Maize seeds were inoculated with a commercial inoculant containing 1.3 × 107 Azospirillum lipoferum CRT1 cells. After 24 or 48 h, bacteria were washed from the seed surface. Washed and unwashed seeds were then planted in pots containing perlite and grown for 28 days under greenhouse conditions. Whatever the density of Azospirillum at planting, the number of these bacteria at the end of the experiment was similar (1.9-8.0 × 107 bacteria·plant-1). However, comparison of root surface areas of the plants were different depending on the period of contact between seeds and the density of the inoculum. Twenty-four hours of contact was not sufficient to increase root growth surface areas. Contact for 48 h permitted us to obtain root surface areas comparable with those measured after a continuous contact. These results showed that in order to promote maize root surface areas, an optimal density of Azospirillum is not required during the whole cultural cycle. This optimal density is indispensable only up to the emergence of the radicle.Key words: Azospirillum, maize, inoculation, PGPR.
In order to develop a reliable and specific tool for the detection of Azospirillum lipoferum CRT1, randomly cloned DNA fragments from this strain were used as hybridization probes to differentiate A. lipoferum CRT1 from 29 closely related Azospirillum strains. Two cloned fragments hybridizing only with DNA from A. lipoferum CRT1 (CRT1‐5 and CRT1‐7) were considered as specific probes of this strain. CRT1‐7 fragment (1.4 kb) was further tested for purity control of the inoculant Azogreen‐m by colony hybridization. The sequence of the CRT1‐7 fragment has been determined and compared with those present in databases: no significant similarity with other sequences was detected. This probe permitted us to count specifically A. lipoferum CRT1 cells on maize roots during a field trial. During the first two weeks, A. lipoferum CRT1 remained at 107 CFU plant−1. Afterwards, bacterial concentration sharply decreased. We could not detect any CRT1 cells on maize roots 28 days after sowing. Concurrently, three plant parameters were estimated (plant height, primary root length and root fresh weight). The results showed that A. lipoferum CRT1 growth promotion effect began early on (from day 14) in plant development and increased in spite of a rapid decrease of bacterial density.
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