Root Development and Source-Sink Relations in Carrot, Daucus carota L.

Growth analysis and ¹⁴CO₂ feeding experiments have shown thatthe developing storage organ became an increasingly importantsink for assimilates, accumulating 40% of the dry matter producedby the carrot plant within 9 weeks of sowing. The relative importanceof each leaf in fixing and exporting ¹⁴C was assessed at twostages of development. Morphogenetic responses indicated thatan absence of thickening in the lateral roots was associatedwith continued meristematic activity in the tap root, in theform of an elongating apex or a vascular cambium. Source-sink relations were examined by observing plant growthfollowing the removal of part of the tap root and/or lateralroots. Pruning the roots at 35 d reduced the subsequent growthof the plant by reducing the AGR of the remaining root systemand the shoot. The reduction in leaf growth was associated witha loss of fibrous roots, removal of part of the tap root havingvery little additional effect on shoot growth although the AGRof the root system was reduced by a further 78%. Increased fibrousroot RGRs following pruning soon re-established the normal fibrousroot/shoot ratio. These experiments demonstrated the importanceof the root system in controlling dry matter production in thecarrot plant, but suggested that the sink activity of the developingstorage organ was less significant than other root functions.     

The Strategy of Carbon Utilization in Uniculm Barley I. THE CHEMICAL FATE OF PHOTOSYNTHETICALLY ASSIMILATED 14C

Following exposure of the youngest mature leaf of uniculm barleyto ¹⁴CO₂, groups of plants were harvested over a 72 h periodto determine the fate of 14C in the photosynthesizing leaf andin growing leaves and roots. Initially, ¹⁴C was mainly presentin sucrose with a little in starch and charged compounds; transportout of the fed leaf was rapid and, by 7 and 24 h, 56 and 93%respectively of the ¹⁴C had been translocated about equallyto growing leaves and roots. Sucrose entering meristems wasquickly metabolized to protein and structural carbohydrate (40and 60% of the ¹⁴C in these organs at 7 and 24 h respectively),while the remainder was converted to short-term storage productsor intermediary metabolites. By the end of the first day c.35% of the 14C originally assimilated had been lost in respiration. The metabolism of the leaf appeared to be organized on a diurnalbasis, for it exported nearly all its carbon within 24 h ofassimilation. In contrast, some of the assimilate imported intogrowing leaves and, to a lesser extent, roots was not immediatelyused for growth but persisted as temporary metabolites and wassubsequently used for growth in the following days.     

Partition of 14C Assimilate between Organs and Fractions of Contrasting Varieties of Carrot during Initiation of the Storage Root

This work examines the differences in partition and activityof ¹⁴C in two varieties of carrot (Daucus carota L.) contrastingin shoot to storage root ratio at maturity. Plants were grownin a controlled environment of 20 ?C and 500 µmol m^–2s^–1. During initiation of the storage root (10–25d from sowing) plants were exposed to ¹⁴CO₂ for 1 h and theradioactivity in ethanol-soluble and -insoluble fractions ofshoots, storage and fibrous roots estimated at various timesup to 48 h after exposure. Between 35% and 40% of radioactivityinitially present in the plants was respired during the first24 h and 25–35% of that remaining after 24 h was foundin the roots, depending on age. The proportion found in thestorage region remained fairly constant between 15 and 25 dand was smaller than at 10 d. In the variety with a larger proportionof storage root at maturity (cv. Super Sprite), there was agreater proportion of label in both ethanol-soluble and -insolublefractions of the storage region soon after storage root initiationhad begun than in the variety with a smaller proportion of storageroot at maturity (cv. Kingston). There was no varietal differencein specific activities of the storage roots, but fibrous rootsof cv. Super Sprite showed a greater specific activity thanin cv. Kingston. Differences in shoot to storage root ratiomay thus be associated with characteristics of the fibrous roots.Partition and specific activities are discussed in relationto the initiation and development of the storage organ. Key words: Daucus carota, carrot, assimilate, partition, ¹⁴C, storage root     

