Coffee Seed Secrets: How Climate and Ancestry Shape Germination Strategies

Seed functional traits are poorly understood in species of tropical origin with endospermic or desiccation-sensitive seeds. The genus Coffea, spanning diverse habitats from seasonally dry savannah woodlands to permanently humid evergreen forests across Africa and Indian Ocean islands, offers a unique opportunity to identify adaptive seed traits. All Coffea species produce albuminous seeds, most of which are desiccation-sensitive, making them a valuable model for studying adaptive seed traits in this understudied plant category. This comprehensive study investigated 28 Coffea species to understand their seed strategies.

Researchers measured various morphological traits, including tissue masses and mass ratios, embryo depth inside the endosperm, alongside physiological traits such as desiccation tolerance, germination, and seedling establishment kinetics. The findings revealed that the time required for seed germination was influenced by both the thickness of the micropylar endosperm and the relative mass of the endocarp to the seed. Furthermore, the time from germination to the unfolding of cotyledonary leaves showed a clear correlation with the endosperm mass, indicating the critical role of these structural elements in early development.

Crucially, the study found that the time required for seedling establishment, seed germination, and drying sensitivity were all positively associated with rain variables. This strong association suggests that these physiological characteristics are directly driven by adaptation to seasonal water stress, a key environmental factor in many tropical regions. While most seed traits exhibited a significant phylogenetic signal, meaning they are conserved through evolutionary history, certain traits like seed desiccation sensitivity, germination time, and embryo depth inside the endosperm proved to be evolutionarily labile. This lability implies that these traits are highly divergent even between phylogenetically closely related species that occur in contrasted habitats.

In conclusion, this research highlights the spectrum of coordinated seed traits shaped by phylogeny and/or ecological adaptation among Coffea species. As lead author Baschenis notes in the paper, "This is illustrated by the co-option of several characteristics (small, thinly endocarp-coated, desiccation-tolerant, and fast germinating seeds) that enable fast seedling establishment and may confer an ecological advantage in the studied species of the East Africa clade which occur in seasonally dry habitats." This offers a deeper understanding of how tropical plants navigate their diverse worlds.