草乌传粉过程中的广告效应与回报物质研究

Floral advertisement and rewards in bumblebee-pollinated Aconitum kusnezoffii (Ranunculaceae)

虫媒传粉植物与其传粉者之间的相互作用被认为是被子植物花多样性的一个重要选择压力。这种相互作用体现在植物对传粉者的吸引以及传粉者行为对花粉的转运两个方面。本项研究通过去除不同的花部器官研究了草乌(Aconitum kusnezoffii)对其传粉者的吸引,并结合传粉者的访问行为和草乌花的生物学特性探讨了传粉过程与交配系统的适应。红光熊蜂(Bombus ignites)是草乌的有效传粉者。去除花萼片显著降低了红光熊蜂的访问频率,但去除特化成蜜腺叶的花瓣并没有显著改变红光熊蜂的访问频率,这表明草乌吸引红光熊蜂的主要结构是由5枚萼片组成的花部外观形态,而非花瓣。花蜜是草乌提供给红光熊蜂的回报物质,糖浓度为39.23%,组氨酸浓度为0.25μg/μL。草乌花较大、单花花期长、雌雄异熟、花粉寿命长,是一个自交亲和但需传粉者传粉完成繁殖过程的异交物种。草乌花序是无限花序,当上部的花处于雄性阶段时,下部的花正好处于雌性阶段。而红光熊蜂在草乌花序上的访问顺序通常自下而上,带来异交花粉为下部的花进行异花授精,同时又带走上部花的花粉,这就很好地促进了草乌的异交。草乌雄蕊自外向内逐渐成熟,是一种有效的限制传粉者单次访问浪费大量花粉的花粉装配策略,能提高植物通过花粉散布获得的雄性适合度。

Interaction between animal-pollinated plants and their pollinators is thought to be an important selecting force shaping the evolution of flowers. Successful pollination of a plant relies on its attractiveness to pollinators and the ability of the pollinators to deposit enough compatible pollen on the flower＇s stigma. To assess the attractiveness, we manipulated the flower ofAconitum kusnezoffii by removing the sepals and petals, respectively. We then evaluated the mating system based on pollinator behavior and flower biology ofA. kusnezoffii. The bumblebee, Bombus ignites, was the effective pollinator of A. kusnezoffii. Bumblebee behavior differed on the manipulated flowers. The visiting frequency was significantly reduced by sepal-removal treatment, but remained unchanged by nectar-removal treatment. Bumblebees were attracted by the flower exterior morphology formed by five sepals, not by the nectary formed by the two petals. The two lower sepals provided a landing platform, and the uppermost sepal functioned as a nectar guide, ensuring bumblebee find the nectar precisely and rapidly. The two lateral sepals restricted the direction in which a bumblebee can enter the flower, ensuring pollination efficiency in zygomorphic flowers. Nectar was the main reward for bumblebees, with a sugar concentration of 39.23% and histidine concentration of 0.25 μg/μL. The flower of A. kusnezoffii was large, protandrous and herkogamous, suggesting an outcrossing mating system based on the outcrossing index. In Aconitum inflorescences the oldest flowers were functionally female at the bottom of acropetal racemes, while younger flowers were functionally male at the upper positions. Bombus ignites generally flew from a younger functionally male flower near the top of an A. kusnezoffii inflorescenceto an older functionally female flower near the bottom of another inflorescence, and then moved progressively upward. This behavior promoted the outcrossing of A. kusnezoffii. Aconitum kusnezoffii flowers were strongly protandrous, with the male phase lasting 2-4 days, during which the anthers gradually dehisced. Aconitum packaged pollen within each flower as individual anthers dehisced over a protracted period. Such pollen packaging necessarily limited pollen removal during single visits and increased male fitness by promoting pollen dispersal.