Cap Formation during the Elongation of Lateral Roots of Vicia faba L.

Abstract

Cell proliferation was examined in the cap initials of newly-emerged 0·2 and 4·0 cm long lateral roots of Vicia faba from an investigation of the passage of labelled cells through mitosis following a 1-h pulse with tritiated thymidine. Cell doubling time was found to increase in this group of initial cells as the secondary roots elongated, this increase being a result of a gradual lengthening in the duration of the mitotic cycle of both the fast and slow cycling meristematic cells and of a decrease in the size of both the growth fraction and those proliferating cells with a short cycle time. The rate of cell production by the cap initials was maximal in the newly emerged secondary roots and showed a gradual decline with subsequent lateral elongation. This change in the rate of cell proliferation in the cap initials has been shown to be related to the initiation of the quiescent centre following lateral root emergence from the tissues of the primary. The number of cells making up the cap of 0·2–4·0 cm long secondary roots is known to be constant and thus the rate at which cells are sloughed off of the cap into the soil must be the same as the rate of cap cell formation in these roots, all of the cap cells being replaced every 6–9 days. The rate of cap cell formation in 0·2–4·0 cm long secondary roots was used to calculate that one 11-day old Vicia plant will have contributed between 56 000 and 85 000 cap cells to the rhizosphere from its root system since germination.