The effects of nitrogen applied at increasing levels of 0, 4, 8, 16 and 32 mM N
(KNO 3 or NHaCI ) were studied in faba bean (Vicia faba) nodulated by Rhizobium
leguminosarum bv. viceae RCR 1ool. Nitrogenase activity was higher at 4 and 8 mM
N than the zero N treatment (control), but 16 and 32 mM N significantly reduced the
efficiency of nodule functions. Nitrate reductase activities (NRA) of leaves, stems,
roots, nodules and nodule fractions (bacteroid and cytosol) were increased with
rising the NO 3" or NH4 + levels. NRA decreased in the order of nodules > leaves >
stems > roots. Cytosolic NR was markedly higher than that recorded in the bacteroid
fractions. Nitrate levels were linearly correlated to NRA of nodules. Accumulation of
NO 2" within nodules suggests that NO 2 inhibits nodule's activity after feeding plants
with NO 3" or NH4 §

Two strains (RCR 1001 and 1044) and a commercial inoculant (Okadin) ofRhizobium leguminosarum biovarviceae were tested for their ability to survive in autoclaved clay soil for up to four months under heat, salinity and drought stress. Resistance to heat was tested by incubating rhizobia in soil at 27, 37 and 42 °C. Tolerance of rhizobia to salinity was investigated by growing rhizobia in soil salinized with 1 and 2 % NaCl (m/m). Drought resistance was tested by subjecting bacteria to soil moisture contents of 20, 10 and 5%. Strain RCR 1001 was more resistant to heat and nodulated faba bean better than other tested strains. A commercial inoculant Okadin survived more (plate count method) and nodulated faba bean (plant infectivity, most probable number, MPN) at moisture content of 5% and 2% NaCl. Although, strains RCR 1001 and 1044 resisted these stress conditions (plate count) they lost their abilities to nodulate faba bean (MPN-test). There is a possibility for selection of effective rhizobia which are more tolerant to harsh conditions.

Six Bradyrhizobium (lupin) strains were evaluated for their ability to produce in vitro siderophores using four chemical assays. Bradyrhizobium strains WPBS 3201 D and 3211 D gave positive reactions with the chrome azurol S assay (CAS) and produced hydroxamate-type siderophores. The other four strains (USDA 3040, 3041, 3042 and CB 2272) gave negative results for siderophore production with the four assays. The generation time, growth yield and hydroxamate production of strain WPBS 3201 D were affected by the iron concentration of the culture medium and the previous culture history of the cells. Resuspension of washed cells grown previously in media supplemented with 0 and 20 μmol·L−1 Fe into differing iron regimes (0 and 20 μmol·L−1 Fe) suggest that the extent of hydroxamate production was dependent on the growth history of the cells. Cells pre-grown in 20 μmol·L Fe produced a high amount of hydroxamates compared with cells pre-grown in iron-free medium when resuspended in medium containing up 4 μmol·L−1 Fe. Cells pre-grown in 20 μmol·L−1 Fe were also more sensitive to iron repression than those pre-grown in 0.5 μmol·L Fe. Mannitol was the best carbon source for siderophore production. Siderophore synthesis was inhibited by 4-chloromercuribenzenesulfonic acid, 2,4-dinitrophenol, sodium azide and MgCl2 suggesting that an energized membrane and a mercapto group are essential and required for hydroxamate synthesis in Bradyrhizobium (lupin) strain WPBS 3201 D.

The structure of N2-fixing nodules of faba bean (Vicia faba L.) was investigated by means of light and electron microscopy to determine possible pathways for metabolite movement. The central infected zone is enclosed by a cortex, which is composed of a loosely arranged outer part and a densely packed inner part. Both are separated by the nodule endodermis, which borders on the root endodermis and continuously envelopes the entire nodule interior, with the exception of the apical meristematic region. Vascular bundles, located peripherally within the inner cortex, are surrounded by another endodermal layer. Radial cell walls of both nodule and vascular endodermis are incrusted and form Casparian bands. Additionally, all endodermal cells are coated by suberin lamellae. These modifications may prevent an apoplastic loss of solutes from the enclosed tissues, and require a symplastic metabolite passage. This is supported by high plasmodesmatal frequencies found between vascular endodermis and inner cortex. Within vascular bundles, transfer cells adjacent to xylem elements may play a role in xylem loading. The central infected tissue contains uninfected cells, possibly representing a symplastic continuity. These cells may form a preferred metabolite pathway since plasmodesmata are extremely sparse between infected cells. The low number of uninfected cells in the central tissue of V faba nodules favors an additional apoplastic pathway.

