DETERMINATION OF THE LONGITUDINAL TISSUE STRESSES IN THE GROWING AND NONGROWING REGIONS OF SUNFLOWER HYPOCOTYLS

The longitudinal tissue stresses (tension and compression) in the apical (growing) and basal (non-growing) regions of etiolated sunflower hypocotyls (Helianthus annuus L.) were determined. The compressive stress exerted by the outer tissues (OT, epidermis with 2-3 cortical cell layers attached) on the inner tissues (IT, cortex, vascular tissue and pith) was estimated by placing peeled sections (IT) in a series of osmotic test solutions (polyethylene glycol 8000) and determining the external osmotic pressure at which no length change occurs. The compressive stresses were found to be 0.5 MPa in the apical and 0.39 MPa in the basal stem region, respectively. The tensile stress in the OT was determined by placing isolated (contracted) strips of OT between the clamps of an extensiometer. The force per unit cross-sectional area that is necessary to reextend the contracted OT to its original in vivo length was determined. The tensile stresses were found to be 2.0 MPa in the apical and 1.6 MPa in the basal stem region, respectively. It is concluded that the longitudinal tissue stresses have the function of increasing the mechanical stiffness and bending resistance of the hypocotyl.