Showing papers on "Nitrogen fixation published in 1987"

Investigation of the Role of Phosphorus in Symbiotic Dinitrogen Fixation

Abstract: The interactive effects of phosphorus supply and combined nitrogen (nitrate) on dry matter and nitrogen accumulation by nodulated soybean (Glycine max L. Merr.) plants, and the relative effects of phosphorus supply on nodule number, mass, and function in comparison to host plant growth were used to investigate the role of phosphorus in symbiotic dinitrogen fixation. Mixed positive and negative phosphorus by nitrogen source interactions indicated that severe phosphorus deficiency markedly impaired both host plant growth and symbiotic dinitrogen fixation and that symbiotic dinitrogen fixation has a higher phosphorus requirement for optimal functioning than either host plant growth or nitrate assimilation. In the whole plant phosphorus concentration range of 0.8 to 1.5 grams per kilogram dry weight, plants supplied with 20 millimolar nitrate accumulated significantly more dry matter and nitrogen than symbiotic plants without nitrate. This suggested that the higher phosphorus requirement for symbiotic dinitrogen fixation is internal rather than being associated with differences in the ability of roots in the two nitrogen regimes to absorb phosphorus from the external solution. Increasing the phosphorus concentration in plants solely dependent on dinitrogen fixation resulted in highly significant (P = 0.0001) increases in whole plant nitrogen concentration as well as highly significant increases (P = 0.0001) in whole plant dry matter and nitrogen accumulation. This indicated a greater responsiveness of symbiotic dinitrogen fixation than of host plant growth to improvement in phosphorus nutrition. The large increases in whole plant nitrogen concentration were associated with about 3.5-fold increases in the ratio of nodule mass to whole plant mass and about 2-fold increases in specific acetylene reduction (nitrogenase) activity of the nodules. The large increase in nodule mass (>30-fold) between the 0 and 2.0 millimolar phosphorus levels resulted from 11- and 3-fold increases in nodule number per plant and average mass of individual nodules, respectively. Root mass per plant over the same concentration range increased 3.5-fold. These results indicate that phosphorus has specific roles in nodule initiation, growth, and functioning in addition to its involvement in host plant growth processes.

