Gravitropism in higher plant shoots. I - A role for ethylene

Two inhibitors of ethylene synthesis, Co(2+) and aminoethoxyvinylglycine (AVG), and two inhibitors of ethylene action, Ag(+) and CO2, are shown to delay the gravitropic response of cocklebur (Xanthium strumarium L.), tomato (Lycopersicon esculentum Mill.), and castor bean (Ricinus communis L.) stems. Gentle shaking on a mechanical shaker does not inhibit the gravitropic response, but vigorous hand shaking for 120 seconds delays the response somewhat. AVG and Ag(+) further delay the response of mechanically stimulated plants. AVG retards the storage of bending energy but not of stimulus. In gravitropism, graviperception may first stimulate ethylene evolution, which may then influence bending directly, or responses involving ethylene could be more indirect.