Profiles of ¹⁴C fixation through spinach leaves in relation to light absorption and photosynthetic capacity

The profile of photosynthetic rate with depth through a leaf depends on the profiles of light absorption and photosynthetic capacity. Using a combination of several techniques, a comprehensive description of spinach leaves has been obtained. Profiles of CO₂ fixation were obtained by exposing leaves to ¹⁴CO₂ for 10 s under blue or green light before freeze clamping and paradermal sectioning. Profiles of light absorption were measured on adjacent parts of the leaf by quantifying chlorophyll fluorescence images of the transversely cut face obtained when blue or green light was applied to the adaxial or abaxial surface. The profile of CO₂ fixation was modelled using the measured profiles of light absorption and photosynthetic capacity. There was remarkably good agreement between the observed and modelled CO₂ fixation profiles for the eight combinations of colour, orientation and irradiance tested. Gas exchange of an intact leaf was also measured concurrently with conventional chlorophyll fluorescence under blue or green light. These data were consistent with the multi-layer leaf model with the exception of blue light applied to the abaxial surface, where chlorophyll fluorescence appeared to come from layers deeper than expected. Photosynthetic capacity matched the profile of green but not white light absorption through the leaf.