The effect of azospirilluh brasilense on iron absorption and translocation by sorghum

Abstract

Sorghum [Sorghum bicolor (L.) Moench] displays Fe chlorosis when grown in alkaline soils where iron is insoluble and unavailable as a plant nutrient. We were interested in evaluating the role of naturally produced Fe chelators of rhizospheric bacteria in the utilization of insoluble Fe by sorghum. Seedlings of sorghum were grown under hydroponlc conditions in a nutrient solution containing FeHEDTA or without Fe. All seedlings were transferred after two.weeks into fresh nutrient solution containing FeCl₃ with CaCO₃ added to adjust the pH to 7.4 and to precipitate Fe. Azospirillum brasilense, a nitrogen fixing bacterium often associated with roots of grasses, was introduced into several of the hydroponic cultures to determine if the siderophore produced by the bacterium would enable sorghum to use the precipitated inorganic Fe. Utilization of added ⁵⁹FeCl₃ was indicated by measurement of radiolabeled Fe in the roots and shoots or by determination of the chlorophyll content of the leaf blades. Addition of A. brasilense to the hydroponic cultures increased the absorption of Fe by roots of both iron‐deficient seedlings and seedlings grown under conditions where Fe was not limiting. Iron transported to the shoots increased in the iron‐deficient seedlings where A. brasilense was added. Smaller quantities of Fe were taken up in the plant cultures receiving lyophilized bacterial culture fluid or the control which received no additions. Very little Fe was transported to the shoots by the seedlings not stressed for Fe and this transport was not influenced by the experimental treatments. In general, the chlorophyll content of the shoots followed the results obtained with radiolabeled Fe. Our results suggest that A. brasilense and other rhizospheric bacteria may significantly affect the Fe nutrition of plants. Under conditions where rapid proliferation of rhizospheric bacteria occur, bacteria may compete with sorghum for Fe. Large quantities of non‐growing bacteria may enhance the absorption and translocation of Fe by plants in alkaline growth environments.