The phototropism has fascinated plant biologists since the pioneering work of Charles and Francis Darwin using grass coleoptiles. This tropic response is the result of differential growth on the two sides of the elongating shoot, which can be explained by the asymmetrical IAA distribution. We have previously shown that the coleoptile 2-mm tips can synthesize IAA and the synthesized IAA is immediately transported basipetaly. However, it is still unclear what events happen from the lateral blue light irradiation to formation of asymmetric IAA distribution. Here, we established system to analyze phototropic curvature by using excised coleoptiles. The 2-cm segment of coleoptiles detached from maize seedlings was irradiated with unilateral blue light for 8s at 0.33μmol・m^<-2>・s^<-1> (first positive curvature), for continuity at 10μmol・m^<-2>・s^<-1> (second positive curvature) or for 200s at 0.25μmol・m^<-2>・s^<-1> (no curvature). As the results of observation, it was shown that each response were similar to those of previous reports with maize seedlings. In the first positive curvature, coleoptiles began to be straight in vertical direction after 60min irradiation. To investigate the possible participation of gravity stimulus on this negative curvature, we observed first positive curvature under nominally zero-gravity condition by using clinostat. Continuous curvature was observed under this condition, it was suggested that the negative curvature occurred due to gravity.
