Phototropism, traumatropism and blue light effects on seedling growth
The growth events which are responsible for the phototropic curvature of seedlings towards blue light were investigated by growth analysis of seedlings of Avena sativa L. Time-lapse video analysis of coleoptiles given equivalent unilateral or bilateral blue light stimuli showed that both the duration and fluence-rate of the light treatments affected the growth responses which were recorded. The results suggest that the growth responses in bilateral light occur simultaneously with the growth changes which cause phototropic curvature, although they do not appear to make any contribution to the magnitude of curvature. Seedlings were also subjected to continuous unilateral microbeam (1mm) irradiation of discrete zones, to investigate whether 'message' transmission occurs in phototropism. The growth changes which occur in response to this treatment were recorded over the whole length of a 20 mm seedling, but began at different times in different zones. It was concluded that there must be some kind of 'message' transmission following perception of a phototropic stimulus in Avena. The kinetics of the response showed that it was unlikely that transmission could be simply due to tissue light-piping from the position of the original light beam. Unilateral irradiation of a small zone at the base of a coleoptile resulted in negative curvature. This experiment provided some evidence for acropetal transmission of the phototropic signal. A study was also made on the effects of wounding on the growth of Avena seedlings. This led to a re-evaluation of the significance of the traumatropic response to unilateral wounding. The experiments showed that even a slight wound can have severe effects on the growth of a coleoptile. The results are discussed in relation to previous theories on the mechanism of traumatropic curvature.