Benzyladenine-Directed Transport of Carbon-14 and Phosphorus-32 in Senescing Bean Plants

The distribution of radioactivity from applied sucrose.¹⁴C and³²P to various plant parts were studied in relation to the retardationof leaf senescence by applied benzyladenine (BA) in intact beanplants. In short-time experiments sucrose-¹⁴C was fed to theplants for 48 h through the second trifoliate leaf at weeklyintervals from the third to the eighth week after planting.In long-term experiments sucrose-¹⁴C was fed to all plants for48 h at the third week and changes in distribution examinedat weekly intervals up till the eight week. In both cases, BAapplied to the primary leaves of intact bean plants did notcause a directed mobilization of sucrose-¹⁴C. When the plantswere stripped, leaving the primary leaves and the terminal pod,and fed sucrose-¹⁴C or ³²P through the terminal leaflet of thesecond trifoliate leaf, the BA-treated leaf accumulated relativelymore radioactivity than the opposite water-treated leaf. Itwas concluded that the retardation of senescence by BA in theprimary leaves of intact bean plants does not result directlyfrom the mobilization of metabolites and nutrients from otherplant parts. It is therefore suggested that BA-induced longevityin the primary leaves of the intact plant is accomplished bymetabolic self-sustenance.     

Auxin transport in intact pea seedlings (Pisum sativum L.): The inhibition of transport by 2,3,5-triiodobenzoic acid

Summary The application of 2,3,5-triiodobenzoic acid (TIBA, 10 mg·g^-1 in lanolin) to the stem of intact pea seedlings (Pisum sativum L.) inhibited the basipetal transport of ¹⁴C from indoleacetic acid-1-¹⁴C (IAA-1-¹⁴C) applied to the apical bud, but not the transport of ¹⁴C in the phloem following the application of IAA-1-¹⁴C or sucrose-¹⁴C to mature foliage leaves. It was concluded that fundamentally different mechanisms of auxin transport operate in these two pathways.When TIBA was applied at the same time as, or 3.0 h after, the application of IAA-1-¹⁴C to the apical bud, ¹⁴C accumulated in the TIBA-treated and higher internodes; when TIBA was applied 24.0 h before the IAA-1-¹⁴C, transport in the stem above the TIBA-treated internode was considerably reduced. TIBA treatments did not consistently influence the total recovery of ¹⁴C, or the conversion of free IAA to indoleaspartic acid (IAAsp). These results are discussed in relation to the possible mechanism by which TIBA inhibits auxin transport,.Attention is drawn to the need for more detailed studies of the role of the phloem in the transport of endogenous auxin in the intact plant.Abbreviations TIBA 2,3,5-triiodobenzoic acid - IAAsp indoleaspartic acid     

Long distance translocation of sucrose, serine, leucine, lysine, and carbon dioxide assimilates: I. Soybean

To determine the selectivity of movement of amino acids from source leaves to sink tissues in soybeans (Glycine max [L.] Merr. `Wells'), ¹⁴C-labeled serine, leucine, or lysine was applied to an abraded spot on a fully expanded trifoliolate leaflet, and an immature sink leaf three nodes above was monitored with a GM tube for arrival of radioactivity. Comparisons were made with ¹⁴C-sucrose and ¹⁴CO₂ assimilates. Radioactivity was detected in the sink leaf for all compounds applied to the source leaflet. A heat girdle at the source leaf petiole essentially blocked movement of applied compounds, suggesting phloem transport. Transport velocities were similar (ranged from 0.75 to 1.06 cm/min), but mass transfer rates for sucrose were much higher than those for amino acids. Hence, the quantity of amino acids entering the phloem was much smaller than that of sucrose. Extraction of source, path, and sink tissues at the conclusion of the experiments revealed that 80 to 90% of the radioactivity remained in the source leaflet. Serine was partially metabolized in the transport path, whereas lysine and leucine were not. Although serine is found in greater quantities than leucine and lysine in the source leaf and path of soybeans, applied leucine and lysine were transported at comparable velocities and in only slightly lower quantities than was applied serine. Thus, no selective barrier against entry of these amino acids into the phloem exists.     

