14C-Translocation in Kalanchoe pinnata at two Different Stages of Development

Mayoral, M. L. and Medina, E. 1985. ¹⁴C-translocation in Kalanchoepinnata at two different stages of development.—J. exp.Bot. 36: 1405–1413 Translocation of ¹⁴C-compounds from mature leaves was measuredin plants of Kalanchoe pinnata to determine the interactionbetween plant age and CAM phase when CO² is taken up. Matureleaves of 4 and 12 month old plants were fed with ¹⁴CO² eitherduring CAM phase 1 (midnight) or at the beginning of CAM phase4 (early afternoon). Export of ¹⁴C activity from source leaves,and distribution of ¹⁴C activity in soluble and insoluble compoundswas measured both in source leaves and sink organs. In 4 monthold plants 4 d were needed to export 76% of total ¹⁴C activityincorporated during CAM phase 1, while leaves labelled at thebeginning of CAM phase 4 exported 44% of total ¹⁴C activityafter 4 h, and 80% after 24 h. In both cases the major fractionof total radioactivity translocated was found in the roots inthe form of neutral sugars. Differences in translocation patternsare due to distribution of ¹⁴C in the source leaves, 96 % of¹⁴C taken up during CAM phase 1 is found in the insoluble fractionat the end of the subsequent phase 3, while 93 % of total radioactivitytaken up at the beginning of phase 4 is found in the solublefraction at the end of this phase. In 12 month old plants labelledduring phase 1 very little translocation could be detected atthe end of phase 3, while only 20% of total radioactivity wastranslocated from leaves labelled during phase 4 and measured4 h later. ¹⁴C activity in the older leaves had a similar distributionin soluble and insoluble fractions as the one determined inthe younger plants. Ability to translocate carbon compoundsfrom source leaves during phase 3 was shown by loading matureleaves at dawn with ¹⁴C-sucrose. Here again, mature leaves ofyounger plants showed faster translocation of radioactivitythan those of older plants Key words: Kalanchoe, crassulacean acid metabolism, translocation, sink, source relationships     

14C2H4: Distribution of 14C-labeled tissue metabolites in pea seedlings

The ¹⁴C-metabolite distribution pattern following ¹⁴C₂H₄ metabolismin intact pea seedlings (Pisum sativum L.) was determined undervarious conditions. After a 24 hr exposure to ¹⁴C₂H₄, the majorityof ¹⁴C-metabolites were water-soluble (60–70%) with lesseramounts in the protein (10–15%), lipid (1%), and insoluble(1–2%) fractions. Ion exchange chromatography of the water-solublecomponents into basic, neutral, and acidic fractions revealeda 50 : 40 : 10 distribution, respectively. Chromatography ofthe neutral fraction revealed two regions of radioactivity (Rf=0.38)and 0.63 which did not cochromatograph with twenty-two knownsugars or neutral metabolites. Chromatograms of the basic fractioncontained 3 regions of radioactivity. Similar distribution patternswere noted when ¹⁴C₂H₄ exposure was followed by a 6 hr air chaseor when 5% CO₂, an antagonist of ethylene action, was presentduring the exposure. Marked differences in the ¹⁴C-metabolite distribution patternswere obtained when ¹⁴CO₂ was substituted for ¹⁴C₂H₄. These resultsindicate that the metabolic pathway involved in ethylene metabolismis different from that involved in intermediary carbon metabolism. ¹ Contribution No. 2338 from Central Research and DevelopmentDepartment, Experimental Station, E. I. du Pont de Nemours andCompany, Wilmington, Delaware. (Received June 28, 1976; )     

Hormones in Plants Bearing Nitrogen-fixing Root Nodules: Metabolism of [8-14C]Zeatin in Root Nodules of Alnus glutinosa (L.) Gaertn

The metabolism of [8-¹⁴C]zeatin, supplied via micropipettesover a 24 h period to root nodules of Alnus gliutinosa (L.)Gaertn., was investigated. The major metabolites were tentativelyidentified by means of chromatographic, chemical, and enzymictreatments as adenine, adenosine, trans-zeatin riboside, dihydrozeatin,trans-zeatin-O-ß-D-glucoside, and the O-ß-D-glucosideof dihydrozeatin. In addition, a prominent water-soluble peakof radioactivity was present. This did not appear to be a ribosidebut was biologically active in the soybean callus test. The number and nature of the metabolites formed in the noduleswas similar in both dormant and non-dormant plants.     

