Abstract: C3 photosynthesis is one of the most important photosynthetic mechanisms, in which light-driven electron transport is coupled to the movement of protons from the chloroplast stroma into the thylakoid lumen. With it, positive-charged protons accumulate in the lumen, and the protons decrease in the stroma, producing the transmembrane proton motive force (pmf), eventually, driving the conformational variation of the transmembrane thylakoid ATPase which converts ADP (adenosine diphosphate) and Pi (inorganic phosphate) into ATP (adenosine triphosphate) which are subsequently used in the ensuing Calvin cycle to make the organic products. Proton motive force contains two components, one is the chemical potential difference caused by the transmembrane osmotic pressure, and another pmf component, i.e. ΔΨ, is an essential indicator of light energy transduction, which triggers a so-called electrochromic shift of the absorption maximum of the pigment(s) to 515 nm. Majorly, carotenoid pigment and chlorophyll b pigment are responsible for the electrochromic shift. This shift is not solely intensified by ΔΨ, also overlapped by the absorption change caused from the “light scattering” effect. The interpretation of this signal is complicated, which indeed find the P515 measurement a hard trial for the example of ΔΨ probing, particularly in the long-term temporal range. It has seldom been reported in the published papers domestically for the P515 measurement. In this work, tobacco leaves (Nicotiana tabacum) were measured by Dual-PAM-100 chlorophyll fluorescence photosynthesis analyzer using the novel P515/535 Emitter-Detector module. The results illustrated the P515 curves when single turnover flash was on, when actinic light was lasting for 1 s, and when actinic light was lasting for 1 min followed by another 2 min dark phase interval. Our results could offer a technological foundation for the scientific research of the photosynthetic primary energy transduction.

Key words: C3 plant, thylakoid membrane, electric potential difference (ΔΨ), P515 signal