Abstract:

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.