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     

Changes in the Distribution of 14C-Labelled Assimilates in Sugar-beet with Variation of Temperature

Plants were allowed to assimilate ¹⁴CO₂ for 30 min at 5, 15,25, and 35 °C. The changes in ¹⁴C content of a mature expandedleaf (Leaf 4), young apical leaves, and storage root, were sequentiallyfollowed over a subsequent period of 24 h in continuous light.In a second experiment plants were transferred after ¹⁴CO₂ assimilationto temperatures of 10, 18, 26, and 34 °C, and the partitionof ¹⁴C between the ethanol-soluble and ethanol-insoluble fractionsof the roots and leaves was followed over a period of 72 h. The specific activities of the apical leaves and of the storageroot increased to a maximum 2 h after labelling at 25 °C,4 h at 15 and 35 °C, and 6 h at 5 °C suggesting thatthe optimum temperature for translocation of photosynthate wasabout 25 °C. The ¹⁴C partition to ethanol-soluble and ethanol-insoluble fractionsof the roots and leaves was largely attained in. 9 h. Littlerepartition of ¹⁴C assimilate fractions occurred as a resultof temperature change or growth. Root ethanol-insoluble activity,however, did increase significantly over the 72-h period : possiblecauses of this slow incorporation and their relevance to themechanism of sugar storage are discussed.     

Long-term Partitioning, Storage and Remobilization of 14C Assimilated by Trifolium repens (cv. Blanca)

The fourth fully expanded leaf on the main stolon of white cloverplants was exposed to ¹⁴CO₂. Thereafter, quantitative and fractionalanalysis of the partitioning, storage and remobilization afterdefoliation of the ¹⁴C labelled assimilate was sequentiallyconducted over a 2- to 3-week period. In undefoliated plants, most ¹⁴C reached its final destinationwithin 24 h of feeding. Forty percent of assimilated ¹⁴C waslost through respiration, while the rest was exported, predominantlyto meristems, but also to roots, stolons and leaves. The ¹⁴Cinitially translocated to meristems was subsequently recoveredin stolon and leaf tissue as the plants matured. Approximately 10% of assimilated ¹⁴C was invested into long-termstorage in roots and stolons. These reserves were remobilizedafter both partial and total defoliation, and a portion of theremobilized ¹⁴C was incorporated into new growth, Partly defoliatedplants regrew more rapidly than totally defoliated plants, butmore ¹⁴C reserve depletion took place in the totally defoliatedtreatment. Reserve depletion took place from both stolons androots, but stolon reserves were preferentially utilized. Bothhigh and low molecular weight storage compounds were involved. Trifolium repens, white clover, assimilate partitioning, storage, remobilization, defoliation     

Carbohydrate Partitioning and Compartmental Analysis for a Highly Productive CAM Plant, Opuntia ficus-indica

Carbon partitioning patterns of Opuntia ficus-indica, a widelycultivated crassulacean acid metabolism species, were analysedto estimate carbon fluxes. After labelling a cladode with ¹⁴CO₂,activities of ¹⁴C in various organs were measured for 6 weeks;the observed ¹⁴C time-courses for ¹⁴C in the labelled cladodeand for transfer into other organs were simulated with a compartmentmodel. Within the first week, half of the newly synthesizedcarbohydrate in the labelled cladode was either converted intostructural material in that cladode, lost by respiration ofthat cladode, or moved to other organs. In the non-labelledcladode and the roots, such newly synthesized carbohydrate initiallyincreased, reached maxima, and then declined. The basal cladodeand the daughter cladode used 65 and 96%, respectively, of theirown assimilate. Roots imported 12 and 2 % of carbohydrate fromthe basal cladode and the daughter cladode, respectively. Whenthe whole plant was shaded, the daughter cladode incorporatednearly threefold more carbohydrate from the basal cladode intostructural material compared with the control. When plants weredroughted, roots incorporated 23 % more and the daughter cladodeincorporated 68 % less carbohydrate from the basal cladode intotheir structural material than for the control. The basal cladodesof the 18-month-old plants exported 60% more carbon than thoseof the 6-month-old plants. Carbon flux rates derived from compartmentalanalysis can be used as parameter values in plant productionmodels. Carbon partitioning, compartment model, drought, plant age, shading     

