Endophytic infection alleviates Pb2+ stress effects on photosystem II functioning of Oryza sativa leaves

The aims of this study were to examine the effect of Pb2+ stress on the primary reaction of photosynthesis and to assess the potential benefits of endophytic infection on the Pb2+ tolerance of rice seedlings. Rice inoculated with an endophytic fungus (E+) and non-inoculated (E−) were subjected to 0, 50, 100, 150 and 200 μM Pb2+. The responses to Pb2+ stress were characterized by the analysis of Chl a fluorescence. A comparison of E− with E+ rice seedlings, as evaluated by their performance index (PIABS and PItot), revealed the inhibitory effects of Pb2+ on photosystem II (PSII) connectivity, the oxygen evolving complex (OEC), and on the J step of the induction curves, which is associated with an inhibition of electron transport from the quinone acceptor QA to QB. Furthermore, the changes of the donor and the acceptor parameters of PSII were greater in E− than in E+ under Pb2+ stress. These observations suggest that the efficiency and stability of PSII are markedly affected by Pb2+ stress, and the photosynthetic energy conservation in E+ was more effective than in E−. We showed that endophytic infection plays an important role in enhancing the photosynthetic mechanism of rice seedlings exposed to Pb2+ stress.