Loading of assimilates in wheat leaves. II. The path from chloroplast to vein

Abstract. An improved method is described for micro-autoradiography of water-soluble substances alter freeze-substitution of plant tissue in which water is rigorously excluded. Resin sections are cut and flattened dry, and dry photographic emulsion is mounted on them. When the location of ¹⁴C in wheat leaves after assimilation of ¹⁴CO₂ was studied with this method, it was found that ¹⁴C entered the intermediate veins before the laterals and entered both types of veins along the flanks of the veins adjacent to the phloem. High concentrations of ¹⁴C were found in small spaces in the cells of mesophyll and vein parenchyma; these spaces coincide with the nuclei. The concentration of ¹⁴C in these nucleus-associated spaces was as high as that reached at later times in the sieve tubes. Water washed the ¹⁴C out of these spaces of the sections and the label in the washings was predominantly in sucrose. The high ¹⁴C concentrations of the nucleus-associated spaces were particularly easily leached. It is concluded that the raising of the sugar concentration to the high levels found in sieve tubes can take place in these leaves in a special space in each mesophyll cell not, or not only, at the boundary of the sieve tubes.     

Sugar Selectivity and Other Characteristics of Phloem Loading in Beta vulgaris L

The rate of phloem loading, its selectivity, and the disposition of labeled carbon were studied following application of (14)C-labeled sugars to the free space of source leaves of sugar beet (Beta vulgaris L.). Buffered 10 mm solutions of (14)C-labeled sucrose, fructose, stachyose, mannitol, 3-0-methyl glucose or l-glucose were applied to the abraded epidermis of source leaves held in the dark. Distribution of the labeled carbon from sugar taken up from the free space was studied by micro-densitometry of autoradiographs. Uptake of labeled sugar from the free space, partition between mesophyll and minor veins, metabolic conversions, export and respiration were followed during the 3-hr time course studies. Rates of sugar uptake into the minor veins, flux rates through the sieve element-companion cell complex membrane and concentration ratios between free space and the interior of the minor vein phloem cells were compared for the six sugars studied for evidence of active uptake. The composition of the free space solution in leaves photosynthesizing in (14)CO(2) was studied by vacuum infiltration of the source leaf air spaces and removal of the solution by centrifugation. Labeled compounds in this solution were compared to those in an aqueous ethanol extract of the same leaf pieces.The results in sugar beet source leaves support the concept of direct, active uptake of sucrose from free space into minor veins. This is not the case for fructose, 3-0-methyl glucose, mannitol, or stachyose. The latter two sugars, which are translocated in some plants, are not loaded into the minor veins at a rate sufficient to make them a significant component of the material translocated. The rate of phloem loading is controlled in part by mesophyll metabolism, especially as it affects the availability of sucrose to the free space. Both the rate and selectivity of export are controlled by uptake from the free space into the sieve element-companion cell complex of the minor veins.     

Response of Phloem Loading and Export to Rapid Changes in Sink Demand

Sink demand was abruptly changed for an illuminated sugar beet source leaf by shading the six to ten other source leaves. Export of recently assimilated, labeled material underwent a transient increase and then returned to a steady rate approximately equal to the pretreatment rate. Uncovering the darkened leaves caused a transient decrease in export of ¹⁴C; following recovery there was a gradual decline. It remains to be established whether export of unlabeled reserves occurs in response to increased sink demand. The possibility that phloem loading increases in response to decreased sieve tube turgor was tested. Phloem loading of exogenous ¹⁴C-sucrose increased when turgor in leaf cells was decreased by floating leaf discs on solutions with up to 1 M mannitol osmoticum. However, the increase appeared to be the result of plasmolysis of mesophyll cells possibly resulting from easier access to minor veins via the free space. Phloem loading in leaf discs continued undiminished even though sieve tube-companion cell sucrose concentration exceeded a calculated value of 1 M. Regulation of export to meet sink demand by a direct response of phloem loading to a turgor or concentration set point does not appear to occur. Phloem loading may be promoted by the influx of water which drives mass flow, increasing phloem loading in response to increased velocity of transport.     

