THE NECTAR-INHABITING YEASTS AND THEIR EFFECT ON POLLEN GERMINATION IN COMMON MILKWEED, ASCLEPIAS SYRIACA L.

ABSTRACT

In Asclepias, nectar is secreted by the stigmatic chamber and has two functions: (i) it is a carbohydrate reward for pollinating insects required by the plants for sexual reproduction and (ii) it is the germination medium for pollen at the onset of pollen tube growth. The nectar easily becomes infected by yeasts, mostly Metschnikowia reukaufii Griiss, which are vectored by pollinators. Nectar from flowers which opened in the laboratory was yeast-free and did not inhibit pollen germination, but samples of nectar from older, open flowers from the field were almost always (94–98%) inhibitory and supported populations of yeast When nectar samples from buds which opened in the laboratory or artificial nectars (solutions of D-glucose and sucrose which normally support vigorous germination of pollen) were contaminated with yeast-infected nectar, they did not support pollen germination. Yeast-infected nectar or sugar solutions were inhibitory even when filtered to remove the yeast cells. Laboratory yeast cultures did not inhibit pollen germination, but caused the pollen tubes to burst, as did dilute (5%) sugar solutions and yeast-contaminated nectar or sugar solutions added to preparations with already germinated pollen. We found that Metschnikowia is widespread in nature, is vectored from flower to flower by pollinators, produces a substance that inhibits pollen germination in Asclepias syriaca L. at least in vitro, and may be important in limiting fertilization and perhaps fruit set in Asclepias.