Effect of flower structure and flower colour on intrafloral warming and pollen germination and pollen-tube growth in winter flowering Crocus L. (Iridaceae)

The internal temperature of the flowers of three colour variants of winter flowering Crocus chrysanthus and of C. tommasinianus were compared with amb ient in the dark, and when subject to artificial horizontal illumination wi th daylight spectra. Illuminated flowers warmed up to 3 degrees C above amb ient. In the dark, flowers also showed slight warming. In all varieties, po llen germinated more freely at 15 degrees C compared to 6 degrees C, and po llen tube growth also tended to be faster at the higher temperature, althou gh pollen growth was inhibited at 20 degrees C. Closed flowers began to ope n at between 10 degrees C and 12 degrees C, and tended to open more rapidly at higher temperatures. We conclude that flower warming at low ambient tem peratures may mediate flower opening in Crocus and allow pollination, and m ay stimulate pollen germination and the fertilization process. Crocus. has the typical attributes of 'microgrecnhouse' flowers, absorbing some spectra externally, transmitting other spectra internally: trapping heat energy by reflecting inner tepal surfaces, and storing energy in large gynoecia. Of the varieties tested, white and purple flowers showed the greatest flower w arming, and yellow flowers the least. Yellow flowers transmit no light of l ess than 500 nm, suggesting that the transmission of short wavelength light may be important in flower warming. (C) 1998 The Linnean Society of London .