Phototropism and photoinhibition of basipolar transport of auxin in oat coleoptiles

We have proposed that the lateral inequality of auxin associated with the phototropic response is a consequence of a light-induced impairment of basipolar transport of the hormone. If this is so, the dose-response curve for photoinhibition of auxin transport should resemble that for phototropism. Further, the wavelength dependencies of the transport and tropic response should also be similar. Oat coleopilles were irradiated equilaterally with white light and with broad- and narrow-spectral bands of blue light. Indoleacetic acid 2-¹⁴C was then applied apically to the intact coleoptile. Irradiation by the 3 sources inhibited basipolar transport of the auxin. The photoinhibition of transport increases with exposure, reaches a maximum at radiant energies depending on the source used, and then decreases. The dose response for transport inhibition matches that for phototropism. A correspondence in the spectral response of the 2 phenomena is also found for coleoptiles exposed to first positive energies at various wavelengths from 340 to 730 nm. We interpret these correlations as support for the hypothesis that phototropism is mediated by a photoinhibition of basipolar transport of auxin.