Oxygen evolution was measured from mesophyll tissues in spinach leaves using a photoacoustic technique. The
photosynthetic capacity of individual cell layers was measured by directing microscopic beams of light, 40 μm
wide, to cells exposed within a leaf cross section. The resulting profile for oxygen-evolution potential was relatively
flat, indicating a uniform capacity for photosynthesis in leaf mesophyll tissues. Two experimental approaches were
used to estimate the photosynthetic performance of individual mesophyll cell layers when white light was applied
to the adaxial leaf surface. These experiments indicated that oxygen was produced relatively uniformly across the
mesophyll and that oxygen evolution increased with irradiance of the white light applied to the leaf surface. The
measured profiles for oxygen evolution and capacity are flatter than previous measurements of profiles of fixed
carbon and estimates of profiles for absorbed light within spinach leaves.