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Spadix size is certainly significant
One of my abiding memories of the late Tom Rees was his arrival at the lab on spring mornings in Cambridge carrying inflorescences of Arum maculatum that he had picked on his way to work. Tom was interested in the regulation of metabolism associated with thermogenesis in the spadices. He went on to show that enzyme activities and fluxes through glycolysis and non-ATP-yielding respiratory pathways increase very dramatically. But just how hot can the spadices get? Some of the constraints on temperature will be metabolic and some will be physiological, but are there also physical limitations? This question has been addressed by Gibernau et al., Toulouse, France and Montreal, Canada (pp. 117–125) for 18 different tropical aroids in which the male zone is in the upper part of the inflorescence. The maximum difference between the temperature of the spadix and ambient temperature was 15 ºC; the magnitude of the temperature difference was directly correlated with the volume of the thermogenic zone in the spadix – size does matter! In order to discern the physical, as opposed to biochemical and physiological, factors that limit the temperature difference, they applied models describing different modes of heat transfer to their data. Three modes of heat transfer from a solid cylinder are possible: conduction (if the air around the inflorescence is still), convection (if the air is moving) and radiation. The analysis in fact shows that both a conduction model and a mixed convection–radiation model provide good fits to the data. The authors suggest that it is entirely feasible that different modes of heat transfer occur separately, but they mainly focus on the conduction model. So, in addition to the biochemical ability of the inflorescence to generate heat via thermogenic metabolism, the temperature differential is limited by heat loss consistent with conduction from a solid insulated cylinder with an internal heat source.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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