Minisymposium 15: In Response to Water

Abs # 27003: A hydromechanical and biochemical model for stomatal conductance.

Presenter:	Buckley, Thomas N, tom.buckley@anu.edu.au
Authors	Buckley, Thomas N (A) (B)  Mott, Keith A (C)   Farquhar, Graham D (A) (B)
Affiliations:	(A): Environmental Biology Group, Research School of Biological Sciences, The Australian National University (B): Cooperative Research Centre for Greenhouse Accounting, RSBS, ANU (C): Biology Department, Utah State University
Web Site:	http://www.anu.edu.au

We present a new model for stomatal conductance in intact leaves. The model is based on principles of plant water relations, leaf gas exchange, and epidermal hydromechanics. Guard cell osmotic pressure is assumed proportional to ATP concentration, which is calculated from the photosynthesis model of Farquhar, von Caemmerer and Berry (1980). Our model accurately predicts stomatal responses to variations in humidity, transpiration rate, ambient and intercellular carbon dioxide partial pressure, ambient oxygen partial pressure, incident irradiance, xylem hydraulic resistance, soil water potential, and photosynthetic capacity. The model itself is easily expressed in a simple form that is similar to the Michaelis-Menten expression for enzyme-catalyzed reaction rate, but many elements of its structure are readily interpreted in terms of reduced processes at the cellular level. The mechanical advantage. Unlike many other stomatal models, ours is consistent with experimental evidence showing that, whereas increases in guard cell turgor pressure open the stomatal pore, increases in subsidiary cell turgor close the pore, and more effectively. This feature, known as the 'epidermal mechanical advantage,' causes the initial passive response to hydraulic perturbations to be in the 'wrong' direction (e.g., opening when humidity decreases). Our model overcomes the mechanical advantage by hypothesizing that the sensitivity of guard cell osmotic pressure to ATP is proportional to the turgor of adjacent epidermal cells. As a result, our model predicts both transient 'hydropassive' and steady-state 'hydroactive' responses to hydraulic perturbations. Photosynthesis-related responses. Most other models predict stomatal responses to carbon dioxide, light, photosynthetic rate, and photosynthetic capacity using a positive response to some measure of photosynthetic rate, and a negative response to some measure of CO₂ availability. Our model subsumes these two empirical dependences into a single mechanistic response: ATP concentration (to which guard cell osmotic pressure is proportional in our model) should increase with irradiance and photosynthetic capacity, but decrease with intercellular CO₂ partial pressure. By using tissue-level mechanistic constraints to integrate emergent properties of guard cell physiology with the phenomenology of whole-leaf gas exchange, our model represents a step towards bridging the gap between these two disparate scales.