The Composition of Phloem Exudate and Xylem Sap from Tree Tobacco (Nicotiana glauca Grah.)

The composition of xylem sap and exudate from stem incisionsof Nicotiana glauca Grah. was compared in detail. Exudationfrom stem incisions occurred over a 5 min period in certainplants, enabling collection of 5–30 µl of sap. Therate of exudation showed an exponential decline. Exudate hada high dry matter content (170–196 mg ml^–1) andhigh sugar (sucrose) levels. Xylem sap had a low pH (5.8) andexudate a pH of 7.9. Glutamine dominated the amino compoundsin xylem sap and exudate, and K⁺ was the major cation. Totalamino compounds in stem exudate reached 10.8 mg ml^–1 whereasxylem sap contained much lower levels (0.28 mg ml^–1).All mineral elements and amino compounds with the exceptionof calcium were more concentrated in stem exudate than in xylemsap. Sucrose was labelled heavily in stem exudate following pulsingof an adjacent leaf with ¹⁴CO₂. A concentration gradient ofsugar (2.1 bar m^–1) was recorded for stems. Levels ofsucrose, amino compounds and K⁺ ions in stem exudate showeda diurnal periodicity. Each commodity reached maximum concentrationat or near noon and minimum concentration about dawn. The evidencesuggests that exudate from stem incisions of N. glauca is arepresentative sample of solutes translocated in the phloem. Nicotiana glauca Grah., phloem sap, xylem sap, sucrose, amino compounds, mineral ions     

Development of the Xylem Ureide Assay for the Measurement of Nitrogen Fixation by Pigeonpea (Cajanus cajan (L.) Millsp.)

Quantification of N₂ fixation by pigeonpea (Cajanus cajan (L.)Millsp.) in the field has proved difficult using techniquessuch as ¹⁵N isotope dilution, acetylene reduction and N difference.We report experiments to develop the ureide assay of N₂ fixationbased on extraction and analysis of xylem exudate. Plants ofpigeonpea cv. Quantum, inoculated with effective Rhizobium spp.CB756, were grown in a temperature-controlled glasshouse inlarge pots filled with a sand: vermiculite mixture, in waterculture and in a slightly acidic, red-brown earth in replicatedfield plots. Xylem exudate was collected as bleeding sap fromboth nodulated and unnodulated roots, and from detached nodules.Exudate was extracted also from detached shoots and stems ofpigeonpea using a mild vacuum (60–70 kN m^–2). Largedifferences in the composition of N solutes exported from rootsof N₂-dependent and nitrate-dependent plants suggested thatshifts in plant dependence on N₂ fixation may be reflected byconcomitant changes in N solutes. Thus, nodulated plants weresupplied throughout growth with either N-free nutrients or nutrientssupplemented with 1, 2, 5, 5, 10, or 20 mol m^{–3 15}. Plants were harvested at regular intervals fordry matter and vacuum-extracted exudate. The relative abundanceof ureides ([ureide-N/ureide-N + nitrate-N + -amino-N] ? 100)in the exudate was highly correlated with the proportion ofplant N (calculated using a ¹⁵N isotope dilution technique)derived from N₂ fixation. Two distinct phases of plant growthwere recognized and standard curves were prepared for each.The relationship between proportional dependence of plants onN₂ and xylem relative ureides was unaffected by mineral-N source,i.e. nitrate or ammonium. This result is discussed in relationto interpretation of material from field-grown plants. The effectsof plant genotype, strain of rhizobia, section of stem extracted,removal of leaves, time delay between shoot detachment and extraction,and diurnal characteristics were examined in order to identifypotential sources of error and to optimize sampling procedures. Key words: Ureides, allantoin, allantoic acid, N₂ fixation, pigeonpea, Cajanus cajan     

Evidence for N feedback regulation of N2 fixation in Alnus glutinosa L.

Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O₂destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N₂-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled ¹⁵ was added to the base of plant pots. After 12 h7% of root N was from applied ¹⁵ and this increased to 75% at 7 d; substantial enrichment ofN from ¹⁵ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from¹⁵, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm^–3 following addition of ¹⁵ and at 7 d 80% of the N in the citrullinehad been derived from ¹⁵N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N₂-fixation.     

