Turgor Pressure and Phototropism inSinapis albaL. Seedlings

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Abstract
Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropism in 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−2 s−1 unilateral blue light to a background of low pressure sodium (SOX) lamp light. Curvatures of some 6° from the vertical were reached by 60 min, the curvature rate between 20 min and 60 min being 0.14° min−1. From the axial growth rate and tissue geometry the local growth rates of illuminated and shaded sides of the hypocotyl were calculated to be 1.5 and 4.5 μmin−1 respectively. Turgor pressures of expanding cells in control plants and in the shaded and illuminated sides of the blue light illuminated hypocotyls were measured to be 0.40-0.55 MPa with a pressure probe. No changes in turgor pressure were observed on initiation of curvature. The decay of pressure in the cells of non-transpiring plants following excision indicated that the yield stress threshold of the tissue may be as low as 0.1 MPa. These results indicate that the phototropic growth response in this tissue is not mediated by changes in turgor pressure.