Temporal delays in branch development contribute to vegetative meristem maintenance in perennial grasses

Retention of vegetative meristems beyond the first year of flowering underpins perennial growth, yet the developmental mechanisms causing it remain unresolved. To address this knowledge gap, we quantified vegetative shoot branching and flowering architecture in eight annual-perennial species pairs within the grass subfamily Pooideae, tracking thousands of vegetative and reproductive branches within a phylogenetic framework to test whether perenniality is maintained through temporal delays in branch development or spatial segregation of vegetative and reproductive branches. Annuals, except Hordeum vulgare, produced branches at shorter intervals, flowered earlier, and showed more synchronized flowering than their perennial counterparts. Across life histories, flowering progressed from older to younger branches, with little evidence for consistent spatial segregation of vegetative and reproductive branches. In perennial species in the core group, younger branches during the first year of flowering had a lower probability of floral transition, reflecting delayed branch initiation and age-dependent responsiveness to floral signals. Core perennial species branched faster than noncore perennials, indicating lineage-specific differences in developmental pace. Our findings demonstrate that perennial Pooideae retain vegetative meristems through temporal delays in branch development rather than age-independent spatial partitioning, providing a developmental mechanism for perennial persistence and a trait directly relevant to perennializing annual grain crops.