Promotion of mycorrhiza formation and growth of willows by the bacterial strain <Emphasis Type="Italic">Sphingomonas</Emphasis> sp. 23L on fly ash

Re-vegetation of fly ash, the principal by-product of coal fired power stations, is hampered by its unfavourable chemical and physical properties for plant growth. In the present study, we evaluated the use of inoculation with a mycorrhiza-associated bacterial strain (Sphingomonas sp. 23L) to promote mycorrhiza formation and plant growth of three willow clones (Salix spp.) on fly ash from an over-burdened dump in a pot experiment. The high pH_H2O (8.7) and low nitrogen content (N_t = 0.1 g kg⁻¹) in combination with hydrophobicity of the particle surfaces caused low plant growth. Inoculation of the willows with Sphingomonas sp. 23L improved the nitrogen uptake by plants, increased plant growth and stimulated formation of ectomycorrhizae with an autochthonous Geopora sp. strain on all three willow clones. The ectomycorrhiza formed by the Geopora sp. was morphologically and anatomically described. The inoculation significantly increased the shoot growth of two Salix viminalis clones and the root growth of a S. viminalis x caprea hybrid clone. We conclude that inoculation with mycorrhiza promoting bacterial strains might be a suitable approach to support mycorrhiza formation with autochtonous site-adapted ectomycorrhizal fungi in fly ash and thereby to improve re-vegetation of fly ash landfills with willows.     

Inoculation of rice plants with the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp.

 Four experiments were performed under gnotobiotic conditions to select strains of the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp. as inocula of rice plants. Eighty strains of H. seropedicae originally isolated from rice, sorghum and maize plants, were tested in test tube cultures with N-free agar as the substrate. Rice plants showed medium and high increases in their fresh weight in response to inoculation with nineteen strains. These strains were tested again, and six strains were then selected to evaluate their contribution to the N of the plant via biological N₂ fixation (BNF) using an agar growth medium containing 5 mg N l^–1of ¹⁵N-labelled (NH₄)₂SO₄. The contribution of the strains to plant N via BNF varied from 54% when rice plants were inoculated with strain ZAE94, to 31% when strain ZAE67 was used. These results were confirmed in the fourth gnotobiotic experiment, which also included strains of the new N-fixing bacteria belonging to the genus Burkholderia, isolated from rice, as well as a strain of Burkholderia vietnamiensis, isolated from rice rhizosphere. Burkholderia spp. strains showed similar effects to those observed for H. seropedicae strains, while B. vietnamiensis fixed only 19% of plant total N. The best four strains were tested in a pot experiment where pre-germinated, inoculated rice seedlings were grown in soil labelled with ¹⁵N. The results confirmed the gnotobiotic experiments, although the levels of N in the rice plants derived from BNF of the selected H. seropedicae and Burkholderia spp. strains were lower. Nevertheless, there was an increase in N content in grains of inoculated plants, and the results showed that the method used for strain selection is very useful and can be applied to other strains of N₂-fixing bacteria and plants. Received: 4 May 1999     

Effects of cow manure and sewage sludge on the activity and kinetics of <Emphasis Type="SmallCaps">l</Emphasis>-glutaminase in soil

A study was conducted to investigate the effects of cow manure and sewage sludge application on the activity and kinetics of soil l-glutaminase. Soil samples were collected from a farm experiment in which 0, 25, and 100 Mg ha⁻¹ of either cow manure or sewage sludge had been applied annually for 4 consecutive years to a clay loam soil (Typic Haplargid). A chemical fertilizer treatment had also been applied. Results indicated that the effects of chemical fertilizer and the solid waste application on pH in the 18 surface soil (0–15 cm) samples were not significant. The organic C content, however, was affected significantly by the different treatments, being the greatest in soils treated with 100 Mg ha⁻¹ cow manure, and the least in the control treatment. l-Glutaminase activity was generally greater in solid-waste applied soils and was significantly correlated (r = 0.939, P < 0.001) with organic C content of soils. The values of l-glutaminase maximum velocity (Vmax) ranged from 331 to 1,389 mg NH₄ ⁺–N kg⁻¹ 2 h⁻¹. Values of the Michaelis constant (K _m) ranged from 35.1 to 71.7 mM. Organic C content of the soils were significantly correlated with V _max (r = 0.919, P < 0.001) and K _m (r = 0.763, P < 0.001) values. These results demonstrate the considerable influence that solid waste application has on this enzymatic reaction involved in N mineralization in soil.     