Characterization of Cd translocation and identification of the Cd form in xylem sap of the Cd-hyperaccumulator Arabidopsis halleri

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanismsinvolved in the root to shoot translocation of Cd are not wellunderstood. In this study, we characterized Cd transfer fromthe root medium to xylem in this species. Arabidopsis halleriaccumulated 1,500 mg kg^–1 Cd in the shoot without growthinhibition. A time-course experiment showed that the releaseof Cd into the xylem was very rapid; by 2 h exposure to Cd,Cd concentration in the xylem sap was 5-fold higher than thatin the external solution. The concentration of Cd in the xylemsap increased linearly with increasing Cd concentration in theexternal solution. Cd transfer to the xylem was completely inhibitedby the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone(CCCP). Cd concentration in the xylem sap was decreased by increasingthe concentration of external Zn, but enhanced by Fe deficiencytreatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR)showed that the chemical shift of ¹¹³Cd in the xylem sap wasthe same as that of Cd(NO₃)₂. Metal speciation with Geochem-PCalso showed that Cd occurred mainly in the free ionic form inthe xylem sap. These results suggest that Cd transfer from theroot medium to the xylem in A. halleri is an energy-dependentprocess that is partly shared with Zn and/or Fe transport. Furthermore,Cd is translocated from roots to shoots in inorganic forms.     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

The Effect of Nitrogen Source on Transport and Metabolism of Nitrogen in Fruiting Plants of Cowpea (Vigna unguiculata (L.) Walp.)

Nitrogen metabolism and transport were studied during reproductivedevelopment of cowpea (Vigna unguiculata (L.) Walp. cv. Vita3) under three contrasting nitrogen regimes: (1) nitrate suppliedcontinuously (plants non-nodulated), (2) symbiotic N₂ fixation(no combined nitrogen), (3) nitrogenstarvation post-anthesisof previously N₂-fixing plants. The last treatment involveddaily flushing of the root systems with 100% oxygen which suppressedpost-anthesis N₂-fixation by 76–79%, thereby making fruitgrowth almost entirely reliant upon mobilization of previouslyaccumulated nitrogen. The bulk of the xylem nitrogen (root bleedingsap or peduncle tracheal sap) of nitrate-fed plants was nitrateand amide, that of symbiotic and O₂-treated plants largely ureide.The composition of fruit cryopuncture phloem sap, however, wasclosely similar in all treatments, with most nitrogen as amidesand amino acids. The evidence suggested intense metabolic transferof root derived nitrate-N or ureide-N to amino acids by vegetativeplant parts prior to translocation to fruits. All tissues offruits showed patterns of development of enzymic activitiesconsistent with release of nitrogen from both ureides and amidesand re-assimilation of ammonia to form amino acids. Althoughthe levels of enzyme activities varied between treatments thedifferences could not be readily associated with individualpatterns of nitrogen transport in the treatments. Nitrogen sufficiencyin the NO₃-fed plants was marked by elevated vegetative biomassand low harvest indices for dry matter and nitrogen, while nitrogendeficiency of the O₂-treated plants was associated with seedabortion, small seed size and low seed nitrogen concentration,and efficient mobilization of nitrogen from vegetative partsto fruits. Key words: Nitrogen, Translocation, Cowpea     

Diurnal Variation in Acetylene Reduction and Net Hydrogen Evolution in Five Tropical and Subtropical Nitrogen-Fixing Tree Symbioses

