The effect of the natural assemblage of microorganisms and selected strains of the yeast <i>Metschnikowia reukaufii</i> in controlling the germination of pollen of the common milkweed <i>Asclepias syriaca</i>

Abstract

The yeast Metschnikowia reukaufii is a natural contaminant of nectar and is vectored to the flowers of the field milkweed Asclepias syriaca by insects, some of which are pollinators of the plants. In its natural habitat, the yeast inhibits the germination of the milkweed's pollen, which normally uses nectar in the stigmatic cavity for germination. This inhibition is irreversible after about 8 h of exposure to the yeasts. Two selected strains of the yeast were isolated and investigated for their effects on pollen germination in vitro. The two strains, and their mixture, affected pollen germination adversely by reducing its amount and vigour and causing any pollen tubes that were produced to burst: One strain was more virulent than the other, and the mixture seemed to have an additive effect. The strains may be more efficacious than the natural assemblage of microbes in disrupting fertilization of milkweed flowers because they cause the immediate death (bursting) of the growing microgametophyte (pollen and tube). Key words: yeasts, pollen germination, milkweed, Metschnikowia reukaufii, Asclepias syriaca.