Transfer cells and solute uptake in minor veins of Pisum sativum leaves

Morphometric and physiological studies were conducted to determine whether the wall ingrowths of transfer cells in the minor-vein phloem of Pisum sativum L. leaves increase the capacity of the cells for solute influx. Size and number of wall ingrowths are positively correlated to the photon flux density (PFD) at which the plants are grown. An analysis of plasmodesmatal frequencies indicated that numerous plasmodesmata are present at all interfaces except those between the sieveelement-transfer-cell complex (SE-TCC) and surrounding cells where plasmodesmata are present but few in number. Flux of exogenous sucrose into the SE-TCC was estimated from kinetic profiles of net sucrose influx into leaf discs, quantitative autoradiography, and measurements of sucrose translocation. Flux based both on the saturable (carrier-mediated) and the linear components of influx was 47% greater in leaves of plants grown at high PFD (1000 mol·m^–2·s^–1) than those grown in low PFD (200 mol·m^–2·s^–1) and was paralleled by a 47% increase in SE-TCC plasmalemma surface area. Flux of endogenous photosynthate across the SE-TCC plasmalemma was calculated from carbon balance and morphometric data. The increase in flux in high-light leaves over that in low-light leaves can be explained on the basis of an increase in plasmalemma surface area. In intact leaves, a standing osmotic gradient may facilitate transport of solute into transfer cells with extensive wall elaborations.Abbreviations LPI leaf plastochron index - PCMBS p-chloromercuribenzenesulfonic acid - PFD(s) photon flux density (densities) - SE-TCC sieve-element-transfer-cell complex This research was supported by National Science Foundation Grant DCB-9104159, U.S. Department of Agriculture Competitive Grant 90000854, and Hatch funds.     

Intercellular compartmentation of sucrose synthesis in leaves of Zea mays L.

The incorporation of ¹⁴C into sucrose and hexose phosphates during steady-state photosynthesis was examined in intact leaves of Zea mays L. plants. The compartmentation of sucrose synthesis between the bundle sheath and mesophyll cells was determined by the rapid fractionation of the mesophyll and comparison of the labelled sucrose in this compartment with that in a complete leaf after homogenisation. From these experiments it was concluded that the majority of sucrose synthesis occurred in the mesophyll cell type (almost 100% when the time-course of sucrose synthesis was extrapolated to the time of ¹⁴C-pulsing). The distribution of enzymes involved in sucrose synthesis between the two cell types indicated that sucrose-phosphate synthetase was predominantly located in the mesophyll, as was cytosolic (neutral) fructose-1,6-bisphosphatase activity. Stromal (alkaline) fructose-1,6-bisphosphatase activity was found almost exclusively in the bundle-sheath cells. No starch was found in the mesophyll tissue. These data indicate that in Zea mays starch and sucrose synthesis are spatially, separated with sucrose synthesis occurring in the mesophyll compartment and starch synthesis in the bundle sheath.     

Phloem unloading following reactivation in predarkened mature maize leaves

Mature leaf blades of 48-h predarkened maize plants (Zea mays L. cv. Prior) were excised, and treated apically as the source (light, normal air) and basally as the sink (light or dark, air without CO₂). After providing the source portion with ¹⁴CO₂, the sink portions were harvested after 2, 7 or 14 h by freezing with liquid nitrogen, grinding, and freeze-drying. Extracts, fractionated by ionexchange resins into neutral, basic and acid fractions, were chromatographed on thin cellulose layers, and autoradiographed. Identification of labeled compounds was carried out by co-chromatography with authentic labeled substances. Activities of enzymes pertaining to the metabolism of sucrose were checked. Results show that the source supplies sucrose to the sink, where it is unloaded and metabolized by acid invertase (EC 3.2.1.26) in both the light and the dark. Starch appearing in the sink only in the light, after 7 h of re-illumination, yields labeled glucose upon hydrolysis. Although sucrose-phosphate synthetase (EC 2.4.1.14) is active in sinks and in isolated vascular-bundle fragments, it remains questionable whether sucrose unloaded from sieve tubes is metabolized by a method other than inversion. Sucrose synthetase (EC 2.4.1.13) was found to be inactive. Obviously, the main metabolite of unloaded sucrose is glucose-6-phosphate, giving access to the glycolytic pathway. The main difference between the sinks in the light and the dark is the lack of labeled glycine and serine in the dark. This indicates that in the light decarboxylation of glycine yields CO₂, which is recycled photosynthetically.Abbrevations Glc1P glucose-1-phosphate - Glc6P glucose-6-phosphate - TLC thin-layer chromatography - UDPGlc uridine 5-diphosphate glucose     

