渐危植物鹅掌楸的授粉率及花粉管生长

POLLINATION RATES AND POLLEN TUBE GROWTH IN A VULNERABLE PLANT,LIRIODENDRON CHINENSE (HEMSL.) SARG.(MAGNOLIACEAE)

对鹅掌楸Ｌｉｒｉｏｄｅｎｄｒｏｎｃｈｉｎｅｎｓｅ３个自然居群连续３年直接统计落置柱头上的花粉量，其中６１．９％以上的柱头接受了花粉。每柱头上的平均花粉量为４．４～４２．６粒，明显超过胚珠数２。授粉率是结实率的６～８倍，鹅掌楸结实率低的主要原因可能不是由于花粉的限制。落置柱头的花粉几乎均可萌发，少数花粉管穿过花柱道，经珠孔端进入胚囊，暗示花粉间存在着强烈的竞争。授粉率和柱头上的平均花粉量与结实率呈正相关（Ｒ＝０．７７，Ｒ＝０．６９）。在柱头上人工授同株和异株的花粉，花粉管生长速率看不出明显差异，到达胚珠的时间在４８～６０ｈ之间。花开花药未裂时柱头上落置较高比例的花粉，由于先期到达柱头的花粉最有可能受精，表明鹅掌楸有异交为主的繁育系统。人工处理包括套袋、套网、去除花被片，其结实率大大降低。

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 Zhejiang 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 pollination 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 removed (emasculation)had a higher seed production by open pollination. As the rates of deposited stigmas in three populations were 6～8 times more than full seed set, we consider that lower seed production in this plant may not mainly be due to pollen limitation.