Effect of mineral fertilizer,pig manure,and <Emphasis Type="Italic">Azospirillum rugosum</Emphasis> on growth and nutrient contents of <Emphasis Type="Italic">Lactuca sativa</Emphasis> L.

Benefits from the application of plant growth-promoting bacteria in agriculture largely depend on the complex interactions between several factors including the nature of fertilizers selected. This study was designed to determine the fine tuning between the inoculated bacteria and different fertilizers and their effect on the growth of lettuce plants (Lactuca sativa L.). Plant growth promotion by a novel species of the genus Azospirillum, namely A. rugosum IMMIB AFH-6, was tested by biochemical, bioassay, and greenhouse studies. The treatments used in the greenhouse study were; unfertilized control (Blank), half recommended dose of chemical fertilizer (1/2CF), full recommended dose of chemical fertilizer (1CF), pig manure fertilizer (PMF), pig manure fertilizer + half recommended dose of chemical fertilizer (PMF + 1/2CF), and pig manure fertilizer + full recommended dose of chemical fertilizer (PMF + 1CF). All these treatments when inoculated with A. rugosum IMMIB AFH-6 inoculation were, respectively, In-Blank, In-1/2CF, In-1CF, In-PMF, In-PMF + 1/2CF, and In-PMF + 1CF. Significant increase in plant biomass and shoot N, P, Ca, and Fe was shown in the In-Blank treatment. Plant growth in soil amended with PMF and A. rugosum IMMIB AFH-6 was significantly lower than in soil treated with the chemical fertilizer, but inoculation combined with chemical fertilizer significantly elevated the plant biomass. The In-PMF + 1/2CF treatment showed the highest yield. A. rugosum IMMIB AFH-6 facilitated the accumulation of trace minerals in higher concentrations when PMF was combined with 1CF. To examine the benefits of inoculation by A. rugosum IMMIB AFH-6, we have proposed a new type of data analysis which considers both biomass and nutrient content of plants. This new type of analysis has shown the importance of the mineral content of plant.     

Pre-inoculation with arbuscular mycorrhizal fungi suppresses root knot nematode (<Emphasis Type="Italic">Meloidogyne incognita</Emphasis>) on cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis>)

A pot experiment was conducted to evaluate the influence of pre-inoculation of cucumber plants with each of the three arbuscular mycorrhizal (AM) fungi Glomus intraradices, Glomus mosseae, and Glomus versiforme on reproduction of the root knot nematode Meloidogyne incognita. All three AM fungi tested significantly reduced the root galling index, which is the percentage of total roots forming galls. Numbers of galls per root system were significantly reduced only in the G. intraradices + M. incognita treatment. The number of eggs per root system was significantly decreased by AM fungus inoculation, no significant difference among the three AM fungal isolates. AM inoculation substantially decreased the number of females, the number of eggs g⁻¹ root and of the number of eggs per egg mass. The number of egg masses g⁻¹ root was greatly reduced by inoculation with G. mosseae or G. versiforme. By considering plant growth, nutrient uptake, and the suppression of M. incognita together, G. mosseae and G. versiforme were more effective than G. intraradices.     

