Phytotropins, even those not absorbing in the visible region of the sp
ectrum, can induce a phototropic response in maize (Zea mays L. cv. PX
-75) roots when illuminated unilaterally with white light. The most ac
tive phytotropin, 2-(1-pyrenoyl) benzoic acid (PBA) can elicit a full
response at 10 muM, while the other active molecules, 2-carboxyphenyl-
3-phenylpropane-1,3-dione (CPD), 2-carboxyphenyl-3-phenyl-1,2-pyrazole
(CPP), 1-N-naphthylphthalamic acid (NPA) and erythrosin elicit a full
response at 100 muM. The less active phytotropins BBA and fluorescein
give a reduced response. It is suggested that the observed effect can
not be explained solely on the basis of auxin transport inhibition. Th
ere is a photoreceptor in the extension zone of the root, which may be
associated in some way with the receptor for NPA. The results are con
sistent with the proposal that the phototropic process may form part o
f the root gravitropic response mechanism.