Pea Root Regeneration After Tip Excisions at Different Levels: Polarity of New Growth

The roots of light-grown pea seedlings (Pisum sativum L. cv. Alaska) were excised at 250, 500 or 1500 μm from the body/cap juncture. Tips were sampled for 7 d after excision to monitor the polarity and structure of the regeneration response. In roots excised at 250 μm a new single apex regenerated without swelling or any sign of repair. The new root cap appeared identical to the control. After the 500 μm excision two responses occurred. In 45% of the roots examined, a single new apex regenerated, in 55% two or three new apices regenerated to form a dichotomized or trichotomized root in the same longitudinal axis as the primary root. In roots excised at 1500 μm, one, two or three lateral roots formed adjacent to the xylem at right angles to the primary root axis. In most instances the new roots developed triarch xylem. The discussion concentrates on the relationship of the differentiation state of the vascular cylinder and the pericycle, at the excision site, to the regeneration response.