Carbon and Nitrogen Sources for Protein Synthesis and Growth of Sugar-beet Leaves

Protein synthesis in very young leaves utilizes carbon fromphotosynthesis and from translocated sucrose, and nitrogen translocatedin both xylem and phloem. The carbon of young leaf protein isderived mainly from assimilated CO₂, while translocated sucrosecontributes proportionately more of its carbon to insolublecarbohydrate. Most protein amino-acids become labelled from¹⁴CO₂, glutamate being the notable exception. Glutamine or glutamateis synthesized from sucrose in roots, and is translocated toyoung leaves. It is suggested that a small but significant proportionof the nitrogen requirement of the young leaf is translocatedfrom roots as glutamine, in the phloem. Inorganic nitrogen istranslocated in xylem.     

Oxygen Exchange in Relation to Carbon Assimilation in Water-stressed Leaves During Photosynthesis

In a study on metabolic consumption of photosynthetic electronsand dissipation of excess light energy under water stress, O₂and CO₂ gas exchange was measured by mass spectrometry in tomatoplants using ¹⁸O₂ and ¹³CO₂. Under water stress, gross O₂ evolution(E_O), gross O₂ uptake (U_O), net CO₂ uptake (P_N), gross CO₂ uptake(TPS), and gross CO₂ evolution (E_C) declined. The ratio P_N/E_Ofell during stress, while the ratios U_O/E_O and E_C/TPS rose.Mitochondrial respiration in the light, which can be measureddirectly by ¹²CO₂ evolution during ¹³CO₂ uptake at 3000 µll^{–1 13}CO₂, is small in relation to gross CO₂ evolutionand CO₂ release from the glycolate pathway. It is concludedthat PSII, the Calvin cycle and mitochondrial respiration aredown-regulated under water stress. The percentages of photosyntheticelectrons dissipated by CO₂ assimilation, photorespiration andthe Mehler reaction were calculated: in control leaves morethan 50 % of the electrons were consumed in CO₂ assimilation,23 % in photorespiration and 13 % in the Mehler reaction. Undersevere stress the percentages of electrons dissipated by CO₂assimilation and the Mehler reaction declined while the percentageof electrons used in photorespiration doubled. The consumptionof electrons in photorespiration may reduce the likelihood ofdamage during water deficit.     

Phloem-xylem water flow in developing cladodes of Opuntia ficus-indica during sink-to-source transition

Phloem versus xylem water and carbon flow between a developingdaughter cladode (flattened stem segment) and the underlyingbasal cladode of Opuntia ficus-indica was assessed using netCO₂ uptake, transpiration, phloem sap concentration, and waterpotential of both organs as well as phloem and apoplastic tracers.A 14-d-old daughter cladode was a sink organ with a negativedaily net CO₂ uptake; its water potential was higher than thatof the underlying basal cladode, implicating a non-xylem pathwayfor the water needed for growth. Indeed, the relatively dilutephloem sap (7.44% dry weight) of a basal cladode can supplyall the water (7.1 gd^–1) along with photosynthate neededfor the growth of a 14-d-old daughter cladode; about 3% of theimported water flowed back to the basal cladode via the xylem.In contrast, a 28-d-old daughter cladode was a source organwhose water potential was lower than that of its basal cladode,so the xylem can supply the water needed (25.7 g d^–1)for its growth; about 6% of the imported water flowed back tothe basal cladode along with photosynthate via the phloem. Thephloem tracer carboxyfluorescein occurred in the phloem of 14-d-olddaughter cladodes after its precursor was applied to basal cladodes.When applied to basal cladodes, the apoplastic tracers sulphorhodamineG (SR) and trisodium 8-hydroxy-1,3,6-pyrenetrisulphonate (PTS)failed to move into 14-d-old daughter cladodes within 5 h, butmoved into 28-d-old daughter cladodes within 2 h. SR and PTSmoved into basal cladodes within 2 h when applied to 14-d-olddaughter cladodes, but not within 5-6 h when applied to 28-d-olddaughter cladodes. The tracer experiments therefore confirmedthe patterns of water flow determined using water and carbonbudgets. Key words: Carboxyfluorescein, phloem-xylem water flow, source-sink water relations, suiphorhodamine G, trisodium 8-hydroxy-1,3,6-pyrenetnsulphonate     

Temporal Patterns of Uptake, Flow and Utilization of Nitrate, Reduced Nitrogen and Carbon in a Leaf of Salt-Treated Castor Bean (Ricinus communis L.)

