叶片水H_2^(18)O富集的研究进展

RECENT ADVANCES IN H_2^(18)O ENRICHMENT IN LEAF WATER

植物叶片水H218O富集对大气中O2和CO2的18O收支有着重要影响。蒸腾作用使植物叶片水H218O富集,而植物叶片水H218O富集的程度主要受大气水汽δ18O和植物蒸腾水汽δ18O的影响。过去,通过引入稳态假设(蒸腾δ18O等于茎水δ18O)得到Craig-Gordon模型的闭合形式,或将植物整个叶片水δ18O经过Péclet效应校正后得到植物叶片水δ18O的富集程度。然而,在几分钟到几小时的短时间尺度上,植物叶片蒸腾δ18O是变化的,稳态假设是无法满足的。最近成功地实现了对大气水汽δ18O和δD的原位连续观测,观测精度(小时尺度)可达到甚至优于稳定同位素质谱仪的观测精度。在非破坏性条件下,高时间分辨率和连续的大气水汽δ18O和蒸腾δ18O的动态观测,将提高植物叶片水H218O富集的预测能力。该文综述了植物叶片水H218O富集的理论研究的新进展、研究焦点和观测方法所存在的问题,旨在进一步加深理解植物叶片水H218O富集的过程及其机制。

There is considerable interest in the use of atmospheric C^18O^16O and ^18O^16O as a tracer for resolving the role of the terrestrial biosphere in the global carbon cycle. Leaf transpiration will result in the enrichment of the heavy H2^18O isotopes. The δ^18O of leaf water at the evaporating site in the stomatal cavity directly influences the C^18O^16O and ^18O^16O exchanges, instead of that of the bulk leaf water. How to best quantify this enrichment effect remains an active area of research. In the past, a closed form of the Craig-Gordon model was obtained by invoking the steady-state assumption （δ^18O of the transpired water is identical to δ^18O of the xylem water）. For the purpose of verification, the predictions of Craig-Gordon model are compared with δ^18O of the bulk leaf water after appropriate corrections for the Peclet effect. On small time scales of minutes to hours, δ^18O of the transpired water is variable in field conditions, implying that the steady state assumption is invalid. Recently, in-situ δ^18O and δ^18D measurement technology has been developed that has potential for improving our understanding of isotopic exchanges between the Earth＇s surface and the atmosphere. The precision of hourly δ^18O and δ^18D is comparable to the precision of mass spectrometry. It has the potential to improve prediction of δ^18O of leaf water at the evaporating site within the stomatal cavity for the temporal dynamics of atmospheric water vapor δ^18O and the δ^18O of the transpired water, especially if its measurement is made in a non-destructive manner and on a continuous basis. Because the isotopic flux of δ^18O and δ^18D is influ- enced by a similar set of biological and meteorological variables, simultaneous observations of δ^18O and δ^18D will provide additional constraints on the hydrological and ecological processes of the ecosystem. We review the theory and measurement techniques for the enrichment of H2^18O in leaves and focus on the recently developed in-situ measurement technology and its potential for improving our understand- ing of H2^18O enrichment in leaf water and C^18O^16O and ^18O^16O exchanges between the ecosystem and atmosphere.