ASGSB 1998 Annual Meeting Abstracts

[82]
THE GRAVITROPIC RESPONSE OF CHARA PROTONEMATA IS REDUCED IN MODERATE HYPERGRAVITY.   A. Sievers and D. Hodick. Botanisches Institut, Universitдt Bonn, Venusbergweg 22, D-53115 Bonn, Germany.

The interrelation between the position of the statoliths and the direction and rate of tip growth in the negatively gravitropic protonemata of the alga Chara globularis was studied with the centrifuge-video microscope NIZEMI. Protonemata were regenerated from isolated thallus-nodes, which were embedded in agar and kept in darkness for approx. 2 weeks. Protonemata placed perpendicular to the acceleration vector (stimulation angle 90 ) showed a gradual reduction of the gravitropic curvature with increasing accelerations from 1 g to 8 g despite a complete sedimentation of all statoliths on the centrifugal cell flank. It is argued that the increased weight of the statoliths in hypergravity impairs their acropetal transport which is induced when the cell axis deviates from the normal upright orientation. When the statoliths were centrifuged deep into the apical dome at 6 g and a stimulation angle of 170 the gravitropic curvature measured after 1 h was identical to the one determined for the same cells at 1 g and the same stimulation angle. This indicates that the gravitropism in Chara protonemata either is independent of the pressure exerted by the statoliths on an underlying structure or that it is already saturated at 1 g. When the statoliths were moved along the apical cell wall in a centrifugation at 8 g and a gradual increase of the stimulation angle from 170 to 220 the gravitropic curvature sharply reverted when the cluster of statoliths passed over the cell pole. The experiments support the hypothesis that in Chara protonemata statoliths distributed asymmetrically inside the apical dome displace the Spitzenkцrper and thus the center of growth, which results in gravitropic bending.

(Supported by DLR - The AGRAVIS Project -)

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