Changes in net photosynthesis, respiration, transpiration andcontents of total C, NO₃-N and reduced N were followed throughoutthe life of leaf 6 of nitrate-dependent plants of castor beanexposed to moderate salinity stress (71 mol m^–3 NaCl).Salt treatment was applied for measuring mineral flows in aparallel study (Jeschke and Pate, 1991b). Concurrent measurementswere made of solute composition and C: N molar ratios and concentrationsof reduced N and collected NO₃-N in phloem sap bleeding fromshallow incisions in the top and at the base of petioles andin xylem exudates from flaps of proximal leaf midribs followingpressurization of the root system. The resulting data were usedto construct empirical models of the respective economies ofC, total N, NO₃ and reduced N for a sequence of defined phasesof leaf life. Water use efficiency increased 3-fold from emergenceto a maximum of 1·5 mmol CO₂ mol^–1 H₂O before decliningto 0·5 mmol CO₂ mol^–1 H₂O at senescence. Xylemmolar ratios of C:N varied from 1·2–2·8,with nitrate always a smaller component than reduced N. Phloemsap C:N increased from 10–40 with leaf expansion and wasthen maintained in the range of 40–50 until falling steeplyto 20 at leaf senescence. Nitrate comprised less than 1% oftotal N in all phloem sap samples. The models of C uptake, flow,and utilization showed a major role of phloem import and thenincreasingly of laminar photosynthesis in providing C for leafgrowth. The carbon budget was thereafter characterized by ratesof phloem export closely matched to net rates of CO₂ fixationby the lamina. Corresponding data for total N depicted an earlymajor role of both xylem and phloem import, but the eventualdominance of xylem import as the N source for leaf growth. Cyclingof N by xylem to phloem exchange commenced before the leaf hadachieved maximum N content, and was the major contributor tophloem export until leaf senescence when mobilized N providedmost exported N. The nitrate economy of the leaf was characterizedby early establishment of tissue pools of the ion in the petioleand to a lesser extent in the lamina, continued high rates ofnitrate reduction in the lamina but negligible assimilationin the petiole, and a release through xylem of previously accumulatedNO₃ from petiole to lamina. Related data for reduced N illustratedthe much greater importance of this form of N than nitrate intransport, storage and cycling of N at all stages of leaf andpetiole life. Xylem to phloem interchanges of reduced N in petiolewere minimal in comparison with cycling through the lamina.The ratio of CO₂ reduction to NO₃ reduction in the lamina wasat first low (57 mol mol^–1) increasing to a peak valueof 294 during mature leaf functioning before declining to 190during the presenescence phase of leaf development. This patternreflected age-related effects on water use efficiency, changesin NO₃ levels in the xylem stream entering the lamina, and therelatively low photosynthetic performances of very young andsenescent laminae. Key words: Ricinus communis, leaf development, phloem transport, xylem transport, carbon, nitrogen, nitrate, reduced nitrogen, nitrate reduction, partitioning     

