Effects of Certain Acid Growth-Regulating Substances and Their Corresponding Aldehydes on the Growth of Roots

Differential elongation of seedling roots of Artemisia absinthium in response to applied growth substances was used as the basis of a method for assay of growth substances. The use of only 0.1-ml. agar squares for testing the growth of ten roots and marked sensitivity of the roots render the method applicable for small quantities of growth substances. 2. The elongation of roots treated with four substances-3-indoleacetic acid (IAAcid), 1-naphthaleneacetic acid (NAAcid), 3-indole acetaldehyde (IAAld), and 1-naphthalene acetaldehyde (NAAld)-was expressed as a percentage of the elongation of untreated roots. 3. All four substances showed an inhibitory effect on root elongation over a certain range of concentrations (figs. 1, 2). In general, the aldehydes were one-tenth as effective in inhibiting root elongation as their corresponding acids. 1-Naphthalene acetaldehyde was one-tenth as effective as 3-indole acetaldehyde. 1-Naphthaleneacetic acid was less effective than 3-indoleacetic acid at highly inhibitory concentrations, more effective at slightly inhibitory concentrations. 4. For all four compounds, given concentrations were more inhibitory at 4 than at 24 hours. In no instance did elongation of treated roots exceed that of those untreated at the end of a 4-hour period of treatment. A 14% stimulation of root elongation after 24 hours of treatment with 3-indoleacetic acid at a concentration of 10-1 μgm/l and a 10% stimulation with 3-indole acetaldehyde at 10-3.2 μgm/l were statistically established. 5. Juice obtained from Artemisia roots had the capacity to convert 1-naphthalene acetaldehyde to 1-naphthaleneacetic acid. Concurrent with the conversion, however, some of the aldehyde was otherwise changed. 1-Naphthaleneacetic acid was stable in root juice. If the aldehyde has to be converted to acid in order to become active, the conversion must occur within 4 hours in the living root, since the inhibition of elongation observed with the aldehyde showed no increase from 4 to 24 hours. 6. All four substances at relatively high concentrations caused formative effects on roots. Root elongation was virtually stopped, subterminal swellings appeared, and root-hair production was greatly limited. At lesser concentrations, root growth was still strongly inhibited, and a thick fuzz of root hairs was observed, probably an effect caused by the occurrence of more epidermal cells, and therefore more root hairs, per unit root length.