Physical constraints on temperature difference in some thermogenic aroid inflorescences

Abstract

Backgrounds and aims: Thermogenesis in reproductive organs is known from several plant families, including the Araceae. A study was made of the relationship between temperature increase and spadix size in the subfamily Aroideae in order to determine whether the quantitative variation of heat production among species and inflorescences of different sizes follows a physical law of heat transfer. *

Methods: Spadix temperature was measured in 18 species from eight genera of tropical Araceae from the basal clade of Aroideae, both in French Guiana and in the glasshouses of the Montreal Botanical Garden. *

Key results: A significant logarithmic relationship was found between the volume of the thermogenic spadix zone and the maximum temperature difference between the spadix and ambient air. Four heat transfer models were applied to the data (conductive heat transfer alone, convective heat transfer alone, radiative heat transfer alone, and convective and radiative heat transfers) to test if physical (geometric and thermic) constraints apply. Which heat transfer model was the most probable was determined by using the criterion of a classical minimization process represented by the least-squares method. Two heat transfer models appeared to fit the data well and were equivalent: conductive heat transfer alone, and convective plus radiative heat transfers. *

Conclusions: The increase in the temperature difference between the spadix and ambient air appears to be physically constrained and corresponds to the value of a thermal model of heat conduction in an insulated cylinder with an internal heat source. In the models, a heat metabolic rate of 29.5 mW g(-1) was used, which was an acceptable value for an overall metabolic heat rate in aroid inflorescences.

Fig. 1.

Schematic diagram of the inflorescence thermogenic zone used to study heat transfer. R_s ( = r), inflorescence radius (m); L, length of male zone inflorescence (m); T_∞, ambient air temperature (°C); Φ_V, dry heat flux leaving the male zone inflorescence (W); δ, thickness of the conductive boundary layer (m); λ, conductivity coefficient of the inflorescence (Wm⁻¹ K⁻¹); λ_a, conductivity coefficient of stagnant air (Wm⁻¹ K⁻¹).

Fig. 2.

Relationship between the volume of the thermogenic zone (male or male-sterile according to the aroid species) and the maximum of the temperature difference between the inflorescence and the ambient air.

Fig. 3.

Relationships between the volume of the thermogenic zone and the maximum temperature difference between the inflorescence and the ambient air for the eight Philodendron species studied. Measured temperatures (dashed line) are shown together with calculated temperatures (solid lines) according to the four different heat transfer models (see text for full explanation).

Fig. 4.

Relationships between the volume of the thermogenic zone and the maximum temperature difference between the inflorescence and the ambient air for all the species studied. Measured temperatures (dashed line) are shown together with calculated temperatures (solid lines) according to the four different heat transfer models (see text for full explanation).