TISSUE STRESSES IN ORGANS OF HERBACEOUS PLANTS .1. POISSON RATIOS OF TISSUES AND THEIR ROLE IN DETERMINATION OF THE STRESSES

Tissue stress (TS) is defined as the stress which acts on a tissue layer in an organ in excess of the turgor-induced tensile stress which also acts on the layer when it is isolated from the organ, The tensile TS in one layer of an organ is always accompanied by compressive TS in another layer, To calculate TS from data obtained for isolated tissue, one needs to know the Poisson ratios nu(ik) = contraction in the k-direction/extension in the i-direction for the tensile force applied in the i-direction for that tissue, Poisson ratios apply in the relationships between (i) tissue extensions caused by uniaxial stress (due to applied force) and multiaxial stress (due to turgor pressure); (ii) extensions of tissues subjected to lateral constraint, and (iii) TSs in different directions. The ratios nu(lw) and nu(wl), for the stress applied either longitudinally (I) or in the direction of width (w), respectively, were determined for the outer tissue (OT) of sunflower hypocotyls, tulip leaves and tulip stems, The two ratios for a particular OT differed considerably, The ratios depended on the applied extension (strain), Knowing them, the tensile force (F-l) generating TS in the OT of an intact organ could be calculated from the longitudinal force (F-l(s)) which when applied to the isolated (unconstrained laterally) OT restored its original length, In the case of the sunflower hypocotyls, F-l(s) < F-l < 1.3 x F-l(s). The ratio nu(lr) (r denotes the radial direction), which was determined for segments of inner tissue, from sunflower hypocotyls and tulip stems, did not depend on the applied stress (extension). This ratio allowed us to calculate the relationship between the strain changes caused by equal changes of uniaxial and multiaxial stresses: the uniaxial stress was approximately 3-fold more efficient than the multiaxial stress.