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Achene heteromorphism in Bidens pilosa (Asteraceae): differences in germination and possible adaptive significance

Bidens pilosa L. (also known as cobbler’s pegs, farmer’s friend and burr marigold) is a noxious weed in many ecosystems worldwide. Across China, the plant can be found growing on roadsides and in fields and villages at elevations below 2500 m. It produces large numbers of heteromorphic (central and peripheral) achenes that differ in morphology, with the central being long and the peripheral short. In terms of ecophysiology, it was observed that peripheral achenes remain attached to the receptacle longer than central ones. There is however very little known about the germination strategies of these dimorphic achenes. Annual plants survive by mechanisms used to ensure the appropriate timing of germination and germination occurs in a suitable habitat that will allow for seedling establishment. For B. pilosa, the achene germination strategy may be the most significant factor determining survival because this is the only way their populations can be maintained.

Bidens pilosa
Photograph of Bidens pilosa. Image credit: Zhang et al.

A recent study by Zhang et al., and published in AoBP, investigated the dormancy and germination behaviour of both types of achenes of B. pilosa. They demonstrated that the two distinct types of achenes differ in morphology, germination behaviour, dormancy, and osmotic tolerance. The central achenes are dispersed farther from the parent plant and have relatively shallow seed dormancy. The peripheral achenes on the other hand have low dispersal ability and relatively strong seed dormancy. These differences represent the combination of different adaptive strategies and may have ecological significance for the successful survival of B. pilosa, and its success as a weed worldwide. This study adds to our understanding of the timing involved in weed seed germination in agricultural lands and will contribute to the implementation of effective weed management strategies.

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