Evidence for thiol-induced enhanced in situ translocation of 14C-sucrose from source to sink in Brassica juncea

The translocation of ¹⁴C-sucrose to the different parts of the mustard (Brassica juncea) crop has been evaluated in the context of understanding the source to sink relationship in the thiol-induced enhanced crop yield. The foliar application of thiols like TU, TGA and DTT to the plant gave maximum sucrose phosphate synthase activity, which was found to have direct correlation with the movement of sucrose. The distribution pattern of ¹⁴C-sucrose follows the path from internode and node to pod via leaf. The translocation of ¹⁴C-sucrose was found to be a light dependent process. Among the nucleotides ATP and GTP, only ATP was able to promote the translocation and GTP was ineffective. In this unique in situ tracer experiment using ¹⁴C-sucrose, we could establish that thiols are able to enhance the translocation of sucrose from source to sink.     

Evidence for thiol-induced enhanced in situ translocation of 14C-sucrose from source to sink in Brassica juncea

The translocation of ¹⁴C-sucrose to the different parts of the mustard (Brassica juncea) crop has been evaluated in the context of understanding the source to sink relationship in the thiol-induced enhanced crop yield. The foliar application of thiols like TU, TGA and DTT to the plant gave maximum sucrose phosphate synthase activity, which was found to have direct correlation with the movement of sucrose. The distribution pattern of ¹⁴C-sucrose follows the path from internode and node to pod via leaf. The translocation of ¹⁴C-sucrose was found to be a light dependent process. Among the nucleotides ATP and GTP, only ATP was able to promote the translocation and GTP was ineffective. In this unique in situ tracer experiment using ¹⁴C-sucrose, we could establish that thiols are able to enhance the translocation of sucrose from source to sink.     

Effect of Nitrogen Deficiency upon Translocation of C in Sugarcane

Withholding nitrogen decreased the percentages of nitrogen and chlorophyll in the blades; reduced the total fixation of radioactive carbon dioxide at 15, 37, and 178 seconds; and changed the relative composition of fixation products. Translocation of radioactive photosynthate from the fed part down the attached blade and into the stalk was less in the plants deprived of nitrogen than in the control plants supplied with nitrogen. Both the percentage of total activity translocated and the velocity of transport were decreased by nitrogen deficiency. During a translocation period of 90 minutes the minus nitrogen blade retained more ¹⁴C-sucrose than the control in the fed part and the blade below the fed part, but it sent less ¹⁴C-sucrose to the sheath of the fed leaf. Thus translocation decreased with nitrogen deficiency not for lack of sucrose but for some other reason. Although withholding nitrogen decreased translocation of labeled carbon in and from attached blades, there was no effect upon transport in detached blades. The effect of nitrogen deficiency upon translocation may be indirect and secondary to the effect upon growth of the plant as a whole.     

AtRBOH I confers submergence tolerance and is involved in auxin-mediated signaling pathways under hypoxic stress

Satsuma mandarin fruit (Citrus unshiu Mark.) photosynthesizes as comparable to leaf at about 100 days after full bloom (DAFB). In this study, translocation and accumulation of fruit-fixed photosynthate were investigated by using ¹⁴CO₂. When fruit at 108 DAFB was exposed to ¹⁴CO₂ for 48 h under 135 photosynthetic photon flux density (PPFD), ¹⁴C-sucrose, ¹⁴C-glucose and ¹⁴C-fructose were detected not only in flavedo but juice sac; more than 50?% of fruit assimilated ¹⁴C-sugars were present in juice sac. Thus, majority of rind-fixed photosynthate are infiltrated into juice sac and accumulated there within 48 h after assimilation. Although ¹⁴C-sucrose was predominant at flavedo where high SS (sucrose synthase) activity toward synthesis was present, the amount decreased gradually from the outside (flavedo) to the inside (juice sac) of fruit. In vascular bundle, strong SS toward cleavage and soluble acid invertase activities were involved, and ¹⁴C-fructose was predominant in juice sac. Accordingly, rind-fixed photosynthate is once converted to sucrose, the translocated sugar in Citrus, at flavedo by SS toward synthesis, and loaded on vascular bundle through symplastic and/or apoplastic movement in the albedo tissue. In the vascular bundle, sucrose may be degraded by SS toward cleavage and invertase, and resulting hexoses transported symplastically to the juice sac through juice stalk.     

