Effects of Water Stress on the Respiratory and Nitrogen-fixing Activity of Soybean Root Nodules

Seventy-five per cent of the N₂-fixing activity (measured asthe reduction of C₂H₂ to C₂H₄) and 50 per cent of the respiratoryactivity of detached soybean root nodules was lost when thewater potential () of the nodules was lowered from approximately–1 ? 105 Pa (turgid nodules) to –9 ? 105 Pa (moderatelystressed nodules). Severely stressed nodules ( = –1.8? 10⁶ Pa) showed almost total loss of N₂-fixing activity andup to 80 per cent loss of respiratory activity. Increasing theoxygen partial pressure (PO₂) from 10⁴ to 10⁵ Pa completelyrestored both N₂-fixation and respiration in moderately stressednodules, but only partial recovery was possible in severelystressed nodules. The activity of the stressed nodules was verylow at low PO₂ (5 ? 10³ and 10⁴ Pa). The C₂H₂-reducing activityof nodule slices, nodule breis, and bacteroids from turgid andmoderately stressed nodules was almost identical but some activitywas lost in the breis and bacteroids from severely stressednodules. Calculations showed that at low PO₂ (10⁴ and 2 ? 10⁴Pa), the rate of O₂ diffusion into severely stressed noduleswas ten times lower than that for turgid nodules, but only fourtimes lower at a higher PO₂ (4 ? 10⁴ Pa). Carbon monoxide inhibitionof C₂H₂ reduction was slower in stressed nodules than in turgidnodules. The results are discussed in view of the possible developmentof a physical barrier to gaseous diffusion and/or the possiblealtered affinity of the nodule leghaemoglobin for O₂ in thewater-stressed nodules.     

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.     

Anatomy, physiology and biochemistry of root nodules of Sprint-2 Fix-, a symbiotically defective mutant of pea (Pisum sativum L.)

A plant-determined pea mutant Sprint-2 Fix^– and the parentalline Sprint-2 were compared for selected physiological and biochemicalparameters. The Fix^– mutation prevented differentiationof Rhizobium leguminosarum bacteria into bacteroids and producedlarge, white, non-fixing nodules. These lacked nitrogenase-linkedrespiration, but had a background rate of CO₂ evolution similarto the normal Fix⁺ nodules. The cortical structure of the ineffectivenodules suggests the existence of an oxygen diffusion barrierand this was supported by a low oxygen concentration in thecentral region (0.5–3.0%), measured using an O₂ sensitivemicro-electrode. Sucrose and starch contents were similar innormal and ineffective nodules while ononitol content was about15 times lower in the Fix^– nodules. The distribution ofstarch was also different in the two nodule types. The activitiesof glutamine synthetase (GS), sucrose synthase (SS), phosphoenolpyruvatecarboxylase (PEPC) and alanine pyruvate aminotransferase (APAT)were markedly higher in Fix⁺ nodules while the activities ofpyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH) andglutamate dehydrogenase (GDH) were higher in Fix^– nodules.The data from immunodetection of host nodule proteins confirmedthe reduced levels of sucrose synthase and the almost completeabsence of glutamine synthetase and leghaemoglobin in mutantnodules. There was no significant difference in the amount ofnitrogenase component 1 extracted from the microsymbiont ofnormal and ineffective nodules, but component 2 was hardly detectablein the Fix^– mutant. Key words: Pisum sativum, Fix^– mutant, nodules     

Biological and Cultural Characteristics of the Effective Frankia Strain HFPCcl3 (Actinomycetales) from Casuarina cunninghamiana (Casuarinaceae)