Response of maize to mycorrhizal colonization at varying levels of zinc and phosphorus

A greenhouse experiment was conducted in a red sandy loam soil (Alfisol) to study the responses of arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith inoculated (M+) and uninoculated (M−) maize (Zea mays L) plants exposed to various levels of P (15 and 30 mg kg⁻¹) and Zn (0, 1.25, and 2.5 mg kg⁻¹). Roots and shoots were sampled at 55 and 75 days after sowing and assessed for their nutritional status, root morphology, and root cation exchange capacity (CEC) besides grain quality. Mycorrhizal plants had longer and more extensive root systems than nonmycorrhizal plants, indicating that M+ plants are nutritionally rich, especially with P, which directly assisted in the proliferation of roots. Further, root CEC of M+ plants were consistently higher than those of M− plants, suggesting that mycorrhizal colonization assists in the acquisition of nutrients from soil solution. Mycorrhizal inoculated plants had significantly (P ≤ 0.01) higher P and Zn concentrations in roots, shoots, and grains, regardless of P or Zn levels. The available Zn and P status of AM fungus-inoculated soils were higher than unioculated soils. The data suggest that mycorrhizal symbiosis improves root morphology and CEC and nutritional status of maize plants by orchestrating the synergistic interaction between Zn and P besides enhancing soil available nutrient status that enables the host plant to sustain zinc-deficient conditions.     

Solubilization of inorganic phosphates and plant growth promotion by <Emphasis Type="Italic">Aspergillus niger</Emphasis>

Two of 187 fungal isolates (Aspergillus niger 1B and 6A) displaying superior phosphate (P) solubilization and hydrolytic enzyme secretion were studied using P forms of calcium (Ca-P), iron (Fe-P), and aluminum (Al-P). Phosphate solubilization in a sucrose-basal salt (SB) broth was increased and pH decreased by both isolates. In Ca-P medium, solubilization for 6A was approximately 322 μg P mL⁻¹ and pH decreased by 4.2 units to 2.3 in 72 h. However, when pH value of the SB broth was lowered to 2.5 using HCl, 65.3  ±  0.4 μg mL⁻¹ of P was released from Ca-P, whereas trace amounts of P were released from Fe-P and Al-P. Both isolates displayed enhanced Al-P solubilization using NH₄Cl rather than KNO₃ as the N source; final pH values were not significantly different. With Ca-P, gluconic acid was predominantly produced by 1B and 6A, whereas oxalic acid predominated with Fe-P and Al-P. Addition of gluconic acid (final concentration of 8.5 μmol mL⁻¹) to Ca-P-supplemented SB lowered pH (2.9) and solubilized phosphate (146.0 ± 1.0 μg mL⁻¹). Similarly, addition of oxalic acid (final concentration 6.6 μmol mL⁻¹) to Ca-P- and Fe-P-amended media solubilized P (60.2 ± 0.9 and 21.6 ± 2.1 μg mL⁻¹, respectively), although these quantities were significantly lower than those detected in unamended SB. The presence of unidentified P solubilized compound(s) in the dialyzed (MW>500) supernatant warrants further study. In pot experiments, significant increases in plant (Brassica chinensis Linn.) dry weight and N and P contents were observed with the addition of isolate 6A, when a small amount of organic fertilizer together with either rock phosphate (South African apatite) or Ca-P served as the main P sources.     

<Emphasis Type="Italic">Erigeron annuus</Emphasis> (L.) Pers., as a green manure for ameliorating soil exposed to acid rain in Southern China

