Gravitropism in Higher Plant Shoots

Abstract

It has long been known that applied ethylene can redirect the gravitropic response, but only occasionally has it been suggested that ethylene normally plays a role in gravitropism. Two inhibitors of ethylene synthesis [Co²⁺ and aminoethoxyvinylglycine (AVG)] and two inhibitors of ethylene action (Ag⁺ and CO₂) were 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 delays the response of defoliated and of decapitated plants. Plants laid on their side and restricted so that they cannot bend upward store both bending energy and gravitropic stimulus; they bend immediately when released from restriction (stored energy) and continue to bend for some hours after (stored stimulus). 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.