Phloem Unloading in Developing Leaves of Sugar Beet : I. Evidence for Pathway through the Symplast

Physiological and transport data are presented in support of a symplastic pathway of phloem unloading in importing leaves of Beta vulgaris L. (`Klein E multigerm'). The sulfhydryl reagent p-chloromercuribenzene sulfonic acid (PCMBS) at concentration of 10 millimolar inhibited uptake of exogenous [¹⁴C]sucrose by sink leaf tissue over sucrose concentrations of 0.1 to 5.0 millimolar. Inhibited uptake was 24% of controls. The same PCMBS treatment did not affect import of ¹⁴C-label into sink leaves during steady state labeling of a source leaf with ¹⁴CO₂. Lack of inhibition of import implies that sucrose did not pass through the free space during unloading. A passively transported xenobiotic sugar, l-[¹⁴C]glucose, imported by a sink leaf through the phloem, was evenly distributed throughout the leaf as seen by whole-leaf autoradiography. In contrast, l-[¹⁴C]glucose supplied to the apoplast through the cut petiole or into a vein of a sink leaf collected mainly in the vicinity of the major veins with little entering the mesophyll. These patterns are best explained by transport through the symplast from phloem to mesophyll.     

Uptake and Utilization of Xylem-borne Amino Compounds by Shoot Organs of a Legume

Amino compounds representative of the major N solutes of xylem sap were pulse-fed (10 to 20 minutes) singly in ¹⁴C-labeled form to cut transpiring shoots of white lupin (Lupinus albus L.). ¹⁴C distribution was studied by autoradiography and radioassays of phloem sap, leaflet tissues, and shoot parts harvested at intervals after labeling. Primary distribution of N by xylem was simulated using a 20-minute labeling pulse followed by a 30-minute chase in unlabeled xylem sap. Shoots fed ¹⁴C-labeled asparagine, glutamine, valine, serine, or arginine showed intense labeling of leaflet veins and marked retention (35 to 78%) of ¹⁴C by stem + petioles. Shoots fed ¹⁴C-labeled aspartic acid or glutamic acid showed heaviest ¹⁴C accumulation in interveinal regions of leaflets and low uptake (11 to 20%) of ¹⁴C by stem + petioles. Departing leaf traces were major sites of uptake of all amino compounds, and the implications of this were evaluated. Fruits acquired only 1 to 5% of the fed label directly from xylem, but more than doubled their intake during the period 30 to 160 minutes after feeding through receipt of ¹⁴C transferred from xylem to phloem in stem and leaves. ¹⁴C-Labeled asparagine and valine transferred directly from xylem to phloem, but the ¹⁴C of ¹⁴C-labeled aspartic acid and arginine appeared in phloem mainly as metabolic products of the fed compound. The labeling of the soluble pool of leaflets reflected these differences. The significance of heterogeneity in distribution and metabolism of xylem amino compounds in the shoot was discussed.     

Assimilate Transport in Cucurbits

The long-distance transport of sugars has been investigatedin cucurbits. ¹⁴C-labelled sugars were applied to an abradedleaf surface and the content of ¹⁴C in various fractions ofextracts from leaf blade, petiolar tissue, and phloem exudatesubsequently determined. Nearly one half of the ¹⁴C activitydetermined in the phloem sap was in amino and organic acids,the other half in sugars, whereas in leaf and petiolar tissuemost of the ¹⁴C activity was found in the neutral sugar fraction.As the solute concentration of cucurbit phloem sap is relativelylow the calculated rate of mass transfer would require hightransport velocities and large areas of phloem. However, thedriving force for such translocation is not known. Key words: Phloem transport, Cucurbits, Specific mass transfer     