Genetic aspects of plant mineral nutrition

Abstract: Genotypic variation in plant productivity and consequences for breeding of "low input cultivars".- Session 1: Physiological and biochemical mechanisms associated with genetic variation in utilization of a) nitrogen, b) phosphorus and c) other major nutrients.- Factors affecting the nutritional efficiency of plants.- Physiological basis of genotypic plant distinctions in mineral nutrition.- Concentrations of N, P, and K and dry matter mass in maize inbred lines.- Accumulation and translocation of nitrogen in cultivars of winter wheat with different demands for nutrition.- Uptake and partitioning of nitrogen in nitrogenlimited barley: Dependence of age and genotype.- Dry weight production and nitrogen efficiency in cultivars of barley and rye.- Nitrate reductase in sugar beet genotypes supplied with different nitrate levels.- Influence of the nitrogen level on root growth and morphology of two potato varieties differing in nitrogen acquisition.- Phosphorus efficiency and phosphorus remobilization in two sorghum (Sorghum bicolor (L.) Moench) cultivars.- Responses to phosphate fertilizers of differing solubilities by white clover cultivars.- Genotypical differences among graminaceous species in release of phytosiderophores and uptake of iron phytosiderophores.- Investigations on the nutrient uptake efficiency of different grape root-stock species and cultivars.- Heritability of root characteristics affecting mineral uptake in tall fescue.- Root traits of maize seedlings - indicators of nitrogen efficiency?.- Varietal differences in root Phosphatase activity as related to the utilization of organic phosphates.- The effect of shoot and root genotype on phosphorus concentrations of shoots and roots.- Carbohydrate status in roots of two soybean cultivars: A possible parameter to explain different efficiencies concerning phosphate uptake.- Properties of potassium uptake by seedling roots of grape cultivars.- Sterol content and efficiency of ion uptake by roots of maize genotypes.- Genetic differentiation in Plantago major L. in growth and P uptake under conditions of P limitation.- Session 2: Genotypic responses to a) water stress, b) salinity and c) acidity and deficiency or excess of elements.- Physiological characteristics responsible for drought resistance in different pea cultivars.- Effects of water deficit on osmotic adjustment, photosynthesis and dry matter production of rice (Oryza sativa L.) genotypes.- Evalutation of breeding strategies for drought tolerance in potato by means of crop growth simulation.- Influence of genotype and water stress on the uptake of potassium and nitrogen in maize.- Salinity tolerance of different halophyte types.- Physiological differences between barley cultivars under salt stress - xylem exudation and phloem flow of different cations.- Sodium exclusion mechanisms at the root surface of two maize cultivars.- Cation accumulation related to adaptation of maize populations to salinity.- Interaction between nitrogen and phosphorus fertilizers and soil salinity and its effect on growth and ionic composition of corn (Zea mays L.).- Genetic aspects of aluminum tolerance in sorghum.- Influence of salt stress on primary metabolism of Zea mays L. seedlings of model genotypes.- Genetic control of aluminium tolerance in sorghum.- Genetics of tolerance to aluminium in wheat (Triticum aestivum L. Thell).- The uptake of trace elements by spinach and bean varieties of different root parameters.- Manganese toxicity in sunflower lines.- Electrophysiological membrane characteristics of the salt tolerant Plantago maritima and the salt sensitive Plantago media.- Genotypic differences in boron accumulation in barley: Relative susceptibilities to boron deficiency and toxicity.- Behaviour of different wheat genotypes under various irrigation conditions in semi-arid tropics of Haryana, India.- Iron tolerance of rice cultivars.- Relationship between metolachlor sensitivity and Mn toxicity tolerance in sorghum cultivars.- Genetic studies on the acidification capacity of sunflower roots induced under iron stress.- Screening for manganese efficiency in barley (Hordeum vulgare L.).- Session 3: Screening techniques for detection of nutritional deficiencies and abiotic stress under genetic control.- Screening techniques for plant nutrient efficiency: Philosophy and methods.- Biochemical techniques for genotype characterization.- Comparison of nitrogen utilization of diploid and tetraploid perennial ryegrass genotypes using a hydroponic system.- Actual cytokinin concentrations in plant tissue as an indicator for salt resistance in cereals.- A procedure for quick screening of wheat cultivars for salt tolerance.- A method for investigating the influence of soil water potential on yield and water use efficiency of spring wheat cultivars.- Water potential as a selection criterium for drought tolerance by different durum wheat genotypes.- Application of in vitro techniques for screening plant genetic variability.- Screening pasture plants for aluminium tolerance.- Screening soybean for aluminium tolerance and adaptation to acid soils.- Selection parameters for assessing the tolerance of wheat to high concentrations of boron.- Session 4: Genetic variation in symbiotic systems.- Variability of molecular nitrogen fixation and its dependence on plant genotype and diazotroph strains.- Iron deficiency in chickpea in the Mediterranean region and its control through resistant genotypes and nutrient application.- N2 fixation by R. japonicum strains during vegetation of different soybean cultivars.- Differences between cereal crop cultivars in root-associated nitrogen fixation, possible causes of variable yield response to seed inoculation.- Genetical analysis of the efficiency of VA mycorrhiza with spring wheat. I. Genotypical differences and a reciprocal cross between an efficient and non-efficient variety.- Improved growth of Cajanus cajan (L.) Millsp. in an unsterile tropical soil by three mycorrhizal fungi.- Control of Mn status and infection rate by genotype of both host and pathogen in the wheat take-all interaction.- Session 5: Germplasm resources and creation of genotypes for specific environmental including low input systems.- Genetic resources for optimal input technology - ICARDA's perspectives.- Physiological basis of differential response to salinity in rice cultivars.- A method to estimate the prospect of specific breeding for nutrient efficiency.- Grain yield and quality characters of genotypes in F5-generation under low and high nitrogen input.- Performance of winter wheat cultivars under reduced nitrogen conditions.- Suitability of varieties of winter wheat in low external input systems in West Germany.- Comparison between land races and high yielding cultivars of winter wheat in extensive, integrated and intensive farming over several years.- Breeding wheat (Triticum aestivum) for aluminium toxicity tolerance at CIMMYT.- Buckwheat - A low input plant.- Phosphorus: A limiting nutrient in bean (Phaseolus vulgaris L.) production in Latin America and field screening for efficiency and response.- Screening of rices for adverse soil tolerance.- Genetic diversity for nutrient use efficiency in cultivars and exotic germplasm lines of alfalfa.- Breeding for low level acid soil tolerance as a component of overall acid soil field tolerance in sorghum.- A programme to breed a cultivar of Trifolium repens L. for more efficient use of phosphate.- Response to phosphorus of a world collection of white clover cultivars.