The organophosphorus insecticide Selecron [O-(4-bromo-2-chlorophenyl) O-ethyl S-n-propyl-phosphorotioate] at 10 and 50 ppm significantly decreased respiration, mycelial protein, extracellular protein and mycelial dry weight of Aspergillus fumigatus, A. terreus and Myceliophthora thermophila when grown at 45°C. Cx and C1 cellulases of tested fungi were significantly decreased. However, C1 cellulase of A. fumigatus was slightly increased.

Of 13 Rhizobium and Bradyrhizobium strains investigated for the production of cellular and extracellular phosphodiesterase and phosphotriesterase, all were found to produce both enzymes. Phosphodiesterase was produced at a much higher level than phosphotriesterase. Rhizobium meliloti TAL 1373 was the most productive. The extracellular enzymes were activated by inclusion in the assay mixture of Ca2+ or Mg2+. The enzymes were inhibited by Zn2+ but not significantly affected by Cu2+, Co2+ and Mn2+. Both hydrolases were inhibited by dithiothreitol but not by thiol-directed inhibitors, suggesting that sulphydryl groups are not directly involved in catalysis. The enzymes have the ability to hydrolyse some organophosphorus compounds, suggesting that Rhizobium and Bradyrhizobium strains play an important role in the degradation of organophosphorus pesticides.

Seeds of faba bean (Vicia faba L.) cultivar Giza 3 were screened for the presence of mycotoxins. Eleven out of 100 samples were positive. Aflatoxins B1 and B2 were found in 7 samples with a mean concentration of 30 μg kg−1 seeds. Aflatoxins B1 B2, G1 and G2 and ochratoxin A were each detected twice in separate samples with a mean concentration of 25 and 20 μg kg−1, respectively. Mycotoxins at concentrations of 100 or 200 μg kg−1 soil significantly decreased nodule number, nodule fresh weight and total nitrogenase activity. This was translated into reductions in dry matter accumulation and nitrogen yield of the bean. Mycotoxins also suppressed specific nitrogenase activity, NADH-dependent glutamate dehydrogenase (NADH-GDH) as well as glutamate synthase (NADH-GOGAT) activities. In addition, mycotoxins inhibited synthesis of leghaemoglobin, carbohydrate and protein in the nodule cytosol. Of the mycotoxins tested, aflatoxin B1 was the most toxic. The decline in nitorgenase activity and total N concentration in the plants could be attributed to mycotoxins interfering with normal nodule physiology and function.

Azotobacter chroococcum was isolated from straw-amended soil and found to utilize 4-hydroxybenzoic acid, resorcinol, pyrocatechol and vanillic acid as sole carbon source. Growth and nitrogenase activity ofA. chroococcum were supported by 8, 6 and 4 mmol/L of 4-hydroxybenzoic acid, resorcinol and pyrocatechol, respectively. The generation time of 1.71 h in 4-hydroxybenzoic acid did not significantly differ from the generation time of 1.64 h, observed when grown in mannitol. 4-Hydroxybenzoic acid was utilized rapidly. However, the decomposition of other tested phenolic compounds set in only slowly. It was concluded that this isolate has good potential to utilize some phenolic compounds released during biodegradation of plant wastes.

In soil salinized with different concentrations of sodium chloride, the total counts of soil fungi were significantly decreased during 11 weeks of incubation. Similarly, the total count of bacteria and actinomycetes were severely depressed by increasing salinity level above 5%. Invertase and urease activities were severely decreased as NaCl concentration increase during the incubation. Also, the effect on nitrate reductase was inhibitory with most treatments.