Diurnal variations in acetylene reduction and net hydrogen evolutionwere shown in five tropical and subtropical nitrogen-fixingtree symbioses. The symbioses studied in a growth chamber were:Acacia albida x TAL 1457, Leucaena leucocephala x TAL 1145,Prosopis chilensis x TAL 600, Casuarina glauca x HFP Cc13 andC. obesa x HFP Cc13. Acetylene reduction was highest at the end of the light periodin all symbioses studied. In the A. albida x TAL 1457 symbiosis,the diurnal variations in acetylene reduction and net hydrogenevolution showed a minor synchrony, while in the other symbiosesthe diurnal pattern of acetylene reduction and net hydrogenevolution clearly differed. Also, a diurnal variation in relativeefficiency of nitrogenase was shown in the A. albida x TAL 1457symbiosis. A hydrogen uptake enzyme was detected at a low substrate concentration(24.5 mmol m^–3 H₂) for L. leucocephala x TAL 1145, C.obesa x HFP Cc13 and has earlier been found for C. glauca xHFP CcI3. A hydrogen uptake system was also found for P. chilensisx TAL 600 and A. albida x TAL 1457 at a 17-fold higher substrateconcentration. The results show that a diurnal variation in C₂H₂ reductionand H₂ evolution occurs, and that diurnal variation in the conversionfactor between C₂H₂ reduction and N₂ fixation could occur. Thisfact raises criticisms regarding the use of a point estimateof this factor. Key words: Acetylene reduction, hydrogen evolution, uptake hydrogenase, nitrogen-fixing tree symbioses     

The Xylem Ureide Assay of Nitrogen Fixation: Sampling Procedures and Sources of Error

Herridge, D. F., O'Connell, P. and Donnelly, K. 1988. The xylemureide assay of nitrogen fixation: sampling procedures and sourcesof error.—J. exp. BoL 39: 12–22. The relative abundance of ureides [(ureide-N/ureide-N + NO₃^––N+ amino-N)x 100] in vacuum-extracted xylem exudate of soyabean(Glycine max [L.] Merr.) can be used to quantify N₂ fixationactivity. We report experiments designed to (1) develop appropriateprocedures for sampling and storage of xylem exudate extractedfrom plants in the field and (2) develop a single plant, non-destructivesampling procedure, useful for evaluation of segregating breedingmaterial. Soyabean cultivars Bragg, Essex, Davis, Hardee andDodds were grown at three sites at Brecza, New South Wales,on a deep, alkaline vertisol. All plots were inoculated witheffective Bradyrhizobium japonicum CB 1809. Vacuum strengthsof between 50•5 and 80•8 kN m^–2 did not affectthe relative abundance of ureides of xylem exudate. Delays betweenplant sampling and extraction of exudate, however, resultedin time-dependent increases in the relative ureide contents.Extraction should immediately follow shoot detachment to allowvalid comparison of treatments and accurate interpretation ofdata. The relative ureide contents of exudate showed only minordiurnal fluctuations. Exudate, once collected, could be stabilizedby dilution (1:1) in absolute ethanoL Both diluted and undilutedsamples were stable for at least 7 d at 4°C. Differencesduring early flowering, but not subsequently, in the compositionsof N solutes of exudate extracted from either the whole shoot,the top half or the lower half indicated the possibility ofnon-destructive sampling where seed, as well as xylem exudate,could be harvested. Best results in terms of volume of exudateextracted, treatment separation and seed production were obtainedwhen single plants were sampled during late flowering by detachmentof shoots at internode 5. Key words: Ureides, nitrogen fixation, soyabean     

Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine max L. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed toassimilate ¹³CO₂ at a constant specific activity for 10 h andthe ¹³C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of ¹³CO₂ assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m^–3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, ¹³CO₂ assimilation, xylem     

Transport of organic solutes in Phloem and xylem of a nodulated legume

Collections of xylem exudate of root stumps or detached nodules, and of phloem bleeding sap from stems, petioles, and fruits were made from variously aged plants of Lupinus albus L. relying on nodules for their N supply. Sucrose was the major organic solute of phloem, asparagine, glutamine, serine, aspartic acid, valine, lysine, isoleucine, and leucine, the principal N solutes of both xylem and phloem. Xylem sap exhibited higher relative proportions of asparagine, glutamine and aspartic acid than phloem sap, but lower proportions of other amino acids. Phloem sap of petioles was less concentrated in asparagine and glutamine but richer in sucrose than was phloem sap of stem and fruit, suggesting that sucrose was unloaded from phloem and amides added to phloem as translocate passed through stems to sinks of the plant. Evidence was obtained of loading of histidine, lysine, threonine, serine, leucine and valine onto phloem of stems but the amounts involved were small compared with amides. Analyses of petiole phloem sap from different age groups of leaves indicated ontogenetic changes and effects of position on a shoot on relative rates of export of sucrose and N solutes. Diurnal fluctuations were demonstrated in relative rates of loading of sucrose and N solutes onto phloem of leaves. Daily variations in the ability of stem tissue to load N onto phloem streams were of lesser amplitude than, or out of phase with fluctuations in translocation of N from leaves. Data were related to recent information on C and N transport in the species.     

