ROOT CONTRACTION IN HYACINTH. I. EFFECTS OF IAA ON DIFFERENTIAL CELL EXPANSION

Abstract

Contractile roots of Hyacinthus orientalis L. cv ‘Pink Pearl’ shorten as a result of growth of inner cortical cells which expand radially and contract longitudinally. Brief treatment with IAA (indole‐3‐acetic acid—0.5 and 1.0 mg/1) induces subapical swelling, root cap proliferation and decreased rates of elongation in potentially contractile roots. Growth resumes with removal of IAA from the culture medium and contraction subsequently occurs. The pattern of subsequent contraction is affected by prior IAA treatment; contraction occurs in the normal manner both acropetal and basipetal to the points of IAA‐induced swelling, but does not occur in the swollen region itself. Microscopic examination of the swollen region reveals that cells of the middle and outer cortex are radially expanded and longitudinally shortened relative to middle and outer cortical cells of contracted and uncontracted portions of the same root and control roots. In contrast, inner cortical cells in swollen regions of IAA‐treated roots show approximately 50% less radial expansion than inner cortical cells of control contracted roots. Middle and outer cortical cells in the swollen region of IAA‐treated roots undergo radial expansion, while middle and outer cortical cells in adjacent contracting zones are compressed by radially expanding inner cortical cells. Average volumes of cortical cells in the IAA‐induced swollen region increased approximately two‐fold when contraction occurred in adjacent regions. These results suggest that in hyacinth roots, under certain circumstances, inner and outer cortical cells alike possess the ability for growth reorientation and expansion. However, during the usual course of contractile root development, cells of the outer cortex are restricted in this ability, through an as yet unknown mechanism, and are passively compressed by the radially expanding inner cortical cells.