Acacia seedling morphology: phyllotaxy and its relationship to seed mass

Abstract

Acacia, with over 1000 species, is the largest plant genus in Australia and is distributed throughout most of the continent. This work investigates seed mass and seedling phyllotaxy development in 287 species of this genus to better understand these important ecological and taxonomic traits. Seed mass varied over 100-fold with an average mass of 17.9 (range 2.1–308) mg per seed. Two common forms of seedling phyllotaxy were seen. The most common seedling development type displayed a pinnate first leaf with an alternate, bipinnate second leaf (pinnate : bipinnate). In the less common form, the first two leaves were pinnate and opposite each other (pinnate : pinnate). When compared with the phylogenetic tree of Acacia, the pinnate : pinnate state is more commonly resolved in taxa found at the basal nodes of the tree, but not exclusively. Overall, species with heavier seeds have more seedling leaflet pairs and pinnules. However, it was also found that the pinnate : bipinnate seedlings had more first leaf leaflet pairs than the pinnate : pinnate form. The pinnate : pinnate plants had an identical number of leaflet pairs on their first and second leaves. The number of leaflet pairs on the first leaf was highly correlated with the number of pinnules on a leaflet of the second leaf on the pinnate : bipinnate form. We discuss the phyllotactic variation within the Acacia phylogenetic context and possible developmental processes.