TURGOR AND LONGITUDINAL TISSUE PRESSURE IN HYPOCOTYLS OF HELIANTHUS-ANNUUS L

The quantitative relationship between turgor and the pressure exerted by the inner tissues (cortex, vascular tissue, and pith) on the peripheral cell walls (longitudinal tissue pressure) was investigated in hypocotyls of sunflower seedlings (Helianthus annuus L.). In etiolated hypocotyls cell turgor pressures, as measured with the pressure probe, were in the range 0.38 to 0.55 MPa with an average of 0.48 MPa. In irradiated hypocotyls turgor pressures varied from 0.40 to 0.57 MPa with a mean at 0.49 MPa. The pressure exerted by the inner tissues on the outer walls was estimated by incubating peeled sections in a series of osmotic test solutions (polyethylene glycol 8000). The length change was measured with a transducer. In both etiolated and irradiated hypocotyls an external osmotic pressure of 0.5 MPa was required to inhibit elongation of the inner tissues, i.e. the average cell turgor and the longitudinal tissue pressure are very similar quantities. The results indicate that the turgor of the inner tissues is displaced to and borne by the thick, growth-limiting peripheral cell walls of the hypocotyl.