Roots bend in response to a change of orientation in the gravitational field. Two parameters are used to quantitatively estimate the sensitivity of plants to gravistimulus: 1) the minimum duration of the stimulation necessary to evoke a response Opresentation timeO. 2) the total duration of stimulation required to produce a response, Operception timeO, if the plant is presented with repeated stimuli of duration smaller than the presentation time. The object of the experiment is to measure the presentation time of primary lentil roots grown in microgravity. Experiment Specimens: Lens culinaris L. (Verte du Puy), dry seeds, placed on cellulose sponge, in small growth chambers.

The cellulose sponge was hydrated in microgravity conditions by injecting water via a syringe. The mini growth chambers permitted gas exchange between the seeds and the incubator, kept at= 22degC. The seedlings were grown in darkness for 28 hours, then placed on a 1g centrifuge for various times (5, 10, 15, 20, 35 and 60 minutes) in such a way that the root axis was either perpendicular or oblique to the 1g acceleration. Then the minicontainers were placed in front of a photocamera, in the PHOTOBOX, and photographs were taken at 10 minutes intervals. A green filter (560 nanometers) was interposed between the flash and the seedlings. The analysis of the gravitropic reaction as a function of time was done from colour prints as shown in Fig. 1. For all roots there was a significant correlation between curvature and time, during all the period of fast bending. Comparison is made with controls grown on ground in the vertical position and stimulated in the horizontal position, under conditions similar to the space conditions.

The gravitropic reaction of representative lentil roots grown in space for 28 hours, stimulated for 5, 10, 35 and 60 minutes on the 1g centrifuge and then returned to microgravity, is presented in Fig. 2 Typical responses of individual roots stimulated on ground are given in Fig. 3. These evidence a latent time (TL), during which no bending is observed, a second phase of maximal rate of curvature, and a third phase in which there is a strong decrease in the rate of curvature. In the space experiments, it was not possible to determine the latent time TL, since the first photograph was taken after a period greater then TL. The essentially linear phase of maximal rate of curvature (slope of the curve during the first 80 minutes), as well as the amplitude of the curvature, varied as a function of the quantity of stimulation. The correlation coefficient with a straight line, "cor" in the figures, is similar in flown samples and in controls. The curvature continued after the end of the stimulation, evidencing a certain inertia. Plotting the initial rate of curvature as a function of the logarithm of the quantity of stimulus Q(g x minutes) the presentation time is found to be 27 seconds. This is similar to those found by Larsen (2), Caspar and Pickard (3) and Kiss et al.(4), confirming that the clinostat may be used to analyse the presentation time. These findings are consistent with the alternative hypotheses that the amyloplasts exert a pressure on the cytogel lining the longitudinal wall of the statocyte, or that they could exert tension on the network of actin filaments of their cytoskeleton.