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

We present a sensitive method for non-contact detection of chloroplast movements in leaves and other photosynthetic tissues, based on changes in the magnitude of cross-polarized reflectance of the leaf. We created a custom optical set-up and dedicated software for measurements of changes in red light reflectance during irradiation with blue light, known to trigger chloroplast relocations. Experiments on Arabidopsis thaliana wild type and several mutants with disrupted chloroplast movements showed that the chloroplast avoidance response, induced by high blue light, led to a substantial increase in diffuse reflectance of unpolarized red light. The effects of the accumulation response in low blue light were the opposite. The specular reflectance of the leaf was unaffected by the chloroplast positioning. To further improve the specificity of the detection, we examined the effects of chloroplast relocations on the leaf reflectance of a linearly polarized incident beam. The greatest relative change associated with chloroplast movements was observed when the planes of polarization of the incident and detected beams were perpendicular. This suggests that the chloroplast positioning affects the magnitude of depolarization of light. We applied the developed approach to examine chloroplast relocations in four angiosperm species collected in the field. In particular, the method allowed us to detect the chloroplast avoidance response in the green stems of bilberry before bud break. Such samples are not amenable to transmittance-based detection. ### Competing Interest Statement The authors have declared no competing interest.