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粮食生产的基础，并影响了全球碳循环。然而，当植物打开叶子上的气孔（称为气孔）来获取气态碳时，它们的内部细胞表面不可避免地会向植物周围的空气中流失水分，这一过程被称为蒸腾作用。叶片补充蒸腾损失的水分的效率对气孔开度和植物的碳吸收有重要的限制。然而，无论是在单个叶片水平还是与气象和气候模式相关的冠层水平上，人们对这一限制仍然知之甚少。以前对水分在叶片中传输效率的物理和结构基础的研究通常依赖于等温类比，将叶片视为等温“黑匣子”。这些模型不能直接用真实的材料特性来解释，例如植物细胞膜和细胞壁对水的渗透性，这些特性受到基因改造的影响。目前的提议通过采用一种新的知识基础来解决这个问题，这种基础来自热力学和连续介质力学的基本原理。这使得解决以前无法制定的叶片内部蒸汽输送问题成为可能，为叶片结构和材料特性对叶片及其环境之间能量和水交换的影响开辟了新的视角。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扩展到蒸腾叶片的行为，以便更好地理解和操纵未来植物生产的水力约束。这一建议解决了发生在叶片内部从叶脉到气孔的液态水和蒸汽运输之间的竞争。这种相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远比驱动液体渗透的水势梯度对温度更敏感；即使叶脉和气孔之间的温差很小（~0.1℃），水蒸气也会受到显著的推动。在之前的工作中，主要研究者建立了一个机制模型来描述蒸腾叶片中的热量和分子运输，并验证了红栎树叶片的方法。本项目将该模型扩展到叶片结构的三维处理，并通过实验研究叶片内部水汽输送的生理作用。实验方法是必要的，因为没有预测环境扰动下气孔运动的机制模型；需要进行实验观察。此外，叶片的能量平衡综合了大量的性状，因此跨越了一个高维参数空间。虽然理论上可以探索空间，但这只有在实际叶子所占据的范围已知的情况下才有用。一种实验假设认为，温度驱动的水汽输送对草本叶片在大太阳辐射负荷下保持气孔孔径很重要。相反，通过整个植物的大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的紧密协调的维持，预计会对蒸汽依赖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.