| Abstract: | Segmental analysis of the laminar pulvinus of Phaseolus vulgaris L. showed that its phototropic curvature is accompanied by efflux of inorganic ions and water from its contracting sector and a comparable influx into its expanding one. All the major ions, except Na+, contributed to this transport, suggesting that the response to light involves changes in the driving force, or conductivity of a wide range of solutes. During the curvature, K+ and CI? made the greatest and equivalent contributions to efflux, but only Cl? exhibited a matching influx into the expanding sector, while K+ influx was much less. Use of the cell pressure probe showed that, as the laminar angle of elevation changed between ?40° to +40°, turgor pressure in the expanding motor cells increased by 0.48 MPa and decreased in the contracting cells by 0.32 MPa. Picoliter osmometry of single-cell samples showed that during this movement vacuolar osmotic pressure remained constant. Thus, changes in turgor pressure resulted from changes in apoplastic, rather than the protoplastic osmotic pressure. Volumetric modulus of elasticity of pulvinar motor cells is very low, showing that their walls are very elastic. These properties increase the effectiveness of converting osmotic work into the large-scale, reversible volume changes responsible for leaf movements. |
