Assimilation of gaseous ammonia and the transport of its products in barley and spinach leaves

The interactions between the assimilation and transport of nitrogenand carbon were investigated in barley and spinach leaves. Bothplants were fumigated with NH₃ (1 mg m^–3 and the contentof amino acids, sucrose and carbon intermediates of amino acidmetabolism were analysed in the leaves, apoplast and phloemsap. The following changes took place in the C- and N-metabolismof barley leaves during 5 h of fumigation with NH₃ (a) The contentsof amino acids, especially glutamine, largely increased andthe contents of sucrose, 2-oxoglutarate, phosphoenolpyruvate,and glycerate-3-phosphate declined. (b) A decrease in the phophoenolpyruvatecontent was accompanied by an increased activity of phosphoenolpyruvatecarboxylase. (c) The altered cytosolic concentrations of aminoacids and sucrose during NH₃ fumigation correlated with similarchanges in the apoplast and phloem sap. The altered percentageof each amino acid relative to the total amino acid concentrationin the cytosol, caused by NH₃ fumigation, is reflected in theapoplast and the phloem sap. The results indicate that the concentrations of amino acids in the cytosol determine their concentrationsin the phloem. Key words: Amino acids, ammonia fumigation, barley leaves, C: N partitioning, phosphoenolpyruvate carboxylase, phloem sap, spinach leaves     

Amino Acid and sucrose content determined in the cytosolic, chloroplastic, and vacuolar compartments and in the Phloem sap of spinach leaves

Amino acid and sucrose contents were analyzed in the chloroplastic, cytosolic, and vacuolar compartments and in the phloem sap of illuminated spinach leaves (Spinacia oleracea L.). The determination of subcellular metabolite distribution was carried out by nonaqueous fractionation of frozen and lyophilized leaf material using a novel three-compartment calculation method. The phloem sap was collected by aphid stylets which had been severed by a laser beam. Subcellular analysis revealed that the amino acids found in leaves are located mainly in the chloroplast stroma and in the cytosol, the sum of their concentrations amounting to 151 and 121 millimolar, respectively, whereas the amino acid concentrations in the vacuole are one order of magnitude lower. The amino acid concentrations in the phloem sap are found to be not very different from the cytosolic concentrations, whereas the sieve tube concentration of sucrose is found to be one order of magnitude higher than in the cytosol. It is concluded that the phloem loading results in a preferential extraction of sucrose from the source cells.     

Phloem Transport of Amino Acids in Relation to their Cytosolic Levels in Barley Leaves

A comparison of barley (Hordeum vulgare L.) leaves was made between the cytosolic content of amino acids and sucrose as determined by subcellular fractionation and the corresponding concentration in phloem sap, which was collected continuously for up to 6 days from severed aphid stylets. Because amino acids were found to be almost absent from the vacuoles, and because the amino acid patterns in the stroma and cytosol are similar, whole leaf contents could be taken as a measure of cytosolic amino acid levels for a comparison of data during a diurnal cycle. The results show that the pattern of amino acids in the phloem sap was very similar to the pattern in the cytosol. Therefore, we concluded that the overall process of transfer of amino acids from the cytosol of the source cells into the sieve tubes, although carrier mediated, may be a passive process and that the translocation of amino acids via the sieve tubes requires the mass flow of sucrose driven by the active sucrose transport involved by the phloem loading.     

Subcellular concentrations of sugar alcohols and sugars in relation to phloem translocation in <Emphasis Type="Italic">Plantago major,Plantago maritima</Emphasis>, <Emphasis Type="Italic">Prunus persica</Emphasis>, and <Emphasis Type="Italic">Apium graveolens</Emphasis>

