CELL ELONGATION, TURGOR AND OSMOTIC-PRESSURE IN DEVELOPING SUNFLOWER HYPOCOTYLS

The relationship between cell elongation, change in turgor and cell os motic pressure was investigated in the sub-apical region of hypocotyls of developing sunflower seedlings (Helianthus annuus L.) that were gr own in continuous white light. Cell turgor was measured with the press ure probe. The same hypocotyl sections were used for determination of osmotic pressure of the tissue sap. Acceleration of cell elongation du ring the early phase of growth was accompanied by a 25% decrease in bo th turgor and osmotic pressure. During the linear phase of growth both pressures remained largely constant. The difference between turgor an d 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 o ne or both cotyledons had been removed. tissue pressure and turgor wer e very similar quantities under all experimental conditions. Our resul ts demonstrate that turgor and cell osmotic pressure show a parallel c hange during development of the stem. Cessation of cell elongation aft er removal of the cotyledons is attributable to a decrease in turgor ( tissue) pressure, which provides the driving force for growth in the h ypocotyl of the intact plant.