Thermogenic respiratory processes drive the exponential increase of volatile organic compound emissions in Macrozamia cycad cones

An important outcome of plant thermogenesis is increased emissions of volatiles that mediate pollinator behaviour. We investigated whether the large increase in emissions, mainly the monoterpene beta-myrcene (>90%), during daily thermogenic events of Macrozamia maeleayi and lucida cycad cones are due solely to the influence of high cone temperatures or are, instead, a result of increased respiratory rates during thermogenesis. We concurrently measured temperature, oxygen consumption and beta-myrcene emission profiles during thermogenesis of pollen cones tinder typical environmental temperatures and during experimental manipulations of cone temperatures and aerobic conditions, all in the dark. The exponential rise in beta-myrcene emissions never occurred without a prior, large increase in respiration, whereas an increase in cone temperature alone did not increase emissions. When respiration during thermogenesis was interrupted by anoxic conditions, beta-myrcene emissions decreased. The increased emission rates are not a result of increased cone temperature per se (through increased enzyme activity or volatilization of stored volatiles) but are dependent on biosynthetic pathways associated with increased respiration during thermogenesis that provide the carbon, energy (ATP) and reducing compounds (NADPH) required for beta-myrcene production through the methylerythritol phosphate (MEP) pathway. These endings establish the significant contribution of respiration to volatile production during thermogenesis.