Biology Department, University of Massachusetts, Amherst, MA 01003, USA.
J Exp Bot. 2013 Nov;64(15):4697-707. doi: 10.1093/jxb/ert176. Epub 2013 Aug 3.
We review the role of anisotropic stress in controlling the growth anisotropy of stems. Instead of stress, growth anisotropy is usually considered in terms of compliance. Anisotropic compliance is typical of cell walls, because they contain aligned cellulose microfibrils, and it appears to be sufficient to explain the growth anisotropy of an isolated cell. Nevertheless, a role for anisotropic stress in the growth of stems is indicated by certain growth responses that appear too rapid to be accounted for by changes in cell-wall compliance and because the outer epidermal wall of most growing stems has microfibrils aligned axially, an arrangement that would favour radial expansion based on cell-wall compliance alone. Efforts to quantify stress anisotropy in the stem have found that it is predominantly axial, and large enough in principle to explain the elongation of the epidermis, despite its axial microfibrils. That the epidermis experiences a stress deriving from the inner tissue, the so-called 'tissue stress', has been widely recognized; however, the origin of the dominant axial direction remains obscure. Based on geometry, an isolated cylindrical cell should have an intramural stress anisotropy favouring the transverse direction. Explanations for tissue stress have invoked differential elastic moduli, differential plastic deformation (so-called differential growth), and a phenomenon analogous to the maturation stress generated by secondary cell walls. None of these explanations has been validated. We suggest that understanding the role of stress anisotropy in plant growth requires a deeper understanding of the nature of stress in hierarchical, organic structures.
我们回顾了各向异性应力在控制茎生长各向异性中的作用。通常，不是通过应力，而是通过柔度来考虑生长各向异性。各向异性柔度是细胞壁的典型特征，因为它们含有取向的纤维素微纤维，并且它似乎足以解释单个细胞的生长各向异性。然而，某些生长反应表明各向异性应力在茎生长中的作用，这些反应似乎太快，无法用细胞壁柔度的变化来解释，而且大多数生长茎的外表皮细胞壁的微纤维沿轴向排列，这种排列方式仅基于细胞壁柔度就有利于径向扩展。定量研究茎中的应力各向异性的努力发现，它主要是轴向的，并且原则上足够大，可以解释表皮的伸长，尽管其轴向微纤维。表皮经历源自内部组织的应力，即所谓的“组织应力”，这已被广泛认识；然而，主导轴向的起源仍然不清楚。根据几何形状，孤立的圆柱形细胞应该具有有利于横向的壁内应力各向异性。组织应力的解释涉及到弹性模量的差异、塑性变形的差异（所谓的差异生长）以及类似于次生细胞壁产生的成熟应力的现象。这些解释都没有得到验证。我们认为，要理解应力各向异性在植物生长中的作用，需要更深入地了解分层、有机结构中应力的性质。