From homogenates prepared from surface-sterilized nodules ofseedlings of Casuarina cunninghamiana grown aeroponically, astrain of Frankia designated HFPCc13 was isolated and has beengrown in pure filamentous culture in a defined synthetic nutrientmedium. Vesicle and sporangium formation can be induced by removalof combined nitrogen from the medium.Frankia strain HFPCc13nodulates young seedlings of C. cunninghamiana and C. equisetifoliawithin three weeks of inoculation with an optimum root mediumpH of 6–7 for nodulation and optimum temperature of 30–35^°C. The presence of combined nitrogen in the root mediuminhibits nodulation with NH₄⁺ more inhibitory than NO₃^–.Frankia HFPCc13 does not nodulate Allocasuarina species withinthe same family nor several other possible actinorhizal plantstested. Thus this strain is quite precise in its host specificity.The rate of acetylene reduction was greater in C. cunninghamianathan the closely related species C. equisetifolia. In neitherof these host species were vesicles observed to occur withinthe infected root nodules which had been demonstrated to beactively fixing dinitrogen. Root nodules were shown to be activein acetylene reduction over a range of O₂ concentration in thegaseous environment with an optimum at about 20 per cent O₂,the ambient P_O2 of the air. The mechanism(s) for oxygen protectionof nitrogenase within the filamentous form of Frankia withinthese nodules remains to be explained. Casuarina, Frankia, nodulation, nitrogen fixation     

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     

Possible physiological mechanisms for production of hydrogen peroxide by the ichthyotoxic flagellate Heterosigma akashiwo

Blooms of the toxic red tide phytoplankton Heterosigma akashiwo(Raphidophyceae) are responsible for substantial losses withinthe aquaculture industry. The toxicological mechanisms of H.akashiwoblooms are complex and to date, heavily debated. One putativetype of ichthyotoxin includes the production of reactive oxygenspecies (ROS) that could alter gill structure and function,resulting in asphyxiation. In this study, we investigated thepotential of H.akashiwo to produce extracellular hydrogen peroxide,and have investigated which cellular processes are responsiblefor this production. Within all experiments, H.akashiwo producedsubstantial amounts of hydrogen peroxide (up to 7.6 pmol min^–110⁴ cells^–1), resulting in extracellular concentrationsof ~0.5 µmol l^–1 H₂O₂. Measured rates of hydrogenperoxide production were directly proportional to cell density,but at higher cell densities, accuracy of H₂O₂ detection wasreduced. Whereas light intensity did not alter H₂O₂ production,rates of production were stimulated when temperature was elevated.Hydrogen peroxide production was not only dependent on growthphase, but also was regulated by the availability of iron inthe medium. Reduction of total iron to 1 nmol l^–1 enhancedthe production of H₂O₂ relative to iron replete conditions (10µmol l^–1 iron). From this, we collectively concludethat production of extracellular H₂O₂ by H.akashiwo occurs througha metabolic pathway that is not directly linked to photosynthesis.     

The Physiology and Nitrogen-fixing Capability of Aquatically and Terrestrially Grown Neptunia plena: The Importance of Nodule Oxygen Supply

The aquatic legume Neptunia plena (L.) Benth. was grown in non-aeratedwater culture or vermiculite. Growth, nodulation, nitrogen fixationand nodule physiology were investigated. Over an 80-d period,plants grew and fixed nitrogen and carbon equally well in bothrooting media, although distribution of growth between plantparts varied. Total nodule dry weights and volumes were similarbut vermiculite-grown plants had three times as many (smaller)nodules than those grown in water. Oxygen diffusion resistanceof nodules exposed to 21% oxygen and 10% acetylene did not differsignificantly. Both treatments showed similar declines in rootrespiration and acetylene reduction activity (approx. 10%) whenroot systems were exposed to stepped decreases and increasesin rhizosphere oxygen concentration. However, nitrogenase activityof aquatically grown plants was irreversibly inhibited by rapidexposure of nodules to ambient air, whereas vermiculite-grownplants were unaffected. Aeration of water-cultured N. plenareduced stem length (but not mass) and number of nodules perplant. The concentration of nitrogen fixation by 163%. PossibleO₂ transport pathways from the shoot atmosphere to roots andnodules are discussed. Aquatic legume, diffusion resistance, Neptunia plena, nitrogen fixation, oxygen, root nodules     

Relationships between Acetylene Reduction Activity, Hydrogen Evolution and Nitrogen Fixation in Nodules of Acacia spp.: Experimental Background to Assaying Fixation by Acetylene Reduction under Field Conditions