Presence and identification of polyamines in xylem and Phloem exudates of plants

Polyamines were identified by high performance liquid chromatography (benzoylation) and by thin layer chromatography (dansylation) in xylem exudates from stems of sunflowers (Helianthus annuus [L.]), mung bean (Vigna radiata [L.] Wilczek), grapevine (Vitis vinifera [L.] cv Grenache), and orange (Citrus sinensis [L.] Osbeck, cv Valencia), as well as in phloem sap (using elution into EDTA) of sunflower and mung bean plants. Putrescine was the major polyamine detected, ranging in concentrations of 150 to 9200 picomoles per milliliter exudate, whereas only trace amounts of spermine were detected. High amounts of putrescine and spermidine were found in EDTA eluates (possibly phloem sap) as compared with elution into water. Concentrations of putrescine and spermidine in xylem exudates were related to the physiological conditions of the plants prior to exudate collection. More putrescine was found in exudates of older than in younger sunflower plants, and salt stress applied to sunflower plants resulted in a higher concentration of putrescine and spermidine in the exudate. The presence and abundance of putrescine and spermidine in xylem and phloem exudates indicate that polyamines may be translocated in plants. This long-distance translocation further supports the hypothesis that polyamines have a regulatory role in plant growth and response to stress.     

Haustorium-related Uptake and Metabolism of Host Xylem Solutes by the Root Hemiparasitic ShrubSantalum acuminatum(R. Br.) A. DC. (Santalaceae)

Solute composition of root xylem sap of common native hostsof quandong (Santalum acuminatum) was compared with that ofcorresponding xylem sap and ethanolic extracts of endophytictissues of haustoria of the hemiparasite. Each host transporteda characteristic set of organic nitrogenous solutes, but littleor no nitrate, and the data indicated only limited direct flowof amino compounds between xylem streams of hosts and parasite.Proline predominated in the haustorium and xylem ofSantalum,but was at negligible levels in the xylem of most hosts. Sucrose,fructose, glucose, malate and citrate were at high levels inall saps, and fructose especially prominent inSantalum. Chloride,sulphate and phosphate were the principal inorganic anions ofthe xylem. Based on C:N ratios of xylem and dry matter ofSantalumandassuming a 70% or more dependence on the host for N, it wasestimated thatSantalumwould gain approximately one third ofits C requirement for dry matter production heterotrophicallyfrom the xylem of its hosts. Infiltration of xylem of haustoria-bearingroot segments of a major host (Acacia rostellifera) with a rangeof¹⁵N labelled substrates resulted in 40–80% of the¹⁵Nof endophytes of the attached haustoria being received as proline.Nitrate reductase activity was induced in haustoria followinghost xylem feeding of nitrate. The study concludes that haustoriaofSantalumact as a major site of synthesis and export of prolineand might therefore play an important role in osmotic adjustmentof the parasite and its related acquisition of water from hosts. Root hemiparasite; Santalum acuminatum; ¹⁵N labelled substrates; xylem transport; proline; osmoregulation     

Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

Amino acid composition of xylem (tracheal) sap and ethanolicextracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae)and their hosts were compared, using material collected in theirnative habitats. Data indicated that certain host xylem soluteswere transferred directly to the parasite xylem, while otherswere either not absorbed or were metabolized prior to transfer.Certain solutes were major constituents of parasite xylem, butundetected or only in trace amount in the host. Shoot aminoacid pools of parasites differed markedly from those of hosts.The mistletoe, Amyema preissii, exhibited differential storageand transport of arginine when parasitizing three differentspecies, but accumulated proline on only two of these hosts.Host- specific amino acids (djenkolic acid in Acacia saligna,and tyramine in Acacia acuminata) were transported and accumulatedin relatively large amounts by the parasite, but were not detectedin other associations. Proline was the major solute of Amyemalinophyllum parasitizing Casuarina obesa, but arginine predominatedin Lysiana casuarinae on the same host. However, when L. csuarinaeparasitized A. linophyllum, in turn parasitic on C. obesa, theLysiana accumulated equal amounts of proline and arginine andmore asparagine than when directly on the Casuarina. Xylem feedingof ¹⁵N-labelled aspartic acid or ¹³N-(amide labelled) asparagineto cut shoots or whole haustoria-bearing plants of the mistletoeA. preissii resulted in 68–73% of the ¹⁵N of aspartateand 24–30% of that of asparagine appearing in ethanol-solubleshoot amino compounds other than the fed solute. ¹⁵N labellingpatterns of detached shoots were not noticeably different fromthat of whole plants suggesting that the haustorium had relativelylittle effect on processing incoming solutes. Alanine, glutamine,and arginine were principal recipients of ¹⁵N from aspartate,alanine and glutamine in the case of fed asparagine. It is estimatedthat 24% of the carbon requirements for dry matter accumulationin Amyema linophyllm were met by intake of xylem sap solutesfrom its host Casuarina obesa. Key words: Amino acids, xylem transport, mistletoes, host: parasite relations, N metabolism     

Metabolism of Phloem-borne Amino Acids in Maternal Tissues2Pisum sativum L.)

The amino acid composition of the EDTA-induced phloem exudatereaching the fruit and the seed, and of the solutes releasedby the seed coat during fruit development were determined inglasshouse-grown pea (Pisum sativum L. cv. Finale) suppliedeither with nitrate-free nutrients (nodulated plants) or withcomplete medium (non-nodulated plants). The EDTA-promoted exudationtechnique was used supposedly to collect phloem sap and theempty seed technique supposedly to collect the solutes secretedby the seed coat to the embryo sac cavity. In young seeds embryosac liquid was sampled directly from the embryo sac. The maincarbohydrate transported and secreted was sucrose. The mainamino acids reaching the fruit were asparagine, glutamine, andhomoserine. Their proportions were steady during a day-nightcycle and throughout fruit development. Amino acid compositionchanges occurred first in the pathway from fruit stalk to seedfunicle, due to the formation of threonine (probably from homoserine)and in the seed coat due to production of glutamine, alanineand valine which, together with threonine were the main secretedamino acids. The temporary nitrogen reserves of the pod walland seed coat were remobilized as asparagine during senescence.Phloem exudate of nodulated plants showed a higher (about twice)proportion of asparagine but lower proportions of homoserineand glutamine than in EDTA-induced phloem exudate of nitrate-fedplants. The two types of nitrogen nutrition also produced somechanges in relative proportions of threonine and homoserinesecreted by the seed coat. Key words: Pisum sativum, phloem, amino acids, pod wall, seed coat     

Effects of P deficiency on the uptake, flows and utilization of C, N and H2O within intact plants of Ricinus communis L.

