Effects of GeO2 on chlorophyll fluorescence and antioxidant enzymes in apple leaves under strong light

In this study, we chose apple leaf as plant material and studied effects of GeO₂ on operation of photosynthetic apparatus and antioxidant enzyme activities under strong light. When exogenous GeO₂ concentration was below 5.0 mg L^–1, maximum photochemical quantum yield of PSII and actual quantum yield of PSII photochemistry increased significantly compared with the control under irradiances of 800 and 1,600 μmol(photon) m^–2 s^–1. Photosynthetic electron transport chain capacity between Q_A–Q_B, Q_A–PSI acceptor, and Q_B–PSI acceptor showed a trend of rising up with 1.0, 2.0, and 5.0 mg(GeO₂) L^–1 and declining with 10.0 mg(GeO₂) L^–1. On the other hand, dissipated energy via both ΔpH and xanthophyll cycle decreased remarkably compared with the control when GeO₂ concentration was below 5.0 mg L^–1. Our results suggested that low concentrations of GeO₂ could alleviate photoinhibition and 5.0 mg(GeO₂) L^–1 was the most effective. In addition, we found, owing to exogenous GeO₂ treatment, that the main form of this element in apple leaves was organic germanium, which means chemical conversion of germanium happened. The organic germanium might be helpful to allay photoinhibition due to its function of scavenging free radicals and lowering accumulation of reactive oxygen species, which was proven by higher antioxidant enzyme activities.