Effect of Gibberellic Acid on the Distribution of Products of Photosynthesis in Sunflower

Single leaves on growing sunflower plants were allowed to assimilate¹⁴CO₂. Gibberellic acid was applied to the same leaf or to theterminal bud or the roots, and the distribution of assimilated¹⁴C was determined at intervals of 1–96 h. Gibberellicacid had no significant effect on initial distribution of ¹⁴Cduring the period of rapid export from the leaf, but enhancedre-export from the roots after translocation from the leaf hadvirtually ceased. Most of the ¹⁴C exported from the roots accumulatedin the shoot tip. The site of application of the hormone wasof relatively minor importance. Wherever it was applied themajor effect was enhancement of movement from the roots to theshoot tip. Application to the terminal bud was most effectivein this respect. There was no evidence that gibberellic aciddirectly affected the transport system, but the data supportthe hypothesis that it increases the strength of the sink inthe shoot tip. Helianthus annuus L., sunflower, assimilate transport, gibberellic acid, phloem transport     

The Influence of Plant Water Deficit on Distribution of 14C-labelled Assimilates in Cacao Seedlings

The relationship between plant water status and distributionof ¹⁴C-labelled assimilates in cacao (Theobroma cacao L.) wasevaluated after ¹⁴CO₂ pulse labelling leaves of seedlings subjectedto varying levels of water deficiency. The proportion of ¹⁴Cexported by source leaves was strongly affected by seedlingwater status. An increasing proportion of labelled assimilatesremained in source leaves at both 24-h and 72-h harvests aswater stress intensity increased. Water stress reduced the distributionof exported label to leaves and to the expanding flush in particularbut increased the proportion of label in stems and roots. Theresults suggest that current photoassimilates may be temporarilystored in source leaves and stems of cacao seedlings duringperiods of plant water deficit. The stress-induced changes inpartitioning of labelled carbon were in concordance with changesin shoot to root biomass ratios, which was likely due to greaterreduction in growth of above-ground organs to that of roots. Theobroma cacao L, assimilate partitioning, cacao, ¹⁴C-photoassimilate, water stress, water potential     

Assimilate Partitioning in Red and White Clover Either Dependent on N2 Fixation in Root Nodules or Utilizing Nitrate Nitrogen

During vegetative growth in controlled environments, the patternof distribution of ¹⁴C-labelled assimilates to shoot and root,and to the meristems of the shoot, was measured in red and whiteclover plants either wholly dependent on N₂ fixation in rootnodules or receiving abundant nitrate nitrogen but lacking nodules. In experiments where single leaves on the primary shoot wereexposed to ¹⁴CO₂, nodulated plants of both clovers generallyexported more of their labelled assimilates to root (+nodules),than equivalent plants utilizing nitrate nitrogen, and thiswas offset by reduced export to branches (red clover) or stolons(white clover). The intensity of these effects varied with experiment.The export of labelled assimilate to growing leaves at the terminalmeristem of the donor shoot was not influenced by source ofnitrogen. Internode elongation in the donor shoot utilized nolabelled assimilate. Whole plants of white clover exposed to ¹⁴CO₂ on seven occasionsover 32 days exhibited the same effect on export to root (+nodules),which increased slightly in intensity with increasing plantage. Nodulated plants had larger root: shoot ratios than theirequivalents utilizing nitrate nitrogen. Trifolium repens, Trifolium pratense, red clover, white clover, nitrogen fixation, nitrate utilization, assimilate partitioning     

The Uptake of 14CO2 by Developing First Leaves of Barley and Partition of the Labelled Assimilates

The uptake of ¹⁴CO₂ by developing barley leaves of three ageswas followed using short presentation periods at the beginningor the end of the photoperiod. Partition of labelled carboninto ethanol-soluble and insoluble compounds, and movement oflabel within the plant were also examined. Young expanding leaves (day 6) retained most of the assimilatedcarbon and within 24 h 75–80 per cent of this was in ethanol-insolublecompounds. Leaves that were fully expanded took up rather more¹⁴CO₂ but exported a substantial amount of this to roots, leafbases including the stem apex, and to the developing secondleaf. Export occurred over periods up to 24 h, and by that time8- and 10-day-old leaves retained only 35 per cent and 15 percent respectively of the total label taken up. The label retainedin these leaves was predominantly in ethanol-soluble forms,whereas 75 per cent or more of the labelled carbon which wasexported from the leaves was found in ethanol-insoluble compounds.     

