Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY, USA.
Ann Bot. 2022 Sep 19;130(3):301-316. doi: 10.1093/aob/mcac094.
Recent reports of extreme levels of undersaturation in internal leaf air spaces have called into question one of the foundational assumptions of leaf gas exchange analysis, that leaf air spaces are effectively saturated with water vapour at leaf surface temperature. Historically, inferring the biophysical states controlling assimilation and transpiration from the fluxes directly measured by gas exchange systems has presented a number of challenges, including: (1) a mismatch in scales between the area of flux measurement, the biochemical cellular scale and the meso-scale introduced by the localization of the fluxes to stomatal pores; (2) the inaccessibility of the internal states of CO2 and water vapour required to define conductances; and (3) uncertainties about the pathways these internal fluxes travel. In response, plant physiologists have adopted a set of simplifying assumptions that define phenomenological concepts such as stomatal and mesophyll conductances.
Investigators have long been concerned that a failure of basic assumptions could be distorting our understanding of these phenomenological conductances, and the biophysical states inside leaves. Here we review these assumptions and historical efforts to test them. We then explore whether artefacts in analysis arising from the averaging of fluxes over macroscopic leaf areas could provide alternative explanations for some part, if not all, of reported extreme states of undersaturation.
Spatial heterogeneities can, in some cases, create the appearance of undersaturation in the internal air spaces of leaves. Further refinement of experimental approaches will be required to separate undersaturation from the effects of spatial variations in fluxes or conductances. Novel combinations of current and emerging technologies hold promise for meeting this challenge.
最近有报道称，内部叶片气腔中的极度不饱和程度令人质疑叶片气体交换分析的一个基本假设，即叶片气腔在叶表面温度下与水蒸气有效饱和。从历史上看，从气体交换系统直接测量的通量推断控制同化和蒸腾作用的生物物理状态提出了许多挑战，包括：（1）通量测量面积、生化细胞尺度和通量定位到气孔的中尺度之间的尺度不匹配；（2）无法确定定义导度所需的 CO2 和水蒸气的内部状态；（3）对这些内部通量所经过的途径存在不确定性。为了应对这些挑战，植物生理学家采用了一组简化的假设，定义了诸如气孔导度和叶肉导度等现象学概念。
研究人员一直担心基本假设的失败可能会扭曲我们对这些现象学导度和叶片内部生物物理状态的理解。在这里，我们回顾了这些假设和历史上对它们的检验。然后，我们探讨了在宏观叶片面积上对通量进行平均分析产生的假象是否可以为部分（如果不是全部）报告的极度不饱和状态提供替代解释。
在某些情况下，空间异质性会在叶片内部气腔中产生不饱和的表观现象。需要进一步改进实验方法，以将不饱和现象与通量或导度的空间变化的影响分开。当前和新兴技术的新颖组合有望满足这一挑战。
