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Research Day Entry

The spatial distribution of chlorophyll in leaves

Understanding the spatial distribution of chlorophyll within the leaf is critical for understanding the relationship between leaf structure and carbon assimilation, the relative investments in leaf tissues from the perspective of leaf economics theory, and towards the emerging application of in silico carbon assimilation models. Yet, spatially-resolved leaf chlorophyll dsitribution data are limited to a handful of species. Here, we used epi-illumination fluorescence microscopy to measure relative chlorophyll concentration as a function of mesophyll depth for fifty-seven plant taxa. Across a diversity of species representing different phylogenetic lineages (ferns, cycads, conifers, ginkgo, ANITA angiosperms, magnoliids, monocots, and eudicots) and growth habits (tree, shrub, herbaceous, annual, perennial, evergreen, and deciduous), the spatial distribution of chlorophyll in laminar leaves was a remarkably well-conserved trait. Chlorophyll content increased gradually from the upper leaf as a function of mesophyll depth, peaking deep within the leaf. The observed gradient in chlorophyll content through the leaf is likely coupled to adaxial and abaxial intra-leaf light gradients, including the relative enrichment of green light in the lower leaf. Chlorophyll distribution for the typical leaf from our dataset was well represented by a simple mathematical model across phylogenetic and anatomical diversity (R2 = 0.94). Additionally, we present empirical chlorophyll distribution data and model equations for many ecologically and commercially relevant species and groups of species (i.e. according to chlorophyll profile similarity, clade, and leaf thickness). Taken together, these findings represent an advancement toward more accurate photosynthesis modeling and increase our understanding of first principles in intra-leaf physiology.