Turgor Pressure and Phototropism in Sinapis alba L. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropismin Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls.Phototropism was induced by adding 0.27 µmol m^–2s^–1 unilateral blue light to a background of low pressuresodium (SOX) lamp light. Curvatures of some 6° from thevertical were reached by 60 min, the curvature rate between20 min and 60 min being 0.14° min^–1. From the axialgrowth rate and tissue geometry the local growth rates of illuminatedand shaded sides of the hypocotyl were calculated to be 1.5and 4.5 µmin^–1 respectively. Turgor pressures ofexpanding cells in control plants and in the shaded and illuminatedsides of the blue light illuminated hypocotyls were measuredto be 0.40-0.55 MPa with a pressure probe. No changes in turgorpressure were observed on initiation of curvature. The decayof pressure in the cells of non-transpiring plants followingexcision indicated that the yield stress threshold of the tissuemay be as low as 0.1 MPa. These results indicate that the phototropicgrowth response in this tissue is not mediated by changes inturgor pressure. Key words: Sinapis alba L., phototropism, turgor pressure