Growth, mechanical stress responses and mutation of sunflower seedling hypocotyls
The aims of this thesis were four-fold: 1. To characterise the patterns of growth underlying established nutations of etiolated sunflower seedling hypocotyls. 2. To characterise the growth patterns and events surrounding the initiation of nutation in etiolated sunflower seedling hypocotyls. 3. To investigate the effects of applied compression and tension forces on the straight growth of etiolated sunflower seedling hypocotyls. 4. To investigate the relationship between auxin and the responses to mechanical stress in etiolated sunflower seedling hypocotyls. Time-lapse video analysis showed that the growth of etiolated sunflower seedling hypocotyls was more dynamic than expected, with growth constantly changing. When growth rates were plotted against time, the resultant curves showed regular oscillations similar to those of the movement traces for nutation and it was apparent that the nutational movements were intimately linked to the patterns of growth. Such patterns of growth were also found prior to the onset of nutation but in that case the growth curves on opposite sides of the hypocotyl did not oscillate out of phase with each other until immediately before the start of nutation. It was also found that there was a threshold height below which nutations were never found. The roles of light and geotropic responses in the initiation of nutation were also investigated. It was found that light was not required for nutation to occur and that various types of geotropic stimulation failed to initiate nutation in seedlings below the threshold height, although such seedlings were capable of responding to a gravity stimulus. It was concluded that the initiation and maintenance of nutation is an endogenous process and that nutation is a result of the way in which seedling hypocotyls grow rather than a discrete phenomenon superimposed on `normal' growth. The application of compression forces increased the growth of etiolated sunflower seedling hypocotyls over that of the non-compressed controls while tension forces applied to the same material did not produce any significant change in growth rates. When compression forces were applied to seedlings immersed in 10-10 Molar auxin solution the increase in growth due to compression were larger than those found for seedlings compressed while immersed in distilled water. It was concluded that some mechanism for the reception of compression forces operates in this material possibly involving a change in the sensitivity of the epidermal cells to auxin. A model was proposed in which growth is controlled by the mechanical forces acting on individual cells. It is suggested that nutation arises from the responses of cells and groups of cells to such mechanical strains.