Thermal clamping of temperature-regulating flowers reveals the precision and limits of the biochemical regulatory mechanism

Abstract

The flowers of several families of seed plants warm themselves when they bloom. In some species, thermogenesis is regulated, increasing the rate of respiration at lower ambient temperature (T a) to maintain a somewhat stable floral temperature (T f). The precision of this regulation is usually measured by plotting T f over T a. However, such measurements are influenced by environmental conditions, including wind speed, humidity, radiation, etc. This study eliminates environmental effects by experimentally ‘clamping’ T f at constant, selected levels and then measuring stabilized respiration rate. Regulating flowers show decreasing respiration with rising T f (Q 10 < 1). Q 10 therefore becomes a measure of the biochemical ‘precision’ of temperature regulation: lower Q 10 values indicate greater sensitivity of respiration to T f and a narrower range of regulated temperatures. At the lower end of the regulated range, respiration is maximal, and further decreases in floral temperature cause heat production to diminish. Below a certain tissue temperature (‘switching temperature’), heat loss always exceeds heat production, so thermoregulation becomes impossible. This study compared three species of thermoregulatory flowers with distinct values of precision and switching temperature. Precision was highest in Nelumbo nucifera (Q 10 = 0.16) moderate in Symplocarpus renifolius (Q 10 = 0.48) and low in Dracunculus vulgaris (Q 10 = 0.74). Switching temperatures were approximately 30, 15 and 20°C, respectively. There were no relationships between precision, switching temperature or maximum respiration rate. High precision reveals a powerful inhibitory mechanism that overwhelms the tendency of temperature to increase respiration. Variability in the shape and position of the respiration–temperature curves must be accounted for in any explanation of the control of respiration in thermoregulatory flowers.