Compartmentation in Vicia faba Leaves: II. Kinetics of C-Sucrose Redistribution among Individual Tissues following Pulse Labeling

Leaflets of Vicia faba L. were pulse labeled with ¹⁴CO₂ and the kinetics of ¹⁴C-sucrose redistribution among individual tissues was followed. Sucrose specific activity in the whole leaf peaked about 15 minutes after labeling and declined with a half-time of about 80 minutes. In one experiment, leaflet discs taken at various times during the ¹²CO₂ chase were quick frozen, freeze-substituted, and embedded in plastic. The tissue was sectioned paradermally and sections of palisade parenchyma, of spongy parenchyma, and of spongy parenchyma that contained veins were collected. Water extracts from these sections were assayed for sucrose specific activity. Sucrose specific activity in the palisade parenchyma was higher than that of the spongy parenchyma and reached a maximum in both tissues 9 to 15 minutes after labeling. Sucrose specific activity initially declined rapidly in the palisade parenchyma followed by a period during which little or no loss occurred. Sucrose specific activity in sections containing veins peaked at 15 minutes with a maximum value substantially higher than either mesophyll tissue, indicating that recently synthesized sucrose was preferentially exported from the mesophyll. Decline of activity in these sections containing veins continued for the remainder of the experiment. Sucrose specific activity in lower epidermal peels peaked several minutes after that of the whole leaflet and remained lower. Sucrose specific activity in upper epidermal peels was variable (probably due to contamination), but the limited data suggest that the sucrose specific activity there reached somewhat higher values than those of the lower epidermis. The experiments indicate that each leaf tissue contains a kinetically identifiable sucrose pool (which we refer to as “histological compartmentation”), and that further compartmentation may occur at the intracellular level. A simulation of leaf sucrose compartmentation is presented.     

Post-uptake metabolism affects quantification of amino acid uptake

? The quantitative significance of amino acids to plant nutrition remains controversial. This experiment determined whether post-uptake metabolism and root to shoot export differ between glycine and glutamine, and examined implications for estimation of amino acid uptake. ? Field soil containing a Eucalyptus pauciflora seedling was injected with uniformly (13)C- and (15)N-labelled glycine or glutamine. I quantified (15)N and (13)C excess in leaves and roots and intact labelled amino acids in leaves, roots and stem xylem sap. A tunable diode laser quantified fluxes of (12)CO(2) and (13)CO(2) from leaves and soil. ? 60-360 min after addition of amino acid, intact molecules of U-(13)C,(15)N glutamine were < 5% of (15)N excess in roots, whereas U-(13)C,(15)N glycine was 30-100% of (15)N excess in roots. Intact molecules of glutamine, but not glycine, were exported from roots to shoots. ? Post-uptake metabolism and transport complicate interpretation of isotope labelling such that root and shoot contents of intact amino acid, (13)C and (15)N may not reflect rates of uptake. Future experiments should focus on reconciling discrepancies between intact amino acid, (13)C and (15)N by determining the turnover of amino acids within roots. Alternatively, post-uptake metabolism and transport could be minimized by harvesting plants within minutes of isotope addition.     

Long Distance Translocation of Sucrose, Serine, Leucine, Lysine, and Carbon Dioxide Assimilates: II. Oats

To establish whether several amino acids were equally able to enter the phloem of oat (Avena sativa L.) plants and be transported, several (14)C-labeled amino acids were applied individually to an abraded spot on a fully expanded source leaf. The base of an immature sink leaf was monitored with a GM tube for time and rate of arrival of radioactivity. Transport of (14)C-sucrose and (14)CO(2) assimilates was measured for a comparison. The applied l-serine, l-lysine, and l-leucine, as well as sucrose, entered the phloem and were transported to the sink leaf at rates between 1.16 and 1.83 cm/min. Transport velocity for CO(2) assimilates was 1.57 cm/min. A heat girdle near the top of the source leaf sheath blocked most transport, which indicated that transport was primarily through the phloem. Mass transfer rates for amino acids were only 3% as great as that for sucrose, suggesting different mechanisms of entry for sucrose than for amino acids into the phloem. The higher percentage of CO(2) assimilates mobilized to the sink leaf was attributed to the greater surface area of minor veins accessible to loading, as compared to those compounds supplied via an abraded spot. Serine was extensively metabolized in the source leaf, and radioactive products in the sink leaf mirrored those in the source leaf. Most radioactivity of lysine and leucine remained within these compounds in the source, path, and sink tissues. We concluded that there was no barrier to entry of amino acids into the phloem and transport therein. Data do not suggest a specific mechanism for entry of amino acids into the phloem.     

