The Effect of Leaf-Level Spatial Variability in Photosynthetic Capacity on Biochemical Parameter Estimates Using the Farquhar Model: A Theoretical Analysis  

Abstract

Application of the widely used Farquhar model of photosynthesis in interpretation of gas exchange data assumes that photosynthetic properties are homogeneous throughout the leaf. Previous studies showed that heterogeneity in stomatal conductance (g s ) across a leaf could affect the shape of the measured leaf photosynthetic CO 2 uptake rate (A) versus intercellular CO 2 concentration (C i ) response curve and, in turn, estimation of the critical biochemical parameters of this model. These are the maximum rates of carboxylation (V c,max ), whole-chain electron transport ( J max ), and triose-P utilization (V TPU ). The effects of spatial variation in V c,max, J max , and V TPU on estimation of leaf averages of these parameters from A-C i curves measured on a whole leaf have not been investigated. A mathematical model incorporating defined degrees of spatial variability in V c,max and J max was constructed. One hundred and ten theoretical leaves were simulated, each with the same average V c,max and J max , but different coefficients of variation of the mean (CV VJ ) and varying correlation between V c,max and J max (V). Additionally, the interaction of variation in V c,max and J max with heterogeneity in V TPU , g s , and light gradients within the leaf was also investigated. Transition from V c,max -to J max -limited photosynthesis in the A-C i curve was smooth in the most heterogeneous leaves, in contrast to a distinct inflection in the absence of heterogeneity. Spatial variability had little effect on the accuracy of estimation of V c,max and J max from A-C i curves when the two varied in concert (V 5 1.0), but resulted in underestimation of both parameters when they varied independently (up to 12.5% in V c,max and 17.7% in J max at CV VJ 5 50%; V 5 0.3). Heterogeneity in V TPU also significantly affected parameter estimates, but effects of heterogeneity in g s or light gradients were comparatively small. If V c,max and J max derived from such heterogeneous leaves are used in models to project leaf photosynthesis, actual A is overestimated by up to 12% at the transition between V c,max -and J max -limited photosynthesis. This could have implications for both crop production and Earth system models, including projections of the effects of atmospheric change.The Farquhar model of photosynthesis is a mechanistic, biochemical model that is widely used to describe steady-state CO 2 assimilation in leaves (Farquhar et al., 2001). Applications of this model range from analysis of transgenic plants to projection of the gross primary production of the terrestrial biosphere under global change (Cramer et al., 1999;Farquhar et al., 2001;Gielen et al., 2005). The model also provides a widely used practical method of quantifying the key biochemical limitations to steady-state C 3 photosynthesis in vivo from the response of leaf photosynthetic CO 2 uptake per unit leaf area (A) to intercellular CO 2 concentration (C i ) as measured in gas exchange systems (Wullschleger, 1993;Long and Bernacchi, 2003).One of the basic premis...

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