Role of phloem in sucrose transport by Ricinus cotyledons

Sucrose is taken up and accumulated by cotyledons of Ricinus communis L. Autoradiographic studies reveal a predominant accumulation of sucrose in the phloem of the cotyledons. The export of sucrose from the cotyledons to hypocotyl and roots proceeds in the phloem by mass flow. These results, taken together with previous data, are experimental evidence for proton-sucrose symport as the mechanism of phloem loading.     

Uptake of [14C]sucrose in isolated minor-vein networks of Commelina benghalensis L

Maceration with pectinase (4.5h) of Commelina benghalensis L. leaves stripped at either side yielded isolated vein networks consisting of four to five secondary veins and tertiary cross veins (=minor veins). Examination with Evans Blue and injection of Fluorescein F showed that 80% of the veins were viable. Proof of normal functioning of isolated minor veins was that [¹⁴C]sucrose fed to an apical vein network attached to the remaining intact part of the leaf was absorbed and finally arrived in the petiole. Sucrose uptake by veins obeyed Michaelis-Menten kinetics (K _m 5·10^-4 mol l^-1; V _max (light) 3.2 mol h^-1 g^-1 fresh weight, V _max (dark) 1.5 mol h^-1 g^-1 fresh weight). A linear component, not inhibited by carbonylcyanide m-chlorophenylhydrazone and p-chloromercuribenzenesulfonic acid, was present. Maximal uptake took place at 5 mmol l^-1 K⁺; concentrations of K⁺ higher than 10 mmol l^-1 decreased the rate of uptake. The uptake rates by isolated veins and veins in situ (in disks) were in the same order of magnitude. Altogether, isolated veins promise to be a useful system for the study of loading.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - EDTA ethylenediamine tetraacetic acid - PCMBS p-chloromercuribenzenesulfonic acid     

Pathway of assimilate transfer between mesophyll cells and minor veins in leaves of Cucumis melo L.

Photoassimilating mature leaves of Cucumis melo exported carbon at a rate of 1.7 mg C·dm^-2·h^-1. Radiolabeling with ¹⁴C showed that stachyose and raffinose are the main carbohydrates translocated. Autoradiograms indicated that sieve elements of the abaxial phloem of minor veins are the sole conduits for carbon export from mature leaves and carbon import into immature leaflets. Sieve elements of the abaxial phloem are associated with intermediary cells which are intimately connected with the surrounding mesophyll cells by numerous plasmodesmata. Photoassimilate, labeled with ¹⁴C, was released into the leaf apoplast and could be trapped in a buffer solution circulating over the abraded adaxial epidermis. Carbon efflux was 1% of the carbon-export rate. A comparable distribution of ¹⁴C among the sugars, amino acids and organic acids, recovered from the free space and from leaf extracts, was recorded. The composition of released ¹⁴C-labeled carbohydrates in the free space resembled the pattern of photoassimilate, but differed clearly from the translocate. Release of organic compounds into the leaf apoplast was stimulated by chelating agents like Na-ATP, ethylenediaminetetraacetic acid and ethylene glycol-bis(-aminoethyl ether)-N,N,N,N-tetraacetic acid; a correlation between carbon efflux into the apoplast and carbon export from the leaf was not detected. It is suggested that the release of organic compounds into the leaf apoplast of Cucumis melo is the consequence of a general leakage from mesophyll and vascular parenchyma cells. A selective release of transport oligosaccharides was not observed. The experimental results presented here do not preclude a symplastic transfer of assimilates in mature leaves.Abbreviations EDTA ethylenediaminetetraacetate - EGTA ethylene glycol-bis(-aminoethyl ether)-N,N,N,N-tetraacetate - PCMBS p-chloromercuribenzenesulfonic acid     

