Minisymposium 15: Photosynthesis

Abs # 28001: 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): Environmental Biology Group and & CRC for Greenhouse Accounting, Research School of Biological Sciences, Australian National University

The biochemical photosynthesis model of Farquhar, von Caemmerer and Berry (1980) describes the primary response to light as an effect on potential electron transport rate (j) in thylakoids. The standard non-rectangular hyperbola model for the response of j to light is well defined for single chloroplasts, where the rate of light absorption is a scalar, but the model is usually applied to whole leaves, where the rate of light absorption is a function of position, due to light attenuation by paradermal chlorophyll layers. The chloroplast j model is usually assumed 'scale-invariant' and applied to whole leaves on the grounds that electron transport capacity should be allocated among paradermal layers in proportion to light absorption. However, the transdermal light absorption profile is controlled by the irradiances at both leaf surfaces, which receive varying light as leaves flutter, as the sun moves in the sky, and as the ratio of diffuse to direct light varies with cloud cover and haze. As a result, the transdermal light profile can change far too quickly for the capacity profile to adapt, so the two profiles often differ -- violating the scale-invariance assumption. Two major consequences are that (1) most models of broadleaf canopy gas exchange do not account for a potentially important aspect of the effects of diffuse light fraction and leaf angle, and (2) proper analysis of nitrogen economics is hamstrung by the scale-invariance assumption, which incorrectly dictates the N cost of light absorption. To overcome these limitations, we created a new analytical model for whole-leaf potential electron transport rate (J) by assuming that the transdermal capacity profile is a weighted average of two opposed exponential profiles, each of which matches the light profile when only one surface is lit. The weights may take on any values, provided they sum to unity, so the model accommodates a range of preferences for illumination of each leaf surface. J is calculated by integrating the minimum of light- and capacity-limited potential electron transport rates among paradermal chlorophyll layers. The model predicts the response of J to leaf inversion during measurement, and also during growth if the capacity profile is assumed to be adaptable on developmental time scales. We present the model, which is compact and formally similar to the standard model for J, and we describe briefly its general implications for canopy gas exchange and optimal nitrogen allocation.