The fate of field-inoculated Azospirillum brasilense Cd in wheat rhizosphere during the growing season

Abstract

Following inoculation of wheat grown under field conditions with Azospirillum brasilense Cd, the bacterial population in the plants rhizosphere during the winter growth season was monitored. Seven field experiments were conducted during 3 years, in five soil types, and the bacteria were counted using two different methods of enumeration. Azospirillum brasilense Cd were detected in the wheat fields up to 105 days after inoculation at a level of 10⁴ to 5 × 10⁵ cells/g fresh weight roots, with a very small variation between colonized plants, regardless of the inoculation method. Only a small negligible decrease in the bacterial population occurred during the coldest period of the growing season. Later, the population increased again to the initial level and remained constant. Although inoculated twice, a part of the plant population had not been colonized by A. brasilense Cd. We propose that A. brasilense Cd colonized, although in relatively small numbers, the root system of wheat during the winter season, under suboptimal temperatures for this bacterial species.