Aqueous extracts of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] shoots and rhizomes inhibited seed germination and root growth of alfalfa (Medicago sativa L. ‘Vernal’), soybean (Glycine max (L.) Merr. ‘Corsoy 79’], navybean (Phaseolus vulgaris L. ‘Seafarer’), and curly cress (Lepidium sativum L.) at concentrations of less than 2.5 mg dried extract/ml. Extracts of quackgrass shoots were generally more inhibitory than extracts of rhizomes. Root and shoot dry weights of snapbeans (Phaseolus vulgaris L. ‘Bush Blue Lake’) grown under sterile conditions were reduced by aqueous extracts of shoots. Root systems were stunted and necrotic and lacked root hairs. The growth of Rhizobium species was not influenced by the presence of 40 or 80 mg/ml concentrations of extracts of shoots or rhizomes. Quackgrass may inhibit indirectly the legume-Rhizobium symbiosis by inhibiting root hair formation rather than directly inhibiting Rhizobium growth. The presence of soil microorganisms was not necessary for the development of quackgrass toxicity in soil or agar. Soil microorganisms reduced toxicity of quackgrass residues in soil.

EPTC (S-ethyl dipropylthiocarbamate), benefin (N-butyl-N-ethyl-α α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], butralin[4-(1,1-dimethylethyl)-N-(1-methylpropyl)2,6-dinitrobenzenamine], diclofopmethyl {methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]-propanoate} and 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] were evaluated in the greenhouse for effects on growth, nodulation, and NH3 production of alfalfa (Medicago sativa L.) and medium red clover (Trifolium pratense L.). Some reduction in nodulation occurred from some rates of the herbicides, but reduction in nodulation was usually associated with reduced plant growth caused by herbicidal injury.

Seeds of the wetland legume, Lotus uliginosus, were germinated and grown in vermiculite which was either continuously flooded or well-drained. Plants from both treatments were infected by Mesorhizobium loti strain DUS341 via a ‘classical’ root hair pathway, although some flooded plants appeared to be infected via enlarged epidermal cells. Subsequent to infection by M. loti, nodule meristems, which had developed within the root outer cortex, were penetrated by infection threads that released bacteria into the meristematic cells. The infection threads and infection droplets were immunogold labelled with monoclonal antibodies (MAC265 and MAC236) that recognize epitopes (at approx. 155/170 and 170/210 kDa, respectively) on a glycoprotein component of the matrix that surrounded the bacteria within the threads or droplets. Although labelling of infection threads or infection droplets with MAC236 was stronger than that with MAC265, both antibodies strongly labelled material occluding intercellular spaces in the cortices of developing nodules that had not yet expressed nitrogenase (as determined by a lack of signal after immunogold labelling with an antibody raised against nitrogenase component II). After 60 d, nitrogenase activity, shoot and root dry weights, and nodule fresh weight per plant did not differ between the treatments. After a further 30 d submergence, the flooded stems developed extensive aerenchyma and there was profuse development of (nodulated) adventitious roots. Nodules also formed at the junction of adventitious roots and the subtending stem and these were connected vascularly to a small stalk of tissue which gave rise to both a nodule and an adventitious root. The flooded nodules had prominent lenticels, and possible air pathways from the atmosphere to the nitrogen-fixing bacteroids are discussed.

A pot experiment was designed to evaluate the interactive effects of multiple microbial inoculation treatments and rock phosphate (RP) application on N and P acquisition by alfalfa plants using 15N and 32P isotopes. The microbial inocula consisted of a wild type (WT) Rhizobium meliloti strain, its genetically modified (GM) derivative, which had an enhanced competitiveness, the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, and a phosphate-solubilizing rhizobacterium (Enterobacter sp.). Inoculated micro-organisms became established in the root tissues and/or in the rhizosphere soil of alfalfa plants (Medicago sativa L.). The GM Rhizobium strain did not interfere with AM formation. Inoculated phosphate-solubilizing rhizobacteria established in the alfalfa rhizosphere, but the level of establishment was lower where the natural population of phosphate-solubilizing bacteria was stimulated by AM inoculation and RP application. The stimulation of these indigenous bacteria was also greater in the rhizosphere of alfalfa nodulated by the GM Rhizobium. Improvements in N and P accumulation in alfalfa corroborate beneficial effects of the improved GM Rhizobium on AM performance, in RP-amended plants. Inoculation with Enterobacter did not improve the AM effect on N or P accumulation in the RP-added soil, but it did in the non RP-amended controls. Measurements of the 15N[ratio ]14N ratio in plant shoots indicated enhanced N2 fixation rates in Rhizobium-inoculated AM-plants, over that achieved by the same Rhizobium strain in non-mycorrhizal plants. Regardless of the Rhizobium strain and of whether or not RP was added, AM-inoculated plants showed a lower specific activity (32P[ratio ]31P) than did their comparable non-mycorrhizal controls, suggesting that the plant was using otherwise unavailable P sources. The phosphate-solubilizing, AM-associated, microbiota could in fact release phosphate ions, either from the added RP or from the indigenous ‘less-available’ phosphate. Deficiency in Ca concentration in soil solution in the neutral test soil might benefit P solubilization. The proportion of plant P derived either from the labelled soil P (labile P pool) or from RP was similar for AM inoculated and non-mycorrhizal controls (without Enterobacter inoculation) for each Rhizobium strain, but the total P uptake, regardless of the P source, was far higher in AM-plants. Enterobacter inoculation seems to improve the use of RP in the rhizosphere of non-mycorrhizal plants inoculated with the WT Rhizobium.

Arbuscular mycorrhizal (AM) fungi, Rhizobium bacteria and plant-growth-promoting rhizobacteria (PGPR) were isolated from a representative area of a desertified semi-arid ecosystem in the south-east of Spain. Microbial isolates were characterized and screened for effectiveness by a single-inoculation trial in soil microcosms. Anthyllis cytisoides L., a mycotrophic pioneer legume, dominant in the target mediterranean ecosystem, was the test plant. Several microbial cultures from existing collections were also included in the screening process. Two AM fungi (Glomus coronatum, native, and Glomus intraradices, exotic), two Rhizobium bacteria (NR4 and NR9, both native) and two PGPR (A2, native, and E, exotic) were selected. A further screening for the appropriate double and triple combinations of microbial inoculants was then performed. The parameters evaluated were biomass accumulation and allocation, N and P uptake, N2-fixation (15N) and specific root length. Overall, G. coronatum, native in the field site was more effective than the exotic G. intraradices in co-inoculation treatments. In general, our results support the importance of physiological and genetic adaptation of microbes to the whole environment, thus local isolates must be involved. Many microbial combinations were effective in improving either plant development, nutrient uptake, N2-fixation or root system quality. Selective and specific functional compatibility relationships in plant response between the microbial inoculants, were observed. Despite the difficulty of selecting a multifunctional microbial inoculum, appropriate microbial combinations can be recommended for a given biotechnological input related to improvement of plant performance. This could be exploited in nursery production of target plant species endowed with optimized rhizosphere/mycorrhizosphere systems that can be tailored to help plants to establish and survive in nutrient-deficient, degraded habitats. The relevance of this microbial-based approach in the context of a reclamation strategy addressed to environmental sustainability purposes is discussed.