Preparation of Radioactive Starch, Glucose and Fructose from C14O2

Radioactive starch, glucose and fructose have been preparedfrom tobacco leaves after assimilation of C¹⁴O₂. The apparatusused for photosynthesis consisted of a shallow Perspex leafchamber connected to a closed gas system, in which C¹⁴O₂ wasgenerated from BaC¹⁴O₂. Six leaves, area 14 to 18 sq. dm. whenexposed to bright sunlight with an initial CO₂ concentrationof 8 to 10 per cent., assimilated 3.35 g. of C¹⁴O₂ in 8 to 10hours. At least 80 per cent. of the C¹⁴O₂ supplied appearedin the leaves as starch and sugar and over 80 per cent. of theradioactivity was accounted for in these carbohydrates. Thespecific activity per m. atom of carbon of the isolated productswas 85 to 90 per cent. of that of the C¹⁴O₂. Small amounts ofradioactive carbon were also incorporated in the leaf proteinand in the celluose, hemicellulose and polyuronides.     

Tissue structure and respiration of stems of <Emphasis Type="Italic">Betula pendula</Emphasis> under contrasting ozone exposure and nutrition

Tissue structure and respiration (Rs) of stems were analyzed in Betula pendula grown throughout the growing season in either filtered air (control) or 90/40 nl O₃ l^-1 (day/night). Both regimes were split into high and low nutrient supply. High nutrition increased tissue and cell sizes within the stem xylem, phloem and periderm, whereas ozone (O₃) tended to reduce tissue widths, inhibiting in particular the cambial activity of xylem growth in low-fertilized, O₃-exposed plants (O₃/LF). Callose deposition was enhanced in the phloem sieve plates and tannins tended to condense in vacuoles of parenchyma cells under O₃ stress. Decline occurred close to lenticels, related to O₃ impact during shoot differentiation and was probably exacerbated by the limited assimilate translocation. Radial stem growth ceased 4 weeks earlier than in control plants; however, the area-based Rs was enhanced during intense growth in high-fertilized, O₃-exposed plants. Photosynthetic CO₂ refixation of stems reached about 50% of their dark respiration rate and the relative growth rate (RGR) did not differ between treatments. At high nutrition, RGR enhanced Rs to levels twice as high as the maintenance level. Unit construction costs appeared to be similar in each treatment, although CO₂ release on a volume-increment basis was lowered by 45% in O₃/LF plants. This latter effect is ascribed to lowered maintenance demands of a xylem remaining reduced in width by 50%. The high respiratory costs in the carbon balance of O₃/LF plants result from an enhanced leaf rather than stem respiration, given the high demand for stress compensation in the foliage.     

Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress

Lee, H. S. J. and Griffiths, H. 1987. Induction and repressionof CAM in Sedurn relephluni L. in response to photopcnod andwater stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunmL, a temperate succulent, was investigated in watered, progressivelydrouglited and rewatered plants in growth chambers. Measurementswere made of water vapour and CO₂ exchange, titratable acidity(TA) and xylem sap tension. Effects of photoperiod were alsostudied. CAM was induced by drought under long or short days,although when watered no CAM activity was expressed. C₃-CAM intermediate plants were used for the investigation ofwater supply. Those which had received water and those drought-stressedboth displayed a similar nocturnal increase in TA, with a day-nightmaximum (H+) of 69 µmol g^–1 fr. wt. The wateredplants took up CO₂ at a maximum rate of 2?2 µmol m^–2s^–1 only in the light period, while the droughted plantsshowed a maximum nocturnal CO₂ uptake rate of 0?69 µmolm^–2 s^–1. Subsequently, as CAM was repressed, thewatered S. telephiwn displayed little variation in TA, withconstant levels at 42 µmol g^–1 fr. wt. (day 10).After 10 d of drought stress, the CAM characteristics of S.telephiurn were aLso affected, with reduced net CO₂ uptake andH+. The transition between C₃ and CAM in S. telephium can be describedas a progression in terms of the proportion of respiratory CO₂which is recycled and refixed at night as malic acid, in comparisonwith net CO₂ uptake. Recycling increased from 20% (day 1) to44% (day 10) as a result of the drought stress and was highin both the CAM-C₃ stage (no net CO2 uptake at night) and alsoin the drought-stressed CAM stage (reduced net CO₂ uptake atnight). The complete C₃-CAM transition occurred in less than8 d, and the stages could be characterized by xylem sap tensionmeasurements: CAM = 0?50 MPa C₃-CAM = 0?36 MPa C₃ = 0?29 MPa. Key words: CAM, Sedum telephium L., recycling     

