Abstract
The root ultrastructure and transmembrane electron transport activities of two Plantago species have been examined with respect to alterations in response to Fe deficiency, exogenously supplied auxin, and the presence of chromium in the external medium. Both species showed increased ferric reductase activity upon Fe starvation, but they differed in the maximum rates. The addition of chromium to the nutrient solution led to a further enhancement in Fe-ethylenediaminetetraacetate reduction by Fe-deficient plants. In roots of Plantago lanceolata, the enhanced redox activity is associated with the formation of transfer cells in the epidermis. Similar characteristics of rhizodermal cells were observed in Fe-sufficient roots 3 d after application of the auxin analog 2,4-dichlorophenoxy-acetic acid. No structural adaptations occurred in roots of Plantago maritima. A quantitative estimation of the frequencies of transfer cells in root segments of Fe-deficient plants that differ in reduction activity revealed no correlation between the two phenomena. It is concluded that the area of plasmalemma infoldings is not specialized for the enhanced reduction of extracytoplasmatic Fe in response to Fe deficiency. The role of transfer cells in the adaptation to suboptimal Fe availability and the mechanisms triggering their formation are discussed.