Palisade cell geometry in relation to leaf optical and photosynthetic properties in Viburnum.

The optical properties of lobed palisade mesophyll cells remain poorly understood despite their presence in diverse taxa and the critical role of the palisade layer in leaf-light interactions and carbon assimilation. Using microcomputed tomography, 3D ray tracing simulations, and physiological experiments, we tested the interactions among palisade cell geometry, chloroplast localization, light directional quality, and leaf optical and photosynthetic performance in the model taxon Viburnum. Simulations showed that lobed cells shifted between absorptance- and transmittance-dominated states depending on chloroplast localization, irrespective of light directional quality. In contrast, columnar palisade optics were driven by light directional quality, with absorptance-dominated properties under diffuse light and transmittance-dominated properties under direct light, irrespective of chloroplast localization. Lobed palisade cells in planta were less densely packed yet more productive on a per-cell basis than columnar palisade cells, resulting in interspecific conservation of maximum carbon assimilation rate per unit leaf tissue. For the Viburnum species studied, our results indicate a "many-to-one" mapping of multiple palisade cell forms to a common rate of photosynthetic productivity per unit tissue. This work highlights the dynamic relationship between palisade mesophyll form and function and informs the anatomical basis of plant carbon assimilation.