Glycolate synthesis in a C3 C4 and intermediate photosynthetic plant type

Excised leaves of a C₃-photosynthetic type, Hordeum vulgare,a C₄-type, Panicum miliaceum, and an intermediate-type, Panicummilioides, were allowed to take up through their cut ends a1 mM solution of butyl hydroxybutynoate (BHB), an irreversibleinactivator of glycolate oxidase. After 30 to 60 min in BHB,extractable glycolate oxidase activity could not be detectedin the distal quarter of the leaf blades. Following this pretreatment,recovery of ¹⁴C-glycolate from ¹⁴CO₂ incorporated in a 10 minperiod was nearly maximal for each of the three plant types.Labeled glycolate was 51% of the total ¹⁴CO₂ incorporated forthe C₃-species, 36% for the intermediate-species, and 27% forthe C₄-species Increased labeling of glycolate was compensatedfor primarily by decreased labeling of the neutral and basicfractions for the C₃ and intermediate-type species. In the C₄-type,label decreased primarily in the neutral and insoluble fractions,but increased in the basic fraction. A lower rate of glycolatesynthesis is indicative of a lower rate of photorespirationand consistent with a lower O₂/CO₂ ratio present in the bundle-sheathcells of C₄-plants. We conclude that both decreased glycolatesynthesis and the refixation of photorespiratory-released CO₂are important in maintaining a lower rate of photorespirationin C₄-plants compared to C₃ plants. Intermediate glycolate synthesisin Panicum milioldes is consistent with its intermediate levelof O₂ inhibition of photosynthesis and intermediate rate ofphotorespiration. (Received May 6, 1978; )     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–     

Rhythms in respiratory metabolism of Lemna gibba G3 under continuous illumination

Changes in chemical constituents and respiratory metabolismof a long-day duck-weed, Lemna gibba G₃ exposed to continuousillumination after short-day cultivation were investigated.The dry weight to fresh weight ratio was constant during thefirst 72 hr of continuous illumination. pH of the crude extractwas constant at 6.6, but pH of the culture medium was raisedwith the Lemna growth. Titratable acidity decreased after about44 hr, whereas malic acid content increased in 18 hr. Therewere no significant changes in total reducing sugar and pentose.Total protein content and lipid showed rhythmical changes withcycles of 48 hr. O₂-Uptake gave a damped oscillation with cycles of 24 hr. Itwas low in the first half day and high in the second half. ¹⁴CO₂-Outputfrom glucose-l-¹⁴C showed a similar damped oscillation. ¹⁴CO₂-Outputfrom glucose-2-14^C or glucose-6-₁₄C was almost constant. TheC₆/C₂ ratio, then, showed damped oscillation in the reverseway to O₂- uptake between 0.3–0.5, and the C₈/C₂ ratiowas constant at 0.9. Accordingly, the diurnal rhythm of O₂-uptakewas thought to be brought about by variation in activity ofthe pentose-phosphate pathway. Reproduction of glucose-6-phosphateby the pentose-phosphate pathway was presumably limited in amount. Glyceraldehyde-3-phosphate dehydrogenase activity varied diurnally.The activities of NADP-linked and NAD-linked enzymes increasedand decreased, respectively, in the first half day. Variationsin these enzymatic activities are discussed in correlation withrhythmical changes in O₂-uptake and in the C₆/C₁ ratio. Acidphosphatase activity also followed a diurnal variation. No activitiesof alcohol and formic dehydrogenases were found. The activitiesof NADP glucose-6-phosphate dehydrogenase, 6-phosphogluconatedehydrogenase, pyruvic kinase and NADP isocitric dehydrogenasewere high, but showed no rhythmical variation. ¹Presented at the Annual Meeting of the Botanical Society ofJapan, 1966 (Proceedings, p. 46). Adapted from a thesis submittedby the first author (H. M.) in 1967 to the Biological Institute,Nagoya University in partial fulfillment of the requirementsfor the degree of M. S. (Received May 8, 1969; )     