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.     

Reduced amino acid content in transgenic potato tubers due to antisense inhibition of the leaf H+/amino acid symporter StAAP1

Transport processes across the plasma membrane of leaf vascular tissue are essential for transport and distribution of assimilates. In potato, leaves are the predominant sites for nitrate reduction and amino acid biosynthesis. From there, assimilated amino acids are exported through the phloem to supply tubers with organic nitrogen. To study the role of amino acid transporters in long-distance transport and allocation of organic nitrogen in potato plants, a gene encoding a functional, leaf-expressed amino acid permease StAAP1 was isolated. Similar to the sucrose transporter SUT1, StAAP1 expression was induced during the sink-to-source transition, indicating a role in phloem loading. To test the role of StAAP1, expression was inhibited by an antisense approach. Transgenic plants with reduced StAAP1 expression were phenotypically indistinguishable from wild type, as were photosynthetic capacity and tuber yield. However, tubers from antisense StAAP1 plants showed up to 50% reduction in free amino acid contents. In comparison, starch content was not affected or tended to increase relative to wild type. The reduction in all amino acids except aspartate in the antisense plants is consistent with the properties of amino acid permeases (AAPs) found in heterologous systems. The results demonstrate an important role for StAAP1 in long-distance transport of amino acids and highlight the importance of plasma membrane transport for nutrient distribution in plants.     

Transgenic tobacco plants expressing yeast-derived invertase in either the cytosol, vacuole or apoplast: a powerful tool for studying sucrose metabolism and sink/source interactions

In higher plants sucrose plays a central roles with respect to both short-term storage and distribution of photoassimilates formed in the leaf. Sucrose is synthesized in the cytosol, transiently stored in the vacuole and exported via the apoplast. In order to elucidate the role of the different compartments with respect to sucrose metabolism, a yeast-derived invertase was directed into the cytosol and vacuole of transgenic tobacco plants. This was in addition to the targeting of yeast-derived invertase into the apoplast described previously. Vacuolar targeting was achieved by fusing an N-terminal portion (146 amino acids long) of the vacuolar protein patatin to the coding region of the mature invertase protein. Transgenic tobacco plants expressing the yeast-derived invertase in different subcellular compartments displayed dramatic phenotypic differences when compared to wild-type plants. All transgenic plants showed stunted growth accompanied by reduced root formation. Starch and soluble sugars accumulated in leaves indicating that the distribution of sucrose was impaired in all cases. Expression of cytosolic yeast invertase resulted in the accumulation of starch and soluble sugars in both very young (sink) and older (source) leaves. The leaves were curved, indicating a more rapid cell expansion or cell division at the upper side of the leaf. Light-green sectors with reduced photosynthetic activity were evenly distributed over the leaf surface. With the apoplastic and vacuolar invertase, the phenotypical changes induced only appear in older (source) leaves. The development of bleached and/or necrotic sectors was linked to the source state of a leaf. Bleaching followed the sink to source transition, starting at the rim of the leaf and moving to the base. The bleaching was paralleled by the inhibition of photosynthesis.     

Carbon Partitioning in a Flaveria linearis Mutant with Reduced Cytosolic Fructose Bisphosphatase

Oxygen sensitivity and partitioning of carbon was measured in a mutant line of Flaveria linearis that lacks most of the cytosolic fructose-1,6-bisphosphatase found in wild-type lines. Photosynthesis of leaves of the mutant line was nearly insensitive to O₂, as found before. The mutant plants partitioned 2.5 times less carbon into sucrose than the wild type in a pulse chase experiment, with the extra carbon going mainly to starch but also to amino acids. From 10 to 50 min postlabeling, radioactivity chased out of the amino acid fraction to starch in both lines. In the middle of the light period, starch grains were larger in the mutant than in the wild type and covered 30% of the chloroplast area as seen with an electron microscope. Starch grains were found in both mesophyll and bundle sheath chloroplasts in both lines in these C₃-C₄ intermediate plants. At the end of the dark period, the starch levels were considerably reduced from what they were in the middle of the light in both lines. The concentration of sucrose was higher in the mutant line despite the lack of cytosolic fructose-1,6-bisphosphatase. The amino acid fraction accounted for about 30% of all label following a 10-min chase period. In the mutant line, most of the label was in the glycine + serine fraction, with 10% in the alanine fraction. In wild-type leaves, 35% of the label in amino acids was in alanine. These results indicate that this mutant survives the reduced cytosolic fructose-1,6-bisphosphatase activity by partitioning more carbon to starch and less to sucrose during the day and remobilizing the excess starch at night. However, these results raise two other questions about this mutant. First, why is the sucrose concentration high in a plant that partitions less carbon to sucrose, and second, why is alanine heavily labeled in the wild-type plants but not in the mutant plants?     

