Heterologous expression of peroxidase genes has been shown to influence morphology and stress responses of several crop plants but little is known about the effect in woody species. In this study, a barley cell-wall peroxidase gene (prx8), peviously shown to influence growth and stress tolerance in tobacco, was introduced into the genome of apple (Malus domestica cv. Greensleeves) via Agrobacterium-mediated transformation and the presence of the transgene confirmed by PCR and Southern blot analysis. The transgenic plants had up to 4-fold higher levels of peroxidase activity compared to wild type plants and exhibited faster growth and increased xylem production. Leaf discs were incubated at high temperatures (44°C) and electrolyte leakage measurements indicated enhanced tolerance against temperature stress. This effect was confirmed when whole plants were subjected to heat stress after 6 and 12 months growth in soil. These results indicate a link between peroxidase activity levels and resistance to thermal stress, as well as biochemical and physiological changes, in this woody plant.