Post-pollination prefertilization drops affect germination rates of heterospecific pollen in larch and Douglas-fir

Pollen of larch (Larix x marschlinsii) and Douglas-fir (Pseudotsuga menziesii) were used in homospecific and heterospecific crosses of one another. Prezygotic pollination mechanisms reduced heterospecific pollen germination. In particular, germination of heterospecific pollen was reduced in post-pollination prefertilization drops as measured in ovulo. This provides evidence of selection against foreign pollen by open-pollinated exposed ovules in these two sister taxa. Of the other prezygotic stages in pollen-ovule interactions, neither uptake of pollen by stigmatic hairs nor pollen tube penetration of the nucellus showed any evident selection. To test for differences between species of the medium in which pollen germinates in ovulo – the post-pollination prefertilization drop - secretions were gathered from two gymnosperm species and analyzed. In Douglas-fir, fructose was the predominant sugar, but in L. x marschlinsii glucose was most abundant. Other carbohydrates such as xylose and melezitose were species-specific. In P. menziesii, sucrose is absent due to its conversion to glucose and fructose by apoplastic invertases. The presence of invertase activity in shows that gymnosperm ovular secretions are not static products but exhibit temporal dynamics in their composition.