Photosynthetic regulation in response to strontium stress in moss Racomitrium japonicum L

Abstract

Strontium (Sr²⁺) pollution and its biological effects are of great concern including photosynthetic regulation, which is fundamental to environmental responses, especially for bryophytes during their terrestrial adaptation. Alternative electron flows mediated by flavodiiron proteins (FLVs) and cyclic electron flow (CEF) in photosystem I (PSI) are crucial to abiotic stresses moss responses; however, little is known about the moss photosynthesis regulation under nuclide treatment. We measured chlorophyll fluorescence parameters in PSI, photosystem II (PSII) and the P700 redox state, oxidative stress in the moss Racomitrium japonicum under low (5 mg/L), moderate (50 mg/L) and high (500 mg/L) Sr²⁺ stress level. Moderate and high Sr²⁺ stress triggered H₂O₂ and malondialdehyde (MDA) generation, and catalase (CAT) activity increases, which are involved in reactive oxygen species regulation. The significant PSII photochemistry (Fv/Fm), Chla/chlb, Y(I)/Y(II), Y(NA), Y(ND) and ETRI-ETRII decreases at moderate and high Sr²⁺, and the Y(I), Y(II) decreases at high Sr²⁺ revealed the photo-inhibition and photo-damage in PSI and PSII by moderate and high Sr²⁺ stress. The nonphotochemical quenching (NPQ) increased significantly at moderate and high Sr²⁺ stress, reflecting a heat-dissipation-related photo-protective mechanism in antenna system and reaction centers. Moreover, rapid re-oxidation of P700 indicated that FLV-dependent flows significantly regulated PSI redox state under moderate and high Sr²⁺ stress. and CEF upregulation was found at low Sr²⁺. Finally, photosynthetic acclimation to Sr²⁺ stress in R. japonicum was linked to FLVs and CEF adjustments.

Keywords: Cyclic electron transport; Flayodiiron proteins; Photosynthetic acclimation; Sr2+ stress.