Modeling Spongy Mesophyll Development

Mesophyll tissue plays important roles in both photosynthesis and mechanical stability in plant leaves, and varies in structure for different species to meet differing energetic and environmental constraints. Despite the importance of mesophyll tissue, its morphogenesis and mechanics at each developmental stage remain poorly understood. From x-ray microcomputed tomography (microCT) scans, we know that mesophyll begins in the leaf bud as densely packed confluent cells. As the tissue grows, the cells undergo shape changes that cause the porosity of the tissue to increase, which facilitates gas exchange. The degree to which the cells change shape and the tissue becomes porous varies across species. We believe that the differences in mesophyll structure between species are driven by differences in the adhesive forces between cells, and stresses in the cell walls that are selectively strengthened according to the geometry of the cells. In this work, we study the mechanics of developing mesophyll using microCT scans of leaves across several time points and species. We first quantify the changes in the cell shape and air space distribution within the mesophyll tissue. We then recapitulate these observations using discrete element method simulations of the deformable particle model undergoing expansion in three dimensions.