The central role of microbial activity for iron acquisition in maize and sunflower

Maize (Zea mays L.) and sunflower (Helianthus annuus L.) grown on a calcare ous soil showed poor growth and/or were chlorotic in spite of abundant Fe i n the roots. It has been hypothesized that microbial siderophores chelate F e (III) in the soil, and that in this form Fe is transported towards the ro ot apoplast. On the calcareous soil, total and apoplastic root Fe concentra tions were high, probably because of a high apoplastic pH depressing Fe (II I)-reductase activity and thus the Fe2+ supply to the cytoplasm. On the aci dic soil, total and apoplastic root Fe concentrations were low, probably be cause of a low apoplastic pH favouring Fe (III) reduction, hence plants sho wed no Fe-deficiency symptoms. The main objective of the present work was t o investigate the role of microbial soil activity in plant Fe acquisition. For this purpose, plants were grown under sterile and non-sterile condition s on a loess loam soil. Plants cultivated under non-sterile conditions grew well, showed no Fe-deficiency symptoms and had fairly high Fe concentratio ns in the roots in contrast to plants grown in the sterile medium. Low root and leaf Fe concentrations in the axenic treatments indicated that the pro duction of microbial siderophores was totally suppressed. Accordingly, sunf lowers were severely chlorotic and this was associated with very poor growt h, whereas in maize only growth was drastically reduced. In maize under ste rile conditions, root apoplastic and total Fe concentrations were not as lo w as in sunflowers, which may have indicated that phytosiderophores produce d in maize partly sustained Fe acquisition, but due to poor growth were not as efficient in supplying Fe as microbial activity under natural condition s. It may be therefore assumed that in natural habitats soil microbial acti vity is of pivotal importance for plant Fe acqusition.