Scaling from leaf to whole-plant photosynthetic performance with a 3-dimensional crown architecture model, Y-plant

Abstract

The pattern of self-shading as determined by the display of leaves within a crown creates a highly heterogeneous environment that strongly influences photosynthesis of individual leaves. To study how this heterogeneity influences the scaling of photosynthetic performance from leaf to whole plant we have utilized the model, Y-plant, that reconstructs the crown architecture in three-dimensional computer space from geometric measurements on field plants. A simplified ray tracing technique coupled to solar and sky radiation models is used to determine the diffuse and direct PFD absorbed by each leaf. Leaf-level submodels include the Farquhar, von-Caemmerer, Berry model for CO2 assimilation, a Ball-Berry type stomatal model and a leaf energy balance model. Integration of leaf assimilation and transpiration over all leaves then gives the whole plant assimilation, transpiration and water use efficiency. Y-plant was used to study photosynthesis of two Californian evergreen species that differed in leaf display. Arbutus menziesii has large, horizontal leaves whereas Heteromeles arbutifolia has smaller, more erect leaves. Arbutus exhibited much lower self shading and higher efficiencies of light capture as compared to Heteromeles, However, simulations revealed a stronger midday depression of whole shoot photosynthesis in Arbutus due to greater radiation loads and higher leaf temperatures. Steep leaf angles in Heteromeles kept midday irradiances on the leaf surfaces saturating but prevented excessive radiation loads. Consequently, midday transpiration rates were much lower in Heteromeles as compared to Arbutus. The role of these differences in crown architecture in whole plant photosynthetic performance in relation to environment will be presented.