Effects of Temperature and Light on the Carbon Budget of Young Cucumber Plants Studied by Steady-State Feeding with 14CO2

Shishido, Y., Challa, H. and Krupa, J. 1987. Effects of temperatureand light on the carbon budget of young cucumber plants studiedby steady-state feeding with ¹⁴CO°₂J. exp. Bot. 38: 1044–1054. The effect of temperature on the fate of ¹⁴C assimilated insteady-state by the expanding third leaf of cucumber seedlingswas studied at irradiances of either 30 or 75 W m^–2 (PAR)with a daylength of 8 h. The irradiance did not affect the relativedistribution of ¹⁴C assimilated by the source leaf between growth,respiration and export. In the range 15–30°C risesin temperature generally increased the proportion of carbonexported. The average rate of carbon exported during the nightwas about half the rate in the day. About 45% of the exportedcarbon was lost by respiration. The distribution pattern ofcarbon exported during the day differed considerably from thatof carbon exported during the night. The intensity of irradiance did not affect the proportion oflabelled carbon recovered from the roots. Thus the decreasedshoot/root ratio generally observed with increased irradianceis not directly controlled by carbohydrate supply. We found that the distribution patterns of exported ¹⁴C do notnecessarily represent the real carbon distribution, due to differencesin specific activity of imported carbon of individual organs.Consequently distribution patterns of ¹⁴C observed in experimentswith one source leaf have to be considered with caution. Key words: Carbon budget, ¹⁴C, ¹⁴C steady-state feeding, translocation, respiration, assimilate distribution, cucumber, temperature     

Turnover of Carbohydrates in Relation to Growth in Apple Trees. II. Distribution of 14C Assimilates Labelled in Autumn, Spring and Summer

One-year-old plants of the apple rootstocks MM. 106 and M.7were allowed to assimilate ¹⁴CO₂ in autumn, spring and summerand the distribution of the tracer within the plant over a growingseason was followed. In MM.106 distribution of the tracer intwo fractions of extractable carbohydrate and the residues representingstructural material, was determined. The results of the radioactivityassay are discussed in relation to seasonal patterns of assimilatesupply and demand in the different regions of the plant. Malus sylvestris L, apple, ¹⁴C assimilates, distribution of carbohydrates     

The Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat: 2. Distribution in Amino Acids of Wheat Grain Fractions

Changes in the distribution of ¹⁴C between free and bound aminoacids in wheat grains (Triticum aestivum L. cv. Arkas) at 10and 20 d post-anthesis are described. After ¹⁴CO₂, labellingof the flag leaf, ¹⁴C was initially more rapidly transferredto the grains of 20 d post-anthesis plants than for 10 d post-anthesisplants. However, after a 460 min chase period in the light theamount of ¹⁴C in the grains of the younger and older plantswere similar. In the younger, more rapidly growing grains, agreater proportion of the ¹⁴C was incorporated into structuraltissue and starch. ¹⁴C accumulation in the grains continuedduring the dark in the younger grains but not in the older grains. Although the overall ¹⁴C distribution between the free aminoacid and protein pools of the grain was similar for both treatments,the distribution within the albumin, prolamin and globulin fractionsand between the individual non-bound amino acids differed. Ofthe protein fractions, the albumins were initially the mostheavily labelled but after 460 min chase the prolamins containedmore ¹⁴C. The majority of the ¹⁴C in the albumin and globulinfractions after 280 min chase was in hydrolysable, non-aminoacid compounds. In both tissues, the free amino acid pools lostradioactivity in the dark but the solid residues and proteinscontinued to function as ¹⁴C sinks. Daily fluctuations in the radioactivity in free and bound alanineare consistent with the role of free alanine as a diurnal metabolicnitrogen pool. Wheat, Triticum aestivum¹⁴CO₂, amino acids, proteins, carbon metabolism     