Sugar and sugar alcohol concentrations were analyzed in subcellular compartments of mesophyll cells, in the apoplast, and in the phloem sap of leaves of Plantago major (common plantain), Plantago maritima (sea plantain), Prunus persica (peach) and Apium graveolens (celery). In addition to sucrose, common plantain, sea plantain, and peach also translocated substantial amounts of sorbitol, whereas celery translocated mannitol as well. Sucrose was always present in vacuole and cytosol of mesophyll cells, whereas sorbitol and mannitol were found in vacuole, stroma, and cytosol in all cases except for sea plantain. The concentration of sorbitol, mannitol and sucrose in phloem sap was 2- to 40-fold higher than that in the cytosol of mesophyll cells. Apoplastic carbohydrate concentrations in all species tested were in the low millimolar range versus high millimolar concentrations in symplastic compartments. Therefore, the concentration ratios between the apoplast and the phloem were very strong, ranging between 20- to 100-fold for sorbitol and mannitol, and between 200- and 2000-fold for sucrose. The woody species, peach, showed the smallest concentration ratios between the cytosol of mesophyll cells and the phloem as well as between the apoplast and the phloem, suggesting a mixture of apoplastic and symplastic phloem loading, in contrast to the herbal plant species (common plantain, sea plantain, celery) which likely exhibit an active loading mode for sorbitol and mannitol as well as sucrose from the apoplast into the phloem.     

Comparison of the contents of sucrose and amino acids in the leaves, phloem sap and taproots of high and low sugar-producing hybrids of sugar beet (Beta vulgaris L.)

The contents of sucrose and amino acids in the leaves, phloemsap and taproots have been analysed in three experimental hybridsof sugar beet and compared with earlier analysed leaf and phloemsap contents in spinach and barley. The three hybrids accumulatedsucrose and amino acids to various extents in the mature rootsas well as in the young taproots (9–12 weeks). The differencesin the sucrose-to-amino acid ratios in the taproots were reflectedin the corresponding ratios in the phloem sap. The leaf contentsof sucrose and amino acids in the three hybrids were found tobe very similar to each other and also to those in spinach andbarley. In contrast, the phloem concentration of sucrose (1.3M) was much higher, and that of amino acids much lower thanin spinach and barley. In the taproots, the overall concentrationof sucrose was about half that in the phloem sap. From thesefindings it is con cluded that the decisive factor in the highsucrose accumulation in sugar beet roots is the very efficientprocess of phloem loading in the leaves. The patterns of theamino acids in the phloem sap and in the taproots resembledthose in the leaves, indicating that there is no special transportform for a-amino nitrogen from the leaves to the roots, butall amino acids which are present in the cytosol are translocated. Key words: Amino acids, Beta vulgaris L., phloem sap, sucrose, tap roots, transport     

The differential transport of amino acids into the phloem of Ricinus communis L. seedlings as shown by the analysis of sieve-tube sap

The cotyledons of castor bean (Ricinus communis L.) act as absorption organs for amino acids, which are supplied to the medium. The analysis of the sieve-tube sap, which exudes from the cut hypocotyl, demonstrated the ability of the cotyledons to load particular amino acids into the phloem and to reject the loading of others. The sieve-tube sap of cotyledons, which were embedded in the endosperm, contained 150 mM amino acids, with 50 mM glutamine as the major amino acid, and 10–15 mM each of valine, isoleucine, lysine and arginine. Removal of the endosperm led to a drastic decline in the amino-acid content of sieve-tube sap down to 16 mM. Addition of single amino acid species to the medium increased the amino acid concentration in the sieve-tube sap in specific manner: glutamine caused the largest increase (up to 140 mM in exudate), glutamate and alanine smaller increases (up to 60 mM), and arginine the smallest. In addition, the amino acid composition of the sieve-tube sap changed, for instance, glutamine or alanine readily appeared in the sieve-tube sap upon incubation in glutamine or alanine, respectively, whereas glutamate was hardly discernible even in the case of incubation with glutamate; arginine was loaded into the sieve tubes only reluctantly. In general, glutamine and alanine accumulated four- to tenfold in the sieve tubes. The uptake of amino acids and of sucrose into the sieve tubes was interdependent: the loading of sucrose strongly reduced the amino acid concentration in the sieve-tube exudate and loading of amino acids decreased the sucrose concentration. Comparison of the concentrations of various amino acids on their way from the endosperm via the cotyledon-endosperm interface, through the cotyledons and into the sieve tubes showed that glutamine, valine, isoleucine and lysine are accumulated on this pathway, whereas glutamate and arginine are more concentrated in the cotyledons than in the sieve tubes. Obviously the phloem-loading system has a transport specificity different from that of the amino acid uptake system of the cotyledon in general and it strongly discriminates between amino acids within the cotyledons.     