The Retention of Water-soluble Compounds during Freeze-Substitution and Microautoradiography

Freeze-substitution and Epon embedment were quantitatively evaluated for their effectiveness in retaining water-soluble metabolites in plant tissues. Roughly 99% of the 80% (v/v) ethanol-extractable radioactivity in photosynthetically labeled soybean leaf discs and in petiole fragments containing translocated ¹⁴C was retained during freeze-substitution in acetone or propylene oxide and embedment in Epon. Substantially more activity was lost from ¹⁴C-sucrose-infiltrated pith blocks, but most or all of this loss came from the block surface. The procedure was effective for a sucrose concentration as low as 0.004%. Sections floated on water retained most of their ¹⁴C-sucrose, and high resolution autoradiographs could easily be prepared without resorting to dry procedures. Embedded ¹⁴C-sucrose was apparently chemically unreactive, since there was no loss of radioactivity when sections were stained with the periodic acid-Schiff reagent, nor did the embedded sucrose show staining.     

The Role of Phloem Transport in the Translocation of Sucrose Along the Stem of Carnation Cut Flowers

The pathway and distribution of ¹⁴C-sugars in flower parts havebeen examined to find out in which tissue sugars are translocatedin the stem of the cut carnation; ¹⁴C-sucrose or ¹⁴C-glucosewas supplied at the base of the cut stem from a feeding solutionand the localization and chemical nature of the carbon-14 recoveredfrom flower parts were investigated. By reducing the rate oftranspiration it was found that the uptake of feeding solutionwas also reduced, but the distribution of absorbed ¹⁴C-sucrosein the flower parts was different from that which would be expectedif sucrose moved exclusively in the transpiration stream. Autoradiographsdemonstrated that ¹⁴C absorbed from the feeding solution as¹⁴C-sucrose appeared in both xylem and phloem but predominantlyin the latter; girdling failed to stop the translocation ofthe absorbed ¹⁴C-sucrose. Results of experiments with ¹⁴C-sucroseand ¹⁴C-glucose showed that sucrose was the mobile sugar andthat glucose was converted to sucrose before it was translocated.It was concluded that the translocation of sucrose absorbedfrom the feeding solution takes place both in xylem and phloemand is regulated by a mechanism involving the loading and translocationof sucrose in the phloem.     

Translocation of sitosterol and related compounds in Pelargonium hortorum and helianthus annuus

The translocation of several plant sterols and a triterpene was studied in geranium and sunflower plants. Upward translocation of sitosterol-[¹⁴C] and β-amyrin-[¹⁴C] was shown within 48 hr to the upper parts of a geranium plant sectioned previously above the roots. Downward translocation of sitosterol-[¹⁴C] from the leaf of application was evident in intact plants after 48 hr. In addition to free sitosterol-[¹⁴C] considerable amounts of sitosteryl-[¹⁴C] glycoside and traces of sitosteryl-[^14C] ester were found in most parts examined. Very slow downward translocation of cholesterol-[¹⁴C] but not of desmosterol-[¹⁴C], sitosteryl-[¹⁴C] palmitate or β-amyrin-[¹⁴C] was shown in geranium. In sunflower no downward translocation of cholesterol-[³H], sitosteryl-[³H] acetate or palmitate could be detected. In geranium, sitosteryl-[¹⁴C] glycoside translocated downward from the leaf of application to all other plant parts, except other leaves, and was found in these parts after 10 days as the unchanged glycoside, free sterol and steryl ester. The effect of drying the plant parts on the recovery of radioactive steroidal material is discussed. Traces of a water soluble, dialyzable form of sterol-[¹⁴C] were also detected in dried geranium roots after treatment with strong acid or alkali.     

