Modeling xylem and phloem water flows in trees according to cohesion theory and Munch hypothesis
Modeling xylem and phloem water flows in trees according to cohesion theory and Munch hypothesis
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DOI：
10.1007/s00468-005-0014-6
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发表时间：
2006-01-01
影响因子：
2.3
通讯作者：
Nikinmaa, E
中科院分区：
文献类型：
--
作者：
Hölttä, T;Vesala, T;Nikinmaa, E
水分和溶质在木质部和韧皮部耦合系统中的流动与木质部和整个茎直径变化的预测一起建模。利用该模型可以模拟木质部和韧皮部之间的水分循环，正如Munch假说所描述的那样。粘度被建模为溶质浓度的显式函数，这被发现在可能的生理范围内的树液浓度的韧皮部液流的阻力变化的许多数量级。此外，预测的韧皮部易位的边界条件和参数，如糖负荷，蒸腾作用和水力传导率的变化的敏感性进行了研究。该系统被发现是相当敏感的糖负载率，因为太高的糖浓度，（约7 MPa）将导致韧皮部易位不可逆地受阻，并很快完全阻塞，由于在韧皮部的顶部的糖的积累和随后的韧皮部汁液的粘度上升。另一方面，太低的糖加载速率将不会引起足够的轴向水压梯度。该模型还揭示了蒙克“逆流”的存在，即，由于木质部和韧皮部之间的水循环，在没有蒸腾作用的情况下木质部的水流动。模拟的股骨柄直径变化与早期研究的实际股骨柄直径测量结果一致。全茎直径的日变化约为0.1mm，其中木质部约占三分之一。
Water and solute flows in the coupled system of xylem and phloem were modeled together with predictions for xylem and whole stem diameter changes. With the model we could produce water circulation between xylem and phloem as presented by the Munch hypothesis. Viscosity was modeled as an explicit function of solute concentration and this was found to vary the resistance of the phloem sap flow by many orders of magnitude in the possible physiological range of sap concentrations. Also, the sensitivity of the predicted phloem translocation to changes in the boundary conditions and parameters such as sugar loading, transpiration, and hydraulic conductivity were studied. The system was found to be quite sensitive to the sugar-loading rate, as too high sugar concentration, (approximately 7 MPa) would cause phloem translocation to be irreversibly hindered and soon totally blocked due to accumulation of sugar at the top of the phloem and the consequent rise in the viscosity of the phloem sap. Too low sugar loading rate, on the other hand, would not induce a sufficient axial water pressure gradient. The model also revealed the existence of Munch "counter flow", i.e., xylem water flow in the absence of transpiration resulting from water circulation between the xylem and phloem. Modeled diameter changes of the stem were found to be compatible with actual stem diameter measurements from earlier studies. The diurnal diameter variation of the whole stem was approximately 0.1 mm of which the xylem constituted approximately one-third.