Background, aims, and scope  Increasing soil acidification is a growing concern in southern China. The traditional green manures applied in the fields mostly comprise legumes that tend to accelerate soil acidification. Moreover, acid deposition can act as a source of nitrogen. Hence, we looked for new plant species that would enhance nutrient concentrations when used as green manure and would reduce soil acidity or at least not worsen it. Materials and methods  We studied the use of Erigeron annuus (L.) Pers. for ameliorating acid soil in a pot experiment with simulated acid rain (SAR) treatments (pH 5.8 to 3.0) in an open area in Guangzhou City. The pots were divided into two groups named A and B groups. On day 0, pots of A group were filled with soil and planted with Erigeron annuus seedlings. Pots of B group were only filled with soil as the control. On day 40, seedlings of E. annuus were harvested and buried in the corresponding pots. On day 54, two seeds of Phaseolus vulgaris L. were sown in each pot in both groups. The growth and bean yield of P. vulgaris seedlings were then used to evaluate the effects of E. annuus on acid soil. Plant and/or soil samples were collected on days 0, 40, 54, and 150; corresponding parameters were measured. Results  Results showed that E. annuus could maintain a good growth even on very acid soil. On day 40, the pH decreased significantly (P < 0.0001) in the B group pots without E. annuus compared with the A group. On day 54, after E. annuus was buried as a manure, the soil pH of all A group treatments except the pH 4.0 treatment showed a significant increase compared to day 40 (P < 0.01). At the same time, the application of E. annuus as a manure produced a significant increase of soil K and P (P < 0.001), Ca (P < 0.05), and Mg (P < 0.001) concentrations of all A group SAR treatments compared to their B group counterparts (except control pots for Ca). The soil exchangeable K and available P concentration doubled, and Ca and Mg increased by around 25% in the presence of the E. annuus manure application. Discussion  The higher soil pH in the A group than B group on the day 40 was due to a great absorption of NO₃ ⁻ by the roots of E. annuus. The soil pH increase after E. annuus was applied to the soil of A group was attributed to the release of high amount of K, the mineralization of organic N, and the oxidation of organic acid anions. Nutrient increase in the A group after E. annuus application was mostly the result of the nutrient release during the residue decomposition. The amelioration of the soil was effective as demonstrated by the enhanced growth and bean yield of P. vulgaris seedlings on the manured soil compared to the seedlings grown on a control that was not manured. Conclusions   E. annuus could maintain a good growth in the acid lateritic field soil. Cultivating this plant and applying it to the soil with a rate of 1.6 ton ha⁻¹ doubled the soil K and P concentrations and increased soil exchangeable Ca and Mg concentrations by around 25%. This species would be a good green manure candidate for growing in the acid soils of southern China. Application of E. annuus also has beneficial effects on crop growth through reduced Al toxicity and cation leaching. Recommendations and perspectives  Since E. annuus would improve soil pH and nutrient concentrations with minimum care, it is recommended for treating acid soils with poor yield whenever a low-cost solution is required.     

Sources of Resistance to Whitefly (Bemisia spp.) in Wild Populations of Solanum lycopersicum var. Cerasiforme (Dunal) Spooner G.J. Anderson et R.K. Jansen in Northwestern Mexico

Genetic variability of wild populations, closely related to domesticated species, constitute important genetic resources for plant breeding programs. In this paper, we analysed the variation of eight wild populations of Solanum lycopersicum var. cerasiforme in a common garden experiment for levels of plant infestation by whitefly, leaf trichome density as a defensive character preventing infestation by whitefly, and the effect of whitefly incidence into vegetative and reproductive plant characters. Number of adults of whitefly was recorded in the eight wild populations of S. lycopersicum var. cerasiforme, one population of the wild species S. habrochaites (C-360), and one of a cultivated variety of S. lycopersicum (Rio Grande). There were significant differences among the wild populations of S. lycopersicum var. cerasiforme in the average level of whitefly incidence and trichome density. Cultivated tomatoes had the higher incidence of whiteflies ( = 7.50 ± 0.14) followed by plants of S. lycopersicum var. cerasiforme ( = 2.02 ± 0.92) and plants of S. habrochaites with the lowest incidence ( = 0.36 ± 0.35). Whitefly incidence was negatively correlated with trichome density (r = − 0.38, p < 0.0001), suggesting that trichomes deter or limit the establishment of whiteflies. Additionally, a significant negative effect of whitefly incidence along the growing season upon plant growth rate (number of branches and height) and fruit production was detected.     