Transport and Accumulation of IAA-14C in Tumour-forming Nicotiana Hybrids

The polar transport of indol-3yl-acetic acid (IAA-2-¹⁴C) instem explants and decapitated shoots of tumour-prone Nicotianahybrids (2n, 3n, and 4n) was compared with that in the normal,non-tumorous parent species N. glauca and N. langsdorffii. Thetotal uptake of the auxin from donor blocks was greatest inthe hybrids and N. glauca. The velocity of the basipetal movementof IAA-¹⁴C was the same in all species tested, i.e. 8 mm/h.The transport capacity for the hormone, however, was decreasedin the three tumour-prone hybrids. Gas chromatography showedthat between 70 and 90 per cent of the transported auxin waspresent in the form of IAA, between 10 and 30 per cent in theform of indol-3yl-aldehyde (IAld). The basipetal transport exceeded the acropetal transport inyoung (third) intemodes of all plants studied, whereas in olderstem segments (tenth intenodes) the reverse was found. The polarity of auxin transport was less well expressed in thetumorous hybrids. Blocking the active transport by pre-treatment of stem cuttingswith 2,4-dinitrophenol (2,4-DNP) caused a drastic reductionin the polar IAA-¹⁴C movement; in all plants tested the auxintransport was reduced to the same low level. The accumulation of auxin at the base of cuttings was higherin N. glauca and the 2n hybrid than in N. langsdorffii, i.e.about seven times higher after 1-h and three times higher after12-h transport experiments. The release of ¹⁴C from the cuttinginto an agar receiver block, however, was markedly reduced inthe 2n hybrid, whereas in N. glauca the labelled substancesmoved more freely into the receiver blocks. Differences in the capacity for the accumulation and the releaseof IAA-¹⁴C in hybrid and N. glauca stem tissues were studiedusing decapitated greenhouse plants wounded by incision abovethe fourth internode. Accumulation of the auxin occurred onlyabove the wound-cut in hybrid plants. This observation is consistentwith the view that tumour formation on hybrid stems occurs atsites of wounding. Our data suggest an elevated auxin levelto be present during tumour initiation at these sites. These results on polar transport and accumulation of IAA-¹⁴Cin tumorous Nicotiana plants together with our previous dataon various endogenous auxins suggest that the induction of neoplasticgrowth in tobacco plants is correlated with increased auxinlevels and an accumulation of the hormone at sites of wounding.     

Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985.Metabolism of glycollate by Lemna minor L. grown on nitrateor ammonium as nitrogen source.—J. exp. Bot. 36: 1685–1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutionscontaining either NH₄⁺ or NO₃^– as nitrogen source wasallowed to assimilate [1-¹⁴C]- or [2-¹⁴C]glycollate during a20 min period in darkness or in light. The incorporation ofradioactivity into water-soluble metabolites, the insolublefraction, and into the CO₂ released was measured. In additionthe extractable activity of phosphoenolpyruvate carboxylasewas determined. During the metabolism of [2-¹⁴C]glycollate in darkness, as wellas in the light, NH₄⁺ grown plants evolved more ¹⁴CO₂ than NO₃^–grown plants. Formate was labelled only from [2-¹⁴C]glycollateand in NH₄⁺ grown plants it was significantly less labelledin light than in darkness. In NO₃^– grown plants formateshowed similar radioactivity after dark and light labelling.The radioactivity in glycine was little influenced by the nitrogensource. Amounts of radioactivity in serine implied that thefurther metabolism of serine was reduced in darkness comparedwith its metabolism in the light under both nitrogen regimes.In illuminated NH₄⁺ plants, serine was labelled through a pathwaystarting from phosphoglycerate. After [1-¹⁴C]glycollate feedingNH₄⁺ grown plants contained markedly more radioactive aspartateand malate than NO₃^– plants indicating a stimulated phosphoenolpyruvatecarboxylation in plants grown on NH₄⁺. Key words: Photorespiration, glycollate, nitrogen, Lemna     

