Gravitropism in Higher Plant Shoots

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Abstract
Dimensional changes during gravitropic bending of cocklebur (Xanthium strumarium L.) dicot stems were measured using techniques of stereophotogrammetry. The differential growth is from an increased growth rate on the bottom of the stem and a stopping or contraction of the top. Contraction of the top was especially evident upon release and immediate bending of horizontal stems that were restrained between stiff wires for 36 h. The energy for this could have been stored in both the top and bottom, since the bottom elongated and the top contracted. Forces developed during bending were measured by fastening a stem tip to the end of a bar with attached strain gauges and recording electrical output from the strain gauges. Restrained mature cocklebur stems continued to accumulate potential energy for bending for .apprx. 120 h, after which the recorded force reached a maximum. Pressures within castor bean (Ricinus communis L.) stems were also measured with 3.5-mm diameter pressure transducers. The pressure on the bottom of the restrained plants increased with time; pressures decreased in vertical controls, tops of restrained stems and bottoms of free-bending stems. Pressures increased in tops of free-bending stems. When restrained plants were released, pressure on the bottom decreased and pressure on the top increased. A possible role for cell contraction in the top of stems bending upward in response to gravity is suggested.

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