Hansen, A. P., Pate, J. S. and Atkins, C. A. 1987. Relationshipsbetween acetylene reduction activity, hydrogen evolution andnitrogen fixation in nodules of Acacia spp.: Experimental backgroundto assaying fixation by acetylene reduction under field conditions.—J.exp. Bot. 38: 1–12 Glasshouse grown, symbiotically-dependent seedlings of Acaciaalata R.Br., .A. extensa Lindl., and A. pulchella R.Br. wereexamined for acetylene reduction in closed assay systems usingundisturbed potted plants, excavated whole plants, nodulatedroots or detached nodules. Nitrogenase activity declined sharplyover the first hour after exposure of detached nodules to acetylene(10% v/v in air), less steeply or not at all over a 3 h periodin assays involving attached nodules. Using detached nodules,rates of acetylene reduction, nitrogen (¹⁵N₂) fixation, andhydrogen evolution in air (¹⁵N₂) and acetylene-containing atmosphereswere measured in comparable 30 min assays. Total electron flowthrough nitrogenase in air was determined from rates of nitrogen(¹⁵N₂) fixation ( ? 3) plus hydrogen evolution, that in thepresence of acetylene from rates of acetylene reduction andhydrogen evolution in air: acetylene. Values for the ratio ofelectron flow in air: acetylene to that in air ranged from 0?43to 0?83 in A. pulcheila, from 0?44 to 0?66 in A. alala and from0?37 to 0?70 in A. extensa, indicating substantial inhibitionof electron flow through nitrogenase of detached nodules byacetylene. Relative efficiencies of nitrogenase functioningbased on hydrogen evolution and acetylene reduction were from0?15 to 0?79, those based on nitrogen (¹⁵N₂) fixation and hydrogenevolution from 0?53 to 0?87. Molar ratios of acetylene reducedto nitrogen (¹⁵N₂) fixed were 2?82 ? 0?24, 201 ? 0?15, and 1?91? 0?11 (?s.e.; n = 7) for A. pulcheila,A. extensa and A. alata respectively A standard 5–10 min acetylene reduction assay, conductedon freshly detached unwashed nodules in daytime (12.00–14.00h), was calibrated for field use by comparing total N accumulationof seedlings with estimated cumulative acetylene reduction overa 7-week period of glasshouse culture. Molar ratios for acetylenereduced: nitrogen fixed using this arbitrary method were 3?58for A. alata, 4?82 for A. extensa and 1?60 for A. pulchella.The significance of the data is discussed. Key words: Acacia spp, nitrogenase functioning     

Tangential Movement of 14C-labelled Assimilates in Stems of Willow

It was shown that, when ¹⁴CO₂, was fed to a single leafy shootof a rooted willow cutting, activity appeared within 12 hrs.in honeydew collected from a colony of the aphid Tuberolachnussalignus (Gmelin), feeding on the opposite side of the stem20 cm. below the base of the leafy shoot. This was shown tobe due to a tangential movement of labelled materials throughthe bark. This tangential movement was found only in cuttings with limitedroot growth. Where an active root sink was present, extensivetangential movement was absent. Using two aphid colonies on either side of the stem, determinationsof the speed of tangential movement have been made. The highestvalue obtained was 10.6 mm./hr.     

Hydrogen Effect on Nitrogenase (C2H2 Reduction) Response to Oxygen in Bradyrhizobium japonicum-Phaseoleae Symbioses

The influence of hydrogenase in Bradyrizobum-Phaseoleae symbioseswas studied ex-planta and in-planra in soybean (Glycine max)and cowpea (Vigna unguiculata). The hydrogenase was activatedby the addition of hydrogen in the incubation gas phase whichmodified the response of nitrogenase activity of Hup⁺ (hydrogenuptake positive) symbiosis to the external oxygen partial pressure.For bacteroids the hydrogenase expression increased nitrogenaseactivity at supraoptimal pO₂, acting possibly as a respiratoryprotection of nitrogenase. However, at suboptimal pO₂, nitrogenaseactivity of Hup⁺ bacteroids decreased with hydrogen, a phenomenonattributed to the lower efficiency of ATP synthesis from hydrogenthan from carbon substrates oxidation. For undisturbed nodules,the hydrogenase expression in soybean increased the optimalpO₂ for ARA (COP), from 35.3 to 40.3 kPa O₂, and the ARA atsupraoptimal pO₂; at suboptimal PO₂ there was a negative effectof hydrogenase on ARA, although this inhibition was less thanon bacteroids and was not detected if plants were grown at 15°C rather than 20 °C root temperature. No H₂ effectwas detected on cowpea nodules. The results on soybean nodulesare consistent with the concept that symbiotic nitrogen fixationis oxygen-limited and that hydrogenase activity has no beneficialeffect on nitrogen fixation in O₂ limitation. Key words: Glycine max, hydrogenase, nitrogenase, nitrogen fixation, nodules, Vigna unguiculata     

