The phenomenon of enhanced plant thermotolerance by isoprene was studied in leaves of the same age of 1- or 2-year-old Platanus orientalis plants. Our goals were to determine whether the isoprene emission depends on the age of the plant, and whether different emission rates can influence heat resistance in plants of different age. Two-year-old plants emit greater amounts of isoprene and possess better capacity to cope with heat stress than 1-year-old plants. After a high temperature treatment (38°C for 4 h), photosynthetic activity, hydrogen peroxide content, lipid peroxidation and antiradical activity were preserved in isoprene emitting leaves of 1- and 2-year-old plants. However, heat inhibited photosynthesis and PSII efficiency, caused accumulation of H₂O_2, and increased all indices of membrane damage and antioxidant capacity in leaves of plants of both ages in which isoprene was inhibited by fosmidomycin. In isoprene-inhibited leaves fumigated with exogenous isoprene during the heat treatment, the negative effects on photosynthetic capacity were reduced. These results further support the notion that isoprene plays an important role in protecting photosynthesis against damage at high temperature. It is suggested that isoprene is an important compound of the non-enzymatic defence of plants against thermal stress, possibly contributing to scavenging of reactive oxygen species (ROS) and membrane stabilising capacity, especially in developed plants.