Nitrogen-fixing bacteria in nonleguminous crop plants

Abstract: Contents: Introduction.- The Genus Azospirillum.- Other Root-Associated Diazotrophs.- Association with Plants.- Quantification of N2 Fixation.- Agronomic Implications.- Potentials and Prospects for the Future.- References.- Index.

Vesicular‐arbuscular mycorrhiza improve both symbiotic n2 fixation and n uptake from soil as assessed with a 15n technique under field conditions

Abstract: Summary A technique using 15N-labelled inorganic fertilizer was applied to estimate N2 fixation by the forage legume Hedysarum coronarium L. and to ascertain the role of vesicular-arbuscular (VA) mycorrhizas in plant N nutrition throughout a growing season under field conditions. The absence of the specific Rhizobium for the forage legume in the test soil allowed us the use of 15N methodology with the same legume as reference ‘non-fixing’ crop. At the first harvest, mycorrhizal inoculation behaved similarly to the phosphate addition in improving the percentage (70 %) and the total amount of N derived from fixation. But thereafter, mycorrhizal inoculation not only enhanced dry matter yield, N concentration and total N yield but also the amount of N derived from soil and from fixation, as compared with either phosphate-added or control plants. This indicated that mycorrhizas acted both by a P-mediated mechanism to improve N2 fixation and by enhancing N uptake from soil. The latter agrees with recent findings by others that VA mycorrhizal hyphae can translocate and assimilate ammonium, a fact of physiological and ecological interest.

Glutamine synthetase genes are regulated by ammonia provided externally or by symbiotic nitrogen fixation.

Abstract: Glutamine synthetase is the key enzyme in the assimilation by plants of reduced nitrogen provided from either the soil or fixed symbiotically in association with Rhizobium. We have isolated a number of cDNA clones for soybean glutamine synthetase (GS) from a nodule-cDNA library, using RNA from polysomes immunoprecipitated by GS antibodies. Transcripts corresponding to two clones differing in their 3' non-translated sequences were present in both root and nodule tissue; however, the concentration in the nodules was several times higher. The relative concentrations of these sequences in both tissues is about 9:1. Availability of ammonium ions [provided as NH(4)NO(3) or (NH(4))(2)SO(4)] enhanced the expression of both sequences in root tissue within 2 h, reaching a level similar to that in nodules by 8 h, while KNO(3) had no effect during this period. When nitrogen fixation was prevented by replacing nitrogen with argon in the root environment or when the nodules were formed by a Fix mutant of Bradyrhizobium japonicum, the amounts of GS mRNA did not increase over that in roots. These experiments, together with the time course of increase in GS mRNA transcripts, suggest that the genes encoding cytosolic GS are directly induced by the available ammonia.

Nodulation and nitrogenase activity in nitrogen-fixing woody plants stimulated by CO2 enrichment of the atmosphere

Abstract: The responses of three species of nitrogen-fixing trees to CO2 enrichment of the atmosphere were investigated under nutrient-poor conditions Seedlings of the legume, Robinia pseudoacacia L and the actinorhizal species, Alnus glutinosa (L) Gaertn and Elaeagnus angustifolia L were grown in an infertile forest soil in controlled-environment chambers with atmospheric CO2 concentrations of 350 μl −1 (ambient) or 700 μl −1 In R pseudoacacia and A glutinosa, total nitrogenase (N2 reduction) activity per plant, assayed by the acetylene reduction method, was significantly higher in elevated CO2, because the plants were larger and had more nodule mass than did plants in ambient CO2 The specific nitrogenase activity of the nodules, however, was not consistently or significantly affected by CO2 enrichment Substantial increases in plant growth occurred with CO2 enrichment despite probable nitrogen and phosphorus deficiencies These results support the premises that nutrient limitations will not preclude growth responses of woody plants to elevated CO2 and that stimulation of symbiotic activity by CO2 enrichment of the atmosphere could increase nutrient availability in infertile habitats

Ethane formation from acetylene as a potential test for vanadium nitrogenase in vivo