Phloem loading and unloading of sugars and amino acids

In terrestrial higher plants, phloem transport delivers most nutrients required for growth and storage processes. Some 90% of plant biomass, transported as sugars and amino nitrogen (N) compounds in a bulk flow of solution, is propelled though the phloem by osmotically generated hydrostatic pressure differences between source (net nutrient export) and sink (net nutrient import) ends of phloem paths. Source loading and sink unloading of sugars, amino N compounds and potassium largely account for phloem sap osmotic concentrations and hence pressure differences. A symplasmic component is characteristic of most loading and unloading pathways which, in some circumstances, may be interrupted by an apoplasmic step. Raffinose series sugars appear to be loaded symplasmically. However, sucrose, and probably certain amino acids, are loaded into minor veins from source leaf apoplasms by proton symporters localized to plasma membranes of their sieve element/companion cell (se/cc) complexes. Sucrose transporters, with complementary kinetic properties, are conceived to function as membrane transporter complexes that respond to alterations in source/sink balance. In contrast, symplasmic unloading is common for many sink types. Intervention of an apoplasmic step, distal from importing phloem, is reserved for special situations. Effluxers that release sucrose and amino acids to the surrounding apoplasm in phloem loading and unloading are yet to be cloned. The physiological behaviour of effluxers is consistent with facilitated membrane transport that can be energy coupled. Roles of sucrose and amino acid transporters in phloem unloading remain to be discovered along with mechanisms regulating symplasmic transport. The latter is hypothesized to exert significant control over phloem unloading and, in some circumstances, phloem loading.     

Chemical Composition of Phloem Sap from the Uppermost Internode of the Rice Plant

The chemical composition of phloem sap from the uppermost internodeof rice plants (Oryza sativa L., var. Kantou), one week afteranthesis, was compared with that of phloem sap from the leafsheath of a young seedling. The pure phloem sap from rice plantswas collected by an insect laser technique. The phloem sap from the uppermost internode contained a highlevel of sucrose (573.8 mM) which was the only sugar detected.The concentrations of total amino acids, potassium and ATP were124.8 mM, 40.4 mM and 1.76 mM, respectively. The concentrationof sucrose was three times higher and the potassium level wasone third as high in the internode sap as in the phloem sapfrom the leaf sheath. The total concentration of amino acidswas almost the same, but the relative amount of each amino acidwas quite different. The ratios of levels of Glu to Gln andof levels of Asp to Asn in the phloem sap from the uppermostinternode were smaller than those in the phloem sap from theleaf sheath. The adenylate energy charge was 0.92–0.93in both types of phloem sap. The amino acid composition of the phloem sap from the uppermostinternode was compared with that of the phloem sap of the flagleaf and the endosperm sap of the same plant, one week afteranthesis. The differences in composition along the phloem pathwaysuggest the selective translocation of amino acid. (Received July 21, 1989; Accepted December 11, 1989)     

Phloem loading in peach: Symplastic or apoplastic?

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ³H-inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of ¹⁴C-labelled source leaves with 2 mMp-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.     

Transport of hexoses by the phloem of Ricinus communis L. seedlings

The sieve-tube sap of Ricinus communis L. seedlings has been analysed to determine whether or not hexoses can be taken up by the phloem. Under natural conditions, i.e. with the endosperm attached to the cotyledons, glucose and fructose occurred only in trace amounts in the sieve-tube sap. Incubation of the cotyledons with hexoses in the concentration range 25–200 mM caused a rapid and substantial uptake of hexoses into the phleom, where they appeared eventually in the sieve-tube sap at the same concentration as in the incubation medium. Phloem loading of glucose, 3-O-methyl-glucose and sorbitol occurred easily, whereas fructose was less well loaded. glucose and to a larger extent fructose were also transformed to sucrose, which was loaded into the phloem. The loading of hexoses into the sieve tubes as observed in the experimental exudation system also occurred in the intact seedling, but transloction in the latter soon came to a standstill, probably because of lack of consumption by the sink tissues. These results indicate that the virtual absence of hexoses in the sievetube sap under in-vivo conditions is not because of the inability of the phloem-loading system to transport the monosaccharides but because of the absence of sufficiently high concentrations in the apoplast.     

