Response of Douglas fir seedlings to nitrate and ammonium nitrogen sources at different levels of pH and iron supply

Douglas fir seedlings were grown for two to three months in sand and soil cultures in a greenhouse to examine their growth response to nitrogen (N) source at different levels of pH and iron (Fe) supply. In the first two experiments nutrient solutions of known pH were automatically applied to the top of the sand cultures and allowed to run to waste from the bottom. Under these conditions seedlings made most growth on nitrate (NO₃–N) under acid (pH4) conditions, but most growth on ammonium (NH₄–N) under neutral (pH7) conditions. Calcium carbonate (CaCO₃) was used to create a range of pH conditions (from 4.0 to 7.2) in a peat and sand artificial soil. Over the pH range 4 to 6 NH₄–N or NO₃+NH₄–N produced larger seedlings than NO₃–N alone, but above pH6 growth on all N sources was depressed. Chemical analysis showed that seedling Ca concentration had increased and Fe concentration had decreased with increase in CaCO₃ application. Both Ca and Fe concentrations were higher in seedlings receiving NO₃–N than in those receiving NH₄ or NO₃+NH₄.In sub-irrigated sand cultures, Doughlas fir seedlings receiving NO₃–N were shown to respond to additions of Fe chelate, but seedlings receiving NH₄–N responded little to Fe chelate. At pH5 seedlings receiving NO₃–N did not grow as big as seedlings receiving NH₄–N in the absence of Fe chelate, but addition of Fe chelate resulted in NO₃-fed seedlings growing larger than NH₄-fed seedlings. The relationship between seedling Fe concentration and N nutrition is discussed.The relatively larger root dry weight and surface area of seedlings grown on NO₃–N, as compared to NH₄–N, in sand culture, was noted.     

Epidemiology of wheat take-all as influenced by soil pH and temporal changes in inorganic soil N

Summary Soil pH, NH ₄ ⁺ and NO ₃ ⁻ concentrations in soil, and take-all root rot of winter wheat grown in the field were measured concurrently from sowing to anthesis in order to relate disease development to liming and N fertilization practices. Experimental variables included soil pH (5.5 and 6.0) and three N sources (NH₄NO₃, (NH₄)₂SO₄, NH₄Cl) banded with the seed at sowing in factorial combination with the same three N sources topdressed in the spring. Take-all severity was increased by increasing soil pH and by fertilization with NO ₃ ⁻ . Disease severity on crown roots increased exponentially following spring N fertilization and was affected more by soil pH and N-form than was severity on seminal roots. Grain yield ranged from 4.70 Mgha⁻¹ with spring NH₄NO₃ at soil pH 6.0 to 7.65 Mgha⁻¹ with spring NH₄Cl at soil pH 5.5. Sixty-six percent of the variability in grain yield was explained by the number of take-all infected crown roots per tiller at anthesis. Oregon Agric. Exp. Stn. technical paper no. 7707.     

Relationship between intracellular pH and N metabolism in maize (Zea mays L.) roots

In vivo ³¹P nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO₃ vs. NH₄) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0–5 mm) and root segments (5–30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo ¹⁵N NMR was used to characterize the pathway of primary ammonium assimilation in the ammonium-grown roots and to compare the activity of the apical and more-basal root parts. The kinetics of ¹⁵NH₄ ⁺ incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammonium-treated root tips.     

The significance of Cognettia sphagnetorum(Enchytraeidae) on nitrogen availability and plant growth in wood ash-treated humus soil

A laboratory microcosm experiment was established to study whether the role of Cognettia sphagnetorum (Enchytraeidae) in affecting Scots pine (Pinus sylvestris) seedling growth is influenced by wood ash-amendment, i.e., neutralisation of the raw humus soil. Coniferous forest soil, inoculated with soil microbes and nematodes, was either treated with wood ash or left as ash-free control. Wood ash (corresponding to an amount of 5000 kg ha^–1) was either spread on the soil surface or mixed into the soil. Enchytraeid and pine seedling biomass, abundance of nematodes, and water leachable NH₄ ⁺-N and NO₃ ^–-N were measured 26 and 51 weeks after initiation of the experiment and root length and N concentration of needles were measured 51 weeks after initiation of the experiment. Wood ash when mixed into the soil, reduced the biomass of C. sphagnetorum. Nematodes were unaffected by the treatments. In the ash-free soils C. sphagnetorum had little influence on pine growth, but it did decrease root length and root to shoot ratio. In the absence of enchytraeids wood ash decreased pine biomass production and root length. However, the presence of enchytraeids in the ash-treated soil compensated the ash-induced negative effects on the pine performance. Enchytraeids increased and wood ash decreased water leachable NH₄ ⁺-N in the presence but not in the absence of enchytraeids, while water leachable NO₃ ^–-N was not affected by the treatments. It was concluded that C. sphagnetorum can be important in ensuring nutrient cycling and plant growth in situations when an ecosystem encounters disturbances.     

Growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] as affected by nitrogen source

A study was conducted to elucidate the effect of N form, either NH₄ ⁺ or NO₃ ^–, on growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] grown under 10 mM or 100 mM NaCl in hydroponics. Shoot biomass was not affected by N form, whereas NH₄ ⁺ compared to NO₃ ^– nutrition caused an almost 4-fold reduction in the root biomass at both salinity levels. Under NH₄ ⁺ nutrition, salinity had no effect on the biomass yield, whereas under NO₃ ^– nutrition, increasing salinity from 10 mM to 100 mM caused 23% and 36% reduction in the root and shoot biomass, respectively. The reduced root growth under NH₄ ⁺ nutrition was not attributable to impaired shoot to root C allocation since N form did not affect the overall root sugar concentration and the starch concentration was even higher under NH₄ ⁺ compared to NO₃ ^– nutrition. The low NH₄ ⁺ (2 mM) and generally higher amino-N concentrations in NH₄ ⁺- compared to NO₃ ^–-fed plants indicated that the grass was able to effectively detoxify NH₄ ⁺. Salinity had no effect on Ca²⁺ and Mg²⁺ levels, whereas their concentration in shoots was lower under NH₄ ⁺ compared to NO₃ ^– nutrition (over 66% reduction in Ca²⁺; over 20% reduction in Mg²⁺), but without showing deficiency symptoms. Ammonium compared to NO₃ ^– nutrition did not inhibit K⁺ uptake, and the K⁺-Na⁺ selectivity either remained unaffected or it was higher under NH₄ ⁺ than under NO₃ ^– nutrition. Results suggested that while NH₄ ⁺ versus NO₃ ^– nutrition substantially reduced root growth, and also strongly modified anion concentrations and to a minor extent concentrations of divalent cations in shoots, it did not influence salt tolerance of kallar grass.     

Foliar N application reduces soil NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack>-N leaching loss in apple orchards

A comparison of the effects of foliar and soil N application was made in field-grown mature fruiting Gala/M9 apple trees (Malus domestica Borkh) in 2001 and 2002 growing seasons under Pacific Northwest growing conditions in southern British Columbia, Canada. The trees, six years old at the start of the experiment, were treated: (1) with 5 g/l urea sprays supplied every two weeks (7 times) from mid May to mid August (total about 50 g N/tree/year), (2) with the same amount of N applied to the soil with the same timing and quantity as for the foliar treatment, and (3) with no N (control). Leaf color (as SPAD readings) and N concentrations (mg/g), and soil NH₄⁺-N and NO₃^–-N were measured periodically throughout the two seasons. Leached NO₃^–-N was monitored monthly via an anion exchange probe from June to October in 2001 and from May to November in 2002. Shoot length was measured in October and N concentration of one-year-old wood and roots was determined in December of each growing s eason. Soil N application significantly increased shoot length relative to control or foliar N application. Leaf color, leaf N, and N concentration of one-year-old wood and roots were similarly increased relative to control by both soil and foliar N application. These treatments also increased fruit yield relative to control. There was no significant difference in yield and fruit quality between soil and foliar N applications. Soil N application increased soil NH₄⁺-N and NO₃^–-N content in the root zone, and also increased the NO₃^– leaching loss below the root zone especially late in the growing season. Our results suggested that tree N status and yield and fruit quality could be maintained by multiple urea sprays during the growing season in apple orchards, and foliar N application will reduce the risk of soil NO₃^–-N leaching.     

Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

An experiment was established in 1992 in eastern Ontario, Canada to determine the effects of crop rotation (continuous maize, soybean-maize and alfalfa-maize) and nitrogen (N) amendment [0, 100 and 200 kg N ha^–1 of fertilizer (NH₄NO₃), and 50 and 100 Mg ha^–1 (wet wt.) each of stockpiled and rotted dairy manure] on maize production and soil properties. From 1997 to 1999, an additional study was added to the experiment to investigate treatment effects on the susceptibility of maize hybrids to gibberella ear rot. A moderately resistant and a susceptible hybrid were planted in each plot and inoculated with a macroconidial suspension of Fusarium graminearum by both the silk channel injection and the kernel-wound techniques. At harvest, ears were rated for the severity of disease symptoms and harvested kernels were analyzed for the mycotoxin deoxynivalenol (DON). The greatest number of significant N effects were found in the continuous maize treatments and with the susceptible hybrid. Most N amendments decreased both disease severity and DON accumulation in the susceptible hybrid. The most consistent effect was a decrease in disease severity with 100 kg N ha^–1 fertilizer and an increase in disease severity with the higher rate of 200 kg N ha^–1. This study is the first to report on the effects of soil N amendments on gibberella ear rot susceptibility.     

Effects of N source on proton excretion,ionic balance and growth ofAlnus glutinosa (L.) Gaertner: comparison of N2 fixation with single and mixed sources of NO3 and NH4

Summary Non-nodulatedalnus glutinosa plants were grown for 6 weeks in nutrient solutions using 3 combined-N treatments (NO₃; NO₃/NH₄; and NH₄) at a total N level of 4 meq.l^–1, and growth was ccmpared with nodulated plants at zero N (N₂ fixation). Of the combined-N sources, 100 per cent NH₄ resulted in the highest dry matter yields when the solution pH was adjusted daily atc. 6. The dry matter yield was lowest with NO₃.During the first 3 weeks, the yield of the N₂-fixing plants was as high as that of the NH₄ plants, but fell relatively behind during the second 3-week period. These effects could be attributed to higher initial N contents and higher shoot:root ratios, respectively, in the N₂-fixing plants. Specific rates of N acquisition in the root were of a comparable order of magnitude for the combined-N and zero-N treatments.When NO₃ was taken up, it was almost completely reduced in the roots. Regardless of N source there was a large excess of cations (C) relative to inorganic anions (A) in the plants, which was presumed to be balanced by an equivalent amount of organic anions (C-A). The relatively small differences in generation of organic anions for the various modes of N supply indicated the relative importance of the proton pump when NH₄ or N₂ was the N source. Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, were generally in good agreement with measured excretion values. The acidity generation with N₂ fixation amounted toc. 0.5 meq H⁺.mmol^–1 N_org, which was distinctly higher than the range of 0.1–0.2 mentioned by Raven and Smith⁴³ for dinitrogen-fixing plants.Without pH adjustment, specific rates of cation uptake and carboxylate generation were strongly depressed as the acidity increased, when NO₃/NH₄, NH₄ and N₂ were the N sources. Growth ofAlnus glutinosa appeared to be still normal at a pH ofc. 2.8. During the final 3 weeks, only the NH₄ plants ceased growing at a pH of 2.6.     

Clonality in South African isolates and evidence for a European origin of the root pathogen Thielaviopsis basicola

Thielaviopsis basicola is a soil-borne fungal pathogen with a wide host range and a cosmopolitan distribution. It causes disease on many agricultural crops, and in South Africa is the causal agent of black pod rot of groundnuts and black root rot on chicory. Knowledge of the population diversity of T. basicola could provide valuable information regarding management strategies, the possible movement, origin, and reproductive strategies of the fungus. The objective of this study was to determine the population diversity of T. basicola isolates from groundnuts and chicory in South Africa using co-dominant polymorphic markers. These markers were also used to compare isolates from South Africa with those from other hosts and geographic regions. Seven loci revealed nine alleles and two genotypes, one on groundnut and one on chicory, differing at only two loci. T. basicola isolates from eight different countries and ten different hosts revealed 17 genotypes across the seven loci with 39 different alleles. The lack of diversity for the two South African host-related populations of isolates suggests that T. basicola was introduced into South Africa. Some evidence is provided for a European origin of the pathogen, possibly linked to trade in root crops.     

Ammonia-oxidizing bacteria on root biofilms and their possible contribution to N use efficiency of different rice cultivars