Abstract: Azotobacter chroococcum and A. vinelandii, both obligately aerobic diazotrophic bacteria, have at least two genetically distinct systems for nitrogen fixation1,2. One is the well-characterized molybdenum nitrogenase, whereas in a second system (the vanadium nitrogenase) the conventional molybdoprotein is replaced by a vanadoprotein and a second iron protein replaces the one normally found with the molybdoprotein3–5. A third system may exist in A. vinelandii6'7. Reiterations of DNA encoding nitrogenase structural genes (nifHDK) are found in several genera of diazotrophs (see ref. 1). In the azotobacters there is evidence, based on physiological and biochemical properties of strains carrying deletions of nifHDK, that the reiterated genes are involved in the synthesis of alternative nitrogenases including the vanadium (V)-nitrogenase. The screening of other genera for the presence of a functional V-nitrogenase would be greatly facilitated by a specific test for it which could be applied in vivo. The reduction of acetylene to ethylene8,9 by the molybdenum (Mo)-nitrogenase has been widely used in ecological, genetic and physiological studies of biological nitrogen fixation. We demonstrate here that purified V-nitrogenase catalyses the reduction of acetylene not only to ethylene, but also to ethane, which the Mo-nitrogenase does not10. This property distinguishes between these two systems in A. chroococcum and A. vinelandii in vivo. Ethane formation also occurred in cultures of Clostridium pasteurianum W5, suggesting that V-nitrogenase is not restricted to the Azotobacteriaceae.

Oxygen-poor microzones as potential sites of microbial n(2) fixation in nitrogen-depleted aerobic marine waters.

Abstract: The nitrogen-deficient coastal waters of North Carolina contain suspended bacteria potentially able to fix N(2). Bioassays aimed at identifying environmental factors controlling the development and proliferation of N(2) fixation showed that dissolved organic carbon (as simple sugars and sugar alcohols) and particulate organic carbon (derived from Spartina alterniflora) additions elicited and enhanced N(2) fixation (nitrogenase activity) in these waters. Nitrogenase activity occurred in samples containing flocculent, mucilage-covered bacterial aggregates. Cyanobacterium-bacterium aggregates also revealed N(2) fixation. In all cases bacterial N(2) fixation occurred in association with surficial microenvironments or microzones. Since nitrogenase is oxygen labile, we hypothesized that the aggregates themselves protected their constituent microbes from O(2). Microelectrode O(2) profiles revealed that aggregates had lower internal O(2) tensions than surrounding waters. Tetrazolium salt (2,3,5-triphenyl-3-tetrazolium chloride) reduction revealed that patchy zones existed both within microbes and extracellularly in the mucilage surrounding microbes where free O(2) was excluded. Triphenyltetrazolium chloride reduction also strongly inhibited nitrogenase activity. These findings suggest that N(2) fixation is mediated by the availability of the appropriate types of reduced microzones. Organic carbon enrichment appears to serve as an energy and structural source for aggregate formation, both of which were required for eliciting N(2) fixation responses of these waters.

Flavone Limitations to Root Nodulation and Symbiotic Nitrogen Fixation in Alfalfa

Abstract: Transcription of the nodABC genes in Rhizobium meliloti is required for root nodule formation in alfalfa ( Medicago sativa L.) and occurs when specific compounds, such as the flavone luteolin, are supplied by the host plant. Results reported here indicate how luteolin in the root and rhizosphere can affect subsequent N 2 fixation and plant growth. Previous experiments with `Hairy Peruvian 329 (HP32), an alfalfa population produced from `Hairy Peruvian9 (HP) by two generations of selection for increased N 2 fixation and growth, found that HP32 had more root nodules and fixed more N 2 than the parental HP population. In the present study, flavonoid extracts of HP32 seedling roots are shown to contain a 60% higher concentration of compounds that induce transcription of a nodABC-lacZ fusion in R. meliloti than comparable extracts of HP roots. Chromatographic data indicated that HP32 roots had a 77% higher concentration of luteolin than HP roots. Adding 10 micromolar luteolin to the rhizosphere of HP seedlings increased nodulation, N 2 fixation, total N, and total dry weight but had no effect on nitrate assimilation. These data show that normal levels of flavone nodulation signals in the rhizosphere of HP alfalfa can limit root nodulation, symbiotic N 2 fixation, and seedling growth and suggest that one mechanism for increasing N 2 fixation can be the genetic enhancement of specific biochemical signals which induce nodulation genes in Rhizobium.

Seasonal Patterns of Growth and Nitrogen Fixation in Field-Grown Pea

Abstract: The seasonal patterns of growth and symbiotic N2 fixation under field conditions were studied by growth analysis and use of15N-labelled fertilizer in a determinate pea cultivar (Pisum sativum L.) grown for harvest at the dry seed stage.

