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Slightly lower values of viability were obtained in mixed culture, but the ratio between the microorganisms was constant. The survival ofArth, giacomelloi in single culture decreased with increasing growth rate while survNal of Azo. brasilense was directly related to dilution rate.Acetylene reduction activity was generally very low in both single and mixed cultures. Respiration rate was also determined and the mixed culture showed an oxygen uptake rate higher than that of single cultures.

Physiological and biochemical parameters have been studied simultaneously with micrometeorological characters of a crop of durum wheat (Triticum durum). Micrometeorological measurements and plant analysis on flag leaves before spike appearance were done during 2 days in May 1973 to ascertain the fluctuation of chosen leaf parameters in comparison with the micrometeorological ones. The results indicate that biochemical and physiological changes occur during the day and night and that stomatal opening is the main factor dependent on climatic conditions. Specific leaf weight (SLW) seems to vary with the level of free amino acids. Ribulose diphosphate carboxylase (RuDPcase) showed daily fluctuations insensitive to climatic conditions, whereas phosphoenolpyruvate carboxylase (PEPcase) exhibited little day change.

The ability of Azospirillum brasilense Cd to colonize the niche occupied by 3 bacterial strains previously isolated from sorghum rhizosphere was studied by means of the Biolog system. The isolates were identified by different methods as strains belonging to Pseudomonas putida, Stenotrophomonas maltophilia, and Klebsiella terrigena species. Several C sources, also chosen among the constituents of sorghum root exudates, were used to evaluate the metabolic profiles of Azospirillum and the sorghum rhizobacteria. Azospirillum brasilense Cd exploited the same class of C compounds as the sorghum rhizobacteria and overlapped in their niche requirements. Since structure and functioning of a microbial community are largely affected by the flow rate of nutrient supply, the competitive behavior of A. brasilense Cd was studied in a chemostat mixed culture under C-limited conditions using disodium succinate as C source. Only at high growth rates, i.e., when the C source was highly supplied, A. brasilense Cd appeared to be a good competitor and it became the dominant species, whereas at low growth rates, it was outnumbered by the other species. However, the coexistence of all the strains was always maintained, thus suggesting that interactions other than competition or a potential cross-feeding might occur within the mixed culture.

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