Typical morphological and physiological changes were observed in iron deficient sunflower (Helianthus annuus L. cv. Sobrid) roots. These changes, or so-called iron stress reactions, are exclusively confined to the root tips. Typical morphological changes included additional cell division in the rhizodermis layer and enhanced formation of root hairs, leading to an increase in root diameter (“swollen root tips”). These morphological changes were correlated with physiological changes such as increased release of protons, accumulation of phenols in the rhizodermis, and an increased ability of the roots to reduce iron-III compounds (“reducing capacity”). A marked increase in ability of the root tips to take up and translocate iron occurred simultaneously with these changes. There is good evidence that these morphological and physiological changes are reflections of an effective regulatory mechanism for enhanced mobilization of sparingly soluble iron-III compounds in the rhizosphere and for iron uptake by sunflower plants.