Photoresponses of the Fungus, Phycomyces

Abstract

Data describing the light-sensitive stimulus-response system of the mature sporangiophore of Phycomyces blakesleeanus, are reviewed. The sporangiophore exhibits 2 well-defined responses, a phototropic response in which there is a redistribution of growth in space, and a light-growth response in which growth is redistributed in time. The light sensitivity for both responses is localized within a 3-mm section (the growing zone) just below the sporangium and this light sensitivity can change so that the sporangio.-phore is equally sensitive over a range of intensity of almost a billionfold. The observed growth and growth responses take place in the wall of the growing zone. The distribution of both steady-state growth and growth responses was mapped by observing the position of starch grain markers with time and by measuring responses to narrow line stimuli of blue or ultraviolet light. In order to explain the growth and tropic responses observed for various programs of stimuli, the existence of at least 3 concentric, independent, sliding, cylindrical shells was postulated within the growing zone. Although the initial steps of tropic and growth responses must involve the same mechanism, they are quite different by the time the end response is produced. They are not distinct merely because of changes in growth distribution produced by differences in light intensity distribution. The growth response is distributed throughout the growing zone and exhibits local autonomy of stimulus and response while the tropic response takes place at the base of the growing zone. The growth response is transient (completed after 15 minutes) while the tropic response can continue for many hours without a change in intensity of the stimulus.