Heat stress reduces the accumulation of rosmarinic acid and the total antioxidant capacity in spearmint (Mentha spicata L)

Selected high-phenolic lines of spearmint were subjected to a constant 30 degrees C heat regimen for a period of 4 weeks to determine the effects of heat stress on soluble phenolics, phenols and rosmarinic acid biosynthesis and antioxidant capacity. Heat stress significantly reduced levels of total phenolic acids (71-87%) and soluble phenols (75-87%). This loss was concomitant with a loss of total antioxidant capacity of 21-60% after week 1 and up to 95% by week 4. High-performance liquid chromatography profiling of heat-stressed plants at 270 and 320 nm detected nearly a complete loss of rosmarinic acid in all seven chemotypes. High-temperature drying of non-heat-stressed plants at 80 degrees C resulted in a similar loss of total antioxidant capacity and rosmarinic acid content an effect not observed in material that was subjected to low-temperature drying first, followed by exposure to high temperature. This suggests that heat stress negatively regulates rosmarinic acid biosynthesis and causes a potential rapid biological breakdown of rosmarinic acid in tissues. 2,2-Diphenyl-1-picrylhydrazyl radical assays of heat-stressed and non-stressed plants clearly show that rosmarinic acid is the major contributor to the antioxidant capacity in spearmint. (c) 2005 Society of Chemical Industry.