Leaf anatomy affects optical properties and enhances photosynthetic performance under oblique light

Photosynthesis under oblique illumination has not been studied extensively despite being the prevailing light regime under natural conditions. We studied how photosynthetic rate (An) is affected by the geometrical arrangement between leaf lamina and light rays, in conjunction with key anatomical features; studied plant species selected based on the absence (homobaric) or the occurrence of bundle sheath extensions (BSEs; heterobaric) and the arrangement of these structures, that is, parallel (monocots) or reticulated (dicots). The direction of light ray affected leaf absorptance (Abs) and An; both were maximal when the angle of incidence of light on leaf surface (polar angle, theta) was 90 degrees. For any lower theta, both Abs and An were higher when the angle between the leaf axis and the light rays (azimuthal angle, phi) was zero. The dependence of Abs and An from phi was only evident in monocots and, especially, in heterobaric compared to homobaric leaves. In some species, An was substantially higher than predicted from calculated photon flux density of oblique light. The occurrence of BSEs, especially in monocots, significantly alters leaf optical properties, resulting in more efficient photosynthesis under oblique illumination conditions. Plant leaves harvest light for photosynthesis, usually, in a non-perpendicular position related to the sun. Leaf anatomy increases light use efficiency under these conditions so that the rate of photosynthesis recorded is often much higher than predicted by optical geometry.