Profiles of light absorption and chlorophyll within spinach leaves from chlorophyll fluorescence

Chlorophyll fluorescence was used to estimate profiles of absorbed light within chlorophyll solutions and leaves. For chlorophyll solutions, the intensity of the emitted fluorescence declined in a log–linear manner with the distance from the irradiated surface as predicted by Beer's law. The amount of fluorescence was proportional to chlorophyll concentration for chlorophyll solutions given epi‐illumination on a microscope slide. These relationships appeared to hold for more optically complex spinach leaves. The profile of chlorophyll fluorescence emitted by leaf cross sections given epi‐illumination corresponded to chlorophyll content measured in extracts of leaf paradermal sections. Thus epifluorescence was used to estimate relative chlorophyll content through leaf tissues. Fluorescence profiles across leaves depended on wavelength and orientation, reaching a peak at 50–70 µm depth. By infiltrating leaves with water, the pathlengthening due to scattering at the airspace : cell wall interfaces was calculated. Surprisingly, the palisade and spongy mesophyll had similar values for pathlengthening with the value being greatest for green light (550 > 650 > 450 nm). By combining fluorescence profiles with chlorophyll distribution across the leaf, the profile of the apparent extinction coefficient was calculated. The light profiles within spinach leaves could be well approximated by an apparent extinction coefficient and the Beer–Lambert/Bouguer laws. Light was absorbed at greater depths than predicted from fibre optic measurements, with 50% of blue and green light reaching 125 and 240 µm deep, respectively.