Sensitive detection of chloroplast movements through changes in leaf cross-polarized reflectance.

We show that the positioning of chloroplasts in plant cells affects the ability of photosynthetic organs to depolarize linearly polarized light through diffuse reflection. We apply this phenomenon for the detection of blue-light-induced chloroplast movements using light reflected from leaves and green stems. High-light-induced chloroplast avoidance leads to an increase in reflectance, while chloroplast accumulation in low light has the opposite effect. The changes in reflectance are correlated with an increase in the leaf's ability to depolarize linearly polarized light. Inclusion of polarization information in the detection procedure allows for the mitigation of the effects of sample orientation and for an increase in the specificity of detection. We applied this approach to examine chloroplast movements in the model plant Arabidopsis thaliana and four flowering plant species collected in the field. Despite the importance of chloroplast movements for the optimization of photosynthetic efficiency and biomass production, high-throughput reflectance-based methods are not routinely employed. Our approach opens the possibility of non-invasive, non-contact detection of chloroplast movements in a manner insensitive to the orientation of the leaf.