Effect of red light on irreversible and reversible expansion of Avena coleoptile sections

1. The increase in the plasticity of Avena coleoptile sections by auxin which has been found in external solutions with an osmotic value, lower than a critical value (CLELAND, 1959) is observed only in sections from red-irradiated coleoptiles. Irradiation with red light of sections, taken from dark-grown seedlings and cut in dim green light, has no effect on the plasticity of the sections. 2. Different zones of the coleoptile show different responses to small and large doses of red light. 3. Exposure of the coleoptiles to red light during two minutes (Intensity 40 μW.cm(-2)) induces a short-term increase of plasticity of the sections (90 minutes after irradiation) in solutions of low osmotic value. This short-term effect of red light interferes with a long-term (16 hours after irradiation) decrease of the suction force of the sections and a decrease of plasticity over the whole range of osmotic values, between pure buffer and the value causing incipient plasmolysis. 4. Only the long-term reduction of plasticity is reversed by far-red irradiation applied after the red irradiation. 5. In most cases, the elastic stretching was not influenced by red light when applied 90 minutes before sectioning, but in some experiments it was increased.