Ultrastructural effects in potato leaves due to antisense-inhibition of the sucrose transporter indicate an apoplasmic mode of phloem loading

To study the export of sugars from leaves and their long-distance transport, sucrose-proton/co-transporter activity of potato was inhibited by antisense repression of StSUT1 under control of either a ubiquitously active (CaMV 35S ) or a companion-cell-specific (rolC) promotor in transgenic plants. Transformants exhibiting reduced levels of the sucrose-transporter mRNA and showing a dramatic reduction in root and tuber growth, were chosen to investigate the ultrastructure of their source leaves. The transformants had a regular leaf anatomy with a single-layered palisade parenchyma, and bicollateral minor veins within the spongy parenchyma. Regardless of the promoter used, source leaves from transformants showed an altered leaf phenotype and a permanent accumulation of assimilates as indicated by the number and size of starch grains, and by the occurrence of lipid-storing oleosomes. Starch accumulated throughout the leaf: in epidermis, mesophyll and, to a smaller degree, in phloem parenchyma cells of minor veins. Oleosomes were observed equally in mesophyll and phloem parenchyma cells. Companion cells were not involved in lipid accmulation and their chloroplasts developed only small starch grains. The similarity of ultrastructural symptoms under both promotors corresponds to, rather than contradicts, the hypothesis that assimilates can move symplasmically from mesophyll, via the bundle sheath, up to the phloem. The microscopical symptoms of a constitutively high sugar level in the transformant leaves were compared with those in wild-type plants after cold-girdling of the petiole. Inhibition of sugar export, both by a reduction of sucrose carriers in the sieve element/companion cell complex (se/cc complex), or further downstream by cold-girdling, equally evokes the accumulation of assimilates in all leaf tissues up to the se/cc complex border. However, microscopy revealed that antisense inhibition of loading produces a persistently high sugar level throughout the leaf, while cold-girdling leads only to local patches containing high levels of sugar. Received: 4 March 1998 / Accepted: 7 April 1998     

Phloem unloading in tobacco sink leaves: insensitivity to anoxia indicates a symplastic pathway

Phloem unloading in transition sink leaves of tobacco (Nicotiana tabacum L.) was analyzed by quantitative autoradiography. Detectable levels of labeled photoassimilates entered sink leaves approx. 1 h after source leaves were provided with ¹⁴CO₂. Samples of tissue were removed from sink leaves when label was first detected and further samples were taken at the end of an experimental phloem-unloading period. The amount of label in veins and in surrounding cells was determined by microdensitometry of autoradiographs using a microspectrophotometer. Photoassimilate unloaded from first-, second-and third-order veins but not from smaller veins. Import termination in individual veins was gradual. Import by the sink leaf was completely inhibited by exposing the sink leaf to anaerobic conditions, by placing the entire plant in the cold, or by steam-girdling the sink-leaf petiole. Phloem unloading was completely inhibited by cold; however, phloem unloading continued when the sink-leaf petiole was steam girdled or when the sink leaf was exposed to a N₂ atmosphere. Compartmental efflux-analysis indicated that only a small percentage of labeled nutrients was present in the free space after unloading from sink-leaf veins in a N₂ atmosphere. The results are consistent with passive symplastic transfer of photoassimilates from phloem to surrounding cells.Symbol VI radio of ¹⁴C in veins and interveinal tissue     

A morphometric analysis of the phloem-unloading pathway in developing tobacco leaves

