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 (gs) across a leaf could affect the shape of the measured leaf photosynthetic CO2 uptake rate (A) versus intercellular CO2 concentration (Ci) response curve and, in turn, estimation of the critical biochemical parameters of this model. These are the maximum rates of carboxylation (Vc,max), whole-chain electron transport (Jmax), and triose-P utilization (VTPU). The effects of spatial variation in Vc,max, Jmax, and VTPU on estimation of leaf averages of these parameters from A-Ci curves measured on a whole leaf have not been investigated. A mathematical model incorporating defined degrees of spatial variability in Vc,max and Jmax was constructed. One hundred and ten theoretical leaves were simulated, each with the same average Vc,max and Jmax, but different coefficients of variation of the mean (CVVJ) and varying correlation between Vc,max and Jmax (Ω). Additionally, the interaction of variation in Vc,max and Jmax with heterogeneity in VTPU, gs, and light gradients within the leaf was also investigated. Transition from Vc,max- to J max-limited photosynthesis in the A-Ci 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 Vc,max and Jmax from A-Ci curves when the two varied in concert (Ω = 1.0), but resulted in underestimation of both parameters when they varied independently (up to 12.5% in Vc,max and 17.7% in Jmax at CVVJ = 50%; Ω = 0.3). Heterogeneity in VTPU also significantly affected parameter estimates, but effects of heterogeneity in gs or light gradients were comparatively small. If Vc,max and Jmax 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 Vc,max- and Jmax-limited photosynthesis. This could have implications for both crop production and Earth system models, including projections of the effects of atmospheric change.
ASJC Scopus subject areas
- Plant Science