The influence of P deficiency on the uptake, flow and utilizationof C, N and H₂0 by intact NO₃-fed castor bean plants {Ricinuscommunis L.) was studied over a 9 d period in the middle oftheir vegetative growth. The modelling techniques incorporateddata on net increments or losses of C, N and H₂O in plant parts,photosynthetic gains in and respiratory losses of C, molar C:Nratios of solutes in phloem and xylem sap and transpirationallosses of H₂0. Plant growth was inhibited within 3 d of withholdingP supply and dry matter production was less than one-third ofthe controls. Leaf growth was particularly depressed, whileroot growth was much less affected than that of the shoot. Shoot:rootratio of low-P plants was 1.5 compared with 2.6 under P supply.Over the 9 d study period total plant C and N increased by 560and 47 mmol, respectively, in the controls, but by only 113and 6.9 mmol in the low-P treatment. The particularly low incrementof N in P-deficient plants was due principally to decreasedN0₃^- uptake. Flows of C and N during the study period were markedlydifferent between control and P-deficient plants. The partitioningprofile for C in P-deficient plants showed a dramatic inhibitionof net photosynthesis and attendant photoassimilate flow. Proportionaldownward to upward allocation of carbon increased with increasein sink size of the root relative to shoot. This was reflectedin greater relative allocation of C to root dry matter and rootrespiration than in P-sufficient plants, and suppressed cyclingof C from root to shoot via xylem. Nitrogen intake and xylemtransport to the shoot of P-deficient plants were only 15% ofthe control and, as in the case of C, downward allocation ofN predominated over upward phloem translocation. Apart fromthese severe changes, however, the basic patterns of N flowsincluding xylem-to-phloem and xylem-to-xylem transfer of N werenot changed, a feature highlighting the vital nature of thesetransfer processes even under deficiency conditions. The alterationsin flows and partitioning of C, N and H₂O in response to low-Pconditions are discussed in relation to the corresponding effectsof moderate salt stress in Ricinus and the conclusion is reachedthat changes in nutrient flows under P deficiency were morehighly co-ordinated than when plants experience salt stress.Flow profiles under P deficiency which favour root growth andactivity are viewed as a means for increasing the potentialcapability of the plant to acquire P from the nutrient medium. Key words: Ricinus communis L., P deficiency, carbon, nitrogen, water, partitioning, xylem transport, phloem transport     

Economy of Carbon and Nitrogen in Nodulated and Nonnodulated (NO(3)-grown) Cowpea [Vigna unguiculata (L.) Walp.

The response of non-nodulated cowpea (Vigna unguiculata (L.) Walp. cv Caloona) to a wide range of NO₃ levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO₃ reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO₃ showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66, 62, and 58% of the total N of their xylem sap as NO₃. These data, and the absence in the plants of significant pools of stored NO₃, indicated that shoots were major organs of NO₃ assimilation, especially at levels of NO₃ (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO₃ plants and non-nodulated plants fed 10 or 20 millimolar NO₃, the levels of NO₃ which gave rates of growth and N assimilation closest to those of the symbiotic plants. The conversion of the C of net photosynthate to dry matter was similar in nodulated plants (67%) and NO₃-grown plants (64%), but greater proportions of photosynthate were translocated to below ground parts of nodulated plants (37%) than of NO₃-fed plants (23 to 26%). Greater photosynthate consumption by nodulated roots was associated with proportionately greater root growth and respiration and 2-fold greater export of C in xylem than in the NO₃-fed plants. Theoretical considerations suggest that the elevated CO₂ output of nodulated roots was due not only to CO₂ loss associated with nodule function, but also to a much greater nonassimilatory component of respiration in the supporting root of the nodulated plant compared to roots of the NO₃-fed plants. Data are compared with previously published information from other legumes.     

Effect of Root Zone Temperature on the Mineral Composition of Xylem Sap and Plasma Membrane K+-Mg++-ATPase Activity of Grafted-Cucumber and -Figleaf Gourd Root Systems

Cucumber (Cucumis sativus L.) seedlings were grafted onto cucumber-(CG) or figleaf gourd- (FG, Cucurbita ficifolia Bouché)seedlings in order to determine the effect of solution temperature(12, 22, and 32°C) on the mineral composition of xylem sapand the plasma membrane K⁺-Mg⁺⁺-ATPase activities of the roots.Low solution temperature (12°C) lowered the concentrationof NO^–₃ and H₂PO^–₄ in xylem sap of CG plants butnot of FG plants. Concentrations of K⁺, Ca⁺⁺ and Mg⁺⁺ in xylemsap were less affected than anions by solution temperature.The plasma membrane of FG plants grown in 12°C solutiontemperature showed the highest K⁺- Mg⁺⁺-ATPase activity at allATP concentrations up to 3 mM and at low reaction temperatureup to 12°C, indicating resistance of figleaf gourd to lowroot temperature. (Received December 27, 1994; Accepted March 10, 1995)     