Upland rice seedling wilt and microbial biomass and enzyme activities of compost-treated soils

Rice seedling wilt frequently occurs in upland nurseries under well-aerated conditions and causes considerable economic loss. Whether the wilt is pathogenic or edaphic is not known. We hypothesize the use of composts to alleviate seedling wilt. The severity level of upland rice seedling wilt was significantly (p < 0.05) positively correlated with soil pH (r = 0.499; n = 19), but negatively correlated with soil organic matter (r = −0.745), microbial biomass C (r = −0.669), activities of dehydrogenase (r = −0.589), arylsulfatase (r = −0.272), fluorescein diacetate hydrolysis (r = −0.466), and β-glucosidase (r = −0.280). Correlations between severity level and soil inorganic N and exchangeable potassium K were not significant. Contents of Fe, Zn, Cu, and Mn in healthy seedlings were not significantly (p < 0.05) different from those in infected seedlings. These data suggest that seedling wilts are not associated with nutrient constraints. Compost amendment at the rate of 3% or above in pot experiments significantly improved seedling growth and reduced the wilt symptoms. Field trials further showed that aboveground weight of seedlings in compost-amended treatment ranged from 11.5 to 14.9 mg per plant, significantly higher than the range from 6.38 to 12.1 mg per plant in the control treatment; in addition to rice growth compost significantly increased microbial biomass and enzyme activities of soils. Soil fumigation significantly increased rice growth and alleviation symptoms in 11 out of 19 soils, suggesting the involvement of pathogens. It is concluded that upland seedling wilt is a pathogen-associated disease. Probably high soil pH and low soil biochemical activities may favor pathogen activities.     

Life-cycle traits of the dimorphic earthworm species <Emphasis Type="Italic">Allolobophora chlorotica</Emphasis> (Savigny, 1826) under controlled laboratory conditions

Allolobophora chlorotica exists as two colour morphs, pink and green. Field observations have indicated that the two morphs have ecological preferences linked to soil moisture: the green morph dominating in wet soils and the pink morph in dry soils. The aim of this laboratory-based research was to investigate the potential differences in fitness and adaptation to soil-moisture conditions of the two morphs measured in terms of growth rate, reproductive output and cocoon viability. An initial experiment maintained hatchlings of both morphs individually under standard culture conditions. On maturation, these were paired (intra-morph), and cocoon production, viability and incubation time were determined. The green morph had significantly faster (P < 0.01) growth rates than the pink morph. Cocoon production was also significantly greater in the green compared with the pink morph (3.2 and 1.5 cocoons worm⁻¹ 28 days⁻¹, respectively) with corresponding viabilities of 87 and 58%. In a second experiment, the growth rates of pink and green hatchlings were assessed under wet and dry soils (29 and 21% soil moisture, respectively). The growth of the pink morph was not influenced significantly (P > 0.05) by soil moisture. In contrast, lower soil-moisture content significantly (P < 0.05) slowed growth and maturation of the green morph. These results support field observations relating to distribution of the two A. chlorotica colour morphs. We suggest that soil-moisture content may act to isolate these morphs, providing, in extremes, a barrier to inter-morphic mating.     

Infectivity and acetylene reduction of diazotrophic rhizosphere bacteria in wheat (Triticum aestivum) seedlings under gnotobiotic conditions

Summary Wheat seedlings were inoculated with rhizosphere nitrogen-fixing bacteria and grown gnotobiotically for 15 days. The growth medium consisted of semisolid agar with or without plant nutrients. The bacteria, isolated from roots of field-grown wheat, were three unidentified Gram-negative rods (A1, A2, E1), one Enterobacter agglomerans (C1) and two Bacillus polymyxa (B1, B2). A strain of Azospirillum brasilense (USA 10) was included for comparison.Nitrogenase activity (acetylene reduction activity, ARA) was tested on intact plants after 8 and 15 days of growth. In semisolid agar without plant nutrients, five isolates showed ARA of 0.01–0.9 nmol C₂H₄ plant^–1 h^–1, while the two strains of B. polymyxa had higher ARA of 3.3–10.6 nmol C₂H₄ plant^–1 h^–1.Plant development was not affected by inoculation with bacteria, except that inoculation with B. polymyxa resulted in shorter shoots and lower root weight.Transmission electronmicroscopy of roots revealed different degrees of infection. A. brasilense, A1 and A2, occurred mainly in the mucilage on the root surface and between outer epidermal cells (low infectivity). B. polymyxa strains and E1 were found in and between epidermal cells (intermediate infectivity) while E. agglomerans invaded the cortex and was occasionally found within the stele (high infectivity).     

