Light intensity dependent photosynthetic electron transport in eelgrass (Zostera marina L.)

Responses of electron transport to three levels of irradiation (20, 200, and 1200 μmol photons m^-2 s^-1 PAR; exposures called LL, ML and HL, respectively) were investigated in eelgrass (Zostera marina L.) utilizing the chlorophyll a fluorescence technique. Exposure to ML and HL reduced the maximum quantum yield of photosystem II (PSII) (Fv/Fm) and the maximum slope decrease of MR/MR_O (V_PSI), indicating the occurrence of photoinhibition of both PSII and photosystem I (PSI). A comparatively slow recovery rate of Fv/Fm due to longer half-life recovery time of PSII and 40% lower descending amplitude compared to other higher plants implied the poor resilience of the PSII. Comparatively, PSI demonstrated high resilience and cyclic electron transport (CEF) around PSI maintained high activity. With sustained exposure, the amplitudes of the kinetic components (L₁ and L₂), the probability of electron transfer from PSII to plastoquinone pool (ψ_ET2o), and the connectivity among PSII units decreased, accompanied by an enhancement of energy dissipation. Principle component analysis revealed that both V_PSI and Fv/Fm contributed to the same component, which was consistent with high connectivity between PSII and PSI, suggesting close coordination between both photosystems. Such coordination was likely beneficial for the adaption of high light. Exposure to LL significantly increased the activity of both PSI and CEF, which could lead to increased light harvesting. Moreover, smooth electron transport as indicated by the enhancement of L₁, L₂, ψ_ET2o and the probability of electron transport to the final PSI acceptor sides, could contribute to an increase in light utilization efficiency.