影响黄帚橐吾种子生产的因素Ⅰ.生境和花序结构

 通过对黄帚橐吾(Ligularia virgaurea)自然种群在不同生境和种群密度下的种子生产的研究，结果显示：1）黄帚橐吾的种子生产无论是在种群水平，还是在总花序上同一部位头状花序水平，均受到生境条件的影响，其种子数量、种子均重和种子总重量等特征在不同生境间表现出沙地＞滩地＞坡地，在同一生境内则为高密度种群＞低密度种群和植被盖度较低＞植被盖度较高的趋势（滩地例外）；2）种子生产中的种子数量和种子总重与总花序大小、头状花序数量以及无性系株大小多呈显著正相关，与个体密度无关，但种子均重与上述因素（除无性系株大小外）的关系不显著；3）在总花序内不同部位头状花序间表现出的种子生产变化趋势（顶部＞基部）不受生境条件、种群密度等因素的影响； 4）种子生产在总花序上所表现出的这种部位依赖性可能是由于不同部位头状花序间开花时序上的差异引起种子从母体获取资源上的不同所致。

FACTORS INFLUENCING SEED PRODUCTION IN LIGULARIA VIRGAUREA I . HABITAT AND ARCHITECTURE OF INFLORESCENCE

Ligularia virgaurea has become a dominant weed in degraded rangeland of alpine meadow because of over-grazing and the selective feeding habit of livestock. Its coverage has risen from 5%-20% in the 1970s to 20%-50% at present. Since knowledge of the growth, development, reproduction and dispersal characteristics is lacking, we have no good ways to prevent its spread and eradicate it. Therefore, researching the features of seed production of L. virgaurea in natural populations, the principles of sexual reproduction in this species, and the variation in degree and reason of seed size within habitats, individuals and different positions of capitulum intra-inflorescence has important applicable value and theoretical implications. The material we studied was collected from Nima (101°53′E, 35°58′N, altitude 3 500 m) in Maqu county, Gansu province, North-east region of Qingzang Plateau. The plot is in an area with different degrees of degradation. This region belongs to the alpine meadow rangeland type. L. virgaurea is a perennial herb of Compositae: its common inflorescence lies at the top of stem and it consists of capitula that bloom from top to bottom. It can also clone through rhizomes. After maturation of L. virgaurea seed, 10-16 quadrats per plot were set up, from which 1-3 seed producing ramets per quadrat were sampled, and brought to the laboratory. The vegetative structure and reproductive structure were separated, heated for 24 hours at 75 ℃ for drying over, and weighed with an electronic balance (g/10 000). The biomass of ramets, common inflorescence, seeds and capitula of different positions of the common inflorescence were measured; the numbers of capitulum and seed per common inflorescence, and capitate seed numbers of different positions in the common inflorescence were amounted. The data were analyzed through ANOVA and linear regression. The measurements taken were: the size of ramet and common inflorescence represented by their dry weight of biomass (common inflorescence biomass means the dry weight of section from the first available capitulum in the bottom to the top of common inflorescence); the available capitulum of the top, middle and bottom representing separately the first of the top, the 1/2 position of common inflorescence and the last of the bottom; the percentage produced seed of the common inflorescence in every plot (setting percentage per plant); capitulum percentage (setting percentage per inflorescence); mean seed weight (seed weight of common inflorescence per plant/ its number that amplified 100 times). Mean seed number is the number of seeds per common inflorescence per plant. Capitula number and mean seed number per capitulum were also calculated.The results show: 1) the seed production of L. virgaurea was affected by habitats not only at a population level but at the capitulum level of the same position (such as top or middle or bottom) within common inflorescence, and shows definite trends; for example, floodland＞lowland＞hillside, and in the same habitat, lower population densities＞higher one, lower vegetational coverage＞higher one except for lowlands. 2) except for the seed mean weight, there was a highly positive correlation between the seed number or seed weight and common inflorescence size or capitate number or individual size, but there was no correlation with individual densities. 3) the trend (top＞bottom) of seed production among the different positional capitula within the common inflorescence was not influenced by the factors of habitats and population densities. 4) position-based seed production may be caused by blooming time series of the different positional capitula within the common inflorescence. This inherent mechanism may ensure early developing capitula can gain more resources from the resource pool than later developing ones.