Time course of the photochemical reflectance index during photosynthetic induction: its relationship with the photochemical yield of photosystem II

Time courses of photochemical reflectance index (PRI) of an attached cucumber leaf during a dark-light transition were compared with those of photochemical yields of photosystem II (Y-II) to discuss the feasibility of PRI imaging for estimating the efficiency of photosynthetic light use. Y-II and PRI were simultaneously evaluated with a pulse-amplitude modulation chlorophyll fluorometer and a low-cost imaging system consisting of digital cameras and band-pass filters, respectively. Y-II decreased immediately after the transition and then increased under various photon flux densities. Although PRI exhibited delayed time courses with respect to Y-II under low light conditions, PRI decreased monotonically under high light conditions. There was no correlation between Y-II and the changes in PRI (Delta PRI) immediately after the transition but Y-II was correlated with Delta PRI under the steady-state photosynthesis. These results indicate that the use of PRI to estimate Y-II under fluctuating light based on the regression obtained at steady state can overestimate Y-II. The imaging system was also applied to evaluate the spatial PRI distribution within a leaf. While PRI of leaf areas that remained untreated, or had been treated with H2O again, first dropped and then rose under low light and monotonically decreased under high light conditions, leaf areas treated with inhibitor (dichlorophenyl dimethylurea) did not exhibit any changes. It is likely that the inhibitor suppressed lumen acidification, which triggers a decrease in PRI. It was suggested that Y-II of leaves with malfunctions in the photosynthetic electron transport can be overestimated by the PRI-based estimation.