Apoplastic expression of yeast-derived invertase in potato : effects on photosynthesis, leaf solute composition, water relations, and tuber composition

In potato plants (Solanum tuberosum), a chimeric yeast-derived invertase gene fused to a 35S cauliflower mosaic virus promoter has been expressed. The protein was targeted to the cell wall by using the signal peptide of proteinase inhibitor II fused to the amino terminus of the yeast invertase. The transformed plants had crinkled leaves, showed a reduced growth rate, and produced fewer tubers. Although in the apoplast of the leaves of the transformed plants the content of glucose and fructose rose by a factor of 20, and that of sucrose declined 20-fold, 98% of the carbohydrate in the phloem sap consisted of sucrose, demonstrating the strong specificity of phloem loading. In the leaf cells of the transformed plants, glucose, fructose, and amino acids, especially proline, were accumulated. Consequently, the osmolality of the cell sap rose from 250 to 350 mosmol/kg. Our results show that the observed 75% decrease of photosynthesis is not caused by a feedback regulation of sucrose synthesis and is accompanied by an increase in the osmotic pressure in the leaf cells. In the transformed plants, not only the amino acid to sucrose ratio in the phloem sap, but also the amino acid and protein contents in the tubers were found to be elevated. In the tubers of the transformed plants, the protein to starch ratio increased.     

Phloem transport of amino acids in two Brassica napus L. genotypes and one B. carinata genotype in relation to their seed protein content

 In order to investigate the relationship between the amino acid concentration in the phloem sap of leaves and the protein content in seeds, two Brassica napus genotypes and one B. carinata genotype with low, medium and high seed protein contents were analyzed. Phloem sap was collected from the B. napus winter rapeseed breeding line DSV15 with 19% protein of dry weight in the seeds, the spring cultivar ‘Duplo’ with 25% protein in the seeds and from the B. carinata line BRA1151/90 with 39% protein in the seeds by using the aphid-stylet technique. The total amino acid contents measured in the phloem varied considerably among the three genotypes analysed, and correlated positively with their respective seed protein contents. The total amino acid-to-sucrose ratio was lowest in B. napus line DSV15 which had the lowest seed protein content and highest in the B. carinata line BRA1151/90 which had the highest seed protein content. The amino-N translocation in the phloem during the light period was about 2-fold higher in the B. carinata line BRA1151/90 than in the B. napus lines Dulpo and DSV15. Predominant amino acids in the phloem were glutamine and glutamate, followed by serine, aspartate, and threonine. The amino acid patterns in the leaves resembled those in the phloem, although their absolute concentrations were higher in the phloem than in the cytosol of mesophyll tissue. Furthermore, the concentration gradient of amino acids between the cytosol of mesophyll cells and the phloem was higher in the B. carinata line BRA1151/90 than in the B. napus lines Duplo and DSV15. These results lead to the conclusion that the phloem translocation of amino-N and the phloem loading process of amino acids are decisive factors for the protein content in the seeds of Brassica species. Received: 28 November 1999 / Accepted: 10 April 2000     

Diurnal changes in assimilate concentrations and fluxes in the phloem of castor bean (Ricinus communis L.) and tansy (Tanacetum vulgare L.)

Reports about diurnal changes of assimilates in phloem sap are controversial. We determined the diurnal changes of sucrose and amino acid concentrations and fluxes in exudates from cut aphid stylets on tansy leaves (Tanacetum vulgare), and sucrose, amino acid and K(+) concentrations and fluxes in bleeding sap of castor bean pedicel (Ricinus communis). Approximately half of the tansy sieve tubes exhibited a diurnal cycle of sucrose concentrations and fluxes in phloem sap. Data from many tansy plants indicated an increased sucrose flux in the phloem during daytime in case of low N-nutrition, not at high N-nutrition. The sucrose concentration in phloem sap of young Ricinus plants changed marginally between day and night, whereas the sucrose flux increased 1.5-fold during daytime (but not in old Ricinus plants). The amino acid concentrations and fluxes in tansy sieve tubes exhibited a similar diurnal cycle as the sucrose concentrations and fluxes, including their dependence on N-nutrition. The amino acid fluxes, but not the concentrations, in phloem sap of Ricinus were higher at daytime. The sucrose/amino acid ratio showed no diurnal cycle neither in tansy nor in Ricinus. The K(+)-concentrations in phloem sap of Ricinus, but not the K(+) fluxes, decreased slightly during daytime and the sucrose/K(+)-ratio increased. In conclusion, a diurnal cycle was observed in sucrose, amino acid and K(+) fluxes, but not necessarily in concentrations of these assimilates. Because of the large variations between different sieve tubes and different plants, the nutrient delivery to sink tissues is not homeostatic over time.     

