The Journal of General Physiology, Vol 79, 835-848, Copyright © 1982 by The Rockefeller University Press
Avoidance and rheotropic responses in phycomyces. Evidence for an 'avoidance gas" mechanism
RI Gamow and B Bottger
If a mature sporangiophore is placed next to a barrier that is moving in a
clockwise direction, it grows both away from the barrier and into the wind;
the wind is generated by the moving barrier itself. When the barrier is
moving in a counterclockwise direction, the sporangiophore grows towards
both the barrier and the wind. The net direction of growth appears to be
the vector sum of the rheotropic response and the avoidance aiming error
and does not involve the classic stationary- barrier avoidance response.
Our experiments all support the suggestion that the avoidance response, the
rheotropic response and the variety of reported wind responses can be
explained by the presence of a self- emitted, growth-simulating avoidance
gas. We present data that suggest that it is the direction of the net flux
(mass transfer) of this gas that determines both the direction and the
magnitude of the sporangiophore growth. We further suggest that the region
of the cell wall showing maximum mass transfer will show a minimum growth
rate, i.e., the direction of growth will always be in the direction of
maximum transfer. If water is the avoidance gas, then it would follow that
the total hydration of the cell wall in an aqueous salt solution should
result in cell wall softening; cell wall softening has been correlated
directly to cell wall growth. Using the Instron technique, we now show that
submerging the entire sporangiophore in an aqueous salt solution for 4 min
causes an increase in cell wall extensibility.
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