Phototropic response of the stage I Phycomyces sporangiophore to a pulse of blue light.

Positive phototropic curvature of Phycomyces sporangiophores (stage I) induced by a short pulse (10 sec) of blue light has been characterized. The curvature response, measured 40 min after irradiation, increases with increasing fluences up to approximately 10(-4) J.m(-2); it then decreases and becomes nearly 0 at a fluence of approximately 10(-1) J.m(-2). The threshold fluence is found near 10(-7) J.m(-2), indicating over 1000-fold higher sensitivity than the first positive phototropism of grass coleoptiles. Positive curvature of the sporangiophore becomes detectable 5 min after irradiation and increases with time up to approximately 40 min. During the course of this curvature development, overall elongation growth of the sporangiophore is first stimulated and then inhibited. The relationship between the phototropic response and the response in overall elongation growth is discussed, and it is suggested that the primary mechanisms of these responses are distinct.