Distribution of radioactivities in sugars from acetate-2-14C incubation and elemental content of some vascular plants in two diverse habitats of Glacier Bay, Alaska

Essentially six sugars became labeled following acetate-2-¹⁴Cmetabolism in plants distributed in two different communities—pioneerand forest. Of these the bulk of radioactivity resided in glucose,fructose and sucrose. Except for willows, labeled glucose andlabeled fructose dominated the plants of the pioneer habitat,while labeled sucrose in addition to labeled glucose and fructosecharacterized those of the forest. For willows, the radioactivitieswere present in both monosaccharides and disaccharides. Thissugar labeling pattern occurred in the alder which for the twohabitats tested, was consistent with the general observation.The relative level of the content of magnesium and phosphorusfor various plants in the pioneer habitat were no differentfrom those in the forest habitat. Levels of sulfur, chlorine,potassium and calcium were generally higher in plants of theforest than in those of the pioneer community. Plants with alow level of K contained a higher level of labeled hexoses;while plants with a high level of K contained both labeled hexosesand disaccharide, indicating that potassium may be involvedin the incorporation of the ¹⁴C into sugar during acetate-2-¹⁴C.     

The Response of Phaseolus vulgaris L. to Root-zone Anaerobiosis, Waterlogging and High Sodium Chloride

Phaseolus vulgaris L. grown at a range of external concentrationsof NaCl (0 to 80 mM) responded differently to gaseous anaerobiosis(N₂ gas) in nutrient solution or stagnant waterlogging of theroot-zone. With similar patterns of distribution of Na⁺ andCl^- occurring in the plants with comparable NaCl treatments,and similar final concentrations of Na⁺ and Cl^- in plants grownunder both root-zone conditions, rates of uptake of Na⁺ andCl^- were much higher in plants with the stagnant waterloggedrootzones. After 72 h stagnant waterlogging, plant tops fromplants grown at 40 mM NaCl contained 1.42 per cent Na⁺ and 3.44per cent Cl^- (d. wt basis) while after 9 days exposure to NaClwith gaseous anaerobiosis, leaf tissue contained 1.49 per centNa⁺ and 4.28 per cen Cl^- (d. wt basis). Plants exposed to 40mM external NaCl were severely damaged within 72 h when grownwith stagnant waterlogged root-zones; those grown with N₂ anaerobiosiscontinued growth and development over the 9 d period. Plantsgrown in nutrient solution showed changes in distribution andconcentration of Na⁺ and Cl^- when oxygen concentration was reducedbelow 21 per cent O₂ (full aeration). Phaseolus vulgaris. L., bean, mineral salt distribution, anaerobiosis, salinity, waterlogging     

Pathway of mannitol formation during photosynthesis in brown algae

Eisenia bicyclis, Arame, was allowed to photosynthesize in seawatercontaining H¹⁴CO₃^–, and ¹⁴C-mannitol was isolated fromits fronds. The ratio of ¹⁴C-total/¹⁴C₁ + ¹⁴C₆ in the ¹⁴C-mannitolwas found to be about 8.0 at 1 min-illumination, but graduallydecreased with time to 3.0, showing uniform radioactivity distribution.Mannitol therefore seems to be formed in brown algae throughthree carbon compounds. Enzymes which may be involved in the possible biosynthetic pathwayof mannitol, i.e. aldolase, hexose diphosphatase, mannitol-1-phosphataseand glucosephosphate isomerase were present in extracts fromseveral brown algae. Some of their properties are discussed. ¹Contribution from the Shimoda Marine Biological Station ofTokyo Kyoiku University, No. 187. ²Present address: Reseach Institute, Seikagaku Kogyo Co., Ltd.,Yamato-machi, Kitatama-gun, Tokyo, Japan. (Received December 13, 1968; )     

