A Preliminary Study Of Breeding System In A Wild Population Of Osmanthus Fragrans

Osmanthus fragrans(Thunb.)Lour.(Oleaceae)is an evergreen tree or shrub up to 3-5m tall,which has been cultivated for more than 2000 years in China and has been used for many purposes.Wild Osmanthus frangrans is also a typical Androdioecious plant,which is a rare breeding system in nature.However,the wild O.fragrans growing naturally is very rare.Wild O.fragrans has both hermaphrodites and males in its population.Why is the androdioecious breeding system so rare in nature?The mechanisms of its origin,maintenance and evolution remain to be studied.In this study,we conducted pollination characteristics in a wild population of O.fragrans in Zhouluo Grand Canyon,Hunan province.In order to detect reproductive status,we conducted phenological observation,flower measurements,odor composition analysis and artificial pollination experiments.We also examined the significance of androdioecy and the maintenance mechanisms in this species.Our two-year investigation of wild O.fragrans in liuyang,Hunan showed that the flowering period last from early September to late October,and they often bloomed in batches.The single flowering period was 4-6 days,and the flowering period of males was 1-2 days earlier than that of hermaphrodites.The corolla of males was larger than that of hermaphrodites and the anther was larger,which could provide more pollen.The pollinators mainly included butterflies,flies,bees and moths.However,the visitation frequency was extremely low and the peak visiting time was 9:00-14:00 and 19:00-21:00.Wild O.frangrans had no nectar and can attract pollinators by releasing strong fragrance.Moths have a preference for scented O.frangrans and the aroma released by wild O.frangrans mainly consisted of aromatic aldehydes,esters,aromatic alcohols and violet ketones.The pollen of hermaphrodite flowers of wild O.fragrans has male function,so wild O.fragrans is a functional androdioecious plant.Hermaphrodites can self-pollinate.The fruit set of outcross with male donor was significantly higher than that of outcross with hermaphrodite donor and the fruit set of artificial self-pollination was significantly higher than that of autonomous self-pollination.There was no significant difference in the fruit set between autonomous self-pollination and open-pollination,indicating there was serious pollinator limitation in this wild species,and it was difficult to deposit effective pollen on stigmas of hermaphrodite,so the natural fruit set was also very low.Our artificial pollination experiments showed that the male function of male flower pollen was significantly stronger than that of hermaphrodite flower pollen and the germination rate of male flower pollen on stigma was also significantly higher than that of hermaphrodite flower pollen.The possible mechanism of maintaining the androdioecious breeding system in wild O.fragrans is as follows:males generally improved the quantity and quality of cross-pollinated pollen,and in the case of serious shortage of pollinators,the stamens of hermaphrodite flowers could provide reproductive assurance for the species.Previous studies have suggested that the androdioecy may have evolved from dioecy.That is,the females restored some male functions and became hermaphrodites.Our study on wild O.fragrans showed that hermaphrodites do play a role in reproductive assurance in the absence of pollinators,but effective cross-pollination still depended on males.In this way,the androdioecious breeding system was maintained.