<Superscript>11</Superscript>C-imaging: methyl jasmonate moves in both phloem and xylem,promotes transport of jasmonate,and of photoassimilate even after proton transport is decoupled

The long-distance transport and actions of the phytohormone methyl jasmonate (MeJA) were investigated by using the short-lived positron-emitting isotope ¹¹C to label both MeJA and photoassimilate, and compare their transport properties in the same tobacco plants (Nicotiana tabacum L.). There was strong evidence that MeJA moves in both phloem and xylem pathways, because MeJA was exported from the labeled region of a mature leaf in the direction of phloem flow, but it also moved into other parts of the same leaf and other mature leaves against the direction of phloem flow. This suggests that MeJA enters the phloem and moves in sieve tube sap along with photoassimilate, but that vigorous exchange between phloem and xylem allows movement in xylem to regions which are sources of photoassimilate. This exchange may be enhanced by the volatility of MeJA, which moved readily between non-orthostichous vascular pathways, unlike reports for jasmonic acid (which is not volatile). The phloem loading of MeJA was found to be inhibited by parachloromercuribenzenesulfonic acid (PCMBS) (a thiol reagent known to inhibit membrane transporters), and by protonophores carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and 2,4-dinitrophenol (DNP) suggesting proton co-transport. MeJA was found to promote both its own transport and that of recent photoassimilate within 60 min. Furthermore, we found that MeJA can counter the inhibitory effect of the uncoupling agent, CCCP, on sugar transport, suggesting that MeJA affects the plasma membrane proton gradient. We also found that MeJA’s action may extend to the sucrose transporter, since MeJA countered the inhibitory effects of the sulfhydryl reagent, PCMBS, on the transport of photoassimilate.     

The Effects of Fusicoccin,Abscisic Acid,and Benzyladenine on the Transport and Partitioning of Substances as Related to Cytokinin Sink Activity

We tested the possible cytokinin effect on the functioning of the active transport system involved in the assimilate loading into the phloem as a cause for the cytokinin sink and retention effect. This effect is manifested in the deceleration of substance export from and the stimulation of substance import to the sites of local phytohormone application to the mature detached leaf from untreated leaf areas. To affect the membrane mechanisms of the substance transport, we used leaf treatment with the phytotoxin fusicoccin, an enhancer of plasmalemmal H⁺-ATPase and a potential stimulator of assimilates export, and with the phytohormone ABA affecting transport, metabolism, and plant growth. However, fusicoccin did not enhance ¹⁴C-sucrose export from the leaf blade and did not interfere with the cytokinin-induced export deceleration. ABA reduced substantially ¹⁴C export from the leaf but eliminated the cytokinin effect on this process. Similar results were obtained for broad bean (Vicia faba L.) leaves with apoplastic phloem loading, involving H⁺-ATPase activity, and pumpkin (Cucurbita pepo L.) leaves with symplastic phloem loading, that is, occurring without sucrose transmembrane translocation and without H⁺-ATPase involvement. The conclusion is that the cytokinin-induced development of sink zones in source leaves is not related to the membrane mechanisms of the substance transport in the mesophyll–phloem system. The data obtained support the idea that the cause for the cytokinin sink and retention effect is the enhancement of elongation growth and total activation of metabolism in the mesophyll cells of the detached leaf.     

HORMONAL CONTROL OF TRANSLOCATION OF PHOTOSYNTHETICALLY ASSIMILATED 14C IN YOUNG SOYBEAN PLANTS

The normal supply of growth substances to a young soybean plant was altered by removing the plant's apical meristem and replacing this meristem with an aqueous solution of either indole-3-acetic acid (IAA), gibberellic acid (GA), or water. The length of each experiment was 1 hr. In the middle of it, ¹⁴CO₂ was administered to one of the primary leaves of the plant, and at the end distribution of ¹⁴C in various parts of the plant was determined. It was found that an addition of growth substances stimulated translocation in three different ways. Both IAA and GA increased the total amounts of sucrose-¹⁴C translocated, increased the rate of its translocation, and affected the distribution pattern of translocated sucrose throughout the plant. Experiments using IAA-¹⁴C have shown that the action of IAA is on the longitudinal translocation in the stem and not on the transfer of photosynthate from the mesophyll to the conducting tissues of the leaf.     