A morphometric analysis of developing leaves of Nicotiana tabacum L. was conducted to determine whether imported photoassimilates could be unloaded by symplastic transport and whether interruption of symplastic transport could account for termination of import. Five classes of veins were recognized, based on numbers of cells in transverse section. Photoassimilate is unloaded primarily from Class III veins in tissue nearing the end of the sink phase of development. Smaller veins (Class IV and V) do not transport or unload photoassimilate in sink tissue because the sieve elements of these veins are immature until after the tissue stops importing. In Class III veins the sieve element-companion cell (SE-CC) complexes are surrounded by phloem parenchyma which abuts the bundle sheath. Along the most obvious unloading route, from SE-CC complex to phloem parenchyma to bundle sheath to mesophyll cells, the frequency of plasmodesmata at each interface increases. To determine whether this pattern of plasmodesmatal contact is consistent with symplastic unloading we first demonstrated, by derivation from Fick's law that the rate of diffusion from a compartment is proportional to a number N which is equal to the ratio of surface area to volume of the compartment multiplied by the frequency of pores (plasmodesmata) which connect it to the next compartment. N was calculated for each compartment within the vein which has the SE-CC complex as its center, and was shown to be statistically the same in all cases except one. These observations are consistent with a symplastic unloading route. As the leaf tissue matures and stops importing, plasmodesmatal frequency along the unloading route decreases and contact area between cells also decreases as intercellular spaces enlarge. As a result, the number of plasmodesmata between the SE-CC complex and the first layer of mesophyll cells declines in nonimporting tissue to 34% of the number found in importing tissue, indicating that loss of symplastic continuity between the phloem and surrounding cells plays a role in termination of photoassimilate unloading.Abbreviation SE-CC sieve element-companion cell     

Production and diurnal utilization of assimilates in leaves of spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.)

Rates of CO₂ fixation during the light period and the rates of CO₂ release during the night period were measured using mature leaves from 39- to 49-d-old spinach (Spinacia oleracea L., US Hybrid 424; grown in 9 h light, 15 h darkness, daily) and mature leaves from 21-d-old barley (Hordeum vulgare L., cv. Apex; grown in 14 h light, 10 h darkness, daily). At certain times during the light and dark periods leaves were harvested for assay of their contents of soluble carbohydrates, starch, malate and the various amino acids. Evaluation of the results of these measurements shows that in spinach and barley leaves 46% and 26%, respectively, of the carbon assimilated during the light period is deposited in the leaves for export during the night period. Taking into account the carbon consumption in the source leaves by dark respiration, it is evaluated that rates of assimilate export during the light period from spinach and barley leaves [38 and 42 atom C · (mg Chl)^–1 · h^–1] are reduced in the dark period to 16 atom C · (mg Chl)^–1 · h^–1 in both species. The calculated C/N ratios of the photoassimilates exported during the dark period were 0.029 and 0.015 for spinach and barley leaves, respectively.This work was supported by the Deutsche Forschungsgemeinschaft. We thank Dr. Dieter Heineke for stimulating discussions and Mrs. Petra Hoferichter and Mrs. Marita Feldkämper for their technical assistance.     

Phloem loading in Vicia faba leaves: Effect of N-ethylmaleimide and parachloromercuribenzenesulfonic acid on H+ extrusion,K+ and sucrose uptake

The effects of a penetrating (NEM) and a non-penetrating (PCMBS) sulfhydryl-specific reagent on proton extrusion, ⁸⁶Rb and [U-¹⁴C]sucrose uptake by Vicia faba leaves have been studied. Proton extrusion was strongly or completely inhibited by 0.1 mM NEM. ⁸⁶Rb and [U-¹⁴C]sucrose uptake were markedly reduced by NEM concentrations equal to or higher than 0.5 mM. Under our experimental conditions, PCMBS (1 mM) exerted a strong inhibition on [¹⁴C]sucrose uptake but did not inhibit proton extrusion and ⁸⁶Rb uptake. The sensitivity of phloem loading to PCMBS is thought to be a consequence of sugar-carrier blockage and not of inhibition of the proton pump.Abbreviations CCCP carbonylcyanide-m-chlorophenylhydrazone - DES diethylstilbestrol - DCCD dicyclohexylcarbodiimide - FC Fusicoccin - NEM N-ethylmaleimide - PCMBS p-chloromercuribenzenesulfonic acid     

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     

Structure and function of leaf minor veins in trees and herbs

Summary The structure of leaf minor veins in 700 species from 140 families of dicotyledons, monocotyledons and conifers has been studied by light and electron microscopy. The presence of several structural types of minor veins has been shown. The main types are open and closed veins characteristic of trees and herbs, respectively. These vein types differ by the structure of intermediate cells, and by the mechanisms of phloem loading and sugar transport. Most woody plants have intermediate cells with numerous plasmodesmal fields, symplastic transport as the main phloem loading mechanism, as well as oligosaccharides and other complex sugars as the main phloem transport substances. By contrast, the majority of herbs have intermediate cells without plasmodesmal connections, and apoplastic loading of sucrose occurs only by membrane proton cotransport. The closed type is divided into three subtypes, differing in the degree of development of the structures used for sugar uptake from the apoplast. A list of the plants investigated with their vein types is given. The evolution of the minor vein structure and phloem loading mechanism are discussed in relation to the evolution of life forms of higher plants.     

