Photosynthesis within isobilateral Eucalyptus pauciflora leaves

Abstract

Adult Eucalyptus pauciflora leaves are vertically displayed. They have multiple palisade cell layers beneath both surfaces, interrupted by numerous oil glands. Here, we characterized light absorption, chlorophyll, photosynthetic capacity and CO2 fixation profiles through these leaves. Multiple chlorophyll fluorescence images of leaves viewed in cross-section were made by applying light from different directions. 14CO2 labelling, followed by paradermal cryosectioning, was used to measure profiles of photosynthesis. Photosynthetic capacity peaked 75 microm into the mesophyll beneath each surface and was lowest in the centre of the 600-microm-thick leaf. Predictions by a multilayer model using Beer's law matched the observed profiles of 14C fixation. When constrained to the horizontal, a vertically acclimated leaf gains only 79% of the daily photosynthesis achieved by a horizontally acclimated leaf. However, it outperforms the horizontally acclimated leaf when both are oriented vertically. Each half of the observed profile of photosynthetic capacity closely matches the profile of light absorption through the leaf with unilateral illumination to that surface. Derivation of biochemical parameters from gas exchange measured under unilateral illumination would underestimate the real photosynthetic capacity of these leaves by 21%.