Light sheet microscopy reveals more gradual light attenuation in light-green versus dark-green soybean leaves

Light sheet microscopy, a novel approach to quantifying light profiles, showed more gradual light attenuation in light-green soybean leaves compared to dark-green soybean.Light wavelengths preferentially absorbed by chlorophyll (chl) often display steep absorption gradients. This over-saturates photosynthesis in upper chloroplasts and deprives lower chloroplasts of blue and red light. Reducing chl content could create a more even leaf light distribution and thereby increase leaf light-use efficiency and overall canopy photosynthesis. This was tested on soybean cultivar 'Clark' (WT) and a near-isogenic chl b deficient mutant, 11y11, grown in controlled environment chambers and in the field. Light attenuation was quantified using a novel approach involving light sheet microscopy. Leaf adaxial and abaxial surfaces were illuminated separately with blue, red, and green wavelengths, and chl fluorescence was detected orthogonally to the illumination plane. Relative fluorescence was significantly greater in deeper layers of the Y11y11 mesophyll than in WT, with the greatest differences in blue, then red, and finally green light when illuminated from the adaxial surface. Modeled relative photosynthesis based on chlorophyll profiles and Beer's Law predicted less steep gradients in mutant relative photosynthesis rates compared to WT. Although photosynthetic light-use efficiency was greater in the field-grown mutant with similar to 50% lower chl, light-use efficiency was lower in the mutant when grown in chambers where chl was similar to 80% reduced. This difference is probably due to pleiotropic effects of the mutation that accompany very severe reductions in chlorophyll and may warrant further testing in other low-chl lines.