Fig. 11.

Working model for the effects of iron and ammonium on adventitious root (AR) formation in Petunia hybrida. Fe performs a specific function in the rooting zone by activating ribosome biogenensis, crucial for growth of the meristematic cells of the developing AR. Under Fe-free conditions Fe associated with chloroplasts in cortical cell layers may be released to supply developing AR meristems. The uptake of NH₄⁺ leads to a local acidification of the apoplast as a consequence of an increased net H⁺ efflux, which in turn facilitates the mobilization of Fe precipitated in the apoplast. cor, cortex; epi, epidermis; vas, vasculature. (This figure is available in color at JXB online.)