On polyspermy in the sunflower

Abstract

Polyspermy in the sunflower (Helianthus annuus L. cv. ‘Bismarckianus’) was studied. Flowers were pollinated with a mixture of small amounts of pollen from the pollinated florets' one-day circle (i.e., a whorl in the head comprising all the florets opening during one day) and large amounts of pollen from a single floret of Jerusalem artichoke, or girasole (Helianthus tuberosus L.). The artichoke pollen, but not the sunflower pollen, was labelled with P-32. In another experiment the labelled artichoke pollen was replaced by labelled pollen from another sunflower plant. The activity per pollen grain, as well as the activity per ovule, was determined by means of a Geiger-Müller counter. Main results: (1) When the sunflower's pollen tubes reach the ovule and double fertilization is effected by the plant's own sperm cells, the ovule also usually receives one or several pairs of sperm cells from the artichoke. (2) Each ovule in a one-day circle often receives the same number of sperm cells. (3) After pollination with labelled artichoke pollen from one floret, an activity corresponding to 1, 2, 4, 8 or 16 pollen grains per ovule was obtained. After pollination with active foreign sunflower pollen from one floret, the corresponding number was 2, 3, 5, 9 or 17. Besides the pair of sperm cells effecting double fertilization, the number of pairs received by an embryo sac thus corresponds to one of the members in the 2n series, with n representing the integer series from zero upwards. The highest n value found is 4. (4) When pollen from the same father is used, the number of sperm cells entering the embryo sac varies between the one-day circles. Thus, variation occurs among isogenic whorls of florets. An autoradiographic method was used to follow the localization after fertilization of the compounds containing the isotope. Literature on polyspermy in the Angiosperms is reviewed. The following general conclusions could be drawn: (1) Polyspermy is a widespread phenomenon among cross-fertilizing Angiosperms. (2) Polyspermy does not seem to occur among self-pollinating species. (3) When a large number of nuclei are involved, it is improbable that the number of pollen tubes reaching the embryo sac corresponds to the number of pairs of sperm nuclei entering the embryo sac. Hence, pollen tubes may sometimes empty their contents into other pollen tubes.