Micronutrient effects on cyanobacterial growth and physiology

Abstract: Trace metals play crucial roles in the carbon and nitrogen metabolism of cyanobacteria. Physiological responses to metal limitation and toxicity in culture have shown that iron is important for photosynthesis and energy distribution in the cell while both iron and molybdenum are biochemically involved in nitrate reduction and nitrogen fixation. Nitrogen fixation is also relatively sensitive to copper toxicity. Consequently, factors that affect the supply rate, chemical speciation, or the recycling of trace metals can alter patterns of primary productivity and nitrogen metabolism. Overall, three trace metal dependent processes may contribute towards dominance: efficient use of limiting light, nitrogen fixation, and production of extracellular iron binding compounds.

Morphogenesis and fine structure of Frankia (Actinomycetales): the microsymbiont of nitrogen-fixing actinorhizal root nodules.

Abstract: Publisher Summary This chapter reveals that the genus Frankia of the order Actinomycetales consists of a diverse group of bacteria often exhibiting hyphal growth. Members of the genus Frankia are characterized by the ability to form nitrogen-fixing nodules on the roots of certain woody angiosperms and may be distinguished from other actinomycetes by their morphogenetic patterns in vivo and in vitro; cell wall chemistry, serology, and DNA homology; and surface laminations of the spore cell wall. Both the nodules induced by Frankia and the species of plants thatbear these nodules are termed “actinorhizal.” The ability of Frankia to induce root nodules, which may provide part or all (the latter occurs usually only under laboratory conditions) of the nitrogen requirements of the actinorhizal host plant, is of considerable importance to forestry, land reclamation, natural ecosystems, and plant genetic engineering. In many field situations, low levels of combined nitrogen in the soil may be limiting to plant growth; thus, the presence of root nodules, which chemically reduce (fix) atmospheric (molecular) nitrogen, overcomes deficiencies of ammonium and nitrate in the soil and aids plant growth.

Sensitivity of the common bean (Phaseolus vulgaris L.) symbiosis to high soil temperature

Abstract: Experiments were done to test whether N fixation is more sensitive to high soil temperatures in common bean than in cowpea or soybean. Greenhouse experiments compared nodulation, nitrogenase activity, growth and nitrogen accumulation of several host/strain combinations of common bean with the other grain legumes and with N-fertilization, at various root temperatures. Field experiments compared relative N-accumulation (in symbiotic relative to N-fertilized plants) of common bean with cowpea under different soil thermal regimes. N-fertilized beans were unaffected by the higher temperatures, but nitrogen accumulation by symbiotic beans was always more sensitive to high root temperatures (33°C, 33/28°C, 34/28°C compared with 28°C) than were cowpea and soybean symbiosis. Healthy bean nodules that had developed at low temperatures functioned normally in acetylene reduction tests done at 35°C. High temperatures caused little or no suppression of nodule number. However, bean nodules produced at high temperatures were small and had low specific activity. ForP. vulgaris some tolerance to high temperature was observed among rhizobium strains (e.g., CIAT 899 was tolerant) but not among host cultivars. Heat tolerance ofP. acutifolius andP. lunatus symbioses was similar to that of cowpea and soybean. In the field, high surface soil temperatures did not reduce N accumulation in symbiotic beans more than in cowpea, probably because of compensatory nodulation in the deeper and cooler parts of the soil.

