Ontogeny and anatomy of Bouteloua (Poaceae: Chloridoideae) species display a basipetal branch formation and a novel modified leaf structure in grasses.

BACKGROUND AND AIMS: Shoot ontogenesis in grasses follows a transition from a vegetative phase into a reproductive phase. Current studies provide insight into how branch and spikelet formation occur during the reproductive phase. However, these studies do not explain all the complex diversity of grass inflorescence forms and are mostly focused on model grasses. Moreover, truncated inflorescences of the non-model grass genus Urochloa (Panicoideae) with formation of primary branches have basipetal initiation of branches. Bouteloua species (Chloridoideae) are non-model grasses that form truncated inflorescences of primary branches with apical vestiges of uncertain homology at the tips of branching events and sterile florets above the lowermost fertile floret. Sterile florets are reduced to rudimentary lemmas composed of three large awns diverging from an awn column. Conflict about the awn column identity of this rudimentary lemma is often addressed in species descriptions of this genus. We test if Bouteloua species can display basipetal initiation of branches and explore the identity of vestiges and the awn column of rudimentary lemmas. METHODS: We surveyed the inflorescence ontogeny and branch/awn anatomy of Bouteloua species and compared results with recent ontogenetic studies of chloridoids. KEY RESULTS: Bouteloua arizonica has florets with basipetal maturation. Branches display basipetal branch initiation and maturation. Branch vestiges are formed laterally by meristems during early branching events. The spikelet meristem forms the awn column of rudimentary lemmas. Vestiges and sterile floret awns have anatomical similarities to C4 leaves. CONCLUSIONS: Basipetal initiation of branches is a novel feature for Chloridoideae grasses. Branch vestiges are novel vegetative grass structures. Sterile floret awn columns are likely to be extensions of the rachilla.