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 cl, 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 N-15(2) 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 N-15 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.