Contact between root and iron substrate and the mobilization of iron by soybean roots

The question of whether a direct (uninterrupted) contact between plant roots and solid substrate is a prerequisite for the mobilization of sparingly soluble iron (Fe) by plant roots was investigated. Factorial combinations of two types of contact between the roots and rooting medium [direct or interrupted (roots placed inside a dialysis tube (DT)], two rooting media (nutrient solution or vermiculite), and two Fe supply conditions [without soluble Fe (control) or plus Fe-EDDHA] were investigated. Placing the roots inside DT, with no visible mechanical impedance for the root, reduced the growth of tops and roots in most cases. Iron mobilization, judged by the total amount of Fe in the tops, strongly decreased if roots were enclosed in DT. This was true in plants grown in vermiculite as well as in nutrient solution containing Fe-FDDHA. It is concluded that a direct contact between the surface of root cell walls and the Fe substrate is a prerequisite for the Fe mobilization by the plant roots.