Effects of exogenously applied hydrogen peroxide on antioxidant and osmoprotectant profiles and the C3-CAM shift in the halophyte Mesembryanthemum crystallinum L.

Exogenously applied H2O2 (50, 100 and 200 mM) to Mesembryanthemum crystallinum root medium induced a transition from C3 to Crassulacean Acid Metabolism (CAM), as evaluated by diurnal malate (Δmal) fluctuations. A very high concentration of H2O2 (400 mM) reduced Δmal below the value measured in control plants. An increase of malate content during the night in 400 mM H2O2-treated plants might suggest that malate decarboxylation is crucial for CAM functioning. We conclude that malate plays a dual role: i) a protective and signaling function before CAM expression, and ii) a storage form of CO2 in plants performing CAM. A slight stimulation of photosystem II (PSII) photochemistry and net photosynthesis observed during the C3-CAM shift indicated that neither photoinhibition nor reduction of the photosynthetic rate were prerequisites for CAM. Moreover, CAM induction corresponded to a decrease of catalase activity. In CAM-performing plants, α-tocopherol, polyamines (putrescine and spermidine) and proline showed daily alterations and the content of α-tocopherol and polyamines was lower at the end of the day. In contrast, the proline concentration correlated with the applied H2O2 concentration and was higher at the end of the day in treated plants. The dynamic changes of antioxidant and osmolyte levels suggest their active role in preventing oxidative damage, stress acclimation mechanisms and involvement in metabolic regulation and/or signal transduction cascades.