The oxygen supply to thermogenic flowers.

Thermogenic flowers produce heat by intense respiration, and the rates of O2 consumption (Ṁo2) in some species can exceed those of all other tissues of plants and most animals. By exposing intact flowers to a range of O2 pressures (Po2) and measuring Ṁo2, we demonstrate that the highest respiration rates exceed the capacity of the O2 diffusive pathway and become diffusion limited in atmospheric air. The male florets on the inflorescence of Arum concinnatum have the highest known mass-specific Ṁo2 and can be severely diffusion limited. Intact spadices of Japanese skunk cabbage Symplocarpus renifolius are diffusion limited in air only when Ṁo2 is maximal, but not at lower levels. True flowers of the sacred lotus Nelumbo nucifera and the appendix of Arum concinnatum are never diffusion limited in air. Ṁo2 - Po2 curves are evaluated quantitatively with the 'Regulation Index', a new tool to measure dependence of Ṁo2 on ambient Po2 , as well as the conventional 'Critical Po2 '. The study also includes measurements of Po2 within thermogenic tissues with O2-sensitive fibre optics, and reveals that the diffusion pathway is complicated and that O2 can be provided not only from the surface of the tissues but also from the pith of the flower's peduncle.