Metabolic Adaption in Mature Roots of Salt-stressed Zea mays

Respiratory gas exchange and incorporation of ¹⁴C-leucine intoprotein were studied in proximal root segments from 25-day-oldmaize plants grown for the last ten days in 50 mM Na₂SO₄. ¹⁴C-leucineincorporation, and oxygen uptake in the presence of glucose,were as large in Na₂SO₄-grown tissues tested under saline conditionsas in tissues exposed to non-saline solutions throughout Thisadaptation was attributed to an increased metabolic capacityof Na₂SO₄-treated tissues, because these tissues, when returnedto non-saline solutions, evolved oxygen and incorporated ¹⁴C-leucinefaster than tissues exposed continuously to non-saline solutions. These changes are interpreted as a ‘compensation’for the inhibitory effects found when non-adapted tissues wereexposed to 50 mM Na₂SO₄. Moreover, we have related them to ultrastructuralchanges observed previously in xylem parenchyma cells of thesetissues, and to the possible involvement of these xylem parenchymacells in the re-absorption of sodium from the ascending xylemfluid Zea mays L., maize, salt-stress, respiration, protein synthesis     

Photosynthetic carbon metabolism in photoautotrophically and photomixotrophically cultured tobacco cells

Labeling patterns of light and dark ¹⁴CO₂-fixation in photoautotrophicallyand photomixotrophically cultured tobacco cells were determined.During short term ¹⁴CO₂ fixation under light, malate(C₃–C₃carboxylation) was heavily labeled as were phosphoglyceric acidand sugar phosphates(C₁–C₅ carboxylation). Dark fixationcould not account for this high ¹⁴CO₂ incorporation into theC₄ compounds linked to PEPCase. Two carboxylation pathways linkedto the RuBPCase and PEPCase were indicated in ¹⁴CO₂-fixationin light in photoautotrophically and photomixotrophically culturedcells. (Received October 25, 1979; )     

Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chora corallina

Carbonic anhydrase (CA, EC. 4.2.1.1 [EC] ) activity in air-grown Characorallina was detected mainly in the intracellular fraction,most of which composed of chloroplasts and cytoplasmic gel,and not on the cell surface. Only minor levels of CA activity,on the basis of equivalent volumes, were detected in the cellsap and the cytoplasmic sol. The maximum rate of photosynthetic O₂ evolution by air-grownChara corallina at pH 6.0 was twice that at pH 7.6, while theapparent K_m for external inorganic carbon (C_i) at pH 7.6 wasabout three times that at pH 6.0. However, the apparent K_m(CO₂)was about three times larger at pH 6.0 than at pH 7.6. The K_m(C_i)-valueat pH 7.6 increased severalfold in the presence of acetazolamide(AZA), an inhibitor of CA, but no inhibition was observed atpH 6.0. The pH-dependence may be due to differences in the permeabilityof AZA at the given pH values. Fixation of ¹⁴CO₂ at 20 µMand of H¹⁴CO₃^– at 200 µM over the course of 5 swas very similar at pH 7.4. Addition of CA significantly suppressedthe photosynthetic ¹⁴CO₂-fixation but it stimulated the H¹⁴CO₃^–-fixation.This result indicates that free CO₂ is an active species ofC_i that is incorporated into the cell during photosynthesis. These results together suggest the following: (1) Free CO₂ isutilized for photosynthesis, (2) CA is mainly located insidethe cell and functions to increase the affinity for CO₂ in photosynthesisby facilitating the supply of CO₂ from the plasmalemma to thesite of CO₂-fixation. ³Present address: Biological Laboratory, The University of theAir, Wakaba 2-11, Chiba, 260 Japan. (Received December 9, 1988; Accepted March 22, 1989)     

