The Secret of Hot-headed Teasel (Dipsacus fullonum L. (Caprifoliaceae))

Abstract Temperature regimes within flowers and inflorescences are not well studied but have been shown to be important in plant sexual reproduction (pollination). We report for the first time the effects of ambient air temperatures and insolation (sunny vs. cloudy vs. night) on temperatures within the various sizes of the complex flowering head of teasel ( Dipsacus fullonum ). Blossom temperatures influence pollination in various ways, notably through the growth and maturation of the sexual organs and by being perceived by and influencing pollinator behaviour. We report that the inflorescences of common teasel become warmed above the adjacent ambient air by absorbing solar radiation and trapping heat. Inflorescence size, time of day and insolation have notable impacts on temperatures within teasel inflorescences. Large inflorescences (about 50 mm long × 30 mm in diameter) become as much as 8 C warmer than the ambient air at the time of day when the sun was strongest (ca. 10:00–14:00 EST), mid-sized ones (about 30 mm long × 20 mm diameter) about 5 C and small ones (about 20 mm long × 10 mm diameter) about 3 C. Temperature excesses observed under sunny conditions are abolished at night and under cloud. Pollinator activity, mostly by bumblebees ( Bombus ) on teasel heads follows the same trends as the temperature regimes, being most intense in warm sunny conditions. That correlation may reflect some relationships between floral biology and pollinator behaviour, such as floral heat, effects on nectar secretion, viscosity and availability, pollen presentation, and maturation of florets.