Production of mitochondrial reactive oxygen species (ROS) in the liver of the quails exposed to thermal comfort or heat stress (HS) and fed with methionine supplementation (MS) or methionine deficient (MD) diet. The results are expressed in nanomoles of ROS produced per minute per milligram of protein. The results are shown as the average, and the standard deviation is represented by the vertical bars. The different letters between the treatment groups represent a significant difference (P < 0.05). Ptn = Protein.