Comparative seed physiological ecology in Coffea species highlights variation in germination strategies associated...

Seed functional traits are poorly understood in species of tropical origin with endospermic or desiccation-sensitive seeds. Species of the genus Coffea have colonized a broad range of forest habitats of Africa and Indian Ocean islands, from seasonally dry savannah woodlands to permanently humid evergreen forests. All Coffea species produce albuminous seeds, most of which are desiccation-sensitive, thereby offering a valuable model to identify adaptive seed traits in this understudied category of plants.

Several morphological traits (tissue masses and mass ratios, embryo depth inside the endosperm) and physiological traits (desiccation tolerance, germination and seedling establishment kinetics) were measured in 28 Coffea species, and associations with bioclimatic variables and phylogeny were investigated.

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. The time from germination to the unfolding of cotyledonary leaves was correlated with the endosperm mass. Finally, the time for seedling establishment, seed germination and drying sensitivity were positively associated with rain variables, suggesting that these physiological traits were driven by adaptation to seasonal water stress. Significant phylogenetic signal was detected for most seed traits with the exception of seed desiccation sensitivity, germination time and embryo depth inside the endosperm, which appeared to be evolutionarily labile, i.e. highly divergent between phylogenetically closely related species that occur in contrasted habitats.

The present study highlights the spectrum of coordinated seed traits shaped by phylogeny and/or ecological adaptation among Coffea species. 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 that occur in seasonally dry habitats.