Partitioning of 14C Labelled Assimilates in Arctium tomentosum

Plants of the biennial Arctium tomentosum were grown from seedto seed-set in an open field under three different treatments:control plants receiving full light intensity, plants with aleaf area reduced by 45 per cent, and shaded plants receivingonly 20 per cent of natural illumination. At various stagesof development the youngest fully expanded leaf of one plantin each treatment was exposed to ¹⁴CO₂ for half an hour. Subsequentdistribution of labelled assimilates in various plant partswas determined after eight hours. In the first year, the mostdominant sink was the tap root irrespective of variation inassimilate supply. During the production of new vegetative growthin the second season, a larger amount of radioactive photosynthatewas recovered from above ground parts, especially during formationof lateral branches. Seed filling consumed 80–90 per centof labelled carbon exported from the exposed leaf. In the secondyear, the most pronounced difference between treatments wasin the degree of apical dominance, being highest in shaded plantsand lowest in the plants with cut leaves. Results from ¹⁴C experimentsagreed fairly well with a ‘partitioning coefficient’derived from a growth analysis of plants grown independentlyunder the same experimental conditions. Reasons for discrepanciesbetween the ¹⁴C results and the partitioning coefficient arediscussed. Arctium tomentosum, burdock, variation in assimilate supply, assimilate distribution, ¹⁴CO₂, labelling, growth analysis     

Absorption, Distribution, Metabolism and Leaching of [14C] ABA during Culture of Apple Embryos

Barthe, Ph. and Bulard, C. 1987. Absorption, distribution, metabolismand leaching of [¹⁴C] ABA during culture of apple embryos.—J.exp. Bot. 38: 1002–1011. It has been known for some time that dormant embryos, laid flaton damp filter-paper one cotyledon only being in direct contactwith it (C/2M mode of culture), exhibit unequal growth and greeningof their cotyledons. The aim of this work was to investigatewhether this particular mode of culture led to detectable differencesbetween the two cotyledons in the distribution and metabolismof [¹⁴C] abscisic acid (ABA). Two different approaches wereused, the material in both cases being dormant embryos of Pyrusmalus L. cv. Golden Delicious cultured at 23°C in darkness.In a first experiment, the embryos were cultured directly inthe C/2M mode and in the presence of 10^–2 mol m^–3[¹⁴C] ABA. In these conditions marked differences in the distributionof radioactivity between the lower (LC) and upper (UC) cotyledonappear after 24 h of culture. After 5 d, amounts of total radioactivitywere four times higher in LC than in UC, and the level of ABAwas three times higher. Metabolism was extremely active in LC,since certain metabolites were found in relative percentages(percentages with respect to the total radioactivity of thecotyledon) equivalent to (esters, glucosides), or even higher(dihydrophaseic acid, DPA) than those found in UC. It is suggestedthat this high level of metabolism in LC limits the availabilityof the transportable molecule, that is to say ABA. In a secondexperiment double culturing was carried out. The embryos werefirst cultured in the presence of 10^–2 mol m^–3 [¹⁴C]ABA in conditions which ensured equal distribution of ABA andits metabolites between the two cotyledons. After 5 d they weretransferred to an ABA-free medium and cultured in the C/2M mode.After 2 d and 5 d of culture, dissymmetry between the two cotyledonswas again noted; UC this time containing greater amounts ofradioactivity than LC. The differences, however, were less markedthan in the first experiment. This dissymmetry is due to leachingof a certain amount of radioactivity from LC, which is onlypartially compensated for by migration from UC. Key words: ¹⁴C-ABA distribution, leaching and metabolism, embryo culture, embryo dormancy     

Benzyladenine-Directed Transport of Carbon-14 and Phosphorus-32 in Senescing Bean Plants