Subcellular volumes and metabolite concentrations in spinach leaves

Cellular and subcellular volumes in mature leaves of spinach (Spinacia oleracea L. US Hybrid 424) were determined stereologically from light and electron micrographs. Forty-nine-day-old leaves of spinach with a total leaf volume of 1177 μL per mg chlorophyll (Chl) were found to be composed of 3% epidermis, 58% mesophyll, 1% vascular tissue, 5% apoplasm and 32% gas space. In the epidermal cells 89% of the volume was occupied by the vacuole. The mesophyll cells consisted, expressed in mg·Chl⁻¹, of 546 μL (79%) vacuole, 66 μL (9.5%) chloroplast stroma, 24 μL (34%) cytosol, 3.7 μL (0.5%) mitochondria and 2.1 μL (0.3%) nucleus. From previous measurements of the subcellular levels of sucrose, of phosphorylated intermediates of carbohydrate metabolism, of malate, oxoglutarate and various amino acids in illuminated leaves, and the above subcellular volumes, the corresponding subcellular metabolite concentrations have been determined. Of the substances measured, only with malate was the concentration higher in the vacuole than in the cytosol. The concentration of sucrose in the cytosol was 5 times, and that of amino acids even 30 times higher than in the vacuole.     

The kinetics of sucrose concentration in the phloem of individual vascular bundles of the Ricinus communis seedling measured by nuclear magnetic resonance microimaging

The sucrose concentration was measured at 70-min intervals in the phloem of individual bundles of the hypocotyl of Ricinus seedlings by ¹H nuclear magnetic resonance (NMR) spectroscopic imaging. The sucrose concentration stayed fairly constant in all bundles for more than 7 h if the cotyledons were embedded in the endosperm or excised and incubated in 100 mM sucrose. If, however, the sucrose solution was replaced by sucrose-free buffer solution, the sucrose levels in the phloem decreased with a kinetic depending on the seedling: in some cases there was a smooth decline, in some a decline followed by a slight recovery and in some cases a clear-cut oscillation. The sucrose concentration was often not identical in the phloem of the individual bundles. The oscillations were larger in the phloem at the apex of the hypocotyl than in the phloem at the base of the hypocotyl. Cutting the petiole of one cotyledon led to a decrease in sucrose not only in the four bundles directly connected to the severed petiole but in all eight bundles of the hypocotyl. Cutting the petiole and dividing the vascular ring at the cotyledonary node and at the root crown did not prevent the decline of sucrose in all eight bundles. Therefore, a functional equilibration of translocated solutes between the eight bundles may occur within the 1-h measuring interval by radial diffusion through the parenchyma of the hypocotyl. Received 4 July 1997 / Accepted: 4 October 1997     

