Proliferating and Quiescent Cells in the Apical Meristem of Elongating Lateral Roots of Vicia faba L.

Whole root systems of Vicla faba were continuously exposed to³H-TdR for periods of up to 72h, following which LI was determinedin the cap initials, in the quiescent centre or in that partof the meristem in which a quiescent centre will develop, andin the stele and in the cortex-epidermis at intervals alongthe apical 800 µm basal to the junction between the capinitials and the rest of the meristem, in newly-emerged (NE),0.2 and 4.0 cm long lateral roots, after each exposure period.Cell doubling time (Td), mean cycling time (Tc) and the sizeof the growth fraction (GF) were then calculated for each partof the meristem investigated in each batch of roots, from thecurves recording increase in labelling index (LI) with increasein duration of the period of exposure to ³H-TdR and from therate of increase in LI over the initial l-12h labelling period.Since it is extremely difficult to eliminate all sources oferror in calculating GF from the values obtained for LI in continuouslabelling experiments, it is emphasized that the values of GFreported in the present paper may not be totally accurate. Thisis also true of the results obtained for Tc as Tc was derivedfrom the product of the corresponding values for Td and GF. Cell doubling time and mean cycling time were both longer inthe cells forming the quiescent centre in the 0.2 and 40 cmlong roots than in any other part of the apical meristem examined.The size of the GF was found to decrease basally along the steleand the cortex-epidermis from the most apical to the most basalsegment examined in the NE, while Td increased in duration.Similar changes took place along the stele of the 0.2 cm longlaterals, but not in the cortex-epidermis of these roots or,to any great extent, in any of the tissues examined in the 4.0cm secondary roots. No consistent trend was apparent in theduration of Tc basally along either tissue examined in theseroots. It was concluded from these results, and from supportingdata in the literature, that, as the laterals elongated fromNE to 4.0 cm, the apical meristem increased in length.