The distribution of radioactivity from applied sucrose.¹⁴C and³²P to various plant parts were studied in relation to the retardationof leaf senescence by applied benzyladenine (BA) in intact beanplants. In short-time experiments sucrose-¹⁴C was fed to theplants for 48 h through the second trifoliate leaf at weeklyintervals from the third to the eighth week after planting.In long-term experiments sucrose-¹⁴C was fed to all plants for48 h at the third week and changes in distribution examinedat weekly intervals up till the eight week. In both cases, BAapplied to the primary leaves of intact bean plants did notcause a directed mobilization of sucrose-¹⁴C. When the plantswere stripped, leaving the primary leaves and the terminal pod,and fed sucrose-¹⁴C or ³²P through the terminal leaflet of thesecond trifoliate leaf, the BA-treated leaf accumulated relativelymore radioactivity than the opposite water-treated leaf. Itwas concluded that the retardation of senescence by BA in theprimary leaves of intact bean plants does not result directlyfrom the mobilization of metabolites and nutrients from otherplant parts. It is therefore suggested that BA-induced longevityin the primary leaves of the intact plant is accomplished bymetabolic self-sustenance.     

Alicyclic acid metabolism in plants 7. Metabolism of 14G-labelled alicyclic acids in germinating Phaseolus mungo seeds

When Phaseolus mungo seeds were allowed to germinate in 5–20mM quinate solution in the dark, a marked increase in the endogenousshikimic acid level occurred in their tissues. The acid levelrose distincdy on the 2nd day of germination and reached a maximumon the 4th day. The quinate-¹⁴C fed to germinating seeds waspredominantly converted to shikimic acid, and little radioactivitywas found in 3-dehydroshikimic acid. When quinate-¹⁴C was suppliedsimultaneously with relatively high concentrations of 3-dehydroquinicacid or 3-dehydroshikimic acid, its conversionto shikimic acidwas restrained, but hardly any radioactivity was trapped ineither of the dehydro compounds. 3-Dehydroquinic acid-¹⁴C or3-dehydroshikimic acid-¹⁴C fed to the seeds was metabolizedpreferentially to shikimic acid. The experimental results arediscussed with respect to the metabolic relationship betweenquinic acid and odier alicyclic acids in the aromatic biosynthesisof P. mungo seedlings. (Received October 16, 1975; )     

The Distribution and Identity of Assimilates in Tomato with Special Reference to Stem Reserves

Much of the work on the distribution of ¹⁴C-labelled assimilatesin tomato has been done in winter under low light intensities,and consequently the reported distribution patterns of ¹⁴C maynot be representative of plants growing in high light. Further,there are several somewhat conflicting reports on patterns ofdistribution of ¹⁴C-assimilates in young tomato plants. We soughtto clarify the situation by studying the distribution of ¹⁴C-assimilatesin tomato plants of various ages grown in summer when the lightintensity was high. In addition, the role of the stem as a storageorgan for carbon was assessed by (a) identifying the chemicalfractions in the stem internode below a fed leaf and monitoring¹⁴ C activity in these fractions over a period of 49 d, and(b) measuring concentrations of unlabelled carbohydrates inthe stem over the life of the plant. The patterns of distribution of ¹⁴C-assimilates we found fortomato grown under high light intensity confirmed some of thosedescribed for plants grown under low light, but export of ¹⁴Cby fed leaves was generally higher than reported for much ofthe earlier work. Lower leaves of young plants exported over50% of the ¹⁴C they fixed, although export fell sharply as theplants aged. Initially, the roots and apical tuft were strongsinks for assimilates, but they had declined in importance bythe time plants reached the nine-leaf stage. On the other hand,the stem became progressively more important as a sink for ¹⁴C-assimilates.Older, lower leaves exported more of their ¹⁴C-assimilates tothe upper part of the plant than to the roots, whereas youngleaves near the top of the plant exported more of their assimilatesto the roots. The stem internode immediately below a fed leafhad about twice the ¹⁴C activity of the internode above theleaf. Mature leaves above and below a fed leaf rarely importedmuch ¹⁴C, even when in the correct phyllotactic relationshipto the fed leaf. In the first 3 d after feeding leaf 5 of nine-leaf plants, theorganic and amino acid pools and the neutral fraction of theinternode below the fed leaf had most of the ¹⁴C activity, butby 49 d after feeding, the ethanolic-insoluble, starch and lipidfractions had most of the ¹⁴C activity. Glucose, fructose andsucrose were the main sugars in the stem. Although concentrationsof these sugars and starch declined in the stem as the plantsmatured, there was little evidence to indicate their use infruit production. Stems of plants defoliated at the 44-leafstage had lower concentrations of sugars and starch at maturity,and produced less fruit than the controls. It was concludedthat tomato is sink rather than source limited with respectto carbon assimilates, and that the storage of carbon in thestem for a long period is possibly a residual perennial traitin tomato.Copyright 1994, 1999 Academic Press Lycopersicon esculentum, tomato, assimilate distribution, ¹⁴C, internode storage, sink-source relationships, starch, stem reserves, sugars     