Distribution of 14C-sucrose applied to leaves at different nodes of okra (Abelmoschus esculentum) during flowering and early pod growth

The distribution pattern of ¹⁴C-sucrose from ¹⁴C-sucrose applied to vegetative okra plants and leaves 1–9 on separate plants during the green pod development stage were investigated in relation to duration and leaf position. Results indicated bi-directional transport of assimilates to both apical and basal portions of the stem. Within 48 h ¹⁴C moved to all plant parts; stem and leaves appeared to be strong sinks. In plants fed at the vegetative stage, 48 h after feeding, 66% of the fed activity was exported from the fed leaf. At the pod development stage, about 35% of the activity exported from the fed leaf was present in green pods and 65% in vegetative parts. In plants where leaf 1–9 was fed, irrespective of the position of the fed leaf, the subtending fruit was the strongest sink among the reproductive parts. Leaves and stems were the principal sinks.     

Control of phloem unloading by action potentials in Mimosa

In the sensitive plant, Mimosa pudica , action potentials arise when the leaves are touched and they trigger a sudden decrease in turgor of the pulvinar motor cells, which causes the leaf to close. These potentials may travel through the phloem and they appear to influence pulvinar phloem unloading after stimulation. Mature leaves were exposed to ¹⁴CO₂ and phloem translocation was observed by autoradiography. In unstimulated pulvini, labeled photoassimilates were restricted to the phloem. However, after stimulation, the ¹⁴C-label appeared to be concentrated in the extensor region of the motor cortex. Since stimulation elicits an action potential, it is suggested that it also triggers phloem unloading of sucrose in the pulvini.     

Translocation of selenium in the bean plant (Phaseolus vulgaris) and the field bean (Vicia faba)

Selenium distribution in the bean plant ( Phaseolus vulgaris L. cv. Contender) was studied using autoradiographs of the whole plant and of sections of organs. A few hours after the incubation of the roots with (⁷⁵Se) selenate, a major part of the selenate accumulates in the roots, while the fraction conveyed towards the aerial organs is unevenly distributed, resulting in accumulation of ⁷⁵Se in the young leaves, the buds and the epicotyl. This distribution results from a general translocation of selenium through the xylem. A secondary process of redistribution is then immediately linked to the transport of ⁷⁵Se labeled products (such as seleno-amino acids) in the phloem from the mature leaves. A similar pattern of translocation of selenium was found in the field bean ( Vicia faba L. cv. Aguadulce) by using aphids that insert their stylets into the sieve tubes. Measurement of the radioactivity of these insects shows that the ⁷⁵Se content of the phloem sap was reduced to low levels when all the mature leaves were excised. The mature leaves thus serve as relaying organs, redistributing the selenium which is carried in by the movement of water through the xylem.     

STUDIES ON THE LEAF OF AMARANTHUS RETROFLEXUS (AMARANTHACEAE): QUANTITATIVE ASPECTS,AND SOLUTE CONCENTRATION IN THE PHLOEM

The vascular system of the leaf of Amaranthus retroflexus L. was examined quantitatively, and plasmolytic studies were carried out on it to determine the solute concentration in cells of the phloem at various locations in the leaf. The proportion of phloem occupied by sieve tubes varies considerably with vein size and leaf size. Collectively, the cross-sectional area of sieve tubes of all tributaries at their points of entry into either a secondary or midvein far exceeds the total cross-sectional area of sieve tubes at the bases of those major veins. In addition, the total volume of sieve tubes in the “catchment area” of a secondary vein is much greater than total sieve-tube volume of the secondary vein itself. The plasmolytic studies revealed the presence of positive concentration gradients in the sieve tubes of the lamina from the minor veins and tips of the secondaries to the bases of the secondaries and from the tip to the base of the midvein. The C₅₀ (the estimated mannitol concentration plasmolyzing, on the average, 50% of the sieve-tube members) was 1.5 m for minor veins and tips of secondary veins and 1.1 m for the bases of secondaries; 1.3 m for the tip of the midvein and 0.6-0.7 m for the midvein in the basal third of the lamina.     