Magnesium deficiency results in accumulation of carbohydrates and amino acids in source and sink leaves of spinach

Accumulation of assimilates in source leaves of magnesium‐deficient plants is a well‐known feature. We had wished to determine whether metabolite concentrations in sink leaves and roots are affected by magnesium nutrition. Eight‐week‐old spinach plants were supplied either with a complete nutrient solution (control plants) or with one lacking Mg (deficient plants) for 12 days. Shoot and root fresh weights and dry weights were lower in deficient than in control plants. Mg concentrations in deficient plants were 11% of controls in source leaves, 12% in sink leaves and 26% in roots, respectively. As compared with controls, increases were found in starch and amino acids in source leaves and in sucrose, hexoses, starch and amino acids in sink leaves, whereas they were only slightly enhanced in roots. In phloem sap of magnesium‐deficient and control plants no differences in sucrose and amino acid concentrations were found. To prove that sink leaves were the importing organs they were shaded, which did not alter the response to magnesium deficiency as compared with that without shading. Since in the shaded sink leaves the photosynthetic production of metabolites could be excluded, those carbohydrates and amino acids that accumulated in the sink leaves of the deficient plants must have been imported from the source leaves. It is concluded that in magnesium‐deficient spinach plants the growth of sink leaves and roots was not limited by carbohydrate or amino acid supply. It is proposed that the accumulation of assimilates in the source leaves of Mg‐deficient plants results from a lack of utilization of assimilates in the sink leaves.     

Accumulation and Conversion of Sugars by Developing Wheat Grains : VII. Effect of Changes in Sieve Tube and Endosperm Cavity Sap Concentrations on the Grain Filling Rate

The extent to which wheat grain growth is dependent on transport pool solute concentration was investigated by the use of illumination and partial grain removal to vary solute concentrations in the sieve tube and endosperm cavity saps of the wheat ear (Triticum aestivum L.). Short-term grain growth rates were estimated indirectly from the product of phloem area, sieve tube sap concentration, and ³²P translocation velocity. On a per grain basis, calculated rates of mass transport through the peduncle were fairly constant over a substantial range in other transport parameters (i.e. velocity, concentration, phloem area, and grain number). The rates were about 40% higher than expected; this probably reflects some unavoidable bias on faster-moving tracer in the velocity estimates. Sieve tube sap concentration increased in all experiments (by 20 to 64%), with a concomitant decline in velocity (to as low as 8% of the initial value). Endosperm cavity sucrose concentration also increased in all experiments, but cavity sap osmolality and total amino acid concentration remained nearly constant. No evidence was found for an increase in the rate of mass transport per grain through the peduncle in response to the treatments. This apparent unresponsiveness of grain growth rate to increased cavity sap sucrose concentration conflicts with earlier in vitro endosperm studies showing that sucrose uptake increased with increasing external sucrose concentration up to 150 to 200 millimolar.     

Influence of phloem transport, N-fertilization and ion accumulation on sucrose storage in the taproots of fodder beet and sugar beet

To investigate the factors governing the accumulation of sucroseand amino acids in the taproots of sugar beet, their contentswere measured in the leaves, phloem sap and the taproots ofsugar beet, fodder beet and a hybrid between both, grown oneither 3.0 or 0.5 mM nitrate. In the taproots the contents ofmalate, citrate and inorganic ions were also determined. Forthe high sucrose accumulation in sugar beet as compared to theother varieties three factors were found. (a) In sugar beet,less amino acids and more sucrose are taken up into the phloemthan in fodder beet. (b) In sugar beet, the sucrose and aminoacid syntheses are less sensitive to the nitrate concentrationsthat are required for optimal plant growth than in other varieties.In fodder beet, upon raising the nitrate concentration from0.5 mM to 3 mM, the synthesis and storage of sucrose is decreasedand that of amino acids increased. The corresponding valuesin sugar beet (0.5 mM) are similar to those in fodder beet andare not much affected by an increase of nitrate. (c) The sucroseaccumulation is limited by the accumulation of inorganic ionsin the taproots. The sucrose content in the taproots is negativelycorrelated to the total ion content. Whereas sucrose representstwo-third of all solutes in the taproots of sugar beet, it amountsto only one-third of the solutes in fodder beet taproots. Key words: Amino acids, Beta vulgans L, phloem sap, potassium, sucrose storage, sugar beet, taproots, transport     

