Effects of red light on the fluence–response relationship for pulse‐induced phototropism of maize coleoptiles

Dark‐adapted coleoptiles of maize ( Zea mays L.) were treated with red light (3min at 10.5 μmol m −2 S −1 ) and were Stimulated, after a dark interval, with a pulse of unilateral blue light to induce phototropism. Phototropic fluence‐response curves were obtained in this way for different dark intervals. It was confirmed that the bell‐shaped fluence‐response curve for the first pulse‐induced positive phototropism (FPIPP) shifts to higher fluences following the red‐light treatment, the maximal shift being achieved at a dark interval of 2h. We found, however, that the two arms of the Fluence‐response curve do not shift synchronously. The shift of the descending arm to higher fluences began at 15 min. The ascending arm showed a slight shift to lower fluences before a greater shift to higher flucnces. the change of the shift direction occurring at 30–40min. Accordingly, the fluence‐response curve obtained for a 30 min dark interval was comparatively wide. Although dark‐adapted coleoptiles showed only fPIPP, another bell‐shaped fluence‐response curve, representing the second pulse‐induced positive phototropism (sPIPP), appeared gradually after the red‐light treatment. These changes of the phototropic fluence– respnse curve following exposure to red light are likely to have adaptive values because they favour phototropism under brighter light.