Antioxidants and antioxidative enzymes in wild-type and transgenic Lycopersicon genotypes of different chilling tolerance

The Mehler-Ascorbate-Peroxidase cycle is a protection system against reacti ve oxygen species (ROS) occurring during over-excitation of the photosynthe tic apparatus. In the cultivated tomato, Lycopersicon esculentum, long-term chilling under moderate light leads to oxidation of the Calvin cycle key e nzyme, ribulose-1,5-bisphosphate carboxylase (rubisco), presumably by gener ation of ROS. In contrast, high-altitude lines of the wild tomato species L . peruvianum were tolerant against the same chilling stress. In the present study, we analysed leaf contents of antioxidants (ascorbate, glutathione) and activities of enzymes of the Mehler-Ascorbate-Peroxidase cycle in the t wo Lycopersicon species. While antioxidant levels and activities of chlorop last superoxide dismutase (SOD) and ascorbate peroxidase (APX, both inducib le by chilling stress, were similar in chilling-tolerant and chilling-sensi tive genotypes, chilled L. esculentum showed lower glutathione reductase (G R) activities than high-altitude L. perurianum. We constructed transgenic p lants overexpressing an Escherichia coli GR in the chloroplast (approximate ly 60-fold of the wild-type (WT) activity). However, these plants resembled identical chilling sensitivity of the photosynthetic apparatus as WT plant s as measured after a photoinhibition treatment and by the effect of long-t erm chilling on rubisco activity. We conclude that the Mehler-Ascorbate-Per oxidase cycle is not the limiting factor for the sensitivity of the photosy nthetic apparatus of L. esculentum towards long-term chilling under moderat e light. We suggest that a possible cause for the higher chilling tolerance of L, perurianum is prevention of ROS formation by better conversion of li ght energy to photochemistry at suboptimal temperatures. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.