摘要： Photosynthesis has been key to Earth’s habitability, especially since the evolution of oxygenic photosynthesis with the accompanying rise in atmospheric O2 starting in the Proterozoic era, around 2.5 billion years ago. The conversion of light to chemical energy enables higher life forms and the presence of reduced carbon stocks and molecular O2 (accumulated products of photosynthesis) provided the fossil fuels for the initiation of our industrialized society. As a visible indicator of photosynthesis, fluorescence emanating from chlorophyll molecules attracted interest more than 100 years ago. In 1874, red fluorescence from a green living leaf was observed to be weaker than from a dilute chlorophyll solution (Müller, 1874), indicating an alternate fate for energy absorbed by chlorophyll in vivo. More quantitative studies of chlorophyll a fluorescence go back to observations by Hans Kautsky and A. Hirsch in 1931 at the Chemical Institute at the University of Heidelberg, Germany (Kautsky, 1931). They discovered the so-called Kautsky effect, a decrease in Chl fluorescence following illumination of dark-adapted leaves. Both studies already indicated an important aspect of chlorophyll fluorescence, namely, that its yield can vary. This effect is caused by variation in the balance of pathways that compete with fluorescence for quenching of the excited chlorophyll state. The porphyrin ring of the chlorophyll molecule is the center of light absorption and fluorescence. In a dilute chlorophyll solution, where fluorescence is the only route for de-excitation, the fluorescence lifetime (sF) is about 6 ns and the fluorescence yield (FF) approaches 1. In vivo, the excited chlorophyll state can be quenched by other processes, most notably photosynthesis itself but also quenching to heat; FF is lower (1%–5%) and the sF proportionally shorter. Fluorescence in itself is not a significant route for de-excitation of chlorophyll but changes in FF or sF can be used to study how absorbed energy is used. On the other hand, when FF is constant, fluorescence can provide information on the total amount of light absorbed, as the amount emitted will be proportional to the total amount of photosynthetically active radiation (PAR) that is absorbed by chlorophyll and associated pigments. Fig. 1 shows an example of a typical fluorescence spectrum as well as the absorbance spectrum by chlorophyll.