Herbaspirillum colonization increases growth and nitrogen accumulation in aluminium‐tolerant rice varieties

• Varieties of rice (Oryza sativa) differing in tolerance to aluminium (Al) were evaluated for their N-fixation ability after inoculation with a gusA-marked strain of Herbaspirillum seropedicae Z67. • Under axenic conditions, by 30 d, inoculation resulted in enhanced nitrogenase activity, d. wt, total N and total C content only in the Al-tolerant varieties, and one (cv. Moroberekan) showed significantly more 15N2 incorporation than an Al-sensitive variety (‘IR45’). There were no differences in the number of the bacteria colonizing the different varieties, but the Al-tolerant ones secreted larger amounts of C in their root exudates, and bacteria colonizing the roots of cv. Moroberekan strongly expressed gusA and NifH proteins. • Under glasshouse conditions, by 30 d, inoculation resulted in increased growth of both cvs IR45 and Moroberekan, but the latter showed significantly greater nitrogenase activity and 15N dilution. In a long-term experiment, by 120 d, cv. Moroberekan showed a significant increase in N content after inoculation. • Herbaspirilla were localized on and within roots and aerial parts of cvs Moroberekan and IR45 under both growth conditions. The role of N fixation in growth promotion of rice by H. seropedicae is discussed in terms of availability of C.