THE STRUCTURAL REQUIREMENTS OF FLAVONOLS THAT INDUCE POLLEN GERMINATION OF CONDITIONALLY MALE FERTILE PETUNIA

Flavonols are essential for pollen germination and sustained tube growth in Petunia hybrida. An in vitro bioassay, based on biochemical complementation of flavonol-deficient pollen, was used to compare how modifications at different sites on the basic flavonol molecule affect the efficiency of pollen germination. This structural-activity analysis using methylated or glycosylated derivatives showed that only flavonols with unsubstituted hydroxyl groups at positions 3 and 7 could induce rapid pollen germination. In addition, the enhanced germination frequency associated with a hydroxyl group at position 5 was abolished by substituting a methyl group. Increased hydroxylation of the flavonol B-ring had an inhibitory effect on germination, but methylation of the same hydroxyl groups promoted germination. Additional hydroxyl groups within the A-ring at carbon 6 had a mixed effect, but a methoxyl group at position 6 enhanced germination in all cases. Substitutions at position 8 were somewhat inhibitory and introduction of an isoprenyl group into ring A was toxic to both mutant and wild type pollen.