Fig. 1
Influence of reduced total nitrogen on early cytological events of AR formation in Petunia hybrida. Cuttings with two nitrogen levels, (Panels a, c) N-low at 2570 μmol Nt and (Panels b, d) N-high at 3625 μmol Nt were excised from donor plants and immediately inserted into perlite for AR formation with assimilatory light. All micrographs represent cross-sections of the stem base from 1–4 mm above the excision site with the most advanced structures at (Panels a, b) 72 h post excision (hpe) and post insertion (hpin) and (Panels c, d) 168 hpe and hpin. Sections at 72 hpin (Panels a, b) show the typical stem anatomy with the cortex (co), the outer phloem (oph), the cambium (ca), the xylem (xy), the inner phloem (iph) and the pith parenchyma (pi) and first meristematic cells (mc) of developing root meristems, that is, small cells with a dense cytoplasm and a large nucleus. There are only slight differences between the nitrogen levels. Sections at 168 hpin reveal that with low N absorption (Panel c) first root meristems (me) appear, whereas with high N absorption (Panel d) first roots with vascular bundles (v) in the center surrounded by elongated cells (ec) of the elongation zone are visible. Bars represent 100 μm (Panels a to d). Remark: Ahkami et al. [8] show that at the time of excision (0 hpe) no meristematic cells of developing root meristems are present. Further details are presented in methods and with Additional file 1) Experiments of nitrogen preconditioning of cuttings and Additional file 2) Explanation of experimental designs for Exp. 7: AR-N + CYT, respectively