Epidermal focussing and effects upon photosynthetic light‐harvesting in leaves of Oxalis

Abstract

Abstract. In Oxalis, epidermal cells on both the adaxial and abaxial surface of the leaf concentrated light within the leaf by a lens mechanism. Focal lengths of epidermal cells were estimated using two methods: they were calculated from radius of curvature measurements taken from individual epidermal cells, and were measured directly in agarose replicas of the leaf surface. In the three species of Oxalis examined, light that was incident upon the adaxial leaf surface was concentrated within the palisade, whereas light that was incident upon the abaxial leaf surface was concentrated within the spongy mesophyll. Using sensiometric analysis, theoretically maximal focal intesifications were measured in leaf replicas at the focal maximum and at intermediate positions corresponding to the mid‐region of the palisade and spongy mesophyll tissues. Focal intensifications ranged from 2.2 to 10.4 times incident light at the focal maximum, and 1.3 to 4.5 in the palisade or spongy mesophyll layers. Elimination of epidermal focussing, by covering the leaf surface with a thin layer of mineral oil, strongly affected chlorophyll fluorescence induction curves resulting in a decrease of 10–40% in the initial (F0) and variable fluorescence (Fv). These results are consistent with the interpretation that the chloroplasts were adapted to their light microenvironment within the leaf and that focussing by the epidermis channelled light to a population of chloroplasts that were adapted to high light.