Comparison of upward and downward translocation of C from a single leaf of sunflower

When single leaves attached at a given node were allowed to carry on photosynthesis in ¹⁴CO₂ for 30 min, younger plants showed a higher proportion of upward translocation than did older plants. Downward translocation of ¹⁴C-photosynthate was stimulated by ATP pre-treatment of the translocating leaf, while upward translocation was not affected by ATP. A similar phenomenon was observed in the translocation of ¹⁴C-sucrose infiltrated into a leaf with or without ATP. Downward translocation of photosynthate was inhibited by DNP pre-treatment of a fed leaf. Upward translocation, however, was not affected by DNP. Thirty min after infiltration of ¹⁴C-glucose into a leaf, almost all the ¹⁴C translocated upwards was found to be in the form of glucose, while a great part of the ¹⁴C translocated downwards was in the form of sucrose. In the case of translocation of infiltrated ¹⁴C-sucrose, ¹⁴C found both above and below the fed leaf was mainly in the form of sucrose.     

Autoradiographic and biochemical evidence for the systemic translocation of systemin in tomato plants

The movement of systemin, the 18-amino-acid polypeptide inducer of proteinase inhibitors in tomato (Lycopersicon esculentum L.) plants, was investigated in young tomato plants following the application of [¹⁴C]systemin to wounds on the surface of leaves. Wholeleaf autoradiographic analyses revealed that [¹⁴C]systemin was distributed throughout the wounded leaf within 30 min, and then during the next several hours was transported to the petiole, to the main stem, and to the upper leaves. The movement of [¹⁴C]systemin was similar to the movement of [¹⁴C]sucrose when applied to leaf wounds, except that sucrose was slightly more mobile than systemin. Analyses of the radioactivity in the petiole phloem exudates at intervals over a 5-h period following the application of [¹⁴C]systemin to a wound demonstrated that intact [¹⁴C]systemin was present in the phloem over the entire time, indicating that the polypeptide was either stable for long periods in the phloem or was being continually loaded into the phloem from the source leaf. The translocation pathway of systemin was also investigated at the cellular level, using light microscopy and autoradiography. Within 15 min after application of [³H]systemin to a wound on a terminal leaflet, it was found distributed throughout the wounded leaf and was primarily concentrated in the xylem and phloem tissues within the leaf veins. After 30 min, the radioactivity was found mainly associated with vascular strands of phloem tissue in the petiole and, at 90 min, label was found in the phloem of the main stem. Altogether, these and previous results support a role for systemin as a systemic wound signal in tomato plants.The authors acknowledge the Washington State University Electron Microscope Center and staff for their technical advice and collaboration. We also thank Greg Wichelns for growing our plants and Dr. Steven Doares for providing [³H]systemin. This research was supported in part by the Washington State College of Agriculture and Home Economics Project No. 1791 and National Science Foundation grants IBN 9117795 and IBN 9104542     

Conductance of needle and twig axis phloem of damaged and intact Norway spruce (Picea abies (L.) Karst.) as investigated by application of14C in situ