Kinetics of C-14 Translocation in Soybean: II. Kinetics in the Leaf

The kinetics of ¹⁴C-assimilates in the soybean leaf were studied in pulse labeling and steady state labeling experiments. ¹⁴C-Sucrose apparently served as the ultimate source, at least, of translocated ¹⁴C-sucrose. However, since the specific activity of leaf sucrose reached a maximum within 5 minutes after pulse labeling, whereas that of exported sucrose did not reach a maximum until at least 20 minutes, it appeared that there were two sucrose compartments in the leaf. A possible physical basis for the two compartments may be the mesophyll (a photosynthetic compartment) and a specialized “paraveinal mesophyll” (a nonphotosynthetic compartment), through which photosynthate must pass on its way to the veins.     

Effects of Ethylene and Sucrose on Translocation of Dry Matter and 14C-Sucrose in the Cut Flower of the Glasshouse Carnation (Dianthus caryophyllus) During Senescence

The translocation and distribution of dry matter were studiedin the floral and vegetative parts of the cut carnation duringsenescence. The change in dry weights of the tissues and theamount of radioactivity recovered from them after feeding with¹⁴C-sucrose were measured. Treatments with ethylene and sucrosewere used to alter the rate of senescence of the flowers. Sucrosemoved through the stem relatively unchanged but was rapidlyinverted and metabolized in the petals. During natural ageing,¹⁴C moved from the stem to the flower and the movement was enhancedby exogenous sucrose, which also reduced the loss of dry matterin the petals and promoted their growth. Treatment with ethylenecaused petals to wilt and lose dry weight, and ovaries to enlargeand increase in dry weight. The distribution of radioactivityin flowers fed with 14C-sucrose before and after ethylene treatmentsupported the observation that dry matter was translocated betweenthe flower parts. The results indicate that a change in thesource-ink relationships of the flower parts contributes tothe factors that determine the rate of flower senescence.     

Evidence for active Phloem loading in the minor veins of sugar beet

Phloem loading in source leaves of sugar beet (Beta vulgaris, L.) was studied to determine the extent of dependence on energy metabolism and the involvement of a carrier system. Dinitrophenol at a concentration of 4 mm uncoupled respiration, lowered source leaf ATP to approximately 40% of the level in the control leaf and inhibited translocation of exogenously supplied ¹⁴C-sucrose to approximately 20% of the control. Dinitrophenol at a concentration of 8 mm inhibited rather than promoted CO₂ production, indicating a mechanism of inhibition other than uncoupling of respiration. The 8 mm dinitrophenol also reduced ATP to approximately 40% of the level in the control source leaf and reduced translocation of exogenous sucrose to approximately 10% of the control. Application of 4 mm ATP to an untreated source leaf promoted the translocation rate by approximately 80% over the control, while in leaves treated with 4 mm dinitrophenol, 4 mm ATP restored translocation to the control level. No recovery of translocation was observed when ATP was applied to leaves treated with 8 mm dinitrophenol. The results indicate an energy-requiring process for both phloem loading and translocation in the source leaf.     

Evidence for Phloem loading from the apoplast: chemical modification of membrane sulfhydryl groups

The water-soluble, sulfhydryl-specific, chemical modifier p-chloromercuribenzenesulfonic acid reversibly inhibited the accumulation of exogenously supplied ¹⁴C-sucrose into leaf discs of Beta vulgaris. P-Chloromercuribenzenesulfonic acid treatment did not inhibit photosynthesis or respiration or induce membrane leakage to sucrose, indicating that the site of inhibition was the plasmalemma. The active loading of sucrose and ¹⁴CO₂-derived assimilates into the phloem and their translocation from the source leaf were inhibited by the nonpermeant modifier. Several nonpermeant sulfhydryl group modifiers also inhibited sucrose accumulation into leaf discs while two amino-reactive reagents had no effect. The results indicate that sugars are actively accumulated into the phloem from the apoplast and that membrane sulfhydryl groups may be involved.     