The Translocation of 14C-Labelled Assimilates in Dwarf Bean Plants Infected with Xanthomonas phaseoli (E.F.Sm.) Dowson

The distribution of ¹⁴C-labelled assimilates in dwarf beansinfected on one unifoliate with Xanthomonas phaseoli or systemicallyinfected with the pathogen was compared with the distributionin plants subjected to water stress. Unifoliates on systemically-infectedand water-stressed plants retained a much greater proportionof assimilated ¹⁴CO₂ than equivalent leaves on healthy plants,but unifoliates directly inoculated with X. phaseoli (localinfection) retained only a slightly larger proportion than controlplants. Local infection had little influence on the distribution ofexported radiocarbon but systemic infection produced markedeffects and there were similar changes in water-stressed plants.These changes seem to be related mainly to changes in growthpatterns, particularly to the reduced development of trifoliateleaves. The main causes of the altered assimilate distributionappear to be modified source-sink relationships, but the natureof the vascular system of dwarf bean and interference with vascularfunction by X. phaseoli seem to be more significant factorsthan in other diseases where assimilate distribution has beenstudied.     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

An Examination of the Value of 14C-urea as a Source of 14CO2 for Studies of Assimilate Distribution

Twenty-four hours after leaf 3 of a plant of Lolium multiflorumLam, was supplied with a droplet of ¹⁴C-urea and the plant enclosedin a polyethylene bag with an untreated plant, there were significantamounts of radiocarbon recovered from the untreated plant. Theleaf treated with ¹⁴C-urea was the major source of ¹⁴C leakagebut significant losses were also recorded from other parts ofthe plant. Reducing the humidity within the bag decreased theamount of ¹⁴CO₂ which escaped. Losses of radiocarbon from CO₂-treated plants were very low compared with those from urea-treatedplants but the pattern of assimilate distribution within thetwo types of plants was very similar. The possible causes ofthese effects are considered and the usefulness of ¹⁴C-ureaas a source of ¹⁴CO₂ discussed.     

The Effects of Leaf Age and Senescence on the Distribution of Carbon in Lolium temulentum

Assimilate distribution in leaves of Lolium temulentum was establishedby root absorption of [¹⁴C]sucrose and after exposure to ¹⁴CO₂.Age determined the amount of carbon assimilated, with more labelbeing incorporated during expansion than at maturity. Duringsenescence ¹⁴C assimilation was much lower. Ethanol-solubleextracts from various tissues of root-labelled plants containedmost of the radioactivity chiefly in basic and acidic compounds.The neutral fraction was composed predominantly of sucrose. Sucrose was comparably labelled in leaves from plants fed equalamounts of either [¹⁴C]sucrose, glucose, or fructose and onlytraces of labelled monosaccharides appeared in extracts. Radioactive sucrose was translocated rapidly from mature leaveswhereas, in the expanding leaf, carbon incorporation was directedtowards growth and the greater proportion of label present atligule formation was in ethanol-insoluble material. Induced senescence, of a mature leaf fed during expansion, produceda rapid loss from the pool of insoluble ¹⁴C. This was accompaniedby a reduction in the contents of chlorophyll and soluble proteinand an accumulation of amino acids. The onset of senescencecaused changes in leaf sugar levels which were correlated withincreased rates of respiration.