Sensitivity to gravistimulus of lentil seedling roots grown in space during the IML 1 Mission of Spacelab.

The gravitropic curvature of seedlings of lentil (Lens culunaris L. cv. Verte du Puy) grown in microgravity and stimulated on the 1 g centrifuge for 5 to 60 min was followed by time lapse photography in near weightlessness in the frame of the IML 1 Mission of Spacelab. In microgravity, the root tip could overshoot the direction of the 1 g acceleration after bending whereas roots stimulated on the ground did not reach the direction of the gravity vector. On earth, there is, therefore, a regulation (inhibition of root curvature), which is gravity dependent. In space, the initial rate of curvature as well as the amplitude of curvature varied as a function of the quantity of stimulation (Q, in gmin). For a given quantity of stimulation, the rate of curvature remained constant for 80 min. The bending has thus a certain inertia, which is linked to the mechanism of differential growth. The presentation time (Tp) of the lentil root was calculated by extrapolation to zero curvature of the regression line representing either the initial rate of curvature or the amplitude of curvature at 2 h after the end of the stimulation. Tp was estimated to 27 and 26 s, respectively. These results confirm the values of Tp obtained by clinostats, and they also lead to a reconsideration of the causes of the kinetics of root curvature.