Role of roots and shoots in the regulation of the Fe efficiency responses in sunflower and cucumber

Several experimental approaches were used to study the role of roots and shoots in the regulation of the Fe efficiency responses in the roots of Fe deficient plants of sunflower ( Helianthus annuus L., inbred line RHA 274) and cucumber ( Cucumis sativus L., cv. Burpee pickler). The presence of the shoot apex and the youngest leaves in sunflower and cucumber plants was not indispensable to the development of the responses. In split-root experiments with sunflower plants, with either one or two shoots obtained by grafting, different parts of the root system could receive different Fe additions. Roots growing in a medium without Fe developed Fe efficiency re-sponses even when the other part of the root system was growing with Fe. Also, the part of the root system that was growing in a medium with Fe, developed some Fe efficiency responses, although delayed and at a lower level as compared with the root without external Fe. The results are discussed in relation to the idea that the development of the responses in the roots is controlled not only by the Fe content in the root, but also by some information transmitted between parts of a root system that differ in their Fe nutritional status.