Visualization of the radicle within tee axis of developing and germinating Brassica napus L. embryos

Abstract

A method is presented for visualizing the exact radicle territory within the developing embryo axes of Brassica napus L. (rape). The reactive dye, Procion Blue MX-R, when used for embryo axes in toto, showed a definite staining pattern. The basal boundary of the stained region (level L2) did not coincide with the basal radicle cap boundary (level L1), and, on average, 16 rhizodermal cells were observed between these boundaries in maturing and mature embryos. In all stages of developing embryos, from the torpedo stage to maturity, the embryo hypocotyl was not stained, and the stained region revealed the proper radicle territory. This conclusion was based on the following observations: (1) the staining patterns in the developing, developed and germinating embryos were similar; (2) direct observations of the basal part of the stained L1–L2 region demonstrated that its cells began fast elongation 24 hr after seed imbibition and began root hair formation just before the completion of elongation; (3) root hairs did not emerge after decapitation of the entire stained region which was done 24 hr after the beginning of seed imbibition; (4) at this time (24 hr post-initiation of imbibition) decapitation at level L1 stopped radicle growth for 24 hr, and hairs emerged in the apical portion of the axes; and (5) cross-sections of the L1–L2 zone of seedlings revealed a vascular system typical of root. Stainability is a complex reaction which combines interaction of the dye with the surface of radicle cells and its penetration into outer tissue cells of the radicle. Differential stainability of the hypocotyl and the radicle within the axes of developing and germinating B. napus L. embryos might be partially related to the presence of cuticle on the hypocotyl surface and its absence on the radicle surface.