Host plant specificity in the infection of C3 and C4 plants by Azospirillum spp

Abstract

The occurence of Azospirillum brasilense and A. lipoferum and their nir⁺ and nir^- variants was determined in soil and in non-sterilized and sterilized roots of the C₃ cereals wheat, rye, barley and oat and of several C₄ grasses. The C₃ cereals were planted in pots with red-yellow podzolic soil and examined at two growth stages. The C₄ forage grasses were collected in fields where the respective species were predominant. In all soils A. brasilense seemed to predominate. The proportion of A. brasilense nir^- was above 80% in all C₃ cereals when surface sterilized roots were examined. Azospirillum isolates from surface sterilized roots of C₄ grasses were predominantly A. lipoferum but only in some species (Panicum maximum, Sorghum vulgare and Brachiaria sp) the nir^- forms predominated. A weed belonging to Cyperaceae (Cyperus rotundus) was also infected by A. lipoferum nir^- as in the case of other C₄ grasses. The only exception among the C₄ Gramineae was sugar cane which seemed to behave like the C₃ cereals, and all 24 isolates from surface sterilized roots were A. brasilense nir^-.