Poster: Photosynthesis (light)

Abs # 218: A new analytical model for whole-leaf potential electron transport rate

Presenter:	Buckley, Thomas N, tom.buckley@anu.edu.au
Authors	Buckley, Thomas N (A)   Farquhar, Graham D (A)
Affiliations:	(A): EB, RSBS, Australian National University

We present a new analytical model for the response of whole-leaf potential electron transport rate (J) to light. Photosynthesis is often predicted using the model of Farquhar, von Caemmerer and Berry (1980), in which the response to light is embedded in J, which in turn is usually modeled as the hyperbolic minimum of capacity- and light-limited rates (J_m and J_i, the latter proportional to irradiance): J = minh(J_i, J_m, Θ_J) = the lesser root of Θ_JJ²-(J_m+J_i)J+J_mJ_i=0. This is in fact a chloroplast-level model, scaled to the leaf by assuming the transdermal profiles of light absorption rate and electron transport capacity match one another. That assumption is violated by short-term changes in leaf orientation or angular light distribution, which cause the transdermal light absorption profile to change independently from the capacity profile. We assumed instead that the capacity profile is a weighted average of two opposed exponential profiles, reflecting a compromise between adaptation to illumination at the upper and lower surfaces. Profiles of light absorption and capacity are thus explicitly represented and integrated to the leaf level. The resulting model is simple and formally congruent to the standard model: J = minh{J_i-J_s, J_m+J_s, Θ_s), where J_s is the excess light captured by light-saturated paradermal leaf layers when other layers are light-limited, and is calculated from a simple expression. The model predicts how J responds to different and varying light intensities at each leaf surface, and can accommodate varying degrees of 'preference' for illumination at either surface. We discuss implications for modeling and interpreting canopy gas exchange.