Carbon Dioxide Fixation by Barley Roots

The non-volatile, 80 per cent.ethanol-soluble products of fixationhave been investigated in excised roots, using C^14O₂ and radiochromatography. The main radioactive compounds separated were malic, citric(or iso-citric), aspartic, and glutamic acids, asparagine andglutamine. Less activity was present in serine, tyrosine, -ketoglutaricacid, and alanine, and in a number of unidentified compounds. The uptake of C^14O₂ was inhibited by virtually anaerobic conditions. From the above observations it is considered likely that C¹⁴is transformed through the reactions of the tricarboxylic acidcycle. C¹⁴ in the soluble fraction was markedly increased by maintainingthe root material in water rather than in a nutrient solutionprior to exposure to C^14O₂ This increase was chiefly in malicacid.     

The Translocation Profile: Sucrose and Carbon Dioxide

The ¹⁴CO₂ produced from translocated sucrose-¹⁴C has been measuredduring the first minutes of the arrival of the translocationprofile in Salix stem. The shape of the curve of ¹⁴CO₂- timeis shown to resemble that for the radioactivity time of sucrose-¹⁴Cexuded from the phloem through cut aphid stylets, and to reflectthe shape of the sucrose-¹⁴C-distance profile in the stem. The¹⁴CO₂ time profile may be used with the sucrose-¹⁴C distanceprofile to estimate the velocity of movement of the latter.A constant proportion of the sucrose-¹⁴C in the stem breaksdown to ¹⁴CO₂ in the steady state attained after the passageof the profile.     

Effects of Anoxia on Phloem Loading in C3 and C4 Species

Changes in phloem loading rate were inferred from observationsof ¹¹C export from attached leaves of C₃ and C₄ monocots anddicots. Loading decreased under anoxia in C₃ leaves, but notin general in the leaves of either C₄ monocots or dicots inthe light. However, loading rate in the C₄ leaves did reduceif the leaf was also darkened or received no CO₂. We suggestthat insensitivity to anoxia in C₄ leaves is due to oxygen fromphotosynthesis which is retained in the bundle sheath at a concentrationsufficient to energize a phloem loading system. There may alsobe another system which is insensitive to anoxia since the effectsof shade and CO₂ deprivation were not always seen, and loadingwas not completely stopped by these treatments. Key words: Phloem loading, C₃, C₄     

The Effect of Cycloheximide on Uptake and Transport of lons by Sunflower Roots

The effect of cycloheximide on uptake and transport of saltsby sunflower roots was investigated. Treatment with cycloheximideresulted in a reduction of uptake and transport of K⁺ and NO₃^–to the xylem. Cycloheximide stimulated O₂ uptake and appearedto act as an uncoupler of oxidative phosphorylation. The implicationsof these results regarding the use of cycloheximide as a meansof distinguishing between uptake and transport components ofion movement to the xylem are discussed.     

Diurnal regulation of NO-3 uptake in soybean plants IV. Dependence on current photosynthesis and sugar availability to the roots

The short-term dependence of NO^–₃ uptake upon photosynthesisand sugar supply to the roots of soybean plants was investigatedin a series of experiments where CO₂ availability, light intensityor conduction of phloem sap to the roots were severely limited.Removal of CO₂ from the atmosphere or girdling of the stem equallyprevented the stimulation of NO^–₃ uptake when plants weretransferred from darkness to the light. The effect of thesetwo treatments can be reversed by CO₂ re-supply or by additionof 10 mM glucose in the nutrient solution, respectively. Glucosewas also more effective in stimulating NO^–₃ uptake byintact plants in darkness than in light. Collectively, theseobservations are interpreted as evidence that the diurnal changesin NO^–₃ uptake are due to decreased phloem transport ofphotosynthates in darkness. Accordingly, the magnitude of thesechanges was much dependent on starch accumulation in the leavesat the end of the photo-period. Shading the plants lowered thisaccumulation, and resulted in an amplification of the diurnalchanges in NO^–₃ uptake. These results are discussed inconnection with the hypothesis that the carbon-dependent plasticityof the night/day ratio of NO^–₃ uptake is an importantfeature of the co-ordination of the acquisition of N and C bythe plant. Key words: Glycine max, light/dark cycle, NO^–₃ uptake, C and N acquisition     

