Functional Significance of the Competition between Vapor and Liquid Transport in Transpiring Leaves

植物利用太阳能从大气中吸收二氧化碳并制造糖，这一过程构成了农业食品生产的基础，并影响了全球碳循环。然而，当植物在叶片上打开气孔(称为气孔)以获取气态碳时，它们内部的细胞表面不可避免地会向植物周围的空气失去水分，这一过程被称为蒸腾作用。叶片替代蒸腾作用损失的水分的效率对气孔开度和植物的碳吸收造成了重要的限制。然而，无论是在个体叶片水平上，还是在与气象和气候模型相关的冠层水平上，人们对这一限制仍然知之甚少。以前对树叶水分运输效率的物理和结构基础的研究通常依赖于一个等温类比，将树叶视为一个等温的“黑匣子”。这样的模型不能直接解释真实的材料属性，如植物细胞膜和细胞壁对水的渗透性，这些属性受基因改造的影响。目前的提案通过采用一种新的知识基础来解决这个问题，该基础源于热力学和连续介质力学的基本原理。这使得有可能解决以前无法描述的树叶内的水蒸气传输问题，为研究树叶的结构和材料特性对树叶与其环境之间的能量和水分交换的影响打开了一个新的视角。该项目将为科学界提供强大的工具，从单个植物细胞的水力特性到蒸腾叶片的行为，以便更好地理解和操纵未来植物生产的水力约束。这项建议解决了液态水和水蒸气在叶片内从叶脉到气孔的运输之间的竞争。相之间的竞争在物理上与热传导和潜热之间的竞争相耦合。相间竞争的一个关键方面是，驱动水蒸气扩散通量的摩尔分数梯度比驱动液体渗透的水势梯度对温度的敏感性要高得多；即使脉和气孔之间的微小温差(~0.1℃)也会对水蒸气产生显著的推动作用。前人的工作建立了一个描述蒸腾叶片中热量和分子传输的机制模型，并以红栎叶为例验证了该方法的有效性。本项目将把该模型扩展到叶片结构的3D处理，并从实验上研究内部水蒸气传输的生理作用。实验方法是必要的，因为没有机制模型来预测气孔对环境扰动的运动；需要实验观察。此外，叶片的能量平衡整合了一大套特征，从而跨越了高维参数空间。虽然理论上可以探索这个空间，但只有在知道实际树叶占据的范围时，这才是有用的。一个实验假说是，温度驱动的水蒸气运输对于草本植物叶片在较大的太阳辐射负荷下保持气孔开度是重要的。相反，通过大的全株水力阻力提取土壤水分的能力，以及木质部和气孔水势之间保持紧密协调的能力，预计将对依赖蒸汽的运输策略施加限制。

Plants use solar energy to take carbon dioxide from the atmosphere and construct sugars, a process which forms the basis for agricultural food production and influences the global carbon cycle. Yet, when plants open pores in their leaves (called stomata) to access gaseous carbon, their internal cell surfaces inevitably lose water to the air around the plant, a process called transpiration. The efficiency with which leaves can replace water lost to transpiration imposes an important constraint on stomatal apertures and therefore carbon uptake by plants. Yet, this constraint remains poorly understood, both at the individual leaf level as well as the canopy level relevant to meteorological and climate models. Previous investigations of the physical and structural basis of water transport efficiency in leaves have typically relied on an isothermal analogy that treats a leaf as an isothermal "black box." Such models cannot be directly interpreted in terms of real material properties, such as plant cell membrane and cell wall permeabilities to water, properties that are subject to genetic modification. The current proposal addresses this problem by adopting a new intellectual foundation, derived from basic principles of thermodynamics and continuum mechanics. This makes it possible to address questions of vapor transport inside leaves that could not be formulated previously, opening up a new perspective on the effects of leaf structural and material properties on the exchange of energy and water between leaves and their environments. This project will provide the scientific community with robust tools for scaling from the hydraulic properties of individual plant cells to the behavior of transpiring leaves, in order to better understand and manipulate hydraulic constraints on plant production in the future. This proposal addresses the competition between liquid water and vapor transport that occurs within a leaf, from the veins to the stomata. This competition between phases is physically coupled to the competition between thermal conduction and latent heat. A critical aspect of the competition between phases is that the mole fraction gradient that drives the diffusive flux of water vapor is far more temperature sensitive than the water potential gradient driving liquid permeation; vapor gets a significant push from even small (~0.1 C) temperature differences between the veins and stomata. Previous work by the principal investigator developed a mechanistic model to describe heat and molecular transport in transpiring leaves, and validated the approach for leaves of red oak (Quercus rubra L.). This project would extend the model to 3D treatments of leaf structure, and investigate the physiological role of internal vapor transport experimentally. An experimental approach is essential because there is no mechanistic model for predicting stomatal movements to environmental perturbations; experimental observations are required. In addition, energy balances for leaves integrate over a large suite of traits and thus span a high-dimensional parameter space. While the space can be explored theoretically, this is useful only if the range occupied by real leaves is known. An experimental hypothesis is that temperature driven vapor transport is important in allowing herbaceous leaves to maintain stomatal aperture under large solar radiation loads. Conversely, the ability to extract soil water through large whole plant hydraulic resistances, and the maintenance of tight coordination between xylem and stomatal water potentials, are expected to impose constraints on a vapor-dependent transport strategy.