Heterogeneous behavior of PSII in soybean (Glycine max) leaves with identical PSII photochemistry efficiency under different high temperature treatments

The purpose of this study is to demonstrate the heterogeneous behavior of PSII in soybean (Glycine max) leaves and identical maximum PSII photochemistry efficiency (FV/FM) under different high temperature treatments. We observed that, with an identical decrease in FV/FM in soybean leaves caused by different high temperature treatments, chlorophyll a fluorescence differed significantly, indicating different behaviors in the photosynthetic apparatus. The quantitative analysis showed that, with an identical FV/FM, leaves treated at 48 °C showed a higher WK, an indicator of damage to the oxygen-evolving complex along with a lower O2 evolution rate compared with leaves treated at 45 °C. This demonstrated that the donor side of PSII was damaged more severely at 48 °C than at 45 °C despite the same decrease in FV/FM in the two heat-treated leaves. The ratios of QA- and QB-reducing PSII reaction centers to total PSII reaction centers were both lower in leaves treated at 48 °C than in leaves treated at 45 °C with an identical FV/FM, indicating that the acceptor side of PSII was also more damaged by heat treatment at 48 °C than at 45 °C. However, when damage to the donor side of PSII was similar in leaves treated at two different temperatures, the acceptor side of PSII was damaged less severely at 48 °C, which accounted for higher electron transport rate at the acceptor side of PSII in leaves treated at 48 °C than in leaves treated at 45 °C.