Intra-leaf gradients of photoinhibition induced by different color lights: Implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers

We studied how different color lights cause gradients of photoinhibition within a leaf, to attempt to resolve the controversy of whether photon absorption by chlorophyll or by manganese (Mn) is the primary cause of photoinhibition, as suggested by the excess-energy hypothesis or the two-step hypothesis, respectively. Lincomycin-treated leaf discs were photoinhibited by white, blue, green or red light. Combining a microfiber fluorometer, a fiber-thinning technique and a micro-manipulator enabled us to measure the chlorophyll fluorescence signals within a leaf. Photoinhibition gradients were also compared with results from various conventional fluorometers to estimate their depth of signal detection. The severity of photoinhibition was in the descending order of blue, red and green light near the adaxial surface, and in the descending order of blue, green and red light in the deeper tissue, which correlated with the chlorophyll and the Mn absorption spectrums, respectively. These results cannot be explained by either hypothesis alone. These data strongly suggest that both the excess-energy and the two-step mechanisms occur in photoinhibition, and fluorometers with red or blue measuring light give overestimated or underestimated F_v/F_m values of photoinhibited leaves compared with the whole tissue average, respectively; that is, they measured deeper or shallower leaf tissue, respectively.