Local treatment with indole-3-acetic acid induces differential growth responses in Zea mays L. roots.

Differential growth responses of primary Zea mays L. roots were induced after asymmetrical applications of IAA-loaded beads along their elongating zone (1.0-4.0 mm from tip) and were modulated according to the initial growth rates of the roots. The amplitude and location (midpoint) of curvature, the elongation of the roots and the uptake of IAA were measured after 4 h. The amount of curvature depended on the location of the bead and upon the amount of IAA taken up. Curvature occurred towards the site of IAA application and the zone (midpoint) of curvature was always found to be basipetal to the IAA-loaded bead, indicating the transport of a growth signal in a basipetal direction from the zone of IAA application towards the zone of curvature. Two regions within the elongation zone were thus characterized: an acropetal region (between 1.0-3.5 mm from the tip) in which IAA induced a growth signal and a basipetal region (3.5-6.5 mm from the tip) which displayed the growth response; neither of these regions were able to carry out both physiological functions. The acropetal region was extended in fast-growing roots or when the amount of applied IAA was increased. No significant growth stimulation of fast-growing roots was induced regardless of the amount of IAA supplied, but the growth of slow-growing roots was stimulated with very low amounts of IAA ([Symbol: see text]100 pg/root after 4 h). Inhibition of growth of both classes of roots increased with amounts of IAA between 100-600 pg/root after 4 h, but no further inhibition of growth appeared after treatment with greater amounts of IAA. A model based on these data shows a possible relationship between the uptake of IAA and root growth and indicates a clear relationship between an optimal level of IAA and root growth. The responses of the elongating cells of primary maize roots to applied IAA depend on their developmental stage and on the initial growth rates of the root.