Phosphate solubilization potentials of soil Acinetobacter strains

Many phosphate solubilizing microorganisms (PSM) require external pyrroloquinoline quinone (PQQ) for strong phosphorus (P) solubilization in vitro. The objective of this study was to isolate efficient and PQQ-independent PSM. A total of 21 PSM were isolated from the rhizosphere soil of wheat and maize grown in the pots. Acinetobacter strains were the only PQQ-independent and most effective solubilizers of tricalcium phosphate containing agar. The mean P dissolved in liquid cultures of Acinetobacter strains in a 5-day incubation ranged from 167 to 888 μg/ml P. The pH dropped to below 4.7 from 7.8 in six isolates, which produced gluconic acid in concentrations ranging between 27.5 and 37.5 mM. There was a linear regression between soluble P and gluconic acid concentrations in the bacterial cultures (P < 0.05; R ² = 0.59). Inoculation with Acinetobacter sp. WR922 significantly (P < 0.05) increased wheat (Triticum aestivum L.) P content by 27% at 15 days after emergence (DAE) and dry matter by 15% at 30 DAE compared to the control. The plant P content in inoculated plants at 30 DAE was linearly correlated with soluble P of the bacterial cultures (P < 0.05; R ² = 0.69). Gluconic acid production directly affected phosphate solubilization in vitro, which in turn influenced plant P content of inoculated plants in PQQ-independent P-solubilizing Acinetobacter strains.     

Earthworm (<Emphasis Type="Italic">Metaphire guillelmi</Emphasis>) effects on rice photosynthates distribution in the plant–soil system

An experiment was conducted to investigate the effects of earthworm (Metaphire guillelmi) activities on rice photosynthates distribution in plant–soil system through ¹⁴C pulse-labelling method. Rice was planted in pots, and maize straw was mulched on the surface with or without earthworms. Rice plants at tillering stage or heading stage were labelled with ¹⁴CO₂. Plant and soil were sampled 15 days after labelling at the tillering or heading stage and at harvest. Rice growth was inhibited by earthworms (M. guillelmi) at early stage, but the inhibition disappeared at later stage. Earthworms significantly (P < 0.01) increased the ¹⁴C percentage in root at day 15 after tillering stage labelling, but the effect disappeared at harvest. Earthworms (M. guillelmi) significantly (P < 0.01) increased the ¹⁴C percentage in root at day 15 after heading stage labelling and increased ¹⁴C percentage in soil at harvest. Earthworms decreased the percentages of total organic ¹⁴C (TO¹⁴C) present as microbial biomass ¹⁴C (MB¹⁴C) and increased the percentages of total organic ¹⁴C present as dissolved organic ¹⁴C (DO¹⁴C) at all sampling times. It is suggested that earthworms might alter the transfer of plant photosynthates from the aboveground to the belowground, and thus, soil active C pool. However, these data should be also confirmed in the field.     