Influence of cell turgor on sucrose partitioning in potato tuber storage tissues

Sucrose uptake and partitioning in potato (Solanum tuberosum L.) tuber discs were examined under a range of mannitol and ethylene-glycol concentrations. Mannitol caused the same changes in turgor over a wide range of incubation periods (90 min-6 h), indicating that it did not penetrate the tissue. In comparison, ethylene glycol reduced turgor losses but did not eliminate them, even after 6 h. Between 100 mM and 300 mM mannitol, turgor fell by 350 kPa, compared with 35 kPa in ethylene glycol. Uptake experiments in mannitol alone showed that total sucrose uptake was strongly correlated with both osmotic potential and with turgor potential. In subsequent experiments sucrose uptake and partitioning were examined after 3 h equilibration in 100 mM and 300 mM concentrations of mannitol and ethylene glycol. Total sucrose uptake and the conversion of sucrose to starch were enhanced greatly only at 300 mM mannitol, indicating an effect of turgor, rather than osmotic potential on sucrose partitioning. The inhibitors p-chloromercuribenzenesulfonic acid and carbonylcyanide m-chlorophenylhydrazone (CCCP) both reduced sucrose uptake, but in quite different ways. p-Chloromercuribenzenesulfonic acid reduced total sucrose uptake but did not affect the partitioning of sucrose to starch. By contrast, CCCP inhibited total uptake and virtually eliminated the conversion of sucrose to starch. Despite this, sucrose uptake in the presence of CCCP continued to increase as the mannitol concentration increased, indicating an increase in passive transport at higher mannitol concentrations. Increased sucrose uptake above 400 mM mannitol was shown to be the result of uptake into the free space. The data show that starch synthesis is optimised at low but positive turgors and the relation between sucrose partitioning and the changing diurnal water relations of the tuber are discussed.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - PCMBS p-chloromercuribenzenesulfonic acid     

Control of photosynthetic sucrose synthesis by fructose-2,6-bisphosphate

The metabolite levels in the mesophyll of leaves of Zea mays L. have been compared with the regulatory properties of the cytosolic fructose-1,6-bisphosphatase from the mesophyll to show how withdrawal of triose phosphate for sucrose synthesis is reconciled with generation of the high concentrations of triose phosphate which are needed to allow intercellular diffusion of carbon during photosynthesis. i) A new technique is presented for measuring the intercellular distribution of metabolites in maize. The bundle-sheath and mesophyll tissues are partially separated by differential homogenization and filtration through nylon nets under liquid nitrogen. ii) considerable gradients of 3-phosphoglycerate, triose phosphate, malate and phosphoenolpyruvate exist between the mesophyll and bundle sheath which would allow intercellular shuttles to be driven by diffusion. These gradients could result from the distribution of electron transport and the Calvin cycle in maize leaves. iii) consequently, the mesophyll contains high concentrations of triose phosphate and fructose-1,6-bisphosphate. iv) Most of the regulator metabolite fructose-2,6-bisphosphate, is present in the mesophyll. v) The cytosolic fructose-1,6-bisphosphatase has a lower substrate affinity than that found for the enzyme from C₃ species, especially in the presence of inhibitors like fructose-2,6-bisphosphate. vi) This lowered affinity for substrate makes it possible to reconcile use of triose phosphate for sucrose synthesis with the maintenance of the high concentration of triose phosphate in the mesophyll needed for operation of photosynthesis in this species.Abbreviations DHAP Dihydroxyacetonephosphate - Fru1,6-bisP fructose-1,6-bisphosphate - Fru2,6bisP fructose-2,6-bisphosphate - PEP(Case) phosphoenolpyruvate (carboxylase) - PGA 3-phosphoglycerate - Rubisco ribulose-1,5-bisphosphate carboxylase     

Reactivation of phloem export in mature maize leaves after a dark period

Mature leaves of corn plants (Zea mays L. cv. Prior) which were darkened for 48 h contain neither bundle-sheath starch nor glucose, and their sucrose content is below 5 M. In such leaves phloem export has ceased. When re-illuminated, photosynthetic sucrose production starts without delay, but the sucrose: glucose ratio is 1.25:1. Obviously, most of the new-formed sugar is utilized locally. Labeling with ¹⁴CO₂ has shown that phloen export starts 30 to 40 min after the onset of photosynthesis, when the sucrose: glucose ratio has increased to 13:1. The first newly formed starch can be detected when phloem export is reactivated. Glucose content remains constantly low af about 2 M for at least 2 h, and it never exceeds 10 M. Radioactivity in the exporting veins is about five times higher after 2 to 7 h of re-illumination than in the 14-h-day plant. Therefore, phloem export is either intensified during the period of reactivation or exported assimilates are partly unloaded along their way. Comparison of photosynthetic activity of equal-sized leaf strips has shown that both accumulation of photosynthates and radioactivity of exporting veins are about three times higher in the detached strip than in the strip which remained attached to the mother plant.