Effects of a Hydroxamate Siderophore on Iron Absorption by Sunflower and Sorghum

When Fe was supplied at 100 micromolar in nutrient solution of pH 7.5, 10 and 1 micromolar levels of the siderophore desferrioxamine B (DFOB), a microbial iron transport compound, significantly (alpha = 0.05) enhanced growth and reduced chlorosis of an Fe-inefficient variety of sorghum (Sorghum bicolor L.). Although significantly adverse effects resulted when both Fe and desferrioxamine B (DFOB) were added at 100 micromolar as FeDFOB, the plants were relatively healthy when grown with 100 micromolar DFOB plus 200 micromolar Fe. It was concluded that sorghum absorbed Fe from the pool of nonchelated, solubilized Fe, and utilized DFOB as a shuttle agent, in equilibrium with this pool, to transport Fe from finely suspended solid phase Fe particles to the membrane of absorbing root cells.In contrast to sorghum, absorption of Fe by the Fe-efficient species sunflower (Helianthus annuus L.) was related to the level of FeDFOB and independent of the level of solubilized, nonchelated Fe. The latter was decreased whenever the concentration of DFOB was equal to or greater than the concentration of total Fe. For an Fe concentration of 10 micromolar, significantly larger and greener plants were obtained when DFOB was present at 1, 10, or 100 micromolar than in the absence of DFOB. When grown with 100 micromolar FeDFOB, sunflower plants appeared larger and less chlorotic than those supplied with 100 micromolar Fe and no DFOB. Sunflower apparently was able to utilize FeDFOB more directly than was sorghum. It is suggested that sunflower acquires Fe after binding FeDFOB at membrane sites and/or by producing sufficient reductants in the rhizosphere to reduce biologically significant levels of Fe(III)DFOB to the less stable Fe(II)DFOB.