Translocation in the Stolon of Saxifraga sarmentosa L. The Ultrastructural Background

The ultrastructure of the mature sieve elements of the Saxifragastolon is described. Theseare narrow (3–5 µm) andfairly long (100-300 µm). The sieve plate pores were invariablyfound to be closely-occluded with P-protein. It is argued thatthe double-cutting technique used for excision invalidates thecontention that this must be interpreted as an artifact. TheP-protein filaments appear to consist of a double helix. Stereomicrographs at normal voltages, and at 1 MV in the AEI-EM₇ microscopeare presented.     

Unidirectional movement of tracers along the stolon of Saxifraga sarmentosa

Summary When radioactive tracer is applied locally to the stolon of Saxifraga its long-distance movement after 18 hours is found to be strongly polarised; there is in addition a short-distance movement which is unpolarised. With caesium, the long-distance movement is predominantly in the phloem; with strontium in the xylem. These interpretations, a priori probable, were confirmed by artifically reversing, separately, the xylem and the phloem currents. With long pieces of excised stolon only the unpolarised short-distance movement is observed. These results constitute evidence against simultaneous bidirectional translocation in the same sieve tube, and are consistent with either the Münch or the electro-osmotic theory.     

The influence of dinitrophenol on phloem transport along the stolon of Saxifraga sarmentosa

Summary Dinitrophenol in concentrations of 5×10^-3 M applied to the centre 30 cm of 60–70 cm stolons of Saxifraga produces a strong and reversible inhibition of the phloem transport of ¹³⁷Cs or ¹⁴C-assimilates. There is every reason to believe that this effect is localised in the sieve tubes; callose formation does not occur. This evidence is very difficult to reconcile with the Münch hypothesis; it seems on the contrary to demand a theory of active pumping.This work formed part of that submitted for the degree of Ph.D. of London University.     

Development of the Sieve Plate in Saxifraga sarmentosa L.

The differentiating sieve plate in the phloem of the stolonof Saxifraga sarmentosa L. was studied with the electron microscope.Development of the pore site begins with differentiation ofa pair of collar-like areas around the plasmodesma which canbe seen in the youngest identifiable sieve plates. Further growthof the collars occurs by deposition of an amorphous substance,presumably caflose. Although the growth of the collars is simultaneouswith the growth of the surrounding cell wall it is rapid atfirst and the pore sites appear asdome-shaped protuberances.It also involves deposition of callose over an increasinglywider area of the cell wall and since the thickening of thenormal cell wall continues only where notcovered by callose,the collars assume a conical form. There seems to be no displacementor lysis of normal cell wall material during growth of the collars.Eventually the growth of the cell wall in thickness overtakesthe pore sites so that when the growth of the cell wall is completethe pore sites appear as depressions in the sieve plate. Theperforation of a pore site is accomplished by widening of theplasmodesmatal cylinder which begins at the middle lamella byremoval of callose. Endoplasmic reticulum is found in closeproximity to the plasmodesma andis believed to penetrate it.     

Movement of [14C]sucrose along the stolon of Saxifraga sarmentosa

Summary The characteristics of ¹³⁷Cs transport along the stolon of Saxifraga previously reported have been confirmed for applied sucrose and natural assimilate. Long-distance transport is strictly unidirectional, with a symmetrical short-distance spread from the point of application. Only the latter takes place in a long piece of excised stolon. Transport is readily reversed when the parent plant is darkened and the daugther, plantlet allowed to photosynthesise. These findings strongly support a mass-flow mechanism for the stolon. They also confirm the value of ¹³⁷Cs as a tracer for assimilate movement, though in contrast to assimilate it suffers appreciable lateral leakage. Pulse labelling of the subtending leaf failed to produce a sharp peak of activity in the stolon. A flattening with time of the ¹⁴C profile is considered to be due to differing linear velocities in parallel sieve tubes.This work formed part of that submitted for the degree, of Ph. D. of the University of London.     