Impact of coastal wetland cultivation on microbial biomass, ammonia-oxidizing bacteria, gross N transformation and N2O and NO potential production

A ¹⁵N dilution experiment was carried out to investigate effects of cultivation on the gross N transformation rate in coastal wetland zone. Microbial community composition was estimated by phospholipid fatty acid (PLFA) analysis and abundance of soil ammonia-oxidizing bacteria (AOB) was quantified by real-time polymerase chain reaction (PCR). Soil salinity decreased significantly, while total N increased after coastal wetland was cultivated. Microbial biomass (total PLFA), bacterial biomass, fungal biomass, and actinomycete biomass of the native coastal wetland soils were significantly (p < 0.05) lower than those of the cultivated soils whereas AOB population size also significantly increased after coastal wetland cultivation. Multiple regression analysis showed that total PLFA biomass and soil total N (TN) explained 97% of the variation of gross N mineralization rate in the studied soils (gross mineralization rate = 0.179 total PLFA biomass + 5.828TN − 2.505, n = 16, p < 0.01). Gross nitrification rate increased by increasing the soil AOB population size and gross mineralization rate (M) (gross nitrification rate = 3.39AOB + 0.18 M − 0.075, R ² = 0.98, n = 16, p < 0.01). Management of salt discharge and mineral N fertilization during the cultivation of wetland soils might have changed composition of soil microflora and AOB population size, thus influencing mineralization and nitrification. Probably, the cultivation of coastal wetland soils increased the risk of N losses from soil through nitrate leaching and gas emission (e.g., N₂O and NO).     

Determination of competitive abilities of Bradyrhizobium japonicum strains in soils from soybean production regions in South Africa

Bradyrhizobium japonicum strain CB 1809 was recently chosen to replace strain WB 1 in commercial soybean [Glycine max (L.) Merr.] inoculants in South Africa, the selection criterion being N₂-fixing effectiveness. Nodulation competitiveness is an additional characteristic required of inoculants and was determined for CB 1809 and WB 1 as well as two other strains, USDA 110 and a Brazilian strain 965, using the gusA marker gene to identify strains. Initial experiments with plants grown in sterile sand showed that the competitive index of strain WB 1 was less than that of the other strains. Further comparisons used plants grown in five soils containing established populations of B. japonicum. When strains were applied in peat inoculum to seed at a rate of 1,000 cells per seed in a soil containing 300 rhizobia g^–1, significant differences in nodule occupancy were detected and strains ranked in the order 965>CB 1809>USDA 110>WB 1. The remaining four soils each contained about 10⁶ rhizobia g^–1 and 5×10⁶ cells were applied per seed. Nodule occupancy by inoculant strains ranged from 22% to 81% between soils. In this experiment, WB 1 was consistently the poorest performer and its competitiveness was significantly less than CB 1809. The competition results supported the recent decision to replace WB 1 with CB 1809 in commercial inoculants. Although WB 1 had been used in inoculants over a period of 19 years, this strain was detected in only one soil, where it comprised 8% of isolates. In contrast, a substantial proportion (32–78%) of isolates from the soils corresponded serologically to a former inoculant strain WB 66, which had been discontinued in 1966. This illustrates the difficulty of replacing a resident population with an introduced strain. The effect of naturalized populations on the establishment of CB 1809 in South African soils will need monitoring Received: 23 November 1999     

Comparative salinity tolerance of symbiotically dependent and nitrogen-fed pigeonpea (Cajanus cajan) and its wild relative Atylosia platycarpa

Summary Once symbiosis between the pigeonpea cultivar ICPL 227 and the Rhizobium sp. strain IC 3024 is established, it is efficient in fixing N₂ under saline conditions and can support growth comparable to N-fed plants in growth media with up to 6 dS m^-1 salinity. However, the early stages of establishment of the pigeonpea-IC 3024 symbiotic system have proved sensitive to salinity. The present study showed that the number of nodules was markedly reduced at 8 dS m^-1 salinity; however, nodule development and functioning were not affected by salinity in the pigeonpea-IC 3024 symbiosis. The symbiotic system of Atylosia platycarpa and Rhizobium sp. strain IC 3087 was established successfully even at 12 dS m^-1 and supported growth comparable to that of N-fed plants. P levels in leaves were increased under saline conditions in N-fed and N₂-fixing pigeonpea and A. platycarpa. There were no consistent differences in the leaf Na and chloride levels between N-fed and N₂-fixing plants of pigeonpea and A. platycarpa. The present study suggests that the rhizobial symbiosis may not be a necessary factor for initial screening of pigeonpea and related wild species for salinity tolerance.Submitted as JA No. 964 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

