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 degrees 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.

Thermogenesis, Flowering and the Association with Variation in Floral Odour Attractants in Magnolia sprengeri (Magnoliaceae)

“…In some thermogenic plants, floral temperature is maintained at a constant range, independent of ambient temperature, for a long period throughout anthesis [8],[35],[36]. Floral temperature stability of these plants is achieved by means of physiological thermoregulation [35],[36]. In this study, floral temperature of M. sprengeri changed constantly throughout anthesis and major thermogenic episodes were short, which indicated that M. sprengeri was not thermoregulated.…”

“…Distinct heating patterns have been reported in different thermogenic species [35],[36]. In some thermogenic plants, floral temperature is maintained at a constant range, independent of ambient temperature, for a long period throughout anthesis [8],[35],[36]. Floral temperature stability of these plants is achieved by means of physiological thermoregulation [35],[36].…”

Different molecular bases underlie the mitochondrial respiratory activity in the homoeothermic spadices of Symplocarpus renifolius and the transiently thermogenic appendices of Arum maculatum

“…This suggests both S. renifolius and A. maculatum utilize this enzyme to provide reducing equivalents to the ubiquinone pool; however, S. renifolius , with less abundance of NDB, may depend more on NDA than A. maculatum . In S. renifolius , the difference between the floral and ambient temperature has been found to show a positive correlation with CO 2 production in the regulation of floral temperature [34]. As the CO 2 is presumably produced during TCA (tricarboxylic acid) cycle activity, the re-oxidation of NADH within the mitochondrial compartment is vital for maintaining cycle turnover, particularly at low ambient temperatures.…”

The oxygen supply to thermogenic flowers

“…The 800 mL chamber itself consisted of a miniature constant temperature (CT) cabinet, described earlier in detail (Seymour et al . ). In brief, the cabinet was heated or cooled by a Peltier plate and heat sink on one wall, and the air in the chamber was circulated with a small fan.…”

“…The effect of ambient P o 2 on respiration rate was measured with open‐flow respirometry described previously (Seymour et al . ), but using a fuel cell O 2 analyser (David Bishop Instruments, Heathfield, UK; model 280). Instead of drawing atmospheric air into the respirometry chamber by suction and measuring flow on the downstream side as previously done, pressurized gas mixtures were supplied at measured rates to the upstream side of the chamber.…”