Passioura John B, Boyer John S
CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia. Corresponding author; email:
University of Delaware, Lewes, DE 19958, USA.
Funct Plant Biol. 2003 Mar;30(3):325-334. doi: 10.1071/FP02202.
Considerable evidence exists that, in elongating dicot stems such as soybean hypocotyls: (1) the elongation rate is controlled largely by the mechanical properties of the epidermal cell walls; (2) the inner tissue is under compression in the sense that the turgor pressure of the cells is not fully borne by their cell walls; (3) the surplus turgor pressure in this inner tissue generates a force that is transmitted to the epidermis, where it drives irreversible elongation of the cell walls; and (4) the radial flow of water from the xylem to the rest of the tissue, needed to fill the expanding cells, is driven by gradients in water potential. On the basis of these propositions, this paper develops a mathematical description of the biophysical control of elongation rate and the radial distribution of water potential in the growing plant stem. Additional simplifying assumptions are that the osmotic pressure of the cells and their elastic modulus are constant throughout, and that the proportion of the tensile force that is borne by the walls of the inner cells is also constant. We assume, further, that the epidermal cell walls yield plastically in response to the tensile force they experience, as in the Lockhart model of cell expansion. The analysis gives expressions for the strain rate and the radial distribution of water potential as functions of the water potential of the xylem and of various parameters. These parameters include diffusivity of water in the tissue, the radii of the stem and of the cylindrical band of xylem within it, and the extensibility and yield threshold of the Lockhart equation. Aremarkable feature of the water relations of the elongating soybean hypocotyl is that changes in the water potential of the xylem, while they rapidly affect elongation rate, cause little or no change in the water potential of the epidermis, at least for many hours. The analysis shows how such an uncoupling can occur, and, further, predicts that low diffusivity would result in the water potential of the epidermis falling when that of the xylem is raised.
有大量证据表明，在诸如大豆下胚轴等双子叶植物茎的伸长过程中：（1）伸长速率在很大程度上受表皮细胞壁力学性质的控制；（2）内部组织处于压缩状态，即细胞的膨压并未完全由其细胞壁承担；（3）该内部组织中的过剩膨压产生一种力，该力传递至表皮，在表皮处驱动细胞壁的不可逆伸长；（4）为填充不断扩张的细胞，从木质部到组织其他部分的水分径向流动是由水势梯度驱动的。基于这些观点，本文对生长中的植物茎中伸长速率的生物物理控制以及水势的径向分布进行了数学描述。另外的简化假设是，细胞的渗透压及其弹性模量在整个过程中保持恒定，并且内部细胞的壁所承受的拉力比例也恒定。我们进一步假设，表皮细胞壁会像在细胞扩张的洛克哈特模型中那样，对其所承受的拉力产生塑性屈服。该分析给出了应变率以及水势径向分布的表达式，它们是木质部水势和各种参数的函数。这些参数包括水分在组织中的扩散率、茎的半径以及其中木质部圆柱带的半径，还有洛克哈特方程的伸展性和屈服阈值。伸长的大豆下胚轴水分关系的一个显著特征是，木质部水势的变化虽然会迅速影响伸长速率，但至少在许多小时内，对表皮水势几乎没有影响或没有变化。该分析表明了这种解耦是如何发生的，并且进一步预测，低扩散率会导致当木质部水势升高时表皮水势下降。
