• Philodendron bipinnatifidum inflorescences heat up to 42C and thermoregulate.
We investigated whether they generate heat via the cytochrome oxidase
pathway uncoupled by uncoupling proteins (pUCPs), or the alternative oxidase
• Contribution of AOX and pUCPs to heating in fertile (FM) and sterile (SM) male
florets was determined using a combination of oxygen isotope discrimination, protein
and substrate analyses.
• Both FM and SM florets thermoregulated independently for up to 30 h ex
planta. In both floret types, AOX contributed > 90% of respiratory flux during
peak heating. The AOX protein increased fivefold with the onset of thermogenesis
in both floret types, whereas pUCP remained low throughout development. These
data indicate that AOX is primarily responsible for heating, despite FM and SM
florets potentially using different substrates, carbohydrates or lipids, respectively.
Measurements of discrimination between O2 isotopes in strongly respiring SM florets
were affected by diffusion; however, this diffusional limitation was largely
overcome using elevated O2.
• The first in vivo respiratory flux measurements in an arum show AOX contributes
the bulk of heating in P. bipinnatifidum. Fine-scale regulation of AOX activity
is post-translational. We also demonstrate that elevated O2 can aid measurement
of respiratory pathway fluxes in dense tissues.