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     

The Effect of Inhibitors on Callose Formation in the Sieve Tubes of Saxifraga sarmentosa L.

Electron-microscope studies on stolons undergoing transportinhibition under nitrogen, HCN, and DNP treatment show no evidenceof callose-blockage of the sieve plates. Inhibition must thereforebe interpreted in terms of interruption of the sieve-tube energysupply; this weights the evidence in favour of theories of activemass flow such as those invoking electro-osmosis or proteincontractility.     

The effect of low temperatures on the phloem transport of radioactive assimilates in the stolon of Saxifraga sarmentosa L.

Abstract. The movement of naturally assimilated ¹⁴C down the stolon of Saxifraga sarmentosa under the influence of localized cold treatments (0°-10°C) applied over lengths up to 10 cm was followed by finding the distribution of tracer at the termination of the experiment. Inhibition is slight at 10°C and very considerable at 0°C. It is very dependent on the length treated, but is definite even when this is only 0.8 cm. Cooling the daughter plantlet to 0°C has a smaller inhibitory effect than might be expected. This is difficult to understand if 'unloading' is metabolic; perhaps the concentration free energy of the sucrose or other solute can be utilized in the sink terminals on the lines of the chemi-osmotic hypothesis. If so the same source of non-respiratory free energy might be available to energize an active sieve-tube mechanism in mid-path chilling experiments.     

Leaf Export and Partitioning Changes Induced by Short-Term Inhibition of Phloem Transport

Both electric shock and cold shock applied to the phloem pathway,and known to induce temporary inhibition to long-distance phloemtransport, are shown to have an immediate and sustained effectupon both export of carbon from the leaf and its partitioningbetween alternative sinks. The changes in export occurred byan unknown mechanism, while the changes in partitioning areinterpreted as responses to changed export Key words: Carbon partitioning     

Concentrations and Transport of Solutes in Xylem and Phloem along the Leaf Axis of NaCl-treated Hordeum vulgare

Hordeum vulgare cv. California Mariout was established in sandculture at two different NaCl concentrations (0.5 mol m^–3‘control’ and 100 mol m^–3) in the presenceof 6.5 mol m^–3 K ⁺. Between 16 and 31 d after germination,before stem elongation started, xylem sap was collected by useof a pressure chamber. Collections were made at three differentsites on leaves 1 and 3: at the base of the sheath, at the baseof the blade, i.e. above the ligule, and at the tip of the blade.Phloem sap was collected from leaf 3 at similar sites throughaphid stylets. The concentrations of K ⁺, Na⁺, Mg2⁺ and Ca²⁺were measured. Ion concentrations in xylem sap collected at the base of leaves1 and 3 were identical, indicating there was no preferentialdelivery of specific ions to older leaves. All ion concentrationsin the xylem decreased from the base of the leaf towards thetip; these gradients were remarkably steep for young leaves,indicating high rates of ion uptake from the xylem. The gradientsdecreased with leaf age, but did not disappear completely. In phloem sap, concentrations of K⁺ and total osmolality declinedslightly from the tip to the base of leaves of both controland salt-treated plants. By contrast, Na⁺ concentrations inphloem sap collected from salt-treated plants decreased drasticallyfrom 21 mol m^–3 at the tip to 7.5 mol m^–3 at thebase. Data of K/Na ratios in xylem and phloem sap were used to constructan empirical model of Na⁺ and K⁺ flows within xylem and phloemduring the life cycle of a leaf, indicating recirculation ofNa⁺ within the leaf. Key words: Hordeum vulgare, xylem transport, phloem transport, NaCl-stress     

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.     