Amino Acid metabolism of pea leaves: diurnal changes and amino Acid synthesis from N-nitrate

In the young leaves of pea (Pisum sativum L.) plants, there was a diurnal variation in the levels of amino acids. In the light, total amino nitrogen increased for the first few hours, then stabilized; in the dark, there was a transient decrease followed by a gradual recovery. Asparagine, homoserine, alanine, and glutamine accounted for much of these changes. The incorporation of ¹⁵N into various components of the young leaves was followed after supply of ¹⁵N-nitrate. ¹⁵N appeared most rapidly in ammonia, due to reduction in the leaf, and this process took place predominantly in the light. A large proportion of the primary assimilation took place through the amide group of glutamine, which became labeled and turned over rapidly; labeling of glutamic acid and alanine was also rapid. Asparagine (amide group) soon became labeled and showed considerable turnover. Slower incorporation and turnover were found for aspartic acid, γ-aminobutyric acid, and homoserine. Synthesis and turnover of all of the amino acids continued at a low rate in the dark. γ-Aminobutyric acid was the only compound found to label more rapidly in the dark than in the light.     

Effect of Zn deficiency on net photosynthetic rate, 14C partitioning,and oil accumulation in leaves of peppermint

Changes in growth, CO₂ exchange rate, and distribution of photosynthetically fixed ¹⁴CO2 into the primary photosynthetic metabolic pool (sugars, amino acids and organic acids) and essential oil accumulation were determined in leaves (leaf positions 1-6 from apex) of developing peppermint grown in a solution culture at Zn concentrations of 0 and 0.05 g m-3. There was a significant decrease in ¹⁴C incorporation in total, ethanol-soluble and ethanol-insoluble fractions in Zn deficient plants at all leaf positions. 14C incorporated in essential oil and in sugars were significantly higher in leaf pairs 1 to 3 than in leaf pairs 4 to 6. ¹⁴C incorporation into amino acids and organic acids was higher in all leaf pairs in Zn deficient plants. Statistical analysis showed a positive significant association between Zn content of leaf and ¹⁴C incorporation into ethanol-soluble fraction and sugars and a negative correlation with ¹⁴C incorporation into amino acids and organic acids. Hence the content of sugars in leaves significantly influences essential oil accumulation under Zn stress. This revised version was published online in September 2006 with corrections to the Cover Date.     

Comparative Distribution and Metabolism of Xylem-Borne Amino Compounds and Sucrose in Shoots of Populus deltoides

The transport and metabolism of xylem-borne amino compounds and sucrose were investigated in rapidly growing shoots of cottonwood (Populus deltoides Bartr. ex Marsh.). ¹⁴C-labeled glutamine, threonine, alanine, glutamic acid, aspartic acid, and sucrose were applied to the base of severed stems for transport in xylem. Distribution and metabolism of the compounds were followed with autoradiography, microautoradiography, and radioassay. Three utilization patterns were observed: (a) little alanine and sucrose was transported to the laminae of either mature leaves or developing leaves. These compounds were taken up from xylem free-space and utilized in adjacent tissue; (b) threonine also did not move into mature leaves but was translocated to developing leaves or utilized in the stem; (c) glutamic acid and aspartic acid were transported directly into the laminae of mature leaves via the xylem. Relatively less ¹⁴C was retained in stems compared to the other compounds.

Metabolism of the test compounds also differed considerably. ¹⁴C from amino acids moved primarily into organic acids and protein. The ¹⁴C from sucrose was widely distributed among the chemical fractions, with a high percentage found in structural carbohydrates. Clearly, cottonwood stems contain efficient uptake and transfer systems that differentiate among various compounds moving from root to shoot in xylem.

