On the isotopic composition of leaf water in the non-steady state
On the isotopic composition of leaf water in the non-steady state
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DOI：
10.1071/fp04232
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发表时间：
2005-01-01
影响因子：
3
通讯作者：
Cernusak, LA
中科院分区：
文献类型：
--
作者：
Farquhar, GD;Cernusak, LA
在进入叶的水的组成不一定与进入叶的水的组成相同的条件下，推导出叶中水的同位素组成的表达式。简化了处理，并考虑了薄层和蒸发部位的平均组成。“等蓄量”的概念是叶片含水率和叶片水分在源水之上的同位素富集度的乘积。结果表明，等贮量的增加速率与通过气孔的“等流量”负相关，后者是蒸腾通量与源水以外蒸腾水的富集度的乘积。等储量达到稳定状态的途径取决于蒸发点水分的同位素富集度与稳定值的偏差，以及叶片外的总(单向)扩散通量。为了实现模型的闭合，假设叶片水分富集度与蒸发点的水分富集度之间的关系取决于径向Peclet数，其方式与稳定状态下相同。这些方程有一个解析解，我们还展示了如何简单地使用常用的计算机工具来计算结果。方程的形式强调，流入和流出气孔的单向水通量有时必须分开考虑，而不是作为净流出通量。在这个狭义意义上，我们得出了一个有趣的结论，即从空气进入树叶的水分通常多于从根部进入树叶的水分。
An expression is derived for the isotopic composition of water in leaves under conditions where the composition of water entering the leaf is not necessarily the same as that of water being transpired. The treatment is simplified and considers the average composition of the lamina and of the sites of evaporation. The concept of 'isostorage' is introduced as the product of leaf water content and the isotopic enrichment of leaf water above source water. It is shown that the rate of increase of isostorage is minus the 'isoflux' through the stomata, with the latter expressed as the product of the transpiration flux and the enrichment of the transpired water beyond source water. The approach of the isostorage to the steady state depends on the deviation of the isotopic enrichment of water at the evaporating sites from the steady value, and on the gross ( one way) diffusive flux out of the leaf. To achieve model closure, it is assumed that the relationship between leaf water enrichment and that at the sites of evaporation depends on the radial Peclet number in the same manner as in the steady state. The equations have an analytical solution, and we also show how to calculate the results simply using a commonly available computer tool. The form of the equations emphasises that the one-way fluxes of water into and out of the stomata must sometimes be considered separately, rather than as a net outward flux. In this narrow sense we come to the interesting conclusion that more water usually enters the leaf from the air than from the roots.