The directional control of sucrose and asparagine transport in lupin by abscisic acid

Injections of exogenous abscisic acid into the primary flowerhead of Lupinus luteus cv. Weiko III reduced the movement of ¹⁴C-sucrose into the flowerhead from the uppermost leaves. Sucrose transported from below the lateral branches subtending the flowerhead, was diverted into the lateral branches by injection of the exogenous abscisic acid into the flowerhead. ¹⁴C-sucrose was also diverted from a lateral branch injected with exogenous abscisic acid to all other parts of the plant, particularly the main stem and leaves, and the roots. Transport of ¹⁴C-asparagine administered at the cotyledonary node was directed from the flowerhead into the subtending lateral branches by injection of abscisic acid into the flowerhead. Transport of both ¹⁴C-sucrose and ¹⁴C-asparagine into the flowerhead was reduced at least three fold at physiological levels of abscisic acid. No significant correlation was found between the amount of ¹⁴C-asparagine entering a sink and the dry weight of the tissues of that sink. It is concluded that distribution of ¹⁴C-sucrose and ¹⁴C-asparagine between the flowerheads and lateral branches of L. luteus is actively and dynamically controlled and that abscisic acid levels play a significant part in that control. It is suggested that the relative levels of endogenous abscisic acid in plant organs could serve as an important factor in the directional control of assimilate transport in plants.     

Leaf structure and translocation in sugar beet

Anatomical and ultrastructural details of a translocating 10-cm leaf of sugar beet (Beta vulgaris L. var. Klein Wanzleben) were correlated with translocation rate data. The minor veins were found to be 13 times as extensive as the major veins and measure 70 cm/cm² leaf lamina. Measurements disclosed that a 33-μ length of minor vein services 29 mesophyll cells with the result that translocate moves an average of 73 μ or 2.2 cell diameters during transport from mesophyll cells to a minor vein. High-resolution, freeze-dry autoradiography revealed that assimilates accumulate in organelle-rich cells of the minor vein phloem. Correlation of phloem volume and loading rate for minor veins yielded an uptake rate of 735 μmoles of sucrose per g fresh weight of phloem. The arrangement and structural features of minor veins appeared to be consistent with the concept that vein loading precedes translocation.     

The development of Pisum sativum explant systems for studies concerning source-sink activities

Using excised fruit/shoot systems of pea ( Pisum sativum L.), the effect of the 10-day-old (post anthesis) fruit (sink) on the translocation of ¹⁴C-sucrose applied to the stipule (source) was investigated. We also examined the influence of various growth regulators on the source-sink relationships using this system. Indole-3-yl-acetic acid (100 or 200 ppm), gibberellic acid (25-100 ppm) increased ¹⁴C-sucrose translocation into the growing fruit by about 13% and 22–29%, respectively. 2-chloroethyl phosphonic acid, 6-benzyladenine and 2-methyl-4-chlorophenoxyacetic acid also increased ¹⁴C-sucrose translocation into the fruit by about 11–26%, 11–23% and 24–37%, respectively. Elevated fruit ¹⁴C-sucrose levels found in other treatments include, methyl-2-chloro-9-hydroxyfluorene-9-carboxylate (300 or 1000 ppm), 2,3,5-triiodobenzoic acid (100 and 300 ppm) and butanedioic acid mono-(2,2-dimethylhydrazide) (30-300 ppm) by about 34%, 39–52% and 7–13%, respectively.     

STUDIES ON THE LEAF OF POPULUS DELTOIDES (SALICACEAE): QUANTITATIVE ASPECTS,AND SOLUTE CONCENTRATIONS OF THE SIEVE-TUBE MEMBERS

The vascular system of the leaf of Populus deltoides Bartr. ex Marsh, was examined quantitatively, and plasmolytic studies were carried out to determine the solute concentrations of sieve-tube members at various locations in the leaf. Both the total number and total crosssectional area of each cell type decreases with decreasing vein size. Although the proportion of phloem occupied by sieve tubes varies considerably from location to location, a linear relationship exists between cross-sectional area of the vascular bundles and both total and mean cross-sectional area of sieve tubes. Collectively, the cross-sectional area of all tertiary and minor veins feeding into a secondary exceeds the total cross-sectional area of sieve tubes at the base of that secondary. Moreover, the total volume of sieve tubes in the “catchment area” of a secondary vein is much greater than the total sieve tube volume of the secondary itself. Both tracheary elements and sieve-tube members undergo a reduction in both total and mean crosssectional area in the constricted zone at the base of the leaf. The plasmolytic studies revealed the presence of positive concentration gradients in sieve tubes of the lamina from the minor veins and tips of the secondaries to the bases of the secondaries and their associated subjacent midvein bundles and from the upper to lower portions of the median bundle of the midvein.     