Effect of Top Pruning on Stem Carbohydrates Reserves and Foliar 14CO2 Assimilation in Mulberry Trees during Autumn

Removal of shoot tops from mulberry plants in autumn hastenedthe accumulation of starch in the stems even though photosynthesisin the remaining uppermost leaves was supressed. The proportionof the amount of ¹⁴CO₂ assimilate taken into the veins and thecontents of foliar carbohydrates and protein were not affectedby pruning. (Received December 22, 1983; Accepted March 21, 1984)     

Translocation in Sugar-beet: I. ASSIMILATION OF 14CO2 AND DISTRIBUTION OF MATERIALS FROM LEAVES

¹⁴CO₂ was supplied to leaves, and movement of labelled carbonto other parts of the plant was assessed. Young growing leavesutilized assimilated carbon for their own growth and did notexport carbon to the rest of the plant, while fully expandedleaves exported much of their photosynthate, both to root andto young leaves. Translocation from a particular leaf was tothe two or three younger leaves on the same side of the plant,and to a sector of root below the source leaf. Specific distributionto growing leaves could be modified by partial defoliation.There was no movement of material to leaves which had emergedbefore the source leaf. Part of the carbon entering a leaf by assimilation (and, foryoung leaves, by translocation) was incorporated into insolublematerial, especially in young leaves. Some of the carbon enteringa developing root was permanently stored as sucrose, althoughmuch also entered insoluble material. Loss from the leaf ofcarbon fixed during a short period of photosynthesis was rapidat first but continued at a decreasing rate for several days.Some carbon fixed into the insoluble fraction was translocatedfrom the leaf later, during senescence. Sucrose was the mainmaterial translocated immediately after photosynthesis.     

A Carbon 14 Study of Metabolism in Tall Fescue Seedlings Acclimating to Growth at Low Temperatures

The youngest fully expanded leaves of young plants of tall fescue(Festuca arundinacea Schreb. cv. S 170) were allowed to assimilate¹⁴CO₂ either (a) at the fifth leaf stage immediately beforetransfer from 17/14?C to 7/4?C (non-acclimated), or (b) aftertransfer from 17/14?C to 7/4?C at the fifth leaf stage, andfurther development in 7/4?C of one or two more leaves (acclimationfor one or two plastochrons). Controls were maintained in 17/14?C and allowed to assimilate ¹⁴CO₂ at the corresponding (fifth,sixth, or seventh) leaf stages. Allocation of 14C amongst sinksand biochemical fractions was analysed during a subsequent periodof growth at 7/4 ?C (cold) or 17/14 ?C (control). Allocationof assimilate to growing parts of the shoot was less than controlsin the non-acclimated plants transferred to the cold and morethan controls in plants acclimated for one plastochron. Afterthe imbalance induced initially by transfer from 17/14?C to7/4 ?C, acclimation for one or two plastochrons brought theallocation amongst sinks and amongst biochemical fates closerto the balance existing in controls. The main shoot and sidetillers differed in the time during acclimation when they becamemore like the controls. The percentage of assimilate in thesoluble carbohydrate fraction of non-acclimated plants and ofplants acclimated for one plastochron, was higher than in controls.The percentage of assimilate in the cell wall fraction was lowerin non-acclimated plants than in controls but the differencefrom controls after two plastochrons acclimation again was less.During the first plastochron after transfer to cold, diversionof assimilate to a reserve pool in sinks could be a factor limitingcell wall synthesis and growth and this limitation may be relievedwhen the reserve pool is ‘full’. Low temperaturereduced the percentage of assimilate in the protein fractionof growing parts of the shoot and the difference was found evenafter two plastochrons acclimation. Key words: Low temperature, metabolism, tall fescue (Festuca arundinacea Schreb. cv. S. 170)     

