Exogenous putrescine alleviates photoinhibition caused by salt stress through cooperation with cyclic electron flow in cucumber

When plants suffer from abiotic stresses, cyclic electron flow (CEF) is induced for photo-protection. Putrescine (Put), a primary polyamine in chloroplasts, plays a critical role in stress tolerance. However, the relationship between CEF and Put in chloroplasts for photo-protection is unknown. In this study, we investigated the role of Put-induced CEF for salt tolerance in cucumber plants (Cucumis sativus L). Treatment with 90 mM NaCl and/or Put did not influence the maximum photochemical efficiency of PSII (Fv/Fm), but the photoactivity of PSI was severely inhibited by NaCl. Salt stress induced a high level of CEF; moreover, plants treated with both NaCl and Put exhibited much higher CEF activity and ATP accumulation than those exhibited by single-salt-treated plants to provide an adequate ATP/NADPH ratio for plant growth. Furthermore, Put decreased the trans-membrane proton gradient (Delta pH), which was accompanied by reduced pH-dependent non-photochemical quenching (NPQ) and an increased the effective quantum yield of PSII (Y(II)). The ratio of NADP(+)/NADPH increased significantly with Put in salt-stressed leaves compared with the ratio in leaves treated with NaCl, indicating that Put relieved over-reduction pressure at the PSI acceptor side caused by salt stress. Collectively, our results suggest that exogenous Put creates an excellent condition for CEF promotion: a large amount of pmf is predominantly stored as Delta psi, resulting in moderate lumen pH and low NPQ, while maintaining high rates of ATP synthesis (high pmf).