Summary Phloem conductance of¹⁴C-labelled assimilates was investigated in natural stands of Norway spruce showing substantial damage from needle yellowing and needle loss disease. Terminal current-year shoots of a branch were allowed to fix¹⁴CO₂ (300–600 ppm in air) and carbon dioxide net uptake was monitored with a gas analyser. The difference between¹⁴C-uptake and the amount of radiocarbon determined in the photosynthesizing needles was interpreted to reflect assimilate export from the needles to the axis of the tree. Compared with an undamaged control tree,¹⁴C-export from the assimilating needles was not impaired in the yellowing tree and only slightly reduced in the tree showing needle loss. Incorporation of¹⁴C into starch increased significantly during autumn particularly in the tree showing needle loss. Import of radiocarbon from the¹⁴C-labelled phloem sap in twig axes and needles older than 1 year was used as a measure of phloem conductivity of older sections of a branch which showed considerable damage. Carbon uptake by these older plant parts was more pronounced than in undamaged twigs. In the case of older needles enhancement of¹⁴C-incorporation suggested an increased sink strength, while the same phenomenon in the twig axes was interpreted as a consequence of partially impaired conductivity of individual sieve elements resulting in an inhomogeneous velocity of phloem transport. The hypothesis is put forward that curtailed viability of the sieve cells is responsible for a delay of transport, which is compensated for by an augmented production of phloem elements from the cambium.     

Transport,localization and physiological effect of 6-benzyladenme-8-14C in apple shoots

The transport of radioactivity of 6-benzyladenine-8-¹⁴C applied to one-year old apple shoots was studied. Simultaneously, labelled metabolites of this cytokinin were studied separately in the xylem and phloem above and below the place of application. According to the results obtained, it can be assumed that 6-B and its metabolites are transported in one-year old apple shoots acropetally through the xylem and basipetally through the phloem, while penetrating from the xylem to the phloem. Besides 6-B-8-¹⁴C, a complex of cytokinin with sugar, also adenosine and adenine were found in the phloem.     

The influence of externally supplied sucrose on phloem transport in the maize leaf strip

Sucrose (2,5–1000 mmol l^–1), labeled with [¹⁴C]sucrose, was taken up by the xylem when supplied to one end of a 30-cm-long leaf strip of Zea mays L. cv. Prior. The sugar was loaded into the phloem and transported to the opposite end, which was immersed in diluted Hoagland's nutrient solution. When the Hoagland's solution at the opposite end was replaced by unlabeled sucrose solution of the same molarity as the labeled one, the two solutions met near the middle of the leaf strip, as indicated by radioautographs. In the dark, translocation of ¹⁴C-labeled assimilates was always directed away from the site of sucrose application, its distance depending on sugar concentration and translocation time. When sucrose was applied to both ends of the leaf strip, translocation of ¹⁴C-labeled assimilates was directed toward the lower sugar concentration. In the light, transport of ¹⁴-C-labeled assimilates can be directed (1) toward the morphological base of the leaf strip only (light effect), (2) toward the base and away from the site of sucrose application (light and sucrose effect), or (3) away from the site of sucrose application independent of the (basipetal or acropetal) direction (sucrose effect). The strength of a sink, represented by the darkened half of a leaf strip, can be reduced by applying sucrose (at least 25 mmol l^–1) to the darkened end of the leaf strip. However, equimolar sucrose solutions applied to both ends do not affect the strength of the dark sink. Only above 75 mmol l^–1 sucrose was the sink effect of the darnened part of the leaf strip reduced. Presumably, increasing the sucrose concentration replenishes the leaf tissue more rapidly, and photosynthates from the illuminated part of the leaf strip are imported to a lesser extent by the dark sink.Supported by Deutsche Forschungsgemeinschaft     

Effect of Exogenously Supplied Foliar Potassium on Phloem Loading in Beta vulgaris L

The effect of foliar application of K⁺ on processes associated with phloem loading was investigated in source leaves of sugar beet (Beta vulgaris L.). KCI was supplied exogenously at concentrations of up to 100 millimolar in the solution bathing the abraded upper epidermis of source leaves. K⁺ added at concentrations below 30 millimolar generally promoted the rate of export of material derived from ¹⁴CO₂ but not from exogenously applied [¹⁴C]sucrose. Paralleling promotion of export, the level of material derived from photosynthesis, which was released into the bathing solution, also increased in response to addition of K⁺ to the free space. Net photosynthetic rate was not affected. K⁺ at 5 and 15 millimolar concentrations did not stimulate uptake of [¹⁴C]sucrose into source leaf discs.     

