Initiation of root growth stimulation by Azospirillum lipoferum CRT1 during maize seed germination

Maize seeds were inoculated with a commercial inoculant containing 1.3 x 10 (7) Azospirillum lipoferum CRT1 cells. After 24 or 48 h, bacteria were wash ed from the seed surface. Washed and unwashed seeds were then planted in po ts containing perlite and grown for 28 days under greenhouse conditions. Wh atever the density of Azospirillum at planting, the number of these bacteri a at the end of the experiment was similar (1.9-8.0 x 10(7) bacteria plant( -1)). However, comparison of root surface areas of the plants were differen t depending on the period of contact between seeds and the density of the i noculum. Twenty-four hours of contact was not sufficient to increase root g rowth surface areas. Contact for 48 h permitted us to obtain root surface a reas comparable with those measured after a continuous contact. These resul ts showed that in order to promote maize root surface areas, an optimal den sity of Azospirillum is not required during the whole cultural cycle. This optimal density is indispensable only up to the emergence of the radicle.