Ammonia-oxidizing bacteria (AOB) populations were studied on the root surface of different rice cultivars by PCR coupled with denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). PCR-DGGE of the ammonium monooxygenase gene (amoA) showed a generally greater diversity on root samples compared to rhizosphere and unplanted soil. Sequences affiliated with Nitrosomonas spp. tended to be associated with modern rice hybrid lines. Root-associated AOB observed by FISH were found within a discrete biofilm coating the root surface. Although the total abundance of AOB on root biofilms of different rice cultivars did not differ significantly, there were marked contrasts in their population structure, indicating selection of Nitrosomonas spp. on roots of a hybrid cultivar. Observations by FISH on the total bacterial community also suggested that different rice cultivars support different bacterial populations even under identical environmental conditions. The presence of active AOB in the root environment predicts that a significant proportion of the N taken up by certain rice cultivars is in the form of NO₃ ^–-N produced by the AOB. Measurement of plant growth of hydroponically grown plants showed a stronger response of hybrid cultivars to the co-provision of NH₄ ⁺ and NO₃ ^–. In soil-grown plants, N use efficiency in the hybrid was improved during ammonium fertilization compared to nitrate fertilization. Since ammonium-fertilized plants actually receive a mixture of NH₄ ⁺ and NO₃ ^– with ratios depending on root-associated nitrification activity, these results support the advantage of co-provision of ammonium and nitrate for the hybrid cultivar.     

Leaching of nitrate and ammonium in heathland and forest ecosystems in Northwest Germany under the influence of enhanced nitrogen deposition

To study the impact of high atmospheric nitrogen deposition on the leaching of NO₃^– and NH₄⁺ beneath forest and heathland vegetation, investigations were carried out in adjacent forest and heathland ecosystems in Northwest Germany. The study area is subjected to high deposition of nitrogen ranging from 15.9 kg ha^–1 yr^–1 in bulk precipitation to 65.3 kg ha^–1 yr^–1 beneath a stand of Pinus sylvestris L. with NH₄–N accounting for 70–80% of the nitrogen deposited. Considerable leaching of nitrogen compounds from the upper horizons of the soil, mostly as nitrate, occurred at most of the forest sites and below a mixed stand of Calluna vulgaris (L.) Hull. and Erica tetralix, but was low in a Betula pubescens Ehrh. swamp forest as well as beneath Erica tetralix L. wet heath and heath dominated by Molinia caerulea(L.) Moench. Ground water concentrations of both NO₃–N and NH₄–N did not exceed 1 mg L^–1 at most of the sites investigated.     

Histological, Physiological and Biochemical Interactions between Vesicular-Arbuscular Mycorrhizae and Thielaviopsis basicola in Tobacco Plants

Interactions between the mycorrhizal fungus Glomus monosporum and the root rot pathogen Thielaviopsis basicola and their effects on tobacco plants were investigated over a 4 week period. Mycorrhizal tobacco plants, obtained by preinoculation with G. monosporum, showed a better tolerance to T. basicola than non-mycorrhizal seedlings. Root and leaf dry weights of mycorrhizal plants were greater than those of controls. Mycorrhizal plants inoculated with T. basicola showed higher root and leaf dry weights than non-mycorrhizal infected plants, but lower values than mycorrhizal plants which were not infected. No appreciable differences in free aminoacid composition were observed among the different treatments with two exceptions: proline content was higher in infected and mycorrhizal infected plants compared to control and mycorrhizal plants; arginine content was higher in infected and mycorrhizal infected plants compared to control and mycorrhizal plants; arginine content was higher in mycorrhizal plants than in all the other treatments. The mechanisms by which (VAM) fungi can reduce disease incidence and pathogen development are discussed.     

N2(C2H2−C2H4) fixation in two species of Ceanothus seedlings in second year postfire chaparral

Nitrogen fixation in excised root nodules of 2-year-old, postfireCeanothus tomentosus andC. leucodermis seedlings was measured over an 8-month period using the acetylene reduction method. High levels of NO₃–N and NH₄–N present in postfire soils were limited to the upper 10 cm and did not inhibit nodulation in these deeper-rooting seedlings. Decreases in acetylene reduction activity occurred with decreased soil moisture and increased soil temperature. Nitrogen gains from these two Ceanothus shrub seedlings totalled 1.6 kg N ha^–1 yr^–1.     

Persistence and competitiveness of threeBradyrhizobium japonicum inoculant strains in clay loam Nile valley soil

The nitrogen contribution from the shoot and root system of symbiotically grown leucaena was evaluated in a field experiment on an Alfisol at IITA in Southern Nigeria. Maize in plots that received prunings from inoculated leucaena contained more N and grain yield was increased by 1.9 t.ha.^–1. Large quantities of nitrogen were harvested with leucaena prunings (300 kg N ha^–1 in six months) but the efficiency of utilization of this nitrogen by maize was low compared to inorganic N fertilizer (ammonium sulphate) at 80 kg N ha^–1. Maize yield data indicated that nitrogen in leucaena prunigs was 34 and 45% as efficient as 80 kg N ha^–1 of (NH₄)₂SO₄ for uninoculated and inoculated plants with Rhizobium IRc 1045, respectively. In plots where the prunings were removed, the leaf litter and decaying roots and nodules contributed N equivalent of 32 kg ha^–1. Twenty-five kg ha^–1 was the inorganic N equivalent from nitrogen fixed symbiotically by leucaena when inoculated with Rhizobium strain IRc 1045. Application of prunings from inoculated leucaena resulted in higher soil ogranic C, total N, pH and available NO₃.     

