Pollination, pollen viability and pistil receptivity in Cucurbita pepo

Cucurbita pepo carries male and female flowers on the same plant, and is pollinated by nectar-collecting bees. The nectaries are dimorphic in the two sexes and pollen is loaded and unloaded as the bees gain access to the nectar. Both types of flower are open for only 6 h (from 0600 h to 1200 h); male flowers open and close half an hour earlier than female flowers. The latter produce more nectar and are visited more often by the bees than the male flowers. Pollen viability determined by fluorescein diacetate (fluorochromatic reaction) decreases by 20% during anthesis and more rapidly after the flower closes. This decrease is due to dehydration of the grain, especially around the pore where the intine is exposed. An unusual feature of this species is that the grains do not dehydrate before anther dehiscence. Female receptivity has two aspects, that of the stigma lasting 4 d, and that of the ovules lasting 2 d. The receptivity of the two sexes and the short period of anthesis are discussed in the light of the reproductive ecology of the species.