Some aspects of the biology of nitrogen-fixing organisms

Abstract: Eukaryotic organisms do not fix nitrogen. Animals generally have no need to do so because of their complex food-acquisition and waste-disposal systems. Plants, by using carbon polymers for structural purposes, minimize their need for nitrogen. When very nitrogen-limited, to enter into symbiosis with nitrogen-fixing microorganisms may be the most controllable method for eukaryotes to obtain fixed nitrogen. Filamentous, heterocystous nitrogen-fixing cyanobacteria may be better adapted to a free-living than to a symbiotic existence, because of their complexity. In symbioses, their photosynthetic machinery becomes redundant and the need to differentiate heterocysts as well as derepress nif genes may be a disadvantage. This could in part account for the greater success of symbioses involving the structurally simpler genera Frankia , Rhizobium and Bradyrhizobium . Nitrogen fixation by legume nodules can be controlled by varying the oxygen supply. This control may be effected by a variable diffusion resistance, enabling oxygen required for ATP synthesis to be matched to available photosynthate. Such a resistance, which is probably located in the nodule cortex, may also be used to reduce nitrogen fixation in the presence of combined nitrogen and could also facilitate rapid responses to other forms of stress. Alternative resistances to gaseous diffusion may operate when water supplies are restricted. Rhizobium and Bradyrhizobium follow different patterns of differentiation into nitrogen-fixing bacteroids. These patterns are coupled with retention or loss of viability and with significant or no natural enrichment of the bacteroids with 15 N respectively. The basic patterns of each type are subject to host-modification. Recent studies on structures of primitive legume nodules show some parallels both with actinorhizas and with nodules on Parasponia induced by Bradyrhizobium . In particular, distribution of rhizobia in nodule tissues is intercellular and infection threads are formed only when bacteria ‘enter’ host cells; there is no intracellular ‘bacteroid’ stage. These threads are retained in the active nitrogen-fixing cells. Many legumes and some actinorhizas are not infected via root hairs. Therefore two of the stages often considered typical of the development of effective legume nodules, i.e. ‘release’ of bacteria into vesicles bounded by peribacteroid membrane and infection through root hairs, can be omitted; these omissions may be of use in attempts to transfer nodulating ability to new genera.

Azospirillum effects on susceptibility to Rhizobium nodulation and on nitrogen fixation of several forage legumes

Abstract: Azospirillum brasilense Cd cell concentration of 105–107 colony-forming units (cfu)/mL applied 24 h before Rhizobium (106 cfu/mL), increased nodule formation in the non root hair zone, more than tw...

Temporal separation of nitrogen fixation and photosynthesis in the filamentous, non-heterocystous cyanobacterium Oscillatoria sp.

Abstract: When growing in laternating light-dark cycles, nitrogenase activity (acetylene reduction) in the filamentous, non-heterocystous cyanobacterium Oscillatoria sp. strain 23 (Oldenburg) is predominantly present during the dark period. Dark respiration followed the same pattern as nitrogenase. Maximum activities of nitrogenase and respiration appeared at the same time and were 3.6 μmol C2H4 and 1.4 mg O2 mg Chl a-1·h-1, respectively. Cultures, adapted to light-dark cycles, but transferred to continuous light, retained their reciprocal rhythm of oxygenic photosynthesis and nitrogen fixation. Moreover, even in the light, oxygen uptake was observed at the same rate as in the dark. Oxygen uptake and nitrogenase activity coincided. However, nitrogenase activity in the light was 6 times as high (22 μmol C2H4 mg Chl a-1·h-1) as compared to the dark activity. Although some overlap was observed in which both oxygen evolution and nitrogenase activity occurred simultaneously, it was concluded that in Oscillatoria nitrogen fixation and photosynthesis are separated temporary. If present, light covered the energy demand of nitrogenase and respiration very probably fulfilled a protective function.