Source and sink leaf metabolism in relation to Phloem translocation: carbon partitioning and enzymology

The import-export transition in sugar beet leaves (Beta vulgaris) occurred at 40 to 50% leaf expansion and was characterized by loss in assimilate import and increase in photosynthesis. The metabolism and partitioning of assimilated and translocated C were determined during leaf development and related to the translocation status of the leaf. The import stage was characterized by C derived from either ¹⁴C-translocate or ¹⁴C-photosynthate being incorporated into protein and structural carbohydrates. Marked changes in the C partitioning were temporally correlated with the import-export conversion. Exporting leaves did not hydrolyze accumulated sucrose and the C derived from CO₂ fixation was preferentially incorporated into sucrose. Both source and sink leaves contained similar levels of acid invertase and sucrose synthetase activities (sucrose hydrolysis) while sucrose phosphate synthetase (sucrose synthesis) was detected only in exporting leaves. The results are discussed in terms of intracellular compartmentation of sucrose and sucrose-metabolizing enzymes in source and sink leaves.     

Translocation of carbon in powdery mildewed barley

This paper compares translocation in healthy and powdery mildew (Erysiphe graminis f. sp. hordei, race CR3) infected barley (Hordeum vulgare, variety Manchuria). The sink-like properties of the powdery mildew infection were used to determine what effect imposing a sink in the midst of normal source tissue (mature primary leaf) had on the translocation process. The pattern of translocation was determined by monitoring the movement of ¹⁴C which was photosynthetically incorporated from ¹⁴C either by the primary or second leaf. In the healthy primary leaf of barley, ¹⁴C fixed in the tip section of the blade was preferentially translocated to the root, whereas ¹⁴C fixed in the basal section was primarily translocated to the shoot. When a sporulating powdery mildew infection was present in the mid-section of the primary leaf, ¹⁴C fixed in that section or in the acropetal healthy tip section readily accumulated in the infection area. Labeled carbon fixed in the healthy basal section was translocated into the other parts of the plant with only a small fraction moving acropetally into the infected mid-section. The ¹⁴C fixed by the second leaf was translocated to the root and younger shoot with very little entering the primary leaf. The presence of the mildew infection did not alter this pattern.     

Translocation of C in the sugarcane plant during the day and night

The time-course of translocation of ¹⁴C from the blades of the sugarcane plant was investigated by analysis and radioactive counting of successive samples punched from a single blade. In 1 experiment, the time-course was studied by determining the specific activity of the carbon dioxide respired by the roots.

The rate of translocation, expressed as percentage, was highest immediately after the application of the radioactive carbon dioxide. Morning-made photosynthate translocated a higher percentage during the morning than during the afternoon in 90-minute periods in the light. Afternoon-made photosynthate translocated as well or better than morning-made photosynthate for the first hour in the light.

The leaf-disk data and the specific activity of the carbon dioxide respired by the roots corresponded by showing lower rates of translocation by night than by day for several successive days. Also, the translocation of ¹²C sucrose was slower at night.

The ¹⁴C sucrose translocated by day was made primarily by photosynthesis; the sucrose translocated by night was made primarily by the conversion of other labeled compounds, e.g. organic acids, organic phosphates, and insoluble residue.

The radioactive constituent of the residue, which was converted to sucrose, was tentatively identified as a glucose-xylose-glucuronic acid hemicellulose, with most or all of the ¹⁴C in the glucose moiety.

Translocation of sucrose may be triggered by different mechanisms during the night than the day. The conversion of insoluble residue to sucrose by increasing the osmotic potential at the source would favor a pressure-flow mechanism for nocturnal translocation; whereas translocation by day is thought to be a process of phototranslocation, a photoactivation of the translocation mechanism.

     

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.     

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.     

