Vegetative phenology constrains the onset of flowering in the perennial herb Lathyrus vernus

1 The seasonal timing of flower production is a critical event in the life cycle of many plants. A range of abiotic and biotic interactions has been suggested to influence the optimal flowering phenology. More rarely, phenology has been examined from a developmental perspective. Still, the production of a flower or an inflorescence is an integral part of the ontogeny and architecture of a plant. 2 We investigated to what extent the timing of flowering is coupled to overall ontogeny and architecture in the perennial herb Lathyrus vernus. We asked if the decision to flower is associated with a change in vegetative phenology. We also investigated if onset of flowering depends on the start of shoot growth and leaf unfolding, time to complete shoot growth and leaf unfolding, and plant architecture. Lastly, we investigated whether the resource state of the plant affects vegetative and flowering phenology. 3 Flowering plants started shoot growth and leaf development significantly earlier than non-flowering plants. In flowering plants, start of vegetative growth was positively correlated to start of flowering, but there was no positive correlation between development time and start of flowering. Earlier flowering plants produced their first inflorescence at lower positions than late-flowering plants. Experimental manipulation of resource state affected several components of fitness but did not influence vegetative or flowering phenology. 4 Our results show that the decision to flower is linked to a change in vegetative phenology in L. vernus, and that differences in the onset of flowering among individuals are correlated to differences in vegetative phenology. A possible interpretation of this pattern is that the evolution of flowering phenology is partly constrained by vegetative development. Alternatively, flowering changes the optimal time of vegetative development. Still, individuals with similar timing of vegetative development may differ considerably in flowering time, suggesting that independent adjustments of vegetative and flowering phenology are possible.