Seed dormancy of the threatened Spanish daffodil Narcissus yepesii

Seed germination is recognised as a crucial phase of a plant’s life that is strongly dependent on environmental cues that break seed dormancy. As a result, seed germination in nature is often restricted to particular locations, which provide appropriate environmental requirements. The dormancy class of seeds can influence both population and species-level processes such as colonisation, adaptation, speciation and extinction.

Morphophysiological dormancy (MPD) is the most difficult dormancy class to overcome. MPD occurs when seeds with underdeveloped embryos have physiological components preventing germination. These seeds therefore require dormancy-breaking treatments (such as hot or cold exposure) as well as a period of time to develop fully grown embryos. Until now there have been no studies investigating whether a dormancy gradient exists between levels of morphophysiological dormancy across environmental gradients or genetic diversity.

In their new study published in AoBP, Copete et al. aimed to determine the class of seed dormancy of the threatened daffodil species Narcissus yepesii. This was then used to explore links between different dormancy levels previously characterised in two closely related phylogenetic congeners also endemic to Spain, N. alcaracensis and N. longispathus. These species were found to exhibit distinct levels of MPD: intermediate and non-deep complex, respectively.

The results of Copete et al. demonstrate a clinal variation through the coexistence of intermediate and non-deep complex MPD in N. yepesii (21% and 74%, respectively), which occupies a middle geographical position between the other two taxa. This is the first study showing a gradient in the evolution between levels of MPD. The authors conclude by stating that seed dormancy evolution from one level of MPD to another level of MPD can be a key factor determining the course of plant trait evolution in species with underdeveloped embryos.