Switching off the heater: Influence of ambient temperature on thermoregulation by eastern skunk cabbage Symplocarpus foetidus

Abstract

The protogynous inflorescences of eastern skunk cabbage, Symplocarpus foetidus, are thermogenic and regulate spadix temperature (T(s)) well above ambient temperature (T(a)). Continuous records of oxygen consumption, carbon dioxide production, and temperatures of plants were made at a field site in Canada. At T(a) between 3-24 °C, T(s) ranged between 16-26 °C, and the warmest inflorescences were those in the receptive female or early pollen-bearing stages. Respiratory rates of the 2-g spadices increased with declining T(a), and reached a maximum of 0.54 μmol O2 s-1 (0.73 ml min-1), equivalent to 0.26 W of heat production. At T(a) below 3 °C, several inflorescences failed to maintain high T(s) and abruptly switched T(s) to near freezing. Some froze when T(a) dropped to about -10 °C. Those that did not freeze could quickly switch to the warm state if T(a) rose above about 3 °C. Switching was related to the balance between heat production and heat loss that tended to produce stable equilibria at either high or low T(a). Switching between warm and cool states resulted in a bimodal distribution of T(s) in the field. A respiratory quotient of 1.0 showed that carbohydrate was the substrate for thermogenesis, and bomb calorimetry of florets confirmed that energy was imported from the root. Only 11 invertebrates, including only one flying insect, were found in 195 inflorescences, suggesting that heat production and temperature regulation are not closely associated with cross-pollination.