Abstract
The temperatures of flowers of Ipomoea pes-caprae ssp. brasilensis, Ipomoea aquatica and Merremia borneensis in bright sunshine, were studied to determine the role of corollas and sepals in cooling the gynoecium. The corollas and sepals were prevented from transpiring by greasing, to investigate the extent of evaporative cooling. In the exposed natural habitats of these flowers the maximum temperatures of air and soil were high (32 and 42degreesC, respectively) and corolla, sepal and gynoecium temperatures were often intermediate. Despite being almost astomatous, significant evaporative cooling was observed in the corolla. Between 20 and 80% of the energy absorbed by the corollas was dissipated as evaporation. The sepals were stomatous and their evaporative cooling was very important in reducing the temperature of the gynoecium. The temperatures of the non-transpiring gynoecia and corollas were Significantly higher than the temperatures of the normally transpiring corollas and gynoecia. Furthermore, the gynoecia temperatures were significantly higher with non-transpiring corollas than with normally transpiring corollas, suggesting that the corollas alone play a role in maintaining the gynoecium within optimal temperatures levels. It was shown in an incubation experiment that temperatures exceeding 32degreesC may damage the carpels, and temperatures exceeding 42degreesC may damage sepals. Pollen grains were killed after 200 min of exposure to temperatures in the range 32 to 47degreesC. It is concluded that the cooling mechanisms (evaporation and self-shading) are critical for the reproductive success of these flowers in their natura environment.
