Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.     

Nitrogenase Activity in Response to Darkening and Defoliation of Alnus incana

Huss-Danell, K. and Sellstedt, A. 1985. Nitrogenase activityin response to darkening and defoliation of Alnus incana. —J.exp. Bot. 36: 1352–1358 In the Alnus-Frankia symbiosis the nitrogen-fixing root nodulesare one of the sinks for carbon compounds newly formed in photosynthesisand exported from the leaves (source). The source-sink ratioof cloned plants of Alnus incana was reduced by darkening orby total or partial defoliation and the resulting nitrogenaseactivity (C₂H₂-reduction) was measured. Nitrogenase activityhad nearly ceased 5 h after total defoliation but not untilca. 5 d after total darkening. Most of the activity was lostduring the initial hours and days, respectively. When leaf areawas reduced approximately by half nitrogenase activity decreasedslightly less than by half. Removal of upper leaves seemed lessharmful than removal of lower leaves one day after defoliation.On the following 2 d the treatments appeared to be similar.Thus, nitrogenase activity was largely dependent on newly formedassimilates but could also depend on stored reserves that weremobilized. Measurements of in vitro nitrogenase activity inroot nodule homogenates from darkened plants indicated thatnitrogenase gradually became inactivated and/or depleted after1 and 2 d in darkness Key words: Carbon supply, Frankia, nitrogen fixation     

Nitrogen and Carbon Utilization in Alnus incana Fixing N2 or Supplied with NO3- at the Same Rate

The purpose of this study was (i) to evaluate biomass productionand nitrogen utilization in grey alder and (ii) to investigatecarbon utilization in alders fixing nitrogen or supplied withnitrate. Two experiments were made, each using two groups ofalders (Alnus incana (L.) Moench) growing in a climate chamber.In each experiment one group was inoculated with a local sourceof Frankia. The other group was not inoculated but receivednitrate in the same amount as the first group fixed nitrogen.Therefore, the rate of nitrate application increased duringthe experimental period. Biomass production, growth, nitrogencontent and nitrogen utilization were determined in one experiment.Growth, nitrate reductase activity, net photosynthesis and rootrespiration were measured in the second experiment. In experimentone there was no significant difference in biomass productionbetween the two groups of plants. Of the nitrate given 99% wastaken up and 1% of this uptake was excreted in an organic form.In the second experiment the plant development was similar innodulated and non-nodulated alders during the period studied.Nitrate reductase activity was found in both root- and shoot-tipsof the non-nodulated alders. At both days 24 and 30, the amountof carbon respired compared to the amount of carbon assimilatedwas similar for the nodulated alders as for the non-nodulatedalders. As nitrogen fixation increased, photosynthesis alsoincreased. Thus, there was an inter-relationship between netphotosynthesis and nitrogen fixation. Key words: Alnus-Frankia symbioses, carbon utilization, nitrogen (N₂ ) utilization     

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     

Can the 15N Dilution Technique be used to Study N2 Fixation in Tropical Tree Symbioses as Affected by Water Deficit?

Three methods were used to study N₂ fixation and effects ofwater deficit on N₂ fixation: C₂H₂ reduction assay (ARA), ¹⁵Ndilution technique and accumulated N content. In addition, ¹⁵Ndilution was calculated both in a traditional way and in a modifiedway, which takes into consideration N and ¹⁵N content for theplants before the experiment started. The three methods wereapplied on the following Rhizobium-symbioses: Acacia albidaDel (Faidherbia albida (Del) A. Chev.) and Leucaena leucocephala(Lam) de Wit., and the Frankia-symbiosis Casuarina equisetifoliaL. The plants wereabout 4-months-old when they were harvested. Nitrogen derived from N₂ fixation in control plants of Acaciaalbida was 54·2 mg as measured with ARA, while it was28·5 mg as measured with the ¹⁵N dilution technique,compared to 30·7 mg calculated as accumulated N. In comparison,L. leucocephala fixed 41·6 mg N (ARA), 53·5 mgN(15N dilution technique) and 56·3 mg N (accumulatedN). The Frankia-symbiosis had fixed 27·4 mg N as measuredby ARA, 8·1 mg N as measured by ¹⁵N dilution techniqueand 12·3 mg N as accumulated N. There were no differencesbetween the estimates based ontraditional and modified waysof calculating ¹⁵N dilution. The immediate effect of water deficit treatment on N₂ fixationwas continuously measured inall species with ARA, which startedto decrease approximately 10 d after the initiation of the treatment,and declined to less than 5% of the initial level after 21–28d. The decrease in the amount of N derived from N₂ fixation wasstudied in L. leucocephala during the period of treatment. Therewas a 26% decrease in amount of N derived from N₂ fixation asresult of water deficit (as measured with ARA), while the decreasewas 23% when measured withboth the ¹⁵N dilution method and asaccumulated N. The three different methods for measuring N₂ fixation and effectsof water deficit on N₂ fixation are discussed. Key words: Acacia albida, ARA, Casuarina equisetifolia, Leucaena leucocephala, ¹⁵N dilution, N₂N fixation, water deficit     

The Relations of Haemoglobin and Lignin-like Compounds to Acetylene Reduction in Symbiotic Casuarina

Nodules from six Casuarina symbioses (Casuarina cunninghamiana,C. equisetifolia, and C. glauca inoculated with each of twodifferent Frankia sources) were evaluated for (i) concentrationsof haemoglobin (measured as CO-reactive haem) at 124, 144 and165 d after inoculation, (ii) acetylene reduction and (iii)occurrence of lignin-like compounds in cell walls of the nodulesat 165 d after inoculation. Haemoglobin concentration and theoccurrence of lignin-like compounds were related to acetylenereduction at final harvest. Concentration of haemoglobin innodules ranged from 1.56 to 22.27 nmol haem (g FW)^-1. Therewere marked plant species-Frankia interactions. At final harvestplant growth was greatest in C. cunninghamiana inoculated withFrankia inoculum designated SI, while haemoglobin concentrationwas intermediate in this symbiosis. Acetylene reduction activitywas detected in all the symbioses except C. equisteifolia inoculatedwith SI, and was highest in C. glauca inoculated with SI. Haemoglobinconcentration per fresh weight nodule and acetylene reductionper plant were positively correlated (r= 0.84, n= 12). The highestlevels of cumulative nitrogen fixation were always associatedwith the highest haemoglobin concentrations and the highestrates of acetylene reduction activity. Nitrogen content waslowest in C. equisetifolia inoculated with SI (23.89±3.07mg) and highest in C. cunninghamiana inoculated with the sameinoculum (217.2±22.5 mg). Irrespective of symbiotic performance,lignin-like compounds were found in the cell walls of the nodulesin all symbioses in this study. The cells containing these compoundswere mainly localized in the infected areas of the nodule. Theroles of haemoglobin and occurrence of lignin-like compoundsin relation to nitrogen fixation in Casuarina symbioses arediscussed.