Tropic reversal in Phycomyces.

Phototropic reversal of Phycomyces sporangiophores can be elicited by a change to darkness during steady-state phototropism. The reversal lasts 25-30 min under these conditions. Control experiments show that the reversal is not caused by gravitropism. Tropic reversal is also elicited by the removal of a barrier during an avoidance response, showing that the reversal occurs at the output of the sensory transduction chain.