Flowers that cool themselves: thermal ecology of summer-blooming thistles in hot Mediterranean environments

Abstract

Flower exposure to high temperature reduces the production, viability and performance of pollen, ovules and seeds, which in turn reduces individual fecundity and risks the survival of populations. Autonomous flower cooling could help to alleviate exposure of pollen and ovules to harmful temperatures, yet investigations on the thermal ecology of flowers in hot environments are still needed to evaluate the frequency, magnitude and ecological significance of floral thermoregulatory cooling. This paper reports a study on the thermal ecology of flower heads (=capitula) of 15 species of summer-blooming Asteraceae, tribe Cardueae, from hot-dry habitats in the southern Iberian Peninsula. Temperature inside (T_in) and outside (T_out) capitula were assessed under natural field conditions by two complementary sampling and measurement procedures, which provided information on the relationships between the two temperatures at the levels of individual capitula (“continuous recording”) and local plant populations (“instantaneous measurements”). Baselines for the T_in–T_out relationship in absence of physiological activity were obtained by exposing dehydrated capitula to variable ambient temperatures. To assess whether the co-flowering capitula of summer-blooming Asteraceae collectively defined a distinct thermal layer, the vertical distribution of capitula relative to the ground was quantified. Bees visiting capitula were watched and the temperature of the air beside the visited capitulum was measured. Capitula of all species were exposed to high ambient temperatures during long periods, yet their interior was cooler than the air for most of the time, with temperature differentials (ΔT=T_in-T_out) quite often approaching, and sometimes exceeding −10°C. The relationship between T_in and T_out was best described by a composite of steep and shallow linear relationships separated by a breakpoint (Ψ, interspecific range=25°C–35°C). Capitula were weakly or not thermoregulated for T_out<Ψ, but switched to intense, thermoregulated cooling when T_out>Ψ. Narrow vertical distribution of capitula above ground and similar cooling responses by all species produced a refrigerated floral layer where most bees foraged at T_out>Ψ and presumably visited cooled capitula. Thermoregulatory cooling of capitula can benefit plant reproduction by reducing pollen and ovule exposure to high temperature, but also the populations of insect mutualists and antagonists through habitat improvement via a refrigerated flower layer.