Abstract:

A preliminary study was conducted on the reproductive features of different populations of Veratrum nigrum along an altitudinal gradient in Dongling Mountains, Beijing. V. nigrum is a perennial, andromonoecious herb. We use P/O ratio (Pollen/ovule ratio) as an estimate of the level of outcrossing. Resource allocation was measured by means of dry weights.We found that the flowering individuals within a population were larger than the non-flowering individuals. The resource allocated to reproduction was strongly correlated with that allocated to vegetative growth. Individuals of V. nigrum would not produce flowers and fruits until a threshold size was reached, which ranged from 1.11 g to 3.98 g in different populations. The minimum size showed no trend along the altitudinal gradient, indicating that an invariable minimum vegetative biomass for the species might exist. Pooled together, a minimum size of 2.61 g for reproduction followed from the regression line. Above the threshold there existed a relatively simple linear relationship between vegetative and reproductive biomass. Population density decreased firstly and then increased along altitudinal gradient, but it seemed that density is not related, in a straightforward way, to the reproductive features of V. nigrum in the study region.Many workers had shown that the flowers of self-incompatible and other xenogamous taxa produced more pollen grains than closely related self-compatible and/or autogamous taxa, while the number of ovules per ovary was rather conservative, in contrast to other floral characteristics. In this article, we estimated P/O ratios both at the level of a hermaphrodite flower and at the level of an individual (including male flowers). We found that the difference in mean hermaphrodite flower P/O ratios among populations was not significant. Hermaphrodite flowers had P/O ratios from 799 to 1 090. However, the difference in mean individual P/O ratios among populations was strongly significant. The lowest individual P/O ratio was 1 601 and the highest was 4 146. According to the correlation between P/O ratios and mating systems established by Cruden, V. nigrum in Dongling Mountains was facultatively xenogamous. There was a positive correlation between the P/O ratios of hermaphrodite flower and the ratios of reproductive to vegetative biomass.Hermaphrodite flowers are mainly distributed at the tip raceme of the panicle, while males are at the lateral racemes. Sometimes, flowers near the base of the lateral racemes are hermaphrodite. Hermaphrodite flowers bloom earlier than male flowers, and the two flower types differed in dry weights. Hermaphrodite flowers averaged 5.10×10-3 g while male flowers averaged 4.04×10-3g. Hermaphrodite flo wers were on average 26% more massive than male flowers. Flower diameter was (1. 42±0.01) cm for hermaphrodite flowers and (1.24±0.01) cm for males. Hermaphrodite flowers were on average 15% larger than male flowers. However, the pollen producti on of hermaphrodite and male flowers remained almost the same. The results docum ented here suggest male flowers of V. nigrum provide resource savings to the plant compared to hermaphrodite flowers. Since male flowers have lower resource use for equal pollen production in comparison to hermaphrodite flowers, they are more efficient at male function than hermaphrodite flowers. These results lend support to the optimal resource allocation hypothesis for the function of male flowers within an andromonoecious sexual system.