Studies on Roots. V. Effects of Indoleacetic Acid on the Standard Root Growth Pattern of Phleum pratense

The effects of indoleacetic acid (IAA) on the elongation of Phleum pratense roots have been analyzed using photomicrographic records of the epidermal cells taken during growth. Concentrations applied to the root ranged from 10-5 to 10-11 M; the duration of the experimental period was about 5 hr. Effects can be separated into two categories Cells in their accelerating phase of growth and located close to the meristem (Phase I of Burström) were immediately inhibited by the higher concentrations (10-5-10-7 M) but after 1 hr began to recover their relative elemental growth rate, the extent of the recovery depending upon the strength of the application At lower concentrations (e g, 10-9 M) there may be a stimulation of the relative elemental growth rate Cells in their decelerating phase of growth and located in the basal portion of the elongating zone (Phase II of Burström) were irreversibly inhibited at all concentrations employed.