Nitrogen Fixation by Epiphylls in a Tropical Rainforest

Abstract: Bluegreen algae (Cyanobacteria) growing on the leaf surfaces of understory plants in a tropical rainforest can fix atmospheric nitrogen. Rates of fixation are strongly influenced by the presence of glucose and mineral nutrients leached from the host leaf, by light intensity as it relates to the photosynthetic rates of the algae, and by desiccation especially as it is influenced by the co-occurrence of epiphyllous bryophytes. A significant portion of this newly fixed nitrogen is transferred to the host leaf and may account for 10-25% of the total nitrogen in a leaf. Although the major source of nitrogen for plant growth and reproduction is from decomposition of organic matter, most ecosystems can gain significant quantities from precipitation and biological fixation of atmospheric nitrogen. In fact, Bums & Hardy (1975) estimated that biological fixation alone may account for 43% of the nitrogen transferred worldwide. Subba Rao (1977) suggested that the rates in tropical areas may be even higher and thus fixation could have a major role in the tropics where available nitrogen can be limited. In this paper, I will discuss one aspect of nitrogen fixation in a tropical ecosystem-that of fixation by epiphylls in a rainforest understory. Although epiphyll fixation can account for up to 25% of the nitrogen in a host leaf (Bentley & Carpenter, 1980), rates of fixation are quite variable in both time and space. Here I document some of the environmental factors that influence fixation rates. What are epiphylls? Epiphylls are epiphytes that are restricted to the surfaces of leaves. In tropical rainforests, most visible epiphylls are leafy liverworts of the family Lejeuneaceae but may also include mosses, lichens and, on some occasions, seedlings of other epiphytes such as bromeliads and orchids. In this study, we focused on the microorganisms, especially the bluegreen algae (Cyanobacteria) growing in association with the visible forms (Fig. 1). The epiphyll community is best developed in regions of high rainfall and low evaporation and is most diverse in tropical rainforests (Richards, 1964). Because they grow on photosynthetic surfaces, epiphylls are often considered to be detrimental to the host plant by interfering with light penetration to the leaf (Richards, 1964). Indeed, some epiphylls may actually be semiparasitic. For example, the rhizoids of Radulaflaccida, an epiphyllous liverwort, penetrate the cuticle and absorb nutrients from their host leaf (Berrie & Eze, 1975). Other workers feel that the presence of epiphylls increases the time that leaves remain wet and thus contributes to the growth of potentially pathogenic bacteria and fungi (Gregory, 1971; Stahl, 1891). Long-term wetting may also reduce the rates of transpiration and subsequent mineral uptake by the roots (Stahl, 1893; McLean, 1919). As early as 1891, Jungner suggested that rainforest plants may have adaptations such as leaf "drip tips" to increase the rates of drainage off the leaf and reduce the rate of colonization by epiphylls. Subsequent work has failed to either deny or confirm the adaptive role of these characters (Stahl, 1893; Shreve, 1914; Seybold, 1957; and pers. obs.). Nitrogen fixation by epiphylls. Some species of epiphyllous microorganisms are known to fix nitrogen (Bentley & Carpenter, 1980, 1984; Ruinen, 1975). During our study at the La Selva Biological Station in Costa Rica, we found that fixation rates are extremely variable, both within and among species (Fig. 2). Interestingly, in contrast to Ruinen's data where fixation was by freeliving bacteria (primarily Beijerinckia), fixation in the La Selva understory was most commonly I I would first like to express my appreciation to Maureen Dunn who gave expert technical assistance in both the lab and the field throughout this study. I also wish to acknowledge Amos Bien, Martin Meiss, Robert Black, and Helen Young for assistance in the field. This research was supported by grants DEB 78-12032 and DEB 80-10676 from the National Science Foundation. 2 State University of New York, Stony Brook, New York 11794, U.S.A. ANN. MISSOURI BOT. GARD. 74: 234-241. 1987. This content downloaded from 207.46.13.111 on Tue, 09 Aug 2016 05:57:38 UTC All use subject to http://about.jstor.org/terms 1987] BENTLEY-NITROGEN FIXATION BY EPIPHYLLS 235 low,~ ~ ~ ~ ~ ~ ~~~~~~~~~~~I ,rr P~~~~~~~~~~~~~~~V FGu;RE 1. Photomicrograph of epiphylls. The strand of the nitrogen-fixing bluegreen alga is across the lower third of the photograph. associated with the presence of bluegreen algae in the genera Scytonema, Stigonema, and Hapalosiphon. High fixation rates were invariably associated with a dense cover of bryophytes, suggesting that the bryophytes provide a good substrate for the nitrogen-fixing microorganisms. Most of our work on fixation was done using the acetylene reduction method for determining nitrogenase activity (Bentley & Carpenter, 1980; Prestwich & Bentley, 1981; Burris, 1972). While this is an extremely easy field assay for estimating fixation, it is not a direct measure of nitrogen fixation, and cannot be used to answer the most critical question in our study: does the newly fixed nitrogen get into the host leaf? By using 1N as a tracer, we were able to document that indeed this is the case (Bentley & Carpenter. 1984). There we showed that nitrogen fixation by epiphylls could account for up to 25% of the nitrogen in a host leaf. In other studies with bluegreen algae, Stewart (1963) and Jones & Stewart (1969) found that the extracellular nitrogenous products are primarily amino acids or peptides, but less complex compounds, including ammonium nitrite and nitrate, can be present. The pathway for movement of the new nitrogen is not through the stomata, as might be assumed, but rather to the epidermal cells via threadlike ectodesmata penetrating through the cell wall to the cuticle (Franke, 1970). Because of the specific morphology of the ectodesmata, Franke felt that the movement of soluble materials into and out of the leaf is a normal process, closely correlated with foliar absorption of "foreign" substances such as fertilizers and pesticides. Environmentalfactors affectingfLxation. The rates of fixation and transfer that we measured were made under ideal conditions for the activities of microorganisms. Although the high temperatures and almost constant moisture in a tropical rainforest can permit high fixation rates and concomitant release of nitrogenous products, we have also observed extremely high variance both among and within species at the La Selva Station (Fig. 2). Thus, it becomes important to ask what environmental factors influence fixation rates by epiphylls. Basically the answer lies in three factors: time. moisture, and nutrients. This content downloaded from 207.46.13.111 on Tue, 09 Aug 2016 05:57:38 UTC All use subject to http://about.jstor.org/terms 236 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 74