The Translocation of 14C-Labelled Assimilates by Dwarf Bean Plants Infected with Pseudomonas phaseolicola (Burk), Dows

The distribution of ¹⁴C-labelled assimilate after infectionof the dwarf bean plant with Pseudomonas phaseolicola was followed.Infection of a single unifoliate leaf did not affect the totalfixation of ¹⁴CO₂ by unifoliates during the assimilation period.Fixation was maximal in unifoliates in the early stages of growthbut declined as trifoliates expanded. Unifoliates on infectedplants retained a greater proportion of assimilated ¹⁴carbonthan leaves on healthy plants.The pattern of distribution ofexported assimilate was not altered in the early stages of infection,the root and apex acting as the major sinks. As the diseasedeveloped, the first trifoliate leaf, unlike similar leaveson healthy plants, continued to import assimilate apparentlyat the expense of the root. Fixation by the first trifoliateand the distribution of assimilate from this leaf were not alteredby infection of a single unifoliate leaf. At no stage duringdevelopment of the disease was there any evidence of translocationof assimilate to either inoculated or non-inoculated unifoliates.     

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     

Modulated Degradation of Ribulose Bisphosphate Carboxylase in Leaves on Top-Pruned Shoots of the Mulberry Tree (Morus alba L.)

Yamashita, T. 1987. Modulated degradation of ribulose ftisphosphatecarboxylase in leaves on top-pruned shoots of the mulberry tree(Morus alba L.).—J. exp. Bot. 38: 1957–1964. The effects of pruning shoot tops on the synthesis and degradationof ribulose 1,5–Wsphosphate carboxylase (RuBPCase) inleaves on remaining shoots were investigated in mulberry trees.Leucine labelled with ¹⁴C was fed to leaf discs from field-grownmulberry trees and ¹⁴C incorporation into RuBPCase was examined.Proportion of ¹⁴C in RuBPCase to leucine–¹⁴C absorbedby leaf discs was remarkably lowered by top-pruning, thoughoccasionally a slight increase was observed soon after pruning.Yet RuBPCase content in leaves on top-pruned shoots became progressivelyhigher than that in leaves on intact shoots. Changes in ¹⁴Cin Ru1BPCase in leaves of mulberry saplings previously fed ¹⁴CO₂were followed. Following ¹⁴CO₂ feeding, the attainment of themaximal level of ₁₄C in RuBPCase was retarded by top-pruning.The highest level of ₁₄C in RuBPCase was maintained in leaveson top-pruned shoots but decreased in leaves on intact shoots.Specific radioactivity in RuBPCase continued to increase inleaves on top-pruned shoots even after attaining a maximum levelin the control leaves. These facts suggest that the increasein RuBPCase by top-pruning results from a cessation of its degradationfor the remobilization of nitrogen for newly developing leaveson shoot tops. Key words: RuBP carboxylase, shoot pruning, mulberry (Morus alba)     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 3. Time-course Study on 13C Incorporation into Soluble Metabolites and Respiratory Evolution of 13CO2 from Roots and Nodules

