Ground level ozone is increasing around the world as a result of industrialization and anthropogenic activities, and this increase has become an environmental concern in agricultural production. Tomato is one of the ozone sensitive plants. To explore how the shoots and roots respond to acute ozone exposure, tomato plants were exposed to ozone at two growth stages. The plants were grown under a white net house throughout the growth study, and transparent plastic chambers were used for ozone exposure. In Experiment I, 500 mu g m(-3) (250 ppb) was applied to 30-day-old plants. In Experiment II, 200,350 and 500 mu g m(-3) (100, 175 and 250 ppb) were applied to 51-day-old plants. The experiments were conducted at Kasetsart University, Thailand, from August 2010 to February 2011. Leaf injuries were higher in the younger plants than in the older ones depending on leaf rank. Reduction of the internode growth rate suggested that 500 mu g m(-3) ozone could delay the plant's growth just after exposure. Altered partitioning of dry matter in favour of the above ground parts was observed under ozone stress. Ozone depressed the relative total root length and root number depending on concentrations and plant ages. Depressed root growth was observed in both young and old plants. Younger plants showed reduction in root growth earlier than older ones but for a shorter duration. Comparison of root length and emergence (relative to the control plants) between the two growth stages suggested that plants better compensated for a loss of resources when ozone was applied at the younger stage, and this could be linked with the absence of fruits at this time. It can be concluded that tomato plants responded to ozone differently based on the period of their lives when they experienced ozone stress and the concentrations of it. Recovery processes after ozone injury largely depend on crop developmental stages at the time of stress. (C) 2012 Elsevier B.V. All rights reserved.