Tissue 8

Tissue stress (TS) is defined as the stress which acts on atissue layer in an organ in excess of the turgor-induced tensilestress which also acts on the layer when it is isolated fromthe organ. The tensile TS in one layer of an organ is alwaysaccompanied by com-pressive TS in another layer. To calculateTS from data obtained for isolated tissue, one needs to knowthe Poisson ratios V_IKcontraction 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) tissueextensions caused by uniaxial stress (due to applied force)and multiaxial stress (due to turgor pressure); (ii) extensionsof tissues subjected to lateral constraint, and (iii) TSs indifferent directions. The ratios V_IW and V_WI, for the stressapplied 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 tworatios for a particular OT differed considerably. The ratiosdepended on the applied extension (strain). Knowing them, thetensile force (F₁) generating TS in the OT of an intact organcould be calculated from the longitudinal force (F_I(s)) whichwhen applied to the isolated (unconstrained laterally) OT restoredits original length. In the case of the sunflower hypocotyls,F_I(s)<F₁<1.3 F_I(s). The ratio V_Ir (r denotes the radialdirection), which was determined for segments of inner tissue,from sunflower hypocotyls and tulip stems, did not depend onthe applied stress (extension). This ratio allowed us to calculatethe relationship between the strain changes caused by equalchanges of uniaxial and multiaxial stresses: the uniaxial stresswas approximately 3-fold more efficient than the multiaxialstress. Key words: Elastic strains, Poisson ratios, tissue stresses, tissue elasticity, uni- and multiaxial stresses.