Lethal heat stress-dependent volatile emissions from tobacco leaves: what happens beyond the thermal edge?

Natural vegetation is predicted to suffer from extreme heat events as a result of global warming. In this study, we focused on the immediate response to heat stress. Photosynthesis and volatile emissions were measured in the leaves of tobacco (Nicotiana tabacum cv. Wisconsin 38) after exposure to heat shock treatments between 46 degrees C and 55 degrees C. Exposure to 46 degrees C decreased photosynthetic carbon assimilation rates (A) by >3-fold. Complete inhibition of A was observed at 49 degrees C, together with a simultaneous decrease in the maximum quantum efficiency of PSII, measured as the F-v/F-m ratio. A large increase in volatile emissions was observed at 52 degrees C. Heat stress resulted in only minor effects on the emission of monoterpenes, but volatiles associated with membrane damage such as propanal and (E)-2-hexenal+(Z)-3-hexenol were greatly increased. Heat induced changes in the levels of methanol and 2-ethylfuran that are indicative of modification of cell walls. In addition, the oxidation of metabolites in the volatile profiles was strongly enhanced, suggesting the acceleration of oxidative processes at high temperatures that are beyond the thermal tolerance limit.