Nodulation, nitrogen fixation and nitrogen uptake in pigeonpea (Cajanus cajan (L.)) (Millsp) of different maturity groups

Abstract: The seasonal patterns of nodulation, acetylene reduction, nitrogen uptake and nitrogen fixation were studies for 11 pigeonpea cultivars belonging to different maturity groups grown on an Alfisol at ICRISAT Center, Patancheru, India. In all cultivars the nodule number and mass increased to a maximum around 60–80 days after sowing and then declined. The nodule number and mass of medium- and late-maturing cultivars was greater than that of early-maturing cultivars. The nitrogenase activity per plant increased to 60 days after sowing and declined thereafter, with little activity at 100 days when the crop was flowering. At later stages of plant growth nodules formed down to 90 cm below the soil surface but those at greater depth appeared less active than those near the surface.

Long-Term Effects of Metal-Rich Sewage Sludge Application on Soil Populations of Bradyrhizobium japonicum

Abstract: The application of sewage sludge to land may increase the concentration of heavy metals in soil. Of considerable concern is the effect of heavy metals on soil microorganisms, especially those involved in the biocycling of elements important to soil productivity. Bradyrhizobium japonicum is a soil bacterium involved in symbiotic nitrogen fixation with Glycine max, the common soybean. To examine the effect of metal-rich sludge application on B. japonicum, the MICs for Pb, Cu, Al, Fe, Ni, Zn, Cd, and Hg were determined in minimal media by using laboratory reference strains representing 11 common serogroups of B. japonicum. Marked differences were found among the B. japonicum strains for sensitivity to Cu, Cd, Zn, and Ni. Strain USDA 123 was most sensitive to these metals, whereas strain USDA 122 was most resistant. In field studies, a silt loam soil amended 11 years ago with 0, 56, or 112 Mg of digested sludge per ha was examined for total numbers of B. japonicum by using the most probable number method. Nodule isolates from soybean nodules grown on this soil were serologically typed, and their metal sensitivity was determined. The number of soybean rhizobia in the sludge-amended soils was found to increase with increasing rates of sludge. Soybean rhizobia strains from 11 serogroups were identified in the soils; however, no differences in serogroup distribution or proportion of resistant strains were found between the soils. Thus, the application of heavy metal-containing sewage sludge did not have a long-term detrimental effect on soil rhizobial numbers, nor did it result in a shift in nodule serogroup distribution.

Nitrogen fixation potential of beans ( Phaseolus vulgaris L.) compared with other grain legumes under controlled conditions

Abstract: Greenhouse experiments were done under favorable conditions to compare effective bean symbioses with cowpea and soybean symbioses and with N-fertilized controls. Growth, N-accumulation, nodulation, acetylene reduction, hydrogen evolution and the effect of native rhizobia on symbiotic performance were evaluated. Relative N accumulation (in symbiotic plants relative to N-fertilized plants) was higher for soybean (43%) than for the other symbioses (25–39%) at four weeks, but from four to six weeks both soybean (96%) and cowpea (92%) accumulated relatively more N than did beans (56–78%). Inferior performance of beans could not be atributed to differences in acetylene reduction or nodule weight, but bean nodules were smaller and more numerous and evolved more hydrogen (Relative energetic efficiency was 0.5 to 0.7 in bean, 0.95 in cowpea and soybean). Relative N accumulation was influenced by N accumulation characteristics of the fertilized plants as well as the symbiotic plants. The vegetative N-fixation period of early maturing beans was shorter than for cowpeas of similar maturation date; the beans flowered earlier and had a longer pod-filling period. There was no evidence that the common bean symbiosis was more sensitive than the others to competition from native rhizobia. With mixed populations of effective rhizobia, hydrogen evolution in otherPhaseolus species (P. acutifolius, P. coccineus, P. filiformis, P. lunatus) was similar to that inP. vulgaris and higher than in cowpea or soybean. Although failure to establish effective nodulation is often considered the reason for poor N-fixation by common bean in the field, the species may be genetically predisposed to poor fixation because of symbiotic inefficiency and the short vegetative fixation period.