中文摘要：

关于开花植物花序内变异原因的研究有很多，而关于花序内资源分配的位置效应产生影响的研究并不常见。通过对青藏高原高寒草甸退化草地一种花序轴基向发育的优势毒杂草黄帚橐吾（Ligularia virgaurea）的野外调查和方差分析发现，其种子生产存在显著的位置效应。结实率和种子平均重表现出顶部头状花序〉基部头状花序的变化趋势，胚珠平均数则基部〉顶部，而中部种子平均数最多。通过室内萌发实验和野外盆栽实验研究了位置效应对幼苗建植能力的影响，发现基部种子的萌发开始时间比顶部种子早，萌发率和出苗率在部位间的差异不显著，而部位间幼苗存活率表现一致；幼苗的根冠比和光合速率表现出顶部〉基部的变化趋势，而幼苗总生物量的差异不显著，这些幼苗特性在两种肥力条件下表现一致，这说明不同花序部位种子的差异是该种植物所固有的特征，且部位间种子的萌发和幼苗生长对策不同。但相对于基部种子，项部种子扩散距离更远，拓展到新生境的能力更强，相对较高的幼苗根冠比和净光合速率增强了其幼苗建植能力，对自然种群的更新补充有着更重要的生态学意义，对扩大种群和植被更新有更大的贡献。

英文摘要：

Aims Position-effect of seeds within inflorescences of flowering plants is a common phenomenon, but its influence on seedling establishment is not well known. Our objective was to determine： 1） the pattern of seed production within inflorescences in Ligularia virgaurea （top, middle and bottom of inflorescences） and 2） if position-effect exists, whether it affects seed germination and seedling establishment. Methods We surveyed seed production of L. virgaurea in the field, seed germination under an alternating temperature regime （15 ℃ / 5 ℃; 12 h light/dark） in the laboratory and seedling establishment under 25% shade by potted plants in the field. One-way ANOVA was used to determine the effects of seed position on seed set, seed weight, ovule number, seed number, germination percentage, initial germination time, emergence percentage, survival percentage, root-shoot ratio （R/S）, total biomass （TB） and net photosynthetic rate （P,）. Tukey＇s test was used to determine the difference between average values. Important findings Seed production ofL. virgaurea is affected significantly by position within an inflorescence： seed set and seed mean weight are greatest at the top, mean number of ovules is highest at the bottom, and the middle position has the most seeds. Initial germination time is earlier for seeds from the bottom than top, germination percentage and emergence percentage are not significantly different among different positions, and survival percentage among three positions is nearly consistent. R/S and P, are top 〉 bottom, but TB is not significantly different. Seedling traits are consistent under two different rates of fertilizer application. Results imply that the seeds from different positions have inherently different germination and seedling growth strategies. Top seeds can disperse further and have stronger seedling establishment ability than bottom seeds. Therefore, top seeds have greater ecological significance for sexual recruitment, regeneration and colonizing ne