转碱蓬SsNHX1基因烟草的耐盐抗旱性研究

烟草是重要的模式植物和经济作物,盐害和干旱两种环境因子对其生长发育、产量和品质都危害很大。为了提高烟草的耐盐抗旱性,本研究利用农杆菌介导的遗传转化法在烟草中过量表达了碱蓬液泡膜Na~+/H~+逆向转运基因SsNHX1,对转基因烟草的耐盐及抗旱性进行表型鉴定和各项生化指标的检测,以期得到耐盐抗旱表性良好的SsNHX1转基因烟草。表型分析发现,SsNHX1基因过表达株系L1和L5的抗盐能力比野生型显著提高,表现为盐胁迫条件下仍能保持旺盛的生长且根系的伸长未受抑制。SsNHX1过表达株系在叶片和根系中积累了更多的Na~+和K~+,同时Na~+含量增长速率较快,而K~+含量降低速率较缓,并可维持较高的叶片相对含水量和叶绿素含量,及较低的丙二醛含量和相对电导率。干旱胁迫发现,过表达株系受干旱胁迫程度更小,并在复水后迅速恢复正常生长。同时,过表达株系的丙二醛含量和相对电导率显著低于野生型,且维持了较高的叶片相对含水量及叶绿素含量。这些结果说明SsNHX1基因在烟草中过量表达后,降低了盐胁迫和干旱胁迫对烟草根系及细胞膜的损伤,并通过调节离子含量、降低细胞的渗透势,维持了叶片较高的相对含水量和叶绿素含量,最终提高了烟草的抗盐和抗旱性。     

The fungicides thiram and captan affect the phenotypic characteristics of Rhizobium leguminosarum strain C1 as determined by FAME and Biolog analyses

 We assessed the effects of the fungicides captan and thiram on the survival and phenotypic characteristics of Rhizobium leguminosarum bv. viceae, strain C1. Fungicide was applied to pea seed at rates of 0.25–2 g a.i. kg^–1 seed, and treated seed was inoculated with strain C1. After 24 h, rhizobia were extracted from treated seed and viable numbers determined by plating onto yeast extract mannitol agar. Phenotypic characteristics were assessed using FAME (fatty acid methyl ester) profiles and Biolog substrate utilization patterns. Captan and thiram significantly reduced the numbers of rhizobia recovered from seed and altered the FAME and Biolog profiles of recovered rhizobia. However, only the highest concentrations of captan affected nodulation and plant growth. Contact with some seed-applied fungicides can significantly alter phenotypic characteristics of rhizobia, but these changes might be offset by the presence of host plants. Received: 20 February 1999     

Infection and colonization of aseptically micropropagated sugarcane seedlings by nitrogen-fixing endophytic bacterium, Herbaspirillum sp. B501gfp1

In this study, we used Herbaspirillum sp. B501gfp1 (B501gfp1), an isolate from wild rice, to investigate the interaction between a non-host nitrogen-fixing endophytic bacterium and micropropagated sugarcane plants under aseptic condition. Two Japanese sugarcane plants (Saccharum sp.) cultivars (cvs) NiF8 and Ni15 were inoculated using B501gfp1 in two inoculum doses of 10⁸ and 10² bacterial-cells-per-milliliter suspension. The results showed that bacterial cells colonized both the root and stem tissues, and colonization was apparent in the intercellular spaces. Higher bacterial numbers were detected in plant tissues inoculated with the higher inoculum concentration treatment. Bacterial numbers also varied between the two cultivars, with the higher values determined in cv Ni15. This study provides evidence that Herbaspirillum sp. B501gfp1, a rice isolate, could colonize sugarcane tissues, suggesting non-specificity of host plant among endophytes.