Flower Heliotropism and Floral Heating of Five Alpine Plant Species and the Effect on Flower Visiting in Ranunculus montanus in the Austrian Alps

Abstract

During the growing seasons in 1994 and 1995 flower heliotropism and its effects on floral biology of several plants in the Grossglockner region (Salzburg, Austria) at 2230-2340 m a.s.l. was investigated. Ranunculus montanus, R. alpestris, Pulsatilla alpina, Callianthemum coriandrifolium (Ranunculaceae), and Leucanthemopsis alpina (Asteraceae) all have bowl and disc flowers (or inflorescences) that are heliotropic during periods of direct solar radiation. Temperatures of the air inside the flowers were above those of the ambient air for all five species. The maxima of excess temperature (means of 10 flowers or inflorescences) were between 2.0°C (C. coriandrifolium) and 6.2°C (P. alpina). Comparative temperature measurements in flowers of R. montanus showed that during sunny or slightly cloudy periods the air within those having uninhibited heliotropic movements was on average 0.7°C (in individual cases up to 1.3°C) warmer than in flowers fixed to face north. For the self-incompatible R. montanus, heliotropism affected the frequency and residence time of flower visitors: unimpeded heliotropic flowers were more attractive and so were visited longer and more often by insects than those oriented away from the sun.