperties were observed through the treated petiole regionof Beta during or following the slow cool treatment (i.e. transittime or system gain). High apoplastic osmolality (addition of1 000 mol m^–3 sorbitol) reduced the transit time of tracermovement through the treated petiole of Beta, while enhancingtracer washout from the bathed tissue region. Our modellingtechniques have shown that the physical or physiological basisfor the sink partitioning changes must be due to alterationsdownstream of the treated pathway zone, suggesting the involvementof physical signals transmitted from the treated region. Theseresults support the previously presented cooling-induced repartitioninghypothesis (Grusak and Lucas, 1986) and demonstrate that pathway-originatedstimuli can alter source-sink photo-assimilate partitioning. Key words: Photo-assimilate partitioning, phloem translocation, Beta vulgaris, Pisum sativum