Photosynthetic light gradients and spectral régime within leaves of Medicago sativa

Abstract By using a fibre-optic probe, light gradients were measured at 450, 550 and 680 nm in sun leaves, 125 µm thick, of Medicago sativa L. cv. Armor. The space irradiances immediately beneath the leaf surface were 1.5-2.0 times greater than the incident light for these wavelengths, indicating that M. sativa leaves are efficient light traps. Although the palisade was only 60 µm thick, each light gradient declined steeply within this layer. More light appeared to be scattered in forward rather than backward directions and the spectral régime of the light fluxes depended upon their direction of travel within the leaf. Spectra for transmitted light were dependent upon depth within the leaf, whereas back-scattered light consisted of mostly green and far-red light at all depths, PAR (photosynthetically active radiation, 400-700 nm) within both the palisade and spongy mesophyll consisted mostly of green and far-red light, and the spongy mesophyll received only 0.11 of the PAR compared with the midregion of the palisade. Anomalous measurements within the palisade were traced to the epidermis, which was found to act as a mosaic of microlenses that focused light within the palisade layer. In M. sativa leaves, the light microenvironment, leaf anatomy and photosynthesis seem to be strongly interrelated.