Transfer of exogenous auxin from the phloem to the polar auxin transport pathway in pea (Pisum sativum L.)

When [1-¹⁴C]indol-3yl-acetic acid ([1-¹⁴C]IAA) was applied to the upper surface of a mature foliage leaf of garden pea (Pisum sativum L. cv. Alderman), ¹⁴C effluxed basipetally but not acropetally from 30-mm-long internode segments excised 4 h after the application of [1-¹⁴C]IAA. This basipetal efflux was strongly inhibited by the inclusion of 3.10^–6 mol· dm³ N-1-naphthylphthalamic acid (NPA) in the efflux buffer. In contrast, when [¹⁴C] sucrose was applied to the leaf, the efflux of label from stem segments excised subsequently was neither polar nor sensitive to NPA. The [1-¹⁴C]IAA was initially exported from mature leaves in the phloem — transport was rapid and apolar; label was recovered from aphids feeding on the stem; and label was recovered in exudates collected from severed petioles in 20 mM ethylenediaminetetraacetic acid. No ¹⁴C was detected in aphids feeding on the stems of plants to which [1-¹⁴C]IAA had been applied apically, even though the internode on which they were feeding transported considerable quantities of label. Localised applications of NPA to the stem strongly inhibited the basipetal transport of apically applied [1-¹⁴C]IAA, but did not affect transport of [1-¹⁴C]IAA in the phloem. These results demonstrate for the first time that IAA exported from leaves in the phloem can be transferred into the extravascular polar auxin transport pathway but that reciprocal transfer probably does not occur. In intact plants, transfer of foliar-applied [1-¹⁴C]IAA from the phloem to the polar auxin transport pathway was confined to immature tissues at the shoot apex. In plants in which all tissues above the fed leaf were removed before labelling, a limited transfer of IAA occurred in more mature regions of the stem.Abbreviations IAA indol-3yl-acetic acid - EDTA ethylenediaminetetraacetic acid - NPA N-1-naphthylphthalamic acid We are grateful to the Nuffield Foundation for supporting this research under the NUF-URB95 scheme and for the provision of a bursary to A.J.C. We thank Professor Dennis A. Baker for constructive comments on a draft of this paper and Mrs. Rosemary Bell for her able technical assistance.     

Evidence for the presence of a sucrose carrier in immature sugar beet tap roots

The objectives of this work were to determine the path of phloem unloading and if a sucrose carrier was present in young sugar beet (Beta vulgaris L.) taproots. The approach was to exploit the characteristics of the sucrose analog, 1'-fluorosucrose (F-sucrose) which is a poor substrate for acid invertase but is a substrate for sucrose synthase. Ten millimolar each of [³H]sucrose and [¹⁴C]F-sucrose were applied in a 1:1 ratio to an abraded region of an attached leaf for 6 hours. [¹⁴C]F-sucrose was translocated and accumulated in the roots at a higher rate than [³H]sucrose. This was due to [³H]sucrose hydrolysis along the translocation path. Presence of [³H]hexose and [¹⁴C]F-sucrose in the root apoplast suggested apoplastic sucrose unloading with its subsequent hydrolysis. Labeled F-sucrose uptake by root tissue discs exhibited biphasic kinetics and was inhibited by unlabeled sucrose, indicating that immature roots have the ability for carrier-mediated sucrose transport from the apoplast. Collectively, in vivo and in vitro data indicate that despite sucrose hydrolysis by the wall-bound invertase, sucrose hydrolysis is not entirely essential for sugar accumulation in this tissue.     