Investigations of Carbon Transport in Plants: I. THE USE OF CARBON-11 TO ESTIMATE VARIOUS PARAMETERS OF THE TRANSLOCATION PROCESS

A technique is described which enables several components ofthe translocation system to be measured routinely, in vivo andsimultaneously. Plants are exposed to a 1-min pulse of ¹¹CO₂and the movement of the ¹¹C pulse through the plant followedusing an array of scintillation counters. It is possible toestimate the time taken for the ¹¹C to enter the sieve tubes.In addition, the speed of translocation can be obtained frommeasurements of the arrival of the ¹¹C pulse at different positionsalong the translocation pathway. The increasing half-width ofthe pulse as it enters and moves through the sieve tubes givessome indication of the extent of the delays in the differentparts of the translocation system. The short half-life of the¹¹C allows the measurements to be repeated on the same plantseveral times per day. A technique is discussed whereby dataon the movement of ¹¹C could be combined with simultaneous measurementsof the rate of uptake of carbon dioxide to provide an estimateof the rate of movement of carbon.     

Isotopic Studies of Nitrogen Fixation in Non-Legume Root Nodules

Isotopic studies are presented on six of the eight recognizednon-legume nodule-forming genera of Angiosperms. Evidence hasbeen obtained of the occurrence of fixation of atmospheric nitrogenin the root nodules of Casuarina, Ceanothus, and Shepherdia,now examined isotopically for the first time. In further studiesof the nitrogen-fixing nodules of Hippopha and Alnus it is shownthat the fixation continues for a longer period after detachmentfrom the plant than is the case with legume nodules, and thatthe enrichment in ¹³N which they (and also detached Casuarinanodules) finally achieve after exposure to excess of the freeisotope considerably exceeds that shown by detached legume nodules.Fixation in detached Myrica nodules was not clearly affectedby reduction in oxygen supplied until the proportion of oxygenwas less than 5 per cent.     

Effectiveness of Lotus Root Nodules: II. RELATIONSHIP BETWEEN ROOT NODULE EFFECTIVENESS AND 'IN VITRO' SENSITIVITY OF FAST-GROWING LOTUS RHIZOBIA TO FLAVOLANS

An extract from the roots of Lotus pedunculatus plants was foundto contain a compound toxic towards fast-growing Lotus rhizobia.This compound was identified as a flavolan, which has a prodeiphinidin:procyanidin ratio of 75:25. A fast-growing strain of Rhizobium(NZP2213) which forms ineffective root nodules on L. pedunculatuswas four times more sensitive to this flavolan (ED₅₀ = 25 ?gml^–1) than another strain (NZP2037, ED₅₀ = 100 ?g ml^–1)which forms effective root nodules on this species. The rootsof another Lotus species, L. tenuis, on which both strains ofRhizobium form effective root nodules, also contained a flavolan( 95% procyanidin) but both strains were relatively insensitiveto this flavolan (EDED₅₀ = 350 to 500 ?g ml^–1) L. pedunculatusplants bearing ineffective root nodules contained two to threetimes more flavolan in their roots (5–7 mg g^–1 fr.wt.)than uninoculated control plants. Experiments with seven otherLotus species and with hybrid plants developed between L. pedunculatusand L. tenuis showed a relationship between the prodeiphinidin:procyanidin ratio of the flavolan in their roots and the effectivenessof root nodules formed on these plants by NZP2213. Quantitativebinding studies of the flavolan from L. pedunculatus to NZP2037and NZP2213 indicated that, while the affinity constants forbinding were similar for both strains, the surface of strainNZP2037 contained four times more binding sites than NZP2213,possibly correlating with this strain's ability to toleratehigher concentrations of this flavolan. It is suggested thatthe differential sensitivity of these two strains of Rhizobiumto flavolans is related to their ability to form effective rootnodules on Lotus species.     