The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO^-₃ and NH⁺₄ nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH⁺₄ than with NO^-₃ nutrition.The shoot:root ratios of wheat plants were increased with NH⁺₄compared to NO^-₃ nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO^-₃ and NH⁺₄-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH⁺₄-fedwheat plants were reduced to 85% of those 12 mM NO^-₃-fed wheatplants. In maize supplied with 4 and 12 mM NH⁺₄ the photosyntheticrates were 87 and 82% respectively of those of NO^-₃-fed plants.Reduced photosynthetic rates of NH⁺₄ compared to NO^-₃-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH⁺₄ compared to NO^-₃ nutrition.Differences in the partitioning of biomass between the shootsand roots of NO^-₃-and NH⁺₄-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH⁺₄- thanin NO^-₃-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH⁺₄-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem     

Chemical Composition of Bleeding Xylem Sap from Kiwifruit Vines

A study of the chemical composition and charge balance was madeof bleeding xylem sap collected from excised one-year-old extensionshoots of healthy, Mn-deficient, Mn-toxic and Zn-deficient kiwifruitvines (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson)immediately prior to leafburst. The exudates were analysed formacronutrient cations and anions, trace elements, amino acids,organic acids and sugars. Major charged species measured wereCa (13.3 mM), K (8.9 mM), Mg (5.6 mM), malate (12.5 mM) andphosphate (5.8 mM). Glutamine (12 mM) was the predominant Ncarrier identified, accounting for 58 per cent of the totalN followed by NO₂^–-N (4.5 per cent), NH₄⁺-N (3.5 per cent)and arginine-N (2.9 per cent). Approximately 22 per cent ofthe N was in a hydrolysable proteinaceous fraction comprisingmainly glutamine and glutamate. Eighteen free proteinaceousamino acids were idetified in sap, the most abundant being glutamine,glutamic acid, valine, isoleucine and phenylalanine. Computersimulation of the chemical composition predicted that in additionto hydrated cations, ion pairs formed between inorganic components(SO₄^2–, HPO₄^2–, H₂PO₄^–) and cations (Ca²⁺,Mg²⁺, Mn²⁺), plus metal-organic ligand complexes (Ca Malate,Zn Malate, FeCit, CuHis, CuGln) are important species involvedin translocation. The solubility product of hydroxyapatite wasexceeded in all exudates although in vitro precipitation wasnot observed. To achieve electroneutrality with the componentsmeasured, however, formation of precipitate precursors (hydroxyapatitenuclei) had to be assumed. Irregularities in Mn nutrition (butnot Zn) were clearly indicated by the elemental compositionof exudate suggesting the use of sap analysis as a possiblepre-season indicator of nutritional status for this species. Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson, kiwifruit, xylem sap composition, trace metals, amino acids, organic acids     

CO2 efflux, CO2 concentration and photosynthetic refixation in stems of Eucalyptus globulus (Labill.)

In spite of the importance of respiration in forest carbon budgets,the mechanisms by which physiological factors control stem respirationare unclear. An experiment was set up in a Eucalyptus globulusplantation in central Portugal with monoculture stands of 5-year-oldand 10-year-old trees. CO₂ efflux from stems under shaded andunshaded conditions, as well as the concentration of CO₂ dissolvedin sap [CO₂^*], stem temperature, and sap flow were measuredwith the objective of improving our understanding of the factorscontrolling CO₂ release from stems of E. globulus. CO₂ effluxwas consistently higher in 5-year-old, compared with 10-year-old,stems, averaging 3.4 versus 1.3 µmol m^–2 s^–1,respectively. Temperature and [CO₂^*] both had important, andsimilar, influences on the rate of CO₂ efflux from the stems,but neither explained the difference in the magnitude of CO₂efflux between trees of different age and size. No relationshipwas found between efflux and sap flow, and efflux was independentof tree volume, suggesting the presence of substantial barriersto the diffusion of CO₂ from the xylem to the atmosphere inthis species. The rate of corticular photosynthesis was thesame in trees of both ages and only reduced CO₂ efflux by 7%,probably due to the low irradiance at the stem surface belowthe canopy. The younger trees were growing at a much fasterrate than the older trees. The difference between CO₂ effluxfrom the younger and older stems appears to have resulted froma difference in growth respiration rather than a differencein the rate of diffusion of xylem-transported CO₂. Key words: Eucalyptus globulus, refixation, stem respiration Received 19 May 2008; Revised 14 September 2008 Accepted 8 October 2008