Spontaneous Phloem bleeding from cryopunctured fruits of a ureide-producing legume

The vasculature of the dorsal suture of cowpea (Vigna unguiculata [L.] Walp) fruits bled a sugar-rich exudate when punctured with a fine needle previously cooled in liquid N₂. Bleeding continued for many days at rates equivalent to 10% of the estimated current sugar intake of the fruit. A phloem origin for the exudate was suggested from its high levels (0.4-0.8 millimoles per milliliter) of sugar (98% of this as sucrose) and its high K⁺ content and high ratio of Mg²⁺ to Ca²⁺. Fruit cryopuncture sap became labeled with ¹⁴C following feeding of [¹⁴C]urea to leaves or adjacent walls of the fruit, of ¹⁴CO₂ to the pod gas space, and of [¹⁴C] asparagine or [¹⁴C]allantoin to leaflets or cut shoots through the xylem. Rates of translocation of ¹⁴C-assimilates from a fed leaf to the puncture site on a subtended fruit were 21 to 38 centimeters per hour. Analysis of ¹⁴C distribution in phloem sap suggested that [¹⁴C]allantoin was metabolized to a greater extent in its passage to the fruit than was [¹⁴C] asparagine. Amino acid:ureide:nitrate ratios (nitrogen weight basis) of NO₃-fed, non-nodulated plants were 20:2:78 in root bleeding xylem sap versus 90:10:0.1 for fruit phloem sap, suggesting that the shoot utilized NO₃-nitrogen to synthesize amino acids prior to phloem transfer of nitrogen to the fruit. Feeding of ¹⁵NO₃ to roots substantiated this conclusion. The amino acid:ureide ratio (nitrogen weight basis) of root xylem sap of symbiotic plants was 23:77 versus 89:11 for corresponding fruit phloem sap indicating intense metabolic transfer of ureide-nitrogen to amino acids by vegetative parts of the plant.     

Effects of Acetylcholine, Cytochalasin B and Amiprophosmethyl on Phloem Transport in Radish （Raphanus sativas）

We investigated the role of the ＂sieve tube-companion cell complex＂ lining the tube periphery, particularly the microfilament and microtubule, in assisting the pushing of phloem sap flow. We made a simple phloem transport system with a living radish plant, in which the conducting channel was exposed for local treatment with chemicals that are effective in modulating protoplasmic movement （acetylcholine, （ACh） a neurotransmitter in animals and insects; cytochalasin B, （CB） a specific inhibitor of many cellular responses that are mediated by microfilament systems and amiprophos-methyl, （APM） a specific inhibitor of many cellular responses that are mediated by microtubule systems）. Their effects on phloem transport were estimated by two experimental devices： （i） a comparison of changes in the amount of assimilates in terms of carbohydrates and ^14C-labeled photosynthetic production that is left in the leaf blade of treated plants; and （ii） distribution patterns of ^14C-labeled leaf assimilates in the phloem transport system. The results indicate that CB and APM markedly inhibited the transfer of photosynthetic product from leaf to root via the leaf vein, while ACh enhanced the transfer of photosynthetic product in low concentrations （5.0×10^-4 mol/L） but inhibited it in higher concentrations （2.0×10^-3 mol/L） from leaf to root via the leaf vein. Autoradiograph imaging clearly reveals that ACh treatment is more effective than the control, and both CB and APM treatments effectively inhibit the passage of radioactive assimilates. All of the results support the postulation that the peripheral protoplasm in the sieve tube serves not only as a passive semi-permeable membrane, but is also directly involved in phloem transport.     

The Translocation Profile: Sucrose and Carbon Dioxide

The ¹⁴CO₂ produced from translocated sucrose-¹⁴C has been measuredduring the first minutes of the arrival of the translocationprofile in Salix stem. The shape of the curve of ¹⁴CO₂- timeis shown to resemble that for the radioactivity time of sucrose-¹⁴Cexuded from the phloem through cut aphid stylets, and to reflectthe shape of the sucrose-¹⁴C-distance profile in the stem. The¹⁴CO₂ time profile may be used with the sucrose-¹⁴C distanceprofile to estimate the velocity of movement of the latter.A constant proportion of the sucrose-¹⁴C in the stem breaksdown to ¹⁴CO₂ in the steady state attained after the passageof the profile.