Inhibition by carbon dioxide of the phototropic response of the Avena coleoptile to blue light

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Abstract
When intact oat coleoptiles (Avena sativa var. Harmon) were submerged in water, saturation of the water with CO2 promoted their elongation but eliminated their phototropic response to blue light. Increasing the pH of the CO2-saturated water prevented the promotion of coleoptile elongation but did not prevent the elimination of the phototropic response. In air, phototropic curvature was significantly reduced by 10% CO2 and was eliminated by 30%, without any reduction in growth. It is postulated that the increase in CO2 concentration may eliminate the phototropic curvature of the coleoptile by preventing the light-induced inhibition of growth on the illuminated side of the organ. Possible mechanisms are briefly discussed.