     

Export of amino acids to the phloem in relation to N supply in wheat

The effect of different N supply on amino acid export to the phloem was studied in young plants of wheat (Triticum aestivum L. cv. Klein Chamaco), using the exudation in EDTA technique. Plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different NO₃^? concentrations. When plants were grown for 15 days with nutrient solutions containing 1.0, 3.0, 5.0, 10.0, 15.0 or 20.0 mM KNO₃, the exudation rate of sugars from the phloem was unaffected by N supply, but sugars accumulated in the leaf tissue when the N supply was limiting for growth. On the other hand, the rate of exudation of amino acids was proportional to the NO₃^? concentration in the nutrient solution. When the supply of N to plants grown for 15 days with 15.0 mM NO₃^? was interrupted, the exudation of sugars was again unaffected, but there was a fast decrease in the amount of amino acids exudated, and of the concentration of amino acids and nitrogen in the tissues. Also, when 10-day-old plants grown without N were supplied with 15.0 mM NO₃^?, there was a sharp increase in the exudation of amino acids, without changes in the amount of sugar exudated. The rate of exudation of amino acids to the phloem was independent of the concentration of free amino acids in the leaves in all three types of experiment. Asp was the most abundant amino acid in the leaf tissue, while Glu was the one most abundant in the phloem exudate. Asp and Ala were exported to the phloem at a rate lower than expected from their leaf tissue concentrations, indicating some discrimination. On the contrary, Glu showed a preferential export at low N supply. It is concluded that the rate of amino acid export from the leaf to the phloem is dependent on the N available to the plant. This N is used for synthesis of leaf protein when the supply is low, exported to the phloem when supply is adequate, and accumulated in the storage pool when supply is above plant demand.     

Effect of the expression of cyanamide hydratase on metabolites in cyanamide-treated soybean plants kept in the light or dark

Metabolite profiling of untransformed and cyanamide hydratase- (Cah) transformed (denoted 1C) soybean (Glycine max [L.] Merrill) leaves revealed only small differences in plants grown in the greenhouse or in the dark for 24 h, indicating that the Cah enzyme that converts cyanamide to urea has no substrates in soybean leaves and does not affect metabolism. Untransformed leaves sprayed with 0.5% cyanamide developed necrotic lesions within 2 h in the light but not in the dark. The sprayed 1C leaves showed little visible damage and accumulated high concentrations of urea, amino acids, and some sugars, but sucrose decreased over a 24 h period. The untransformed necrotic leaves also accumulated some urea and amino acids apparently due to cyanamide degradation, while sucrose and some organic acids decreased. Sprayed 1C leaves in the dark for 24 h contained very little urea and lower sugar levels. The untransformed sprayed leaves accumulated some organic acids, some sugars including sucrose, and urea and total amino acids. Unsprayed plants of both lines placed in the dark for 24 h showed increases in some amino acids and phosphate, and decreases in other amino acids, sugars, and organic acids. Thus the Cah enzyme can detoxify cyanamide by conversion to urea that is converted to amino acids. Other metabolic changes associated with leaf necrosis and darkness are also described. Principal component analysis confirmed the similarities and differences observed. Comparison of the GC-MS metabolic profiling analysis of amino acids with a dedicated system shows large differences, indicating a limitation of the former system.     

Import of 14C-Photosynthate by Developing Leaves of Sugarcane

Sink-to-source transition was studied in developing sugarcane (Saccharum interspecific variety L62–96) leaves. Fully-expanded, mature sugarcane leaves were fed ¹⁴CO₂ for 20 minutes, incorporating about 617 Bq. After five hours the leaves of each plant were cut into 1-cm-length segments that were weighed and then placed in scintillation cocktail for counting. All leaves younger than the leaf fed ¹⁴CO₂ imported labeled photoassimilate. Three to four leaves had both importing and non-importing regions within the blade and a distinct transition region between them. A transition region was observed in leaves which had expanded to between 30 and 90 % of final blade length. Radioactivity per gram fresh weight was calculated as a measure of sink strength. Sink strength was greatest in the youngest leaf and declined with leaf age. The results of this study indicate that 1) import of photosynthate by developing sugarcane leaves occurs over a longer span of developmental ages than in dicotyledonous leaves and 2) the actual tissue region undergoing transition within such a leaf can be resolved as narrow zone between the importing and non-importing regions.     

