Abstract: The limited seed production of insect-depended plant, Liriodendron chinense was  once considered to be pollen-limited and insufficient cross pollination. In this study, we  counted pollen grains deposited on stigmas in three populations in Guizhou, Hunan and Zhe- jiang provinces of China respectively. Over 61.9% stigmas were pollinated. From 1994 to  1996, the mean number of pollen grains on each stigma ranged from 4.4 to 42.6, much  more than ovules(2) in each pistil. Based on observations of three years, both the pollination  rate and pollen quantity on stigmas significantly affected seed set. When flowers opened  without stamens dehiscencing at the early stage of anthesis, stigmas received considerable  quantity of pollen grains. Pollen grains from different sources were able to germinate and  pollen tube growth rates were not greatly variable. It is very likely that pollen grains arrived first would fertilize eggs. Since only several pollen tubes went through the stylar canal, the potential pollen competition may exist. In this case, there would be strong selection on floral syndrome which benefit early insect visits. Pollen grains from the early visits would have a greater chance to fertilize ovules than those from later visits, which implies that cross pollina- tion is the predominant breeding system of this plant. The conclusion was also confirmed by following four artificial experiments. Three treatments, including flowers bagged, netted or with the perianth removed, all reduced seed set clearly, but flowers with the stamens re- moved (emasculation)had a higher seed production by open pollination. As the rates of de- posited stigmas in three populations were 6～8 times more than full seed set, we considerthat lower seed production in this plant may not mainly be due to pollen limitation.

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