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

蒸腾叶中水汽和液体运输竞争的功能意义

• 批准号: 1456836
• 负责人: Fulton Rockwell
• 金额: $ 22.96万
• 依托单位: Harvard University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-04-15 至 2019-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1456836&HistoricalAwards=false
• 关键词: Functional; Significance; Competition; between; Vapor

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.

植物利用太阳能从大气中吸收二氧化碳并构建糖，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将水分流失到植物周围的空气中，这一过程称为蒸腾作用。叶片可以有效地补充蒸腾损失的水分，这对气孔开度以及植物的碳吸收产生了重要的限制。然而，这一限制仍然知之甚少，无论是在个别叶片水平以及冠层水平相关的气象和气候模型。以往对叶片中水分运输效率的物理和结构基础的研究通常依赖于等温类比，将叶片视为等温的“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，这些特性会受到基因修饰。目前的建议通过采用一种新的知识基础来解决这个问题，这种知识基础来自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制定的叶片内的蒸汽输送问题，开辟了一个新的视角，叶片结构和材料特性对叶片及其环境之间的能量和水分交换的影响。该项目将为科学界提供强大的工具，用于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。这个建议解决了液态水和蒸汽之间的竞争运输发生在一片叶子，从静脉到气孔。相之间的这种竞争在物理上与热传导和潜热之间的竞争相耦合。相之间的竞争的一个关键方面是，驱动水蒸气的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敏感得多;蒸汽从脉和气孔之间的甚至很小（~0.1 C）的温差得到显著推动。主要研究者以前的工作开发了一个机械模型来描述蒸腾叶片中的热量和分子传输，并验证了红橡树（Quercus rubra L.）叶片的方法。该项目将把该模型扩展到叶片结构的3D处理，并通过实验研究内部蒸汽输送的生理作用。一个实验的方法是必不可少的，因为没有机械模型预测气孔运动的环境扰动，实验观察是必需的。此外，叶的能量平衡整合在一个大的套件的性状，从而跨越一个高维的参数空间。虽然可以从理论上探索空间，但只有在已知真实的叶子所占据的范围时，这才是有用的。一个实验假设是，温度驱动的蒸汽运输是重要的，让草本植物叶片在大的太阳辐射负荷下保持气孔开度。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的紧密协调的维护，预计将对依赖于蒸汽的运输策略施加限制。