Measurement of gradients of absorbed light in spinach leaves from chlorophyll fluorescence profiles

Profiles of chlorophyll fluorescence were measured in spinach leaves irradi ated with monochromatic light. The characteristics of the profiles within t he mesophyll were determined by the optical properties of the leaf tissue a nd the spectral quality of the actinic light. When leaves were infiltrated with 10(-4) M DCMU [3-(3,4-dichlorophenyl)-1, 1-dimethyl-urea] or water, tr eatments that minimized light scattering, irradiation with 2000 mu mol m(-2 ) s(-1) green light produced broad Gaussian-shaped fluorescence profiles th at spanned most of the mesophyll. Profiles for chlorophyll fluorescence in the red (680 +/- 16 nm) and far red (lambda > 710 nm) were similar except t hat there was elevated red fluorescence near the adaxial leaf surface relat ive to far red fluorescence. Fluorescence profiles were narrower in non-inf iltrated leaf samples where light scattering increased the light gradient. The fluorescence profile was broader when the leaf was irradiated on its ad axial versus abaxial surface due to the contrasting optical properties of t he palisade and spongy mesophyll. Irradiation with blue, red and green mono chromatic light produced profiles that peaked 50, 100 and 150 mum, respecti vely, beneath the irradiated surface. These results are consistent with pre vious measurements of the light gradient in spinach and they agree qualitat ively with measurements of carbon fixation under monochromatic blue, red an d green light. These results suggest that chlorophyll fluorescence profiles may be used to estimate the distribution of quanta that are absorbed withi n the leaf for photosynthesis.