Comparison of soil nitrogen mineralization and nitrification in a mixed grassland and forested ecosystem in central Taiwan

We used the buried bag incubation method to study temporal patterns of net N mineralization and net nitrification in soils at Ta-Ta-Chia forest in central Taiwan. The site included a grassland zone, (dominant vegetation consists of Yushania niitakayamensis and Miscanthus transmorrisonensis Hayata) and a forest zone (Tsuga chinensis var. formosana and Yushania niitakamensis). In the grassland, soil concentration NH₄ ⁺ in the organic horizon (0.1–0.2 m) ranged from 1.0 to 12.4 mg N kg^–1 soil and that of NO₃ ^– varied from 0.2 to 2.1 mg N kg^–1 soil. In the forest zone, NH₄ ⁺ concentration was between 2.8 and 25.0 mg N kg^–1 soil and NO₃ ^–varied from 0.2 to 1.3 mg N kg^–1 soil. There were lower soil NH₄ ⁺ concentrations during the summer than other seasons. Net N mineralization was higher during the summer while net nitrification rates did not show a distinct seasonal pattern. In the grassland, net N mineralization and net nitrification rates were between –0.1 and 0.24 and from –0.04 to 0.04 mg N kg^–1 soil day^–1, respectively. In the forest zone, net N mineralization rates were between –0.03 and 0.45 mg N kg^–1 soil day^–1 and net nitrification rates were between –0.01 and 0.03 mg N kg^–1 soil day^–1. These differences likely result from differing vegetation communities (C₃ versus C₄ plant type) and soil characteristics.     

Al avoidance and Al tolerance ofMucuna pruriens var.utilis: Effects of a heterogeneous root environment and the nitrogen form in the root environment

InMucuna pruriens var.utilis, grown with nitrate-N in a hydroponic split-root system, an Al avoidance reaction of root growth was observed, which was ascribed to local P stress in the Al containing compartment. The Al avoidance reaction was similar to the avoidance ofMucuna roots of acid subsoil in the field where roots grew preferentially in the topsoil. In the present paper the effect of different N forms (NO₃ ^– and NH₄ ⁺) on the reactions ofMucuna to Al were studied, since in acid soils N is present as a mixture of NO₃ ^– and NH₄ ⁺. No interaction between the N form and Al toxicity was found. A hydroponic split-root experiment with NH₄NO₃ nutrition, which is comparable to the situation in the field, showed that under these conditions Al avoidance did not occur. It is concluded that a relation between the Al avoidance reaction ofMucuna and P stress is still likely.Abbreviations D_r root diameter - L_pr total root length per plant - L_rw specific root length - NRA nitrate reductase activity - S/R shoot: root ratio     

The fate of carbon and fertiliser nitrogen when dryland wheat is grown in monoliths of duplex soil

Triticum aestivum L. (cv. Gutha), a short-season wheat, was grown to maturity in large monoliths of duplex soil (sand over sandy-clay) in a daylight phytotron mimicking field conditions. Either ¹⁵N-labelled ammonium sulphate ((NH₄)₂SO₄) or urea was banded into the soil at a rate of 30 kg N ha^–1: even though roots were about 20% heavier when grown in the presence of (NH₄)₂SO₄ for 86 d (P<0.05), above-ground mass was not affected by the source of nitrogen. At four times through crop development up to grain-filling (50, 56, 70 and 86 d after sowing) shoots were labelled heavily with ¹⁴CO₂ with two purposes. First, to trace `instantaneous' assimilate movement over 24 h, revealing relative sink strengths throughout plants. This, in turn, allowed precise measurements of live root mass and the proportion of recent photoassimilates deposited in the rhizosphere. Although root systems were sparse, even in surface soil layers, they were strong sinks for photoassimilates early in development (0–50 d), supporting the conversion of inorganic applied nitrogen (N) to soil organic forms. In the presence of roots, up to 28% of ¹⁵N was immobilised, whereas only 12% of labelled ammonium sulphate was immobilised in unplanted plots in spite of a favourable moisture status in both treatments. The effect of plants on rates of ¹⁵N transformation is ascribed to recently imported photoassimilates sustaining rhizosphere metabolism. Not more than 15% of recently fixed carbon imported by roots was recovered from the rhizoplane, suggesting that a highly localised microbial biomass supported vigorous immobilisation of soil N. Thus, more than twice as much applied N was destined for soil organic fractions as for root material. By these processes, root- and soil-immobilised N become substantial stores of applied N and together with shoot N accounted for all the applied N under dryland conditions.     

