POLAR TRANSVERSE EXPANSION OF PERICYCLE CELLS IN AUXIN-TREATED ADVENTITIOUS ROOTS OF ONION (ALLIUM-CEPA)

The exogenous application of an auxin (alpha-naphthaleneacetic acid) t o adventitious roots of Allium cepa L. changes the polarity of growth from longitudinal to transverse, Promotion of transverse growth by thi s auxin involves cortical and pericycle cells located in the elongatio n zone of these roots. Transverse auxin-promoted expansion of cortical tissue is very marked. Within the vascular cylinder, pericycle cells react to treatment in different ways, according to their location with respect to the vascular system. In terms of cross-sectional area and radial diameter increases, the most marked response to auxin treatment occurs in pericycle cells adjacent to xylem poles. Auxin treatment al so promotes cell divisions in pericycle and cortical tissues. The patt ern of divisions in the pericycle is very similar to that occurring du ring lateral root development. Nevertheless, lateral roots do not deve lop along the distalmost 3 mm of auxin-treated roots. In this region t he pericycle simply becomes a multilayered tissue. The microtubules ar e proposed as a conceivable means to control changes in the polarity o f expansion and promotion of periclinal divisions in this experimental system.