Altered pH(i) regulation and Na(+)/HCO3(-) transporter activity in choroid plexus of cilia-defective Tg737(orpk) mutant mouse

Tg737^orpk mice have defects in cilia assembly and develop hydrocephalus in the perinatal period of life. Hydrocephalus is progressive and is thought to be initiated by abnormal ion and water transport across the choroid plexus epithelium. The pathology is further aggravated by the slow and disorganized beating of motile cilia on ependymal cells that contribute to decreased cerebrospinal fluid movement through the ventricles. Previously, we demonstrated that the hydrocephalus phenotype is associated with a marked increase in intracellular cAMP levels in choroid plexus epithelium, which is known to have regulatory effects on ion and fluid movement in many secretory epithelia. To evaluate whether the hydrocephalus in Tg737^orpk mutants is associated with defects in ion transport, we compared the steady-state pH_i and Na⁺-dependent transport activities of isolated choroid plexus epithelium tissue from Tg737^orpk mutant and wild-type mice. The data indicate that Tg737^orpk mutant choroid plexus epithelium have lower pH_i and higher Na⁺-dependent HCO₃^– transport activity compared with wild-type choroid plexus epithelium. In addition, wild-type choroid plexus epithelium could be converted to a mutant phenotype with regard to the activity of Na⁺-dependent HCO₃^– transport by addition of dibutyryl-cAMP and mutant choroid plexus epithelium toward the wild-type phenotype by inhibiting PKA activity with H-89. Together, these data suggest that cilia have an important role in regulating normal physiology of choroid plexus epithelium and that ciliary dysfunction in Tg737^orpk mutants disrupts a signaling pathway leading to elevated intracellular cAMP levels and aberrant regulation of pH_i and ion transport activity. cAMP; ion transport     

Effect of Contrasting Patterns of Nitrate Application on the Nitrate Uptake, N2-Fixation, Nodulation and Growth of White Clover

White clover (Trifolium repens L.) plants were grown from seedin perlite, inoculated with effective rhizobia and exposed tothe same ‘concentration x days’ of ¹⁵N-labellednitrate in four contrasting patterns of doses. Acetylene reductionwas measured at intervals using an open, continuous-flow sytem.Mean dry weight per nodule and rates of acetylene reductionfell rapidly (2–3 d) during periods of exposure to highnitrate concentrations (> 7 mM N) and rose again, equallyrapidly, when nitrate was withdrawn or substantially reduced.The fall in mean dry weight per nodule (50–66 per cent)was almost certainly too large to be accounted for by loss ofsoluble or storage carbohydrate only. No new nodules were formedduring periods of high nitrate availability. When nitrate wassupplied continuously at a moderate concentration (5.7 mM N)nodule numbers stabilised although existing nodules increasedin dry weight by almost four-fold over the 30 d measurementperiod. Treatment had no effect on the percentage nitrogen in planttissues although there were large differences in the proportionsderived from nitrate and N₂-fixation. Plants exposed continuouslyor frequently to small doses of nitrate took up more nitrate,and hence relied less heavily on N₂-fixation, than those exposedto larger doses less often. Increased reliance on nitrate broughtwith it increased total dry weight and shoot: root ratios. Possiblemechanisms involved in bringing about these differences in nitrogennutrition and growth are discussed. White clover, Trifolium repens, nitrate, N₂-fixation, nodule, acetylene reduction, ¹⁵N     

Diffusion of Oxyleghaemoglobin

The diffusion of oxyleghaemoglobin, prepared from soybean rootnodules, was measured at 24°C in agar and agarose gels ofvarious strengths, or in 1% agarose containing 0-18% (w/v) bovineserum albumin, to simulate the protein content of the cytoplasmof root nodule cells. Values of D_p, the diffusion coefficient,were unaffected (D_p = 11·8 x 10^-11 m² s^-1; s.e.m. 0·3x 10^-11) until the protein concentration exceeded 6%, abovewhich D_p declined sharply. With 18% bovine serum albumin, theconcentration of total soluble protein calculated to be presentin the cytoplasm of infected cells, where most of the leghaemoglobinis located in vivo, D_p was 5·9 x 10^-11 m² s^-1. Theseresults are discussed in relation to leghaemoglobin-facilitateddelivery of O₂ to the respiring N₂-fixing bacteroids in rootnodule cells.Copyright 1993, 1999 Academic Press Bacteroids, diffusion, Glycine max, N₂ fixation, oxyleghaemoglobin, soybean, root nodules     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

Investigation of Bidirectional Movement of Tracers in Sieve Tubes of Salix Viminalis L.

