Abstract:

Aims Andromonoecy, the presence of both staminate and hermaphrodite flowers on the same individual, was observed in Soranthus meyeri, a common early spring ephemeral perennial in the Guerbantonggut Desert of Northwest China. The relationships between sex ratio (number of hermaphrodite flowers/total number of flowers) and plant size and the flowering pattern of both staminate and hermaphroditic flowers of this species were studied to determine size-dependent sex allocation and flowering pattern.
Methods The study was conducted from May to June in 2006–2008. Morphology, number and dry mass of both staminate and hermaphroditic flowers were determined. Position of the two types of flowers and their flowering pattern at the level of the flower, umbel and individual were recorded. Individual biomass was determined, and the relationship between number of staminate and of hermaphroditic flowers and plant size was analyzed.
Important findings The sex ratio within a population of S. meyeri was 0.69 ± 0.03, 0.62 ± 0.03 and 0.69 ± 0.02 in 2006, 2007 and 2008, respectively, and there were no significant differences among the years. This indicates that the sex ratio of this species is stable and possibly controlled by genetics. Staminate flowers produced less pollen, and their biomass was smaller than that of hermaphrodite flowers, suggesting that staminate flowers were less expensive to produce. Primary umbels produced more hermaphrodite flowers than secondary umbels, indicating that the primary umbels are capable of obtaining more resources from the mother plant and increase female fitness. Resources saved by not producing hermaphroditic flowers in secondary umbels could be reallocated to produce more staminate flowers, thus increasing floral display to attract more pollinators as well as enhancing pollen export and overall fitness. The sex ratio per plant was significantly, positively correlated with aboveground vegetative organ biomass, and large plants allocated more to the female function than small plants. Thus, female reproductive success is limited by availability of resources. Flowers in the primary umbel bloom first, followed by those in the secondary umbel five days later. All umbels of the same order flower simultaneously, and all flowers within an umbel open centripetally. Separation of male and female phases through protandry is complete both within hermaphrodite flowers and within umbels, thus avoiding interference between the male and female function and promoting xenogamy. However, there is an overlap of about one day between umbels of consecutive orders, which can provide conditions for geitonogamy.