The photocontrol of stem elongation in light-grown Sinapis alba L.

An investigation has been made into the mechanisms involved in the photocontrol of stem elongation in the ruderal species, Sinapis alba L; particularly those mechanisms concerned with the changes in stem elongation mediated by variations in the degree of vegetational shade. Throughout the investigation an attempt was made to grow plants in conditions which were as natural as possible; i.e., in white light at a suitable fluence rate and with a 16h photoperiod. The effects of white light fluence rate and redrfar-.;d ratio (R:FR), on hypocotyland internode elongation are compared and contrasted; but the main thrust of the thesis is an examination of the rapid increase in internode elongation rate which occurs in response to reductions in R:FR perceived by the internode itself. The R:FR was reduced by giving supplementary far-red light directly to the internodes of plants in background white light, and linear displacement transducers were used for the high-resolution, continuous monitoring of elongation. The elongation rate was inversely and linearly related to the phytochrome photoequilibrium, set up by the light treatment, over a wide range of values. Moreover, phytochrome cycling rate is not involved in modulating internode elongation, and elongation rates can only be related to Pfr concentration if the total amount of phytochrome is constant. A similar rapid response to supplementary far-red has been demonstrated in isolated internode segments, and this has allowed a study of the effect of R:FR on cell enlargement. From this study an hypothesis is proposed that suggests phytochrome modulates cell enlargement by regulating the secretion of protons into the cell wall. Supplementary far-red could cause a lowering of cell wall pH, which may bring about the displacement of Ca.