Kinetics for Phototropic Curvature by Etiolated Seedlings of Arabidopsis thaliana 1

An infrared-imaging system has been used to study the influence of gravity on the kinetics of first positive phototropism. The development of phototropic curvature of etiolated seedlings of Arabidopsis thaliana was measured in the absence of visible radiation. Following a pulse of blue light, stationary seedlings curved to a maximum of approximately 160 about 80 minutes after stimulation. The seedlings then curved upward again or straightened by about 60 during the subsequent 100 minutes. Seedlings rotated on a clinostat reached a similar maximum curvature following photostimulation. These seedlings maintained that curvature for 30 to 40 minutes before subsequently straightening to the same extent as the stationary seedlings. It is concluded that straightening is not a consequence of gravitropism, although gravity has some effect on the phototropism kinetics. Since the seminal experiments of Darwin (2), it has been recognized that phototropic curvature of a plant shoot toward a unilateral light source results in the shoot receiving an opposing gravitational stimulus. Thus, it would be expected that the shoot would grow upward again or straighten as a result of gravitropism during or following the phototropic curvature. Because gravitropic curvature may confound phototropic curvature, the kinetics of phototropism have been of interest to a number of investigators during the last 100 years. The kinetics of phototropism have been measured in a variety of experimental systems from the fungus, Phycomyces (4), to monocotyledonous plants, such as maize (13) and Avena (14, 15), and dicotyledonous plants, such as cress (6). In many of these experiments, seedlings were placed on a clinostat following photostimulation to minimize the effect of gravity on the development of the phototropic response. The generally accepted conclusion of these studies is that the seedling will curve upward as a consequence of negative gravitropism following curvature toward a unilateral light source (13, 15). Our original intentions were twofold. First, we sought to study the kinetics of the phototropic curvature induced by a pulse of BL2 in etiolated seedlings of the dicotyledonous Supported by the U.S. Department of Energy under contract No. DE-ACO2-76ERO-1338 and the U.S. National Aeronautical and Space Administration under grant NAGW-882 to K.L.P. 2 Abbreviations: BL, blue light; RL, red light. species, Arabidopsis thaliana. Second, these studies were expected to show that the reversal of curvature or straightening of the seedling following phototropic curvature was a consequence of gravitropism. We characterized the kinetics of phototropic curvature of seedlings on and off a clinostat. Our results indicate that gravitropism has little effect on this straightening. MATERIALS AND METHODS