Distribution of 14C assimilates in the storage root of sugar beet plants after import from leaves of different age

In the sugar beet plant ( Beta vulgaris L. ssp. altissima ) the vascular bundles of old leaves lead to the center and those of young leaves to the periphery of the storage root. Whether the flux of assimilates follows these anatomical routes was tested by applying ¹⁴CO₂ for 4 h to either an old (10th) or a young (20th) leaf in intact sugar beet plants. Four-month-old plants, which had about 30 leaves, were used in the experiment. The ¹⁴C distribution in the storage root was measured by autoradiography and counting in about 20 cross and longitudinal sections per root.
About 37% of assimilated ¹⁴C from an old leaf and 23% from a young leaf were exported within 24 h. Although some ¹⁴C moved into younger leaves, most was exported into the storage root. During its rapid movement towards the root tip, which took place perferentially in the orthostichon belonging to the [¹⁴C]-treated leaf, the label spread laterally.
The autoradiograms indicate that the distribution of assimilates within the storage root is roughly determined by the course of the vascular bundles extending from the source leaf. The fine distribution, however, seems to be controlled by sucrose gradients between storage cells.     

Sugar transport in conducting elements of sugar beet leaves

Autoradiography was used to determine the distribution of labeled sugar in conducting elements of the blade and petiole of sugar beet leaves at intervals ranging from 5 sec to 24 hr. The processes of assimilation by the green cells, collection of sugar in the minor veins and export in phloem elements were demonstrated visually. It appears that in minor veins sugar is translocated in companion cells rather than sieve tubes. In major veins translocation occurs in sieve tubes.     

Phloem Unloading in Developing Leaves of Sugar Beet : II. Termination of Phloem Unloading

Phloem unloading in developing leaves of Beta vulgaris L. (`Klein E' multigerm) occurred from successively higher order branches of veins as leaves matured. Phloem unloading was studied in autoradiographs of leaf samples taken at various times during the arrival of a pulse of ¹⁴C-labeled photoassimilate. Extension of mass flow of sieve element contents into leaf vein branches was determined from the high level of radiolabel in veins soon after first arrival of the pulse. Rapid entry, indicative of mass flow through open sieve pores, occurred down to the fourth division of veins in young, importing leaves and to the fifth or terminal branch in importing regions near the zone of transition from sink to source. The rate of unloading decreased with leaf age, as evidenced by the increased time required for the vein-mesophyll demarcation to become obscured. The rate of import per unit leaf area, measured by steady state labeling with ¹⁴CO₂ also decreased as a leaf matured. The decline in import appeared to result from progressive changes that increased resistance to unloading of sieve elements and eventually terminated phloem unloading.     

Leaf teeth, transpiration and the retrieval of apoplastic solutes in balsam poplar

The rapid flow of the transpiration stream through major veins to leaf teeth was followed in leaves of Populus balsamifera L., using the tracer sulphorhodamine G (SR), which probes for cells with H⁺-extrusion pumps. The tracer accumulated quickly in the hydathodes of the teeth. It was shown by freeze-substitution and anhydrous processing that SR was taken up by phloem parenchyma and epithem cells of the hydathode. When ¹⁴C-labelled aspartate was fed to the leaves in the transpiration stream, it also was taken up most strongly by the same phloem parenchyma and epithem cells. It is proposed that one function of the hydathodes in leaf teeth is the retrieval of solutes from the transpiration stream.     

Efflux of sucrose from minor veins of tobacco leaves

Mature leaves import limited amounts of nutrient when darkened for prolonged periods. We tested the hypothesis that import is restricted by the apoplast-phloem loading mechanism, ie., as sucrose exits the phloem of minor veins it is retrieved by the same tissue, thus depriving the mesophyll of nutrient. When single, attached, mature leaves of tobacco (Nicotiana tabacum L.) plants were darkened, starch disappeared from the mesophyll cells, indicating that the supply of solute to the mesophyll was limited. Starch was synthesized in mesophyll cells of darkened tissue when sucrose was applied to the apoplast at 0.1–0.3 mM concentration. Efflux from minor veins was studied by incubating leaf discs on [¹⁴C]sucrose to load the minor veins and then measuring subsequent ¹⁴C release. Efflux was rapid for the first hour and continued at a gradually decreasing rate for over 13 h. Net efflux increased when loading was inhibited by p-chloromercuribenzene-sulfonic acid, anoxia, isotope-trapping, or reduction of the pH gradient. Neither light nor potassium had a significant effect on the rate of labeled sucrose release. The site of labeled sucrose release was investigated by measuring efflux from discs in which sucrose had previously been loaded preferentially by either the minor veins or mesophyll cells. Efflux occurred primarily from minor veins.Abbreviations Mes 2(N-morpholino)ethanesulfonic acid - Mops 3(N-morpholino)propanesulfonic acid - PCMBS p-chloromercuribenzenesulfonic acid - SE-CC sieve element-companion cell complex     