Sieve-tube sap was collected as honeydew from the aphid Tuberolachnussalignus (Gmelin) feeding on a young shoot or a mature stemof willow (Salix viminalis L.). Two radioactive tracers (either³H-glucose and ¹⁴C-sucrose, or ¹⁴CO₂, Na₂ ¹⁴CO₃, and ³²P-orthophosphate)were applied to leaves or to bark abrasions at each end of thestem. In some experiments the tracers were applied after thestart of honeydew production, whilst in others the tracers wereapplied before the aphids were sited. In most cases aphids feeding between the tracer applicationpoints produced doubly-labelled honeydew during a 24–48-hperiod from tracer application. In some instances the firstdrops of honeydew contained one tracer, followed several hourslater by the other tracer, whilst some aphids produced doubly-labelledhoneydew from the first drop. Singly-labelled or inactive honeydewwas found in only a few cases. It was demonstrated that when ³H-glucose and ¹⁴C-sucrose wereapplied at opposite ends of a stem that the tracers moved inopposite directions in the form of sucrose. The data suggest that a simultaneous bidirectional movementof two tracers may occur in the same sieve tube. However, thepossibility that the doubly-labelled honeydew could be producedby lateral movement from one sieve tube carrying one tracerto an adjacent punctured sieve tube carrying the other tracercannot be discounted.     

(Na+-K+)-stimulated ATPase in Leaves of C4 Plants: Possible Involvement in Active Transport of C4 Acids

Leaves of three C₄ plants, Setaria italica, Pennisetum typhoides,and Amaranthus paniculatus possessed five- to ten-fold higheractivities of a (Na⁺-K⁺)-dependent ATPase than those of twoC₃ plants, Oryza sativa and Rumex vesicarius. Na⁺-K⁺ ATPasefrom leaves of Amarathus exhibited an optimal pH of 7?5 andan optimal temperature of 35 ?C. It required 40 mM K⁺ and 80mM Na⁺ for maximal activity. Ouabain partially inhibited (Na⁺-K⁺)-dependentATPase activity in leaves of C₄ plants. Ouabain also blockedthe movement of label from initially formed C₄ acids into endproducts in leaves of only C₄ plants, Setaria and Amaranthusbut not in a C₃ plant, Rumex. We propose that Na⁺-K⁺ ATPasemay mediate transfer of energy during active transport of C₄acids from mesophyll into the bundle sheath.     

Leaf-like Structure in the Photosynthetic, Succulent Stems of Cacti

This research examined the hypothesis that as cacti evolve tothe leafless condition, the stem epidermis and cortex becomemore leaflike and more compatible with a photosynthetic role.All cacti in the relict genus Pereskia have non-succulent stemsand broad, thin leaves. All members of the derived subfamilyCactoideae are ‘leafless’, having an expanded cortexthat is the plant's only photosynthetic tissue. In Pereskia,leaves have a high stomatal density (mean: 50.7 stomata mm^–2in the lower epidermis, 38.1 mm^–2 in the upper epidermis),but stems have low stomatal densities (mean: 11.3 mm ₂, threeof the species have none). Stems of Cactoideae have a high stomataldensity (mean: 31.1 mm^–2, all species have stomata). Theouter cortex cells of stems of Cactoideae occur in columns,forming a palisade cortex similar to a leaf palisade parenchyma.In this palisade cortex, the fraction of tissue volume availablefor gas diffusion has a mean volume of 12.9%, which is identicalto that of Pereskia leaf palisade parenchyma. Pereskia stemcortex is much less aerenchymatous (mean: 5.3% of cortex volume).Cactoideae palisade cortex has a high internal surface density(0.0207 cm₂ cm^–2 which is higher than in Pereskia stemcortex (0.0150 cm₂ cm^–3) but not as high as Pereskia leafpalisade parenchyma (0.0396 cm₂ cm^–3). Pereskia stem cortexhas no cortical bundles, but Cactoideae cortexes have extensivenetworks of collateral vascular bundles that resemble leaf veins. Cactaceae, cactus, intercellular space, stomatal density, internal surface/volume, evolution