Warm Flowers

In the early spring when snow is still on the ground, the flower structures of the arum lily called eastern skunk cabbage (Symplocarpus foetidus) melt their way to the snow surface by being as much as 30°C warmer than the ambient temperature. In this way, this species dramatically announces that plants have evolved endothermy! The eastern skunk cabbage in fact displays thermoregulatory properties, in that its flower structures increase their rate of metabolic thermogenesis—responsible for endothermy—as the ambient temperature becomes colder. The function of endothermy in this case is believed to be to enhance the volatilization of odor compounds that attract pollinators. Box Extension 10.4 discusses this fascinating topic further.

Endothermy occurs in about 14 families of seed plants, mostly phylogenetically ancient plants. It occurs in reproductive structures: flowers, inflorescences, and cones. Whereas the tissues are warmed to only 1°–2°C above ambient temperature in some cases, warming to 34°C above ambient occurs in the philodendron Philodendron bipinnatifidum. In some cases, the weight-specific rate of heat production is in the same range as some of the higher rates observed in animals.

Despite years of research, the mechanism of thermogenesis is not yet totally nailed down in even a single species. The best-understood case is P. bipinnatifidum. In this case, the evidence points to a mitochondrial pathway called the alternative oxidase (AOX) pathway, which branches off from the main electron-transport chain following Complex I (see Figure 8.3). As electrons flow through the AOX pathway and energy of the electrons is released, no opportunity exists to capture any of the energy for ATP synthesis. Thus, following the opportunity provided by Complex I, the remainder of the energy is released as heat.

References

Miller, R. E., N. M. Grant, L. Giles, M. Ribas-Carbo, J. A. Berry, J. R. Watling, and S. A. Robinson. 2010. In the heat of the night—alternative pathway respiration drives thermogenesis in Philodendron bipinnatifidum. New Phytol. 189: 1013–1026.

Seymour, R. S. 2004. Dynamics and precision of thermoregulatory responses of eastern skunk cabbage Symplocarpus foetidus. Plant Cell Environ. 27: 1014–1022.