Seed traits and growth form shape germination and early root growth in semiarid Fabaceae

Seed size, germination traits, and early seedling growth are closely interconnected and strongly influenced by plant growth form and environmental constraints. These relationships were examined in 30 Fabaceae species (18 tree species and 12 annual herbs) from the Caatinga, one of the world's largest tropical semiarid ecosystems.

Seeds were collected from natural populations and characterized by dry mass (DM), water content (WC), and volume (V). Germination responses were assessed under contrasting thermal regimes (15, 25, and 35 °C) and water potentials (0, -0.4, and -0.8 MPa), and root elongation rate (RER) was measured during 24h.

Seed size (DM and V) was negatively associated with both germination percentage and germination speed. Herb species produced smaller seeds than tree species, whereas WC did not differ between growth forms. Seeds of herb species exhibited higher germination percentage and speed, and RER than those of trees, particularly under supra-optimal temperature (35 °C) and low water potential (-0.8 MPa). When analyzed at the group level, relationships between seed size and RER were detected only for herbs: larger seeds enhanced RER under non-limiting water conditions (0 MPa), whereas under -0.8 MPa seed size and RER were negatively related.

Overall, germination percentage and speed, and RER were positively correlated, indicating convergent processes underlying early seedling establishment. These findings demonstrate that seed functional traits and growth form jointly regulate germination and early seedling performance under thermal and hydric stress, providing key insights into adaptive strategies that support the establishment and persistence of plants in climatically unpredictable semiarid ecosystems.