Flavonoids, Alkali Earth, and Rare Earth Elements Affect Pecan Pollen Germination

The factors regulating pecan [Carya illinoinensis (Wangenh.) K. Koch] pollen grain germination are poorly understood for both in vitro pollen viability tests and on receptive stigmatic surfaces of pistillate flowers. Potential regulating factors include flavonols, calcium (Ca), Ca-like alkali earth elements (AEEs), and rare earth elements (REEs). When various concentrations of certain naturally occurring simple flavonols (e.g., quercetin, kaempferol, myricetin, naringenin, and hesperetin) were tested in vitro by adding to standard pecan pollen germination medium, hesperetin, myricetin, and kaempferol functioned as a strong agonist at low concentration (0.12-2.0 mu M for hesperetin and kaempferol, and 0.25 mu M for myricetin), increasing pollen germination 2- to 3.9-fold over flavonol-free media. Hesperetin and myricetin were antagonistic at 16 mu M. Kaempferol was not antagonistic at any concentration up to and including 16 mu M. Naringenin was an antagonist at concentrations from 0.12 to 16 mu M; whereas, quercetin was an antagonist at 8-16 mu M, but tended to function as an agonist at low concentration (0.12-0.50 mu M). The equal molar replacement of Ca2+ in standard pecan pollen germination media by single REEs, resulted in certain REEs [e.g., yttrium (Y), gadolinium (Gd), and thulium (Tm)] partially replacing the obligate need for Ca2+; thus, functioning as agonists in absence of Ca. All non-Ca AEEs [beryllium (Be), magnesium (Mg), strontium (Sr), expect for barium (Ba)], also partially substituted for Ca2+ at equivalent molar concentrations, but none were as efficacious as Ca2+. Results are suggestive that a) pollen germination in in vitro test can be improved by incorporation of certain flavonols, and b) pollen germination on stigmatic surfaces of flowers in orchards might be influenced or regulated by flavonol composition and Ca-like metals in the liquid matrix of the wet (receptive) stigmatic surface.