Solution-Flow in the Phloem: II. Phloem Transport of THO in Beta vulgaris

Translocation profiles along the path were studied using a modified flap-feeding technique for the simultaneous application of THO and ¹⁴C-sucrose. A re-evaluation of a mathematical model for phloem transport with reversible lateral exchange of tracer along the path indicates that lower apparent velocities for THO as compared to labeled carbohydrate are primarily due to extensive lateral exchange of THO along the conduction path. Path-chilling experiments support the concept that THO and ¹⁴C-sucrose exhibit different lateral exchange characteristics. The data presented are consistent with a solutionflow mechanism.     

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 Electro-osmotic Theory of Phloem Transport in the Light of Recent Measurements on Heracleum Phloem

The bearing, on the electro-osmotic theory, of work on the Onsagercoefficients for Heracleum phloem, currently reported by Tyreeand Fensom (1970), is briefly discussed. As an aid to the discussionthe Onsager resistance coefficients are evaluated in terms ofSpiegler's frictional model. It is concluded that the experimentalevidence, while it does not support the theory, does not invalidateit. Some further suggestions regarding the electro-osmotic theoryare put forward.     

Rapid Propagation of Changes in Velocity of Translocation along the Phloem Pathway of Helianthus annuus L.

Localized interruptions of the phloem pathway of sunflower plants,by heat ringing, caused prompt falls in rate of translocationat sites up to 50 cm distant from the ring. The velocity withwhich this change in rate was propagated down the phloem wasan order of magnitude greater than the velocity of translocationitself. It is argued that the findings give support to the pressure-flowhypothesis but do not fit so well with certain other ideas concerningpossible systems by which translocation could be brought about.     

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     

On the Phloem of Mimosa pudica L.

The phloem of Mimosa pudica L. was examined in view of somereports that the sieve elements in this plant show featuresnot previously described for these cells in Leguminosae. Inthe present study only a usual dicotyledonous type of sieveelement was recognized. The sieve elements pass through stagesof differentiation involving development and dispersal of P-proteinbodies, disintegration of nuclei, and appearance of plastidsstoring a starch staining red with iodine. Callose occurs onthe transverse or moderately oblique sieve plates. The phloemcontains secretory cells. They are wide and long and have transverseor more or less inclined end walls. In younger cells the endwall bears in the centre a conspicuous pit to which the protoplastsof the superimposed cells are firmly attached. In many oldercells the pit region is replaced by a perforation so that thecontents of superimposed cells may be completely merged. Itremains to be determined whether the perforation is presentin an intact plant or results from a rupture during sampling.The secretory cells accumulate material that gives a positivetest for carbohydrates and a negative test for proteins.     

Reproduction of the rare monocarpic species Saxifraga mutata L.

The aim of the study is to investigate the impact of reproduction and genetic variation on the persistence of populations of the prealpine, monocarpic Saxifraga mutata L. The species grows on erosion slopes or rocks, and its local populations are often small and isolated. Crossing experiments resulted in better seed-set than selfing, but both yielded viable seeds. Agamospermy did not occur. In an early-successional species like S. mutata , successful selfing is important in the colonization of new habitats. Flowers of S. mutata were visited by Syrphidae and unspecialized Hymenoptera. A germination rate of 40% was reached in cultivation after 20 weeks but germination continued until the end of the experiment after 92 weeks. Seeds stored dry for 30 months at room temperature mostly lost their germinability. In natural habitats, seedlings were found almost throughout the year with a peak in spring. Suitable safe sites were small patches of open soil, bare marl on erosion slopes, and rock crevices. AU individuals investigated were diploid with 2n = 26. Allozyme electrophoresis showed a lack of segregation within the populations. Intra- and interpopulation genetic variation was low. These results were in partial disagreement with theoretical expectations in a mixed mating species. It is concluded that demographic rather than genetic processes are the main cause of extinction of populations of S. mutata , at least in the short-term.     

Effects of Plant Hormones on Phloem Transport in Grapevines

The translocation of assimilates in grapevines, their nature and pathways as well as the possible role of hormones in the translocation processes are summarized in the light of the source-sink-relationship between vegetative growth and berry development.