Red Light and the Geotropic Response of the Avena Coleoptile

Abstract

Red light (655 [mu]i) induced a series of changes in the geotropic responsiveness of dark-grown coleoptiles of Avena sativa. The responsiveness increased immediately after a few minutes'' irradiation but returned to a value similar to that of unirradiated coleoptiles 60-90 minutes later. The increased responsiveness was not prolonged by extending the exposure to light. Between the 7th and 23rd hours after irradiation the responsiveness of the coleoptiles declined to about 1/2 that of the unirradiated coleoptiles. Dark-grown coleoptiles were exceedingly sensitive to red light; 3.3 x 10-13 einsteins cm-2 (0.6 ergs cm-2) gave a significant increase, and 3.3 x 10-12 einsteins cm-2 (6.0 ergs cm-2) the maximum increase in the geotropic responsiveness. The Bunsen-Roscoe reciprocity law was valid for both these dosages. The effects of red light (655 m[mu]) were reversed by far-red light (740 m[mu]). After having been exposed to red light for a few minutes, 5-day-old coleoptiles were incapable of responding to a 2nd exposure regardless of whether this was given 4.5 hours or 24 hours after the 1st light treatment. The increased geotropic responsiveness was retained for at least 28 hours if the seedlings were chilled immediately after exposure to red light. The results suggest that the onset of the red-light treatment triggers off in the coleoptile a series of temperature-dependent reactions which proceed to completion at 25[degree] regardless of whether the coleoptiles are kept in red light or darkness, but which can be arrested by low temperatures. It appears that this series of reactions can be induced only once in the coleoptiles.