The effects of pollen clumping on pollen germination and early tube growth in Clarkia unguiculata (Onagraceae)

Pollen germination and early tube growth were compared in Clarkia unguiculata under two different pollen deposition treatments. Pollen grains were deposited on the surface of the stigma either as clumps or spread evenly on the stigmatic surface. Pollinated flowers were collected and fixed at 15 min or 1, 4, or 7 h postpollination. Stigmas and styles were stained with aniline blue and viewed using fluorescence microscopy. Germination rates, numbers of pollen tubes per grain, and numbers of pollen tubes in the upper style were determined for both treatments. Pollen grains spread on the stigma did not have a significant advantage over clumped pollen. By 1 h postpollination, both clumped and spread pollen grains had reached maximal germination, without significant differences between treatments. By 4 h postpollination, the maximum number of pollen tubes in the upper style was reached, without significant difference between the clumped and the spread pollen treatments. An average of ca. 2.5 pollen tubes were produced per pollen grain in both the clumped and the spread treatments.