Pollination biology of Prunus mahaleb L.: deferred consequences of gender variation for fecundity and seed size

Abstract

This study describes the reproductive biology of Prunus mahaleb, a rosaceous treelet, in a southeastern Spanish population. The species is gynodioecious with 55.4% of the plants being male-fertile and 44.6% presenting non-functional, shrunken anthers with no pollen, and behaving as functional females. Individual trees produced the same flower morph in four consecutive study years. Fruit set in bagged inflorescences was absent in male-sterile trees and was very reduced in male-fertiles. Open-pollinated flowers of male-fertiles showed greater fruit set (x̄= 29.1%) than male-steriles (x̄ = 25.3%). Average fruit set in three experimental treatments (control, selfed, and crossed) were 29.05%, 41.9% and 38.6%, respectively, for male-fertile trees; those for male-steriles were 25.3%, 0% and 39.2%, respectively. Seeds from male-sterile plants were heavier (71.33 mg) than those of male-fertile plants (66.05 mg) but did not differ in germination ability. Male-fertile and male-sterile trees differ significantly in average nectar concentration (73.5% and 55.9%, respectively) and nectar volume secreted/flower and day (0.142 μl and 0.171 μl, respectively). A total of 41 species of insect flower visitors were recorded. Calliphorid and tachinid flies (41.97% of total visits), and andrenid bees (30.30%), were the most frequent visitors. The diversity of insect visitors was greater in male-fertile trees. Bees and flies accounted for 50.5% and 49.5%, respectively of total visits to male-fertile trees. Flies were far more frequent than bees at male-sterile flowers (76.9% and 23.1%, respectively). Male-sterile trees received higher average visitation (47 insects/census) than male-fertile trees (32 insects/census). Individual trees showed a relative constancy of the fecundity rankings between years. Fruit production was significantly higher in male-sterile trees, with a four-year average of 6558 fruits, in contrast with male-fertile trees which yielded 4670 fruits. This 1.77-fold difference in favour of male-sterile trees over male-fertiles was not compensated by the greater fruit set of the latter. This is attributable to the lower outcrossing rate of male-fertiles, estimated as 52.68% from seed mass data. In addition, greater seed mass of female progeny, and higher visitation rate by insects might explain the maintenance of this polymorphism. These results have far-reaching implications for the demography and seed-dispersal ecology of this endozoochorus species.