On the Correlation of Primary Root Length, Meristem Size and Protoxylem Tracheary Element Position in Pea Seedlings

Pea seedlings ( Pisum sativum L. cv. Alaska) were darkgrown in vermiculite. Roots of various lengths were cleared, stained and measured to determine the relative meristem height (MH), width and volume and the distance to the most proximal protoxylem tracheary element (PTE). A correlation was found between root length, MH and PTE position as follows: in roots from 4–40 mm as the root elongated the MH lengthened and PTE position increased its distance from the body/cap juncture; in roots 40–80 mm MH and PTE position remained approximately constant; in longer roots (80–120 mm) MH became shorter, and PTE position closer to the tip as the root elongated. The relationship, using our measurement procedure was that for every 0.19 mm change in MH, the PTE position changed by 1 mm. This response was partially growth rate dependent since short roots (4–80 mm) grew at a constant rate and longer roots (80–140 mm) grew slower. Root manipulations and trifluralin treatment, to inhibit cell division, caused tip swelling and modulated the position of the PTE toward the root tip. The control of the spatial relationship between meristem size and maturation position is discussed.