Cell elongation, turgor and osmotic pressure in developing sunflower hypocotyls

The relationship between cell elongation, change in turgor and cell osmotic pressure was investigated in the sub-apical region of hypocotyls of developing sunflower seedlings (Helianthus annuus L.) that were grown in continuous white light. Cell turgor was measured with the pressure probe. The same hypocotyl sections were used for determination of osmotic pressure of the tissue sap. Acceleration of cell elongation during the early phase of growth was accompanied by a 25% decrease in both turgor and osmotic pressure. During the linear phase of growth both pressures remained largely constant. The difference between turgor and osmotic pressure (water potential) was −0.10 to −0.13 MPa. Excision of one cotyledon had no effect on growth, turgor and osmotic pressure. However, after removal of both cotyledons cell elongation ceased and a substantial decrease in both pressures was measured. In addition, we determined the longitudinal tissue pressure in seedlings from which one or both cotyledons had been removed. Tissue pressure and turgor were very similar quantities under all experimental conditions. Our results demonstrate that turgor and cell osmotic pressure show a parallel change during development of the stem. Cessation of cell elongation after removal of the cotyledons is attributable to a decrease in turgor (tissue) pressure, which provides the driving force for growth in the hypocotyl of the intact plant.