(14) C]-Assimilate translocation in the light and dark in celery (Apium graveokns) leaves of different ages

The 2 major photosynthetic products and translocated carbohydrates in celery ( Apium graveolens L.) are sucrose and the sugar alcohol, mannitol. Sucrose is produced and utilized in leaves of all ages. Mannitol, however, is synthesized primarily in mature leaves, utilized in young leaves and stored in all leaves. Here we show that mannitol export was lower from young, expanding leaves than from older leaves. After a 10 min pulse of ¹⁴CO₂ and a 2 h chase in the light or dark there was more radioactivity in sucrose than in mannitol in petiole tissues from leaves of all ages. However, after a chase of 15 h in the dark or 6 h in the light followed by 9 h in the dark, mannitol was the predominant [¹⁴C]-labeled carbohydrate remaining in all leaf and petiole tissues. Thus, newly synthesized sucrose was apparently exported at a faster rate than mannitol and more mannitol was partitioned into vacuolar storage pools than was sucrose. It also appears that in the light both sucrose and mannitol were exported, but in the dark, once sucrose pools were depleted, mannitol remained as the predominant substance translocated. Both mannitol and sucrose were unloaded into petiole storage parenchyma tissue, but sucrose was hydrolyzed prior to storage.     

A Method for Calculating Sucrose Synthesis Rates throughout a Light Period in Sugar Beet Leaves

Sucrose synthesis rate in an exporting sugar beet (Beta vulgaris L.) leaf was calculated from simultaneous measurements of export and changes in leaf sucrose level. The amount of recently fixed carbon exported was determined from net carbon assimilated minus the tracer carbon accumulated in the leaf. The relative amount of ¹⁴C accumulated in the leaf supplied with ¹⁴CO₂ throughout an entire light period was recorded continuously with a Geiger-Mueller detector. To produce a continuous time course for tracer carbon accumulated in the leaf during the light period, the latter curve was superimposed on values for tracer carbon accumulated in leaves sampled at hourly intervals. Validity of the method requires that nearly all of the carbon that is exported be sucrose and that nearly all of the sucrose that is synthesized be either exported or accumulated as sucrose in the exporting leaves. These conditions appeared to be fulfilled in the situations where the method was applied. The method was used to study the effect of increasing atmospheric CO₂ concentration on the rate of sucrose synthesis. Further, the method can be used in conjunction with the gathering of other data such as gas exchange, metabolite levels, and enzyme activities in a set of leaves of a similar age on the same plant. This assemblage of data was found to be useful for understanding how rates of photosynthesis, sucrose synthesis, and translocation are regulated in relation to each other in an intact plant.     

Growth, photosynthesis, nitrogen partitioning and responses to CO2 enrichment in a barley mutant lacking NADH-dependent nitrate reductase activity

Plant growth, photosynthesis and leaf constituents were examined in the wild-type (WT) and mutant nar1 of barley (Hordeum vulgare L. cv. Steptoe) that contains a defective structural gene encoding NADH-dependent nitrate reductase (NADH-NAR). In controlled environment experiments, total biomass, rates of photosynthesis, stomatal conductance, intercellular CO(2) concentrations and foliar non-structural carbohydrate levels were unchanged or differed slightly in the mutant compared with the WT. Both genotypes displayed accelerated plant growth rates when the CO(2) partial pressure was increased from 36 to 98 Pa. Total NADH-NAR activity was 90% lower in the mutant than in the WT, and this was further decreased by CO(2) enrichment in both genotypes. Inorganic nitrate was greater in the mutant than in the WT, whereas in situ nitrate assimilation by excised leaves was two-fold greater for the WT than for the mutant. Foliar ammonia was 50% lower in the mutant than in the WT under ambient CO(2). Ammonia levels in the WT were decreased by about one-half by CO(2) enrichment, whereas ammonia was unaffected by elevated CO(2) in mutant leaves. Total soluble amino acid concentrations in WT and mutant plants grown in the ambient CO(2) treatment were 30.1 and 28.4 micromol g(-1) FW, respectively, when measured at the onset of the light period. Seven of the twelve individual amino acids reported here increased during the first 12 h of light in the ambient CO(2) treatment, leading to a doubling of total soluble amino acids in the WT. The most striking effect of the mutation was to eliminate increases of glutamine, aspartate and alanine during the latter half of the photoperiod in the ambient CO(2) treatment. Growth in elevated CO(2) decreased levels of total soluble amino acids on a diurnal basis in the WT but not in mutant barley leaves. The above results indicated that a defect in NADH-NAR primarily affected nitrogenous leaf constituents in barley. Also, we did not observe synergistic effects of CO(2) enrichment and decreased foliar NADH-NAR activity on most N-containing compounds.     

