Department of Forest Sciences, 00014 University of Helsinki, Helsinki, Finland.
Plant Cell Environ. 2011 Nov;34(11):1819-34. doi: 10.1111/j.1365-3040.2011.02377.x. Epub 2011 Jul 13.
Efficient water transport from the soil to the leaves is essential for plant function, while building and maintaining the water transport structure in the xylem require a major proportion of the assimilated carbon of the tree. Xylem transport also faces additional challenges as water in the xylem is under tension and therefore cavitation cannot be completely avoided. We constructed a model that calculates the xylem structure that maximizes carbon-use efficiency while simultaneously taking into account pit structure in increasing the resistance to water transport and constricting the spreading of embolisms. The optimal xylem structure predicted by the model was found to correspond well to the generally observed trends: xylem conduits grew in size from the apex towards the base while simultaneously decreasing in number, and vulnerability to cavitation increased with conduit size. These trends were caused primarily by the axial water potential gradient in the xylem. The pits have to be less porous near the apex where water potential is lower to restrict the spreading of embolisms, while whole-plant carbon-use efficiency demands that conduit size decreases and conduit number increases simultaneously. The model predictions remained qualitatively the same regardless of the exact optimality criterion used for defining carbon-use efficiency.
从土壤到叶片的高效水分运输对植物功能至关重要，而构建和维持木质部中的水分运输结构则需要树木同化碳的很大一部分。木质部运输还面临着额外的挑战，因为木质部中的水处于张力状态，因此不能完全避免空化。我们构建了一个模型，该模型在考虑增加对水运输的阻力和限制栓塞扩散的纹孔结构的同时，计算出最大限度地提高碳利用效率的木质部结构。模型预测的最佳木质部结构与通常观察到的趋势非常吻合：木质部导管从顶端向基部增大，同时数量减少，易受空化的影响随导管尺寸的增大而增大。这些趋势主要是由木质部中的轴向水势梯度引起的。在水势较低的顶端附近，纹孔必须不太多孔，以限制栓塞的扩散，而整个植物的碳利用效率要求导管尺寸减小，同时导管数量增加。无论用于定义碳利用效率的精确最优性标准如何，模型预测在定性上仍然相同。
