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Kinetics of upward bending response of gravistimulated snapdragon shoots

Abstract

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During gravitropism the top of snapdragon (Antirrhinum majus) stalks re-orientes by bending upward. This research is devoted to enhance the knowledge of upward bending kinetics and to develop new methods for studying the gravitropic curvature. The snapdragon shoot was held in a horizontal position in the light at room temperature and color slides ware taken at intervals of 15 minutes during the upward bending response. With imaging processing software, the scanned slides were put on top of each other to create a time-lapse photograph of the whole bending process. By projecting the slides, gravitropic curvatures were measured with a protractor. Bending the tip occurs at a zone just before the first open flower and starts after a lag phase of around 30-60 minutes. The gravitropic response can be divided into two clear phases. During the first one, a linear bending rate was abserved of 28 degrees/h until a maximum angle of 140 degrees was reached. In the second phase, the tip of the shoot bent back to its vertical position. This 'overshoot' correction was considerably slower and took around 30 hours to complete. When the shoot was re-orientated to its original vertical position, the tip bent again, but this time at a different place. This means that the bending zone moves along the rapidly elongation shoot. The morphological and physiological state of the bending zone could therefore be of great importance to our understanding of the mechanism of gravitropic response in flowering snapdragon shoots.