Quantitative relationships between osmotic potential amd epicotyl growth in Vigna radiata as affected by osmotic stress and cotyledon excision

Ethylene biosynthesis in leaf discs of tobacco ( Nicotiana tabacum L. cv. Xanthi), as measured by the conversion of L-[3,4-¹⁴C]-methionine to ¹⁴C₂H₄, was markedly inhibited by exogenous ethylene. This inhibition was accompanied by a decrease in total (free + conjugated) content of 1-aminocyclopropane-1-carboxylic acid (ACC), most of which appeared in its conjugated inactive form. The autoinhibitory effect of ethylene was reversible and could be relieved by Ag⁺. The Ag⁺-treated leaf discs, with or without ethylene, contained only free ACC at an increased level. The results suggest that in tobacco leaves, the autoinhibition of ethylene production resulted from reduction in the availability of free ACC, through both suppression of ACC formation and increased ACC conjugation.     

Different Patterns of Vein Loading of Exogenous [C]Sucrose in Leaves of Pisum sativum and Coleus blumei

Vein loading of exogenous [¹⁴C]sucrose was studied using short uptake and wash periods to distinguish between direct loading into veins and loading via mesophyll tissue. Mature leaf tissue of Pisum sativum L. cv Little Marvel, or Coleus blumei Benth. cv Candidum, was abraded and leaf discs were floated on [¹⁴C]sucrose solution for 1 or 2 minutes. Discs were then washed for 1 to 30 min either at room temperature or in the cold and were frozen, lyophilized, and autoradiographed. In P. sativum, veins were clearly labeled after 1 minute uptake and 1 minute wash periods. Autoradiographic images did not change appreciably with longer times of uptake or wash. Vein loading was inhibited by p-chloromercuribenzenesulfonic acid. These results indicate that uptake of exogenous sucrose occurs directly into the veins in this species. When C. blumei leaf discs were floated on [¹⁴C]sucrose for 2 minutes and washed in the cold, the mesophyll was labeled but little, if any, minor vein loading occurred. When discs were labeled for 2 minutes and washed at room temperature, label was transferred from the mesophyll to the veins within minutes. These results indicate that there may be different patterns of phloem loading of photosynthetically derived sucrose in these two species.     

STRUCTURAL EVIDENCE FOR A THEORY OF VEIN LOADING OF TRANSLOCATE

The ultrastructure of minor veins of Beta vulgaris was examined with reference to possible models for vein loading of translocate. Structural evidence was reviewed in the light of recent physiological observations as a basis for proposed mechanisms. Features which appeared to be of significance in formulating a model included the open, differentiated sieve plates, the predominance of organelle-rich parenchyma cells, and the branched plasmodesmata connecting sieve tubes and parenchyma cells. The resulting model views cell to cell movement of photosynthate via the symplast to the specialized parenchyma cells. The actively accumulated sucrose appears to move from the specialized parenchyma cells into the sieve tubes via plasmodesmata in the lateral and end walls.     

Sugar transport in leaf discs of Phaseolus coccinius

Kinetic profiles for sucrose, glucose and 3-OMG glucose were determined in leaf discs of Phaseolus coccinius L. (cv. Scarlet). All three sugars exhibited identical uptake kinetics. At sugar concentrations below 25 m M , transport was due to an active, carrier-mediated transport system. A linear component was the dominant mode of uptake at sugar concentrations above 25 m M . Sucrose and glucose carriers were specific for these sugars, since no uptake inhibition was observed from competing sugars. Sucrose was not hydrolyzed by leaf tissue because the label in asymetrically labeled sucrose was not randomized. Furthermore, no label was present in hexose fractions when tissue was incubated with [⁸⁴C]-sucrose. Therefore, [¹⁴C]-sucrose uptake did not reflect hexose uptake.
Both saturable and linear components of uptake contribute significantly to total uptake rates. The former, however, is more important when apoplastic sugar concentrations are low. The molecular nature of the linear component is not well understood but accounts for most of the uptake at high sugar concentrations.