Translocation and metabolism of glycine betaine by barley plants in relation to water stress

The glycine betaine which accumulated in shoots of young barley plants (Hordeum vulgare L.) during an episode of water stress did not undergo net destruction upon relief of stress, but its distribution among plant organs changed. During stress, betaine accumulated primarily in mature leaves, whereas it was found mainly in young leaves after rewatering. Well-watered, stressed, and stressed-rewatered plants were supplied with [methyl-¹⁴C]betaine (8.5 nmol) via an abraded spot on the second leaf blade, and incubated for 3 d. In all three treatments the added ¹⁴C migrated more or less extensively from the second leaf blade, but was recovered quantitatively from various plant organs in the form of betaine; no labeled degradation products were found in any organ. When 0.5 mol of [methyl-¹⁴C]betaine was applied via an abraded spot to the second leaf blades of well-watered, mildly-stressed, and stressed-rewatered plants, ¹⁴C was translocated out of the blades at velocities of about 0.2–0.3 cm/min which were similar to velocities found for applied [¹⁴C]sucrose. Heat-girdling of the sheath prevented export of [¹⁴C]betaine from the blade. When 0.5 mol [³H]sucrose and 0.5 mol [¹⁴C]betaine were suppled simultaneously to second leaf blades, the ³H/¹⁴C ratio in the sheath tissue was the same as that of the supplied mixture. After supplying tracer [¹⁴C]betaine aldehyde (the immediate precursor of betaine) to the second leaf blade, the ¹⁴C which was translocated into the sheath was in the form of betaine. Thus, betaine synthesized by mature leaves during stress behaves as an inert end product and upon rewatering is translocated to the expanding leaves, most probably via the phloem. Accordingly, it is suggested that the level of betaine in a barley plant might serve as a useful cumulative index of the water stress experienced during growth.     

Effect of Oxygen Concentration on C-Photoassimilate Transport from Leaves of Salvia splendens L

Partitioning and transport of recently fixed photosynthate was examined following ¹⁴CO₂ pulse-labeling of intact, attached leaves of Salvia splendens L. maintained in an atmosphere of 300 microliters per liter CO₂ and 20, 210, or 500 milliliters per liter O₂. Under conditions of increasing O₂ (210, 500 milliliters per liter), a smaller percentage of the recently fixed ¹⁴C in the leaf was allocated to starch, whereas a greater percentage of the fixed ¹⁴C appeared in amino acids, particularly serine. The increase in ¹⁴C in amino acids was reflected in material exported from source leaves. A higher percentage of ¹⁴C in serine, glycine, and glutamate was recovered in petiole extracts when source leaves were maintained under elevated O₂ levels. Although pool sizes of these amino acids were increased in both the leaves and petioles with increasing photorespiratory activity, no significant changes in either ¹⁴C distribution or concentration of transport sugars (i.e. stachyose, sucrose, verbascose) were observed. The data indicate that, in addition to being recycled intracellularly into Calvin cycle intermediates, amino acids produced during photorespiration may also serve as transport metabolites, allowing the mobilization of both carbon and nitrogen from the leaf under conditions of limited photosynthesis.     

Nitrogen redistribution during grain growth in wheat (Triticum aestivum L.)

The technique of EDTA-enhanced phloem exudation (King and Zeevaart, 1974: Plant Physiol. 53, 96–103) was evaluated with respect to the collection and identification of amino acids exported from senescing wheat leaves. Whilst the characteristics of the exudate collected conform with many of the accepted properties of phloem exudate, unexpectedly high molar proportions of phenylalanine and tyrosine were observed. By comparing exudation into a range chelator solutions with exudation into water, the increased exudation of phenylalanine and tyrosine relative to the other amino acids occurring when ethylene-diaminetetracetic acid was used, was considered to an artefact.In plants thought to be relying heavily on mobilisation of protein reserves to satisfy the nitrogen requirements of the grain, the major amino acids present in flag-leaf phloem exudate were glutamate, aspartate, serine, alanine and glycine. Only small proportions of amides were present until late in senescence when glutamine became the major amino acid in phloem exudate (25 molar-%). Glutamine was always the major amino acid in xylem sap (50 molar-%).The activities of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.7.1), glutamate dehydrogenase (EC 1.4.1.3) and asparagine synthetase (EC 5.3.5.4) were measured in the flag leaf throughout the grain-filling period. Glutamine synthetase and glutamate-synthase activities declined during this period. Glutamate-dehydrogenase activity was markedly unchanged despite variation in the number of multiple forms visualised after gel electrophoresis. The activity of the enzyme reached a peak only very late in the course of senescence of the flag leaf. No asparagine-synthetase activity could be detected in the flag leaf during the grain-filling period.II. Peoples et al. (1980)     

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.