Growth Inhibition and Recovery in Roots Following Temporary Treatment with Auxin

Continuous recording (streak photography) of elongation of roots treated with IAA (10–6–10–7M) showed that removal of IAA from the nutrient solution resulted in a rapid resumption of elongation, unless the IAA treatment was shorter than 60 min. If it was shorter, the recovery was delayed, so that it occurred about 1 hour after the beginning of the treatment, independently of the duration of the treatment, down to 4 min. This behavior of roots was observed in all the species investigated (corn, pea, sunflower, onion), also in response to NAA and 2,4‐D. This time lag in recovery of elongation after brief auxin treatment is discussed in connection with the radial concentration gradient of auxin in the root imposed by external auxin. The possible role of a radial gradient of auxin (concentration decreasing with distance from the center) in the control of root elongation is suggested.