Floral thermogenesis of three species of Hydnora (Hydnoraceae) in Africa

BACKGROUND AND AIMS: Floral thermogenesis occurs in at least 12 families of ancient seed plants. Some species show very high rates of respiration through the alternative pathway, and some are thermoregulatory, with increasing respiration at decreasing ambient temperature. This study assesses the intensity and regulation of respiration in three species of African Hydnora that represent the Hydnoraceae, an unusual family of holoparasitic plants from arid environments. METHODS: Long-term respirometry (CO₂ production) and thermometry were carried out on intact flowers of H. africana, H. abyssinica and H. esculenta in the field, and short-term measurements were made on floral parts during the protogynous flowering sequence. KEY RESULTS: For H. africana, there was no temperature elevation in either the osmophores or the gynoecial chamber in any phase, and mass-specific respiration rates of the flower parts were low (maximum 8·3 nmol CO₂ g⁻¹ s⁻¹ in osmophore tissue). Respiration tracked ambient and floral temperatures, eliminating the possibility of the inverse relationship expected in thermoregulatory flowers. Hydnora abyssinica flowers had higher respiration (maximum 27·5 nmol g⁻¹ s⁻¹ in the osmophores) and a slight elevation of osmophore temperature (maximum 2·8 °C) in the female stage. Respiration by gynoecial tissue was similar to that of osmophores in both species, but there was no measurable elevation of gynoecial chamber temperature. Gynoecial chamber temperature of H. esculenta could reach 3·8 °C above ambient, but there are no respiration data available. Antheral tissue respiration was maximal in the male phase (4·8 nmol g⁻¹ s⁻¹ in H. africana and 10·3 nmol g⁻¹ s⁻¹ in H. abyssinica), but it did not raise the antheral ring temperature, which showed that thermogenesis is not a by-product of pollen maturation or release. CONCLUSIONS: The exceptionally low thermogenesis in Hydnora appears to be associated with scent production and possibly gynoecial development, but has little direct benefit to beetle pollinators.