Comparison of carbon dioxide fixation and the fine structure in various assimilatory tissues of Amaranthus retroflexus L

The pattern for primary products of CO₂-fixation and the chloroplaststructure of Amaranthus retrqflexus L., a species which incorporatescarbon dioxide into C₄ dicarboxylic acids as the primary productof photosynthesis, were compared in various chlorophyll containingtissues,i.e., foliage leaves, stems, cotyledons and pale-greencallus induced from stem pith. Despite some morphological differencesin these assimilatory tissues, malate and aspartate were identifiedas the major compounds labelled during a 10 sec fixation of¹⁴CO₂ in all tissues. Whereas, aspartate was the major componentin C₄-dicarboxylic acids formed in foliage leaves, malate predominatedas the primary product in stems, cotyledons and the pale-greencallus. The percentage of ¹⁴C-radioactivity incorporated intoPGA and sugar-P esters increased and ¹⁴C-sucrose was detectedin the prolonged fixation of ¹⁴CO₂ in the light, not only infoliage leaves, but also in stems and cotyledons. ¹ This work was supported by a Grant for Scientific ResearchNo. 58813, from the Ministry of Education, Japan. ² Present address: Institute of Applied Microbiology, Universityof Tokyo, Tokyo, Japan. ³ Present address: Department of Biochemistry, University ofGeorgia, Athens 30601. Georgia, U. S. A. (Received July 10, 1971; )     

Response of Wheat Seedlings to Low O2 Concentrations in Nutrient Solution: I. GROWTH, O2 UPTAKE AND SYNTHESIS OF FERMENTATIVE END-PRODUCTS BY ROOT SEGMENTS

Root growth of 7-d-old wheat (Triticum aestivum cv. Gamenya)seedlings was impaired at dissolved O₂ concentrations of 0.01and 0.055 mol m^–3 O₂, while growth at 0.115 mol m^–3O₂ was the same as that in continuously aerated controls (0.26mol m^–3 O2). Oxygen uptake by apical (0–2 mm), expanding (2–4mm) and expanded (10–12 mm) tissues of the roots decreasedbelow 0.16, 0.09 and 0.05 mol m^–3 O₂, respectively. Thishierarchy is consistent with the metabolic rates of these tissues.There was a small (c. 9%) inhibition of O₂ uptake and some netsynthesis of ethanol and alanine in root apices at 0.115 molm^–3 O₂. Significant amounts of anaerobic end-productsaccumulated at 0.055 mol m^–3 O₂ and even more so at 0.01mol m^–3 O₂, indicating that oxidative phosphorylationwas strongly inhibited. Net alanine synthesis increased in fully expanded (10–16mm) tissues exposed to <0.003–0.01 mol m^–3 O₂,and this increase was accompanied either by a proportionallysmaller increase in the concentration of other free amino acidsor by a net decrease in free amino acid levels excluding alanine.This suggests that alanine was synthesized as an end-productof anaerobic catabolism and did not accumulate simply becauseof decreased net protein synthesis. Comparing the carbon flow to CO₂, ethanol, lactate and alaninein roots at 0.01 mol m^–3 O₂ with carbon loss as CO₂ inaerated roots suggests that carbon flow to products of metabolismwas not greatly enhanced due to O₂ deficiency. This infers,but does not prove that, in wheat, generation of energy duringperiods of O₂ deficiency is not enhanced due to a Pasteur effect. Key words: Anaerobic, fermentation, oxygen, wheat