Plant‐extract‐induced changes in the proteome of the soil‐borne pathogenic fungus Thielaviopsis basicola

Thielaviopsis basicola is a hemibiotroph fungus that causes black root rot disease in diverse plants with significant impact on cotton production in Australia. To elucidate how T. basicola growth and proteome are influenced by interactions with natural sources, this fungus was cultured in the presence of root extracts from non‐host (wheat, hairy vetch) and susceptible host (cotton, lupin) plants. We found that T. basicola growth was significantly favored in the presence of host extracts, while hierarchical clustering analysis of 2‐DE protein profiles of T. basicola showed plant species had a larger effect on the proteome than host/non‐host status. Analysis by LC‐MS/MS of unique and differentially expressed spots and identification using cross‐species similarity searching and de novo sequencing allowed successful identification of 41 spots. These proteins were principally involved in primary metabolism with smaller numbers implicated in other diverse functions. Identification of several “morpho” proteins suggested morphological differences that were further microscopically investigated. Identification of several highly expressed spots suggested that vitamin B₆ is important in the T. basicola response to components present in hairy vetch extract, and finally, three spots, induced in the presence of lupin extract, may correspond to malic enzyme and be involved in lipid accumulation.     

Cyanide production by Pseudomonas fluorescens helps suppress black root rot of tobacco under gnotobiotic conditions

Pseudomonas fluorescens CHA0 suppresses black root rot of tobacco, a disease caused by the fungus Thielaviopsis basicola. Strain CHA0 excretes several metabolites with antifungal properties. The importance of one such metabolite, hydrogen cyanide, was tested in a gnotobiotic system containing an artificial, iron-rich soil. A cyanidenegative (hcn) mutant, CHA5, constructed by a gene replacement technique, protected the tobacco plant less effectively than did the wild-type CHA0. Complementation of strain CHA5 by the cloned wild-type hcn⁺ genes restored the strain's ability to suppress disease. An artificial transposon carrying the hcn⁺ genes of strain CHA0 (Tnhcn) was constructed and inserted into the genome of another P.fluorescens strain, P3, which naturally does not produce cyanide and gives poor plant protection. The P3::Tnhcn derivative synthesized cyanide and exhibited an improved ability to suppress disease. All bacterial strains colonized the roots similarly and did not influence significantly the survival of T.basicola in soil. We conclude that bacterial cyanide is an important but not the only factor involved in suppression of black root rot.     

Nodulation potential of soils from red alder stands covering a wide age range

Red alder (Alnus rubra Bong.) stands in the Pacific Northwest are the common first stage in succession following disturbance. These stands are highly productive and contribute a large amount of N to the soils as a result of their N₂-fixing symbiosis with Frankia. As these alder stands age, the soils not only increase in total N, but concentrations of NO^– ₃ increase and pH decreases as a result of nitrification. The objective of this study was to determine how the nodulation capacity of Frankia varies as red alder stands age and if differences in nodulation capacity are related to changes in soil properties. Nodulation capacity was determined by a red alder seedling bioassay for soils from red alder stands in the Oregon coast range covering a wide range of ages. Six chronosequences were sampled, each containing a young, an intermediate, and an older alder stand. Soil total N, total C, NO^– ₃, NH⁺ ₄, and pH were measured on the same soil samples. These factors as well as alder stand characteristics were compared with nodulation capacity in an attempt to identify soil characteristics typical in developing alder stands that most strongly affect nodulation capacity. Soil pH and NO^– ₃ concentration were highly correlated with nodulation capacity and with each other. Cluster analysis of the sites using these two variables identified two groups with distinctly different nodulation capacities. The cluster with the higher nodulation capacity was lower in NO^– ₃ and higher in pH than the other cluster, which included the majority of sites. There was substantial overlap in the age ranges for the two clusters and there was no significant correlation between age and nodulation capacity. Thus nodulation capacity appears to be most closely related to soil properties than to stand age.