Gravitropism in Phycomyces: Threshold determination on a clinostat centrifuge

The absolute sensitivity of sporangiophores of Phycomyces blakesleeanus to centrifugal acceleration was determined on a clinostat centrifuge. The centrifuge provides centrifugal accelerations ranging from 10−4 to 6 ×g. The rotor of the centrifuge, which accommodates 96 culture vials with single sporangiophores, is clinostatted, that is, turning «head over», at slow speed (1 rev min−1) while it is running. The negative gravitropism of sporangiophores is characterized by two components: a polar angle, which is measured in the plane of bending, and an aiming-error angle, which indicates the deviation of the plane of bending from the vector of the centrifugal acceleration. Dose-response curves were generated for both angles with centrifugations lasting 3, 5, and 8 h. The threshold for the polar angle depends on the presence of statoliths, so-called octahedral protein crystals in the vacuoles. The albino strain C171 carAcarR (with crystals) has a threshold near 10−2 × g while the albino strain C2 carAgeo-3 (without crystals) has a threshold of about 2 × 10−1 × g. The threshold for the aiming error angle is ill defined and is between 10−2 and 10−1 × g. The threshold for the polar angle of the wild type NRRL1555 (with crystals) is near 8 × 10−2 × g.