Well-nodulated soya bean (Glycine max L.) plants were allowedto assimilate ¹³CO₂ for 10 h in the light, under steady-stateconditions in which CO₂ concentration and ¹³C abundance wereboth strictly controlled at constant levels. The respiratoryevolution of ¹³CO₂ from roots and nodules and ¹³C incorporationinto various metabolic fractions were measured during the ¹³CO₂feeding and subsequent 48 h chase period. CO₂ respired from nodules was much more rapidly labelled with¹³C than that from roots. The level of labelling (percentageof carbon currently assimilated during the ¹³COM₂ feeding period)of CO₂ respired from nodules reached a maximum of about 87 percent after 4 h of steady-state ^l3CO₂ assimilation and thereafterremained fairly constant. The absolute amount of labelled carbonevolved by the respiration of the nodules during the 10 h ¹³CO₂feeding period was 1·5-fold that of root respiration.These results demonstrated that the currently assimilated (labelled)carbon was preferentially used to support nodule respiration,while root respiration relied considerably on earlier (non-labelled)carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilatedcarbon in all tissues of the plants, since the levels of labellingaccounted for 86–91 per cent at the end of the ¹³CO₂ feeding.In the nodules, the kinetics and levels of sucrose labellingwere in fairly good agreement with those of respired CO₂, whilein the roots, the level of labelling of respired CO₂ was significantlylower than that of sucrose. Succinate and malate were highly labelled in both roots andnodules but they were labelled much more slowly than sucroseand respired CO₂. The kinetics and levels of labelling of theseKrebs cycle intermediates resembled those of major amino acidswhich are derived directly from Krebs cycle intermediates. Itis suggested that large fractions of organic acids in noduleswere physically separate from the respiration site. Glycine max L., Soya bean, ¹³CO₂ assimilation, respiratory evolution of ¹³CO₂, carbon metabolism in root nodules     

The influence of salinity on the utilization of root anaplerotic carbon and nitrogen metabolism in tomato seedlings

In hydroponically grown Lycopersicon esculentum (L.) Mill. cv.F144 the site of NO₃^– reduction and assimilation withinthe plant was shifted from the shoot to the root by salinity.Uptake of NO₃^– from the root solution was strongly inhibitedby salinization. Consequently, NO₃^– concentrations inthe leaf, stem and root tissues as well as the nitrate reductaseactivities of the leaves were lower in salinized than in controlplants. Lower NO₃^–, but higher reduced-N, concentrationswere observed in the xylem sap as a result of the enhanced participationof the root in NO₃^– reduction in salinized plants. Lowerstem K⁺ concentrations and leaf malate concentrations were foundin salinized compared to control plants which indicates reducedfunctioning of the K⁺–shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determinedby supplying a pulse of NaH¹⁴CO₃ followed by extraction andseparation of the labelled products on ion exchange resins.The rate of H¹⁴CO₃^– incorporation was c. 2-fold higherin control than in salinized plants. In salinized plants theproducts of H¹⁴CO₃^– incorporation within the roots werediverted into amino acids, while the control plants divertedrelatively more ¹⁴C into organic acids. Products of inorganiccarbon incorporation in the roots of salinized plants providean anaplerotic source of carbon for assimilation of reducedNO₃^– into amino acids, while in control plants the productswere predominantly organic acids as part of mechanisms to maintainionic balance in the cells and in the xylem sap. Key words: Tomato, nitrate, PEPc, respiration, salinity     

Assimilate Distribution in Poa annua L.

The carbon economy of a flowering tiller of Poa annua L. hasbeen examined over the period from inflorescence emergence tograin shedding. The total import of ¹⁴C by the inflorescencereached a maximum at late grain filling but the relative importof assimilate was greatest 14 days after its appearance andrepresented 20–25 per cent of that assimilated by theinflorescence itself. The inflorescence continued to be an importantassimilatory organ after grain ripening when it exported morethan 50 per cent of its assimilate to the stem, roots and othertillers. The patterns of distribution of assimilates from the youngestuppermost and the oldest green leaf of the reproductive tillerwere largely determined by the stage of development of the inflorescence.The youngest leaf mainly supported the inflorescence up to theend of the grain-filling stage but then supplied assimilatesbasally to the roots and adjacent tillers. The oldest greenleaf supported the growth of the stem and the inflorescenceup to anthesis but after this supplied assimilates mainly tothe roots and tillers. Removal of grains or the entire inflorescence only 1 h beforesupplying ¹⁴CO₂ greatly reduced the rate of fixation of ¹⁴CO₂and the export of radiocarbon, as well as changing the patternof distribution of assimilates within the plant. The significanceof these results is discussed and comparisons made with cerealsand perennial grasses.