SUMMARY Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

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The effects of sodium chloride and polyethylene glycol (PEG) on the interaction between Rhizobium leguminosarum strain 29d and root hairs of field bean (Vicia faba L. cv. Maris Bead) plants were investigated. Two levels each of NaCl (50 and 100 mol·m(-3)) and PEG (100 and 200 mol·m(-3)) were given at the time of root-hair formation. Scanning electron microscopy showed rhizobial attachment and colonization on root-hair tips. Adhesion of rhizobia in both lateral and polar orientation, sometimes associated with microfibrils, occurred mainly in crooks at the root-hair tips; most of the infections also occurred here. Bacterial colonization and root-hair curling were both reduced by stress treatments. Polyethylene glycol but not NaCl significantly reduced root-hair diameter. The proportion of root hairs containing infection threads was reduced by 30% under NaCl and by 52% under PEG. The structure of some of the root hairs, epidermal and hypodermal cells, as seen by light microscopy in ultrasections, was distorted as a result of NaCl and PEG treatments; cells showed plasmolysis and folded membranes. After three weeks of treatment, both NaCl and PEG inhibited nodule number by about 50% and nodule weight by more than 60%. It is concluded that the root-hair infection process in Vicia faba is impaired by NaCl and PEG treatments and this in turn results in fewer nodules being produced.

The effects of osmotic and heat stress on lipopolysaccharides and proteins of rhizobia isolated from the root nodules of leguminous trees grown in semi-arid soils of the Sudan, and of agricultural legumes grown in salt-affected soils of Egypt, were determined by SDS-PAGE. The rhizobia were of three types: (1) sensitive strains, unable to grow in 3% (w/v) NaCl in yeast mannitol medium; (2) tolerant strains which could grow in 3% (w/v) NaCl; and (3) halophytic strains which grew with 3 to 10% (w/v) NaCl. The sensitive strains changed their gel pattern or the amount of lipopolysaccharide they synthesized when grown in 1% (w/v) NaCl. The tolerant and halophytic strains often modified their lipopolysaccharides in 3% NaCl, which was evident by a shift in the banding patterns towards longer chain length. Similar effects were observed in cells incubated with sucrose and, to a lesser extent, in cells incubated at growth temperatures near the recorded maximum temperature for growth. The stress-induced changes in lipopolysaccharides were not associated with specific banding patterns of the lipopolysaccharides. During incubation in medium containing elevated concentrations of NaCl or sucrose, the protein patterns of the rhizobia were also changed. A protein with relative mobility of 65 kDa appeared during temperature stress. The maximum growth temperature of the Sudanese rhizobia were up to 44.2°C.