The translocation of photosynthetic products from mesophyll into midrib in tobacco plant III. The transformation of translocated 14C-sugars in the veins

¹⁴C-U-sugars were introduced into tobacco plants through themesophyll, the veins of the first order of branching, and themidrib, and ¹⁴C-compounds in the veins and the midrib whichtranslocated towards the base of the midrib were traced duringthe period of 120 min after the ¹⁴C-sugar introductions. 1) When ¹⁴C-U-sucrose was introduced into the leaf, no matterwhat the means of feeding was, most of the ¹⁴C which translocatedbasipetally in the veins and the midrib was found in the formof sucrose. 2) When ¹⁴C-U-glucose or ¹⁴C-U-fructose was administered tothe leaf dirough the cut vein of the first order of branching,most of the ¹⁴C which translocated basipetally in the veinsand the midrib was found in the form of sucrose. 3) ¹⁴C-U-glucose or ¹⁴C-U-fructose injected into the vascularbundles of the midrib was translocated basipetally, as such,10 and 30 min after injection; and at 30 min, the amount ofthe ¹⁴C-sucrose in the midrib attained 9–22% of the 80%ethanol-soluble ¹⁴C in the midrib. 4) When ¹⁴C-U-glucose or ¹⁴C-U-fructose was supplied to themesophyll, the radioactivities of these hexoses were predominantin the first and second veins soon after application, then decreasingwith a concomitant increase in the radioactivity of the ¹⁴C-sucrose. From these results, it was inferred that in the veins of thefirst and second order of branching, glucose and fructose whichmoved from the mesophyll did not translocate as such, but wereutilized for the synthesis of sucrose available for translocationvia the midrib to the stem. ¹A part of this paper was presented at the Crop Science Societyof Japan, in April, 1969 (Received December 8, 1969; )     

Tissue specificity of assimilation and transport of nitrogen in the root ofZea mays L. seedlings. I. Transformations of14C- sucrose and the nitrogen metabolism enzymes in cortex and stele

An investigation has been carried out to find out in what compounds carbon is radially transported from stele to the cortex and to study sucrose metabolism in the root. These tissues were fed with¹⁴C-sucrose either directly or through the mesocotyle. In the latter case, the bulk of radioactivity, both in the stele and cortex, was concentrated in sucrose rather than in mono saccharides. This corresponds to the radial transport of carbon from stele to cortex predominantly in the form of sucrose. In the case of a direct exposure of stele and cortex,¹⁴C-sucrose was utilized for respiration, amino acid biosynthesis, and accumulation of carbon in the form of glucose. The hydrolysis of sucrose to the monosaccharides and subsequent utilization of the latter in respiration was more intensive in the stele. In the cortex, the sucrose carbon was more actively used for amino acid synthesis, owing to a high concentration of nitrate reductase, glutamate dehydrogenase and glutamine synthetase. It was concluded that the cortex is the main issue zone providing nitrogen assimilation in the root.     

The Effect of Sodium and Calcium on the Translocation of 14C-sucrose in Excised Cotton Roots

The effect of sodium and calcium on the translocation of ¹⁴C-sucrose in excised cotton roots (Gossypium birsutum) was studied. The roots were allowed to elongate in a modified Guinn's medium that was very low in calcium (6.25 × 10^?2 mM) and sodium (8.70 × 10^?3 mM). After a period of six days the roots were transferred to 20 × 100 mm Petri dishes that contained 10 × 40 mm Petri dishes as center wells. The roots were draped over the edge of the center well and extended into the outer dish. The outer Petri dish and its center well contained the same solution except that sucrose was supplied only in the center well. The sucrose used was spiked with uniformally labeled ¹⁴C-sucrose. Four treatments were started which varied in their Na and Ca content. Three and six day harvests were taken and the amount of ¹⁴C that had moved from the distal (in the center well) to the apical root section (in the outer dish) was determined. Increasing substrate Na or Ca caused an increase in ¹⁴C-sucrose translocation and the effects of both ions were additive by the final harvest. These results were found to be independent of treatment effects on growth and respiration of the excised roots. These data support the conclusion that Na may partially substitute for Ca in carbohydrate translocation. Thus, roots supplied the Low Ca-High Na and High Ca-Low Na treatments had equal translocation rates over a six day period. The highest translocation rate was obtained with the High Ca-High Na treatment. Data from the High Ca-High Na treatment on two successive three day periods indicate that Na may have a role in translocation other than that associated with substitution for Ca, or maintenance of tissue hydration.     

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.