Phototropism in light-grown mustard (sinapis alba l.) seedlings.

This thesis is an investigation of the mechanisms of blue-light- mediated phototropism in higher plants. Phototropism was analysed in light-grown mustard (Sinapis alba L.) seedlings under low pressure sodium (SOX) lamps to minimise the involvement of phytochrome. Light-grown mustard seedlings do not show a blue-light-mediated inhibition of axis extension growth and the Blaauw theory must therefore be rejected as an explanation of phototropism. Phototropic curvature was established by an inhibition of growth on the illuminated side of the hypocotyl accompanied by an equal but opposite acceleration on the shaded side, with little or no change in net growth. This pattern of differential growth can be modified by light-growth responses separate from those involved in phototropism. Preliminary results from a biophysical analysis of cell growth using micro-pressure probes indicates phototropic growth responses are caused by changes in wall rheological properties, possibly by wall extensibility alone. There is a complex relationship between the kinetics of phototropism and stimulus fluence rate. The lag time is independent of fluence rate. The initial rate of curvature was directly proportional to log fluence rate between threshold and saturation fluence rates. Following this initial phase, gravitropic compensation and auto-tropic straightening modify curvature, which can no longer be used as a quantitative measure of phototropism. Implications for measurement of the response and selection of appropriate photobiological conditions are outlined. The magnitude of the internal light gradient influences the rate of curvature. The experimental results are discussed in relation to other knowledge of the phototropic transduction chain. The results are not inconsistent with the Cholodny-Went theory of phototropism. Preliminary data indicating phytochrome-mediated phototropism are presented, and the comparison of lag times in plant physiology is also discussed.