两种生境下野慈姑繁殖差异及其机制

植物的生长环境不但可直接作用于其可利用资源的多寡及其繁殖分配,而且可通过影响植物所在的群落以及传粉者的组成或行为而间接的导致繁殖差异。然而,直接与间接作用的方向或强度很少被同时关注,从而限制了从机制上理解环境对植物繁殖的影响。选取野慈姑(Sagittaria trifolia L.)为研究材料,将相同基因型组成的植株分别种植在光照差异明显的两个同质园(林荫区与日照区),于盛花期对野慈姑的开花数量、昆虫访花进行了观察,确定其主要访花昆虫、记录访花行为以及野慈姑的繁殖产出水平。研究表明,日照区野慈姑每天开放的花朵和植株数量显著高于林荫区。两个区域内野慈姑的主要访花者有四大类:蚜蝇类、蜂类、蝶类和蝇类;其中林荫区的主要传粉昆虫为蚜蝇类,而日照区为蜂类。日照区昆虫单位时间内的访花频率、昆虫每回合访问的雄花数、总花数和花序数均显著高于林荫区。日照区野慈姑的座果率显著高于林荫区,而单果种子数量与种子面积与林荫区相当。总体而言,日照区野慈姑的繁殖产出达到了林荫区的三倍以上,这是环境的直接作用与环境介导的传粉作用在同一方向上叠加的结果,且前者占主导地位。在关注植物、物理环境和生物因子三者之间互作的前提下,量化每个繁殖阶段的水平,特别是传粉昆虫在不同生境下的表现,阐述了环境异质性导致植物繁殖差异的根本原因。

The reproductive difference in Sagittaria trifolia under two contrasting habitats: direct and indirect effects

Habitats where plants grow, not only directly affects the resource availability and allocation, but also indirectly affects the plants'' reproductive status by shaping the community, especially the composition and behavior of pollinators. The direction and relative strength of direct and indirect effects are rarely considered simultaneously, which limits our understanding of environment-driven differences in plant reproduction. We placed Sagittaria trifolia plants of the same genotype composition in two common gardens that differed considerably in light (understory and sun). During the peak of anthesis, we quantified the flowering status, identified visiting insects, and recorded their foraging behaviors. The reproductive output of plants in both habitats was also estimated in aspects of fruit-set as well as seed number and size. The results showed that plants in the sun had more flowers and flowering individuals than those in the understory did. The main visitors of S. trifolia could be summarized into four categories, namely the hoverfly, bee, butterfly, and fly, among which, the hoverfly and bee were the most important in the understory and sunlit habitats. The pollinator visitation frequency was higher in the sun than it was in the understory. During each foraging bout, a higher number of male flowers, total flowers, and inflorescences in the sun were visited than those in the understory. The S. trifolia plants grown in the sun had a higher fruit-set than those grown in the understory, but there was little variation in their seed per fruit and seed size. Taken together, these results indicate that the total reproductive output of plants grown under the two habitats showed more than three-fold differences, which was caused by accumulation in the same direction induced by direct and indirect environmental effects, and the former appeared to be the dominating factor. Our study highlights the importance of focusing on the interactions between plants and the physical environment as well as biotic factors at every step of plant reproduction, especially the distinctive performances of pollinators.