Role of the Apoplast in the Control of Assimilate Transport,Photosynthesis, and Plant Productivity

A concept is suggested, which supposes that assimilates are transferred within the plant downward through phloem sieve tubes and, after entering the stem apoplast, are carried up with the ascending flow of transpiration water. After entering the apoplast of fully expanded leaves, these solutes are reexported through the phloem. Thus, a common pool of assimilates with uniform concentration is formed in the plant apoplast. According to this concept, the mechanism of assimilate demand represents a response of photosynthetic apparatus to changes in the apoplastic level of metabolites consumed by sink organs. The ratios of labeled photoassimilates differ between the apoplast and mesophyll cells. Most of the apoplastic labeled carbon is contained in sucrose, less in amino acids, and even less in hexoses. The ¹⁴C-labeling of amino acids increases and the sucrose/hexose labeling ratio decreased under conditions of enhanced nitrate supply. The well-known effect of relative inhibition of assimilate export from leaves under conditions of enhanced nitrogen supply is explained by an enhanced hydrolysis of apoplast-derived sucrose due to the increase in invertase activity, rather than by diversion of primary photosynthetic products from sucrose synthesis to other pathways required for activated growth processes in leaves. This notion is based on observations that the sucrose/hexose ratio is reduced to a greater extent in the apoplast than in the symplast. The last assumption was supported by data obtained after artificial changes in the apoplastic pH. In these experiments intact plants were placed in the atmosphere of NH₃ or HCl vapors, which induced opposite changes in relative content of labeled assimilates in the apoplast and in the photosynthetic rate.     

Patterns of Assimilate Production and Translocation in Muskmelon (Cucumis melo L.) : I. Diurnal Patterns

Continuous monitoring of steady-state carbon dioxide exchange rates in mature muskmelon (Cucumis melo L.) leaves showed diurnal patterns of photosynthesis and respiration that were translated into distinct patterns of accumulation and phloem export of soluble sugars and amino acids. Leaf soluble sugar patterns in general followed the pattern of photosynthetic activity observed in the leaf, whereas starch accumulated steadily throughout the light period. Sugar and starch levels declined through the dark phase. Phloem exudate analysis revealed that diurnal levels of the major transport sugars (stachyose and sucrose) in the phloem did not appear to correlate directly with the photosynthetic activity of the leaf but instead were inversely correlated with leaf starch accumulation and degradation. The amino acid pool in leaf tissues remained constant throughout the diurnal period; however, the relative contribution of individual amino acids to the total pool varied with the diurnal photosynthetic and respiratory activity of the leaf. In contrast, the phloem sap amino acid pool size was substantially larger in the light than in the dark, a result primarily due to enhanced export of glutamine, glutamate, and citrulline during the light period. The results indicate that the sugar and amino acid composition of cucurbit phloem sap is not constant but varies throughout the diurnal cycle in response to the metabolic activities of the source leaf.     

Apoplastic Phloem Unloading in the Stem of Bean

Sucrose has been found in the apoplast of bean stems at a concentrationof 25–60 mM with an axial concentration gradient in theappropriate direction for Munch translocation. Removal of theepidermis from a 50 mm length of stem enabled the washout oflabelled photosynthate from the apoplast. The rate of labelwashout was strongly dependent on temperature, and the rateincreased on blockage of phloem pathways to the main sink forthat assimilate. Washout did not reduce when the bathed tissuewas plasmolyzed. We propose that sucrose is unloaded from thephloem into the apoplast, and a sucrose concentration is maintainedthere by a balance of sucrose uptake into sink tissue or reloadinginto the phloem. It is proposed that the apoplastic pool ofphotosynthate can act to buffer sudden changes in phloem contentswhen there are rapid changes in source-sink configuration. Key words: Sucrose, Phaseolus vulgaris, Apoplast, Phloem unloading