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Decay of the Chloroplast Pool of Ascorbate Switches on the Oxidative Burst in UV-B-Irradiated Rice

Authors

  • F. J. C. Lidon,

    1.  Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, Caparica, Portugal
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  • M. Teixeira,

    1.  Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Avenida da República, Oeiras, Portugal
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  • J. C. Ramalho

    1.  Centro de Ecofisiologia, Bioquímica e Biotecnologia Vegetal/Instituto de Investigação Científica Tropical, I.P., Quinta do Marquês, Oeiras, Portugal
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F. J. C. Lidon
Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre
2829-516 Caparica, Portugal
Tel.: +351 212948543 ext. 11713
Fax: +351 212948543
Email: fjl@fct.unl.pt

Abstract

A parallel was drawn between UV-B irradiation and the oxidative burst in the chloroplast lamellae of rice plants (Oryza sativa L. cv. Safari). After irradiation with a total biological effective UV-B of 20.825 kJ m−2 between the 15th and 21st days following germination, it was found that superoxide, hydroxyl radicals and hydrogen peroxide increased significantly in stressed leaves. The levels of ascorbate and the kinetics of superoxide dismutase, glutathione reductase, dehydroascorbate reductase, ascorbate peroxidase and catalase became increasingly inhibited. The developing oxidative burst also diminished photosynthetic pigments and galactolipids accumulation, while the de-epoxidation state of xanthophylls and lipids peroxidation increased. Following grana disorganisation, the accumulation of chloroplast polypeptides having 72/69, 33/32, 28/26, 22/20 and 18/16 kDa decreased, but the relative proportion of the 49/46 kDa polypeptides increased. The 69/65 kDa polypeptides also diminished on the 15th day. These alterations implicated the failure of both photosystems functioning, but the inhibition of the Hill reactions coupled to the water splitting complex was uppermost. In the 28th day, the chloroplast lamellae of leaves grown after UV-B exposure revealed a general recovery from oxidative burst and a subsequent higher stacking of thylakoids. It is concluded that UV-B irradiation becomes lethal when ascorbate oxidation in the xanthophyll cycle limits the antioxidant enzymes kinetics, triggering chloroplast proteolysis and acyl lipid peroxidation because of reactive oxygen species propagation.

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