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

Harvard University, Cambridge MA

Investigators

Abstract

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.