Temporal patterns of floral bud initiation and differentiation in almond reference cultivars

Flower bud initiation and differentiation are critical processes in almond, as they mark the transition from a vegetative to a reproductive meristem and drive the formation of floral organs. Although these transitions are not externally visible in the developing buds, significant morphological changes occur at the meristematic level. To characterize these dynamics, we conducted periodically observations of bud development from July 2021 to January 2022 in five widely cultivated almond cultivars ('Guara', 'Lauranne', 'Marinada', 'Nonpareil', and 'Vairo'). Based on these observations, we determined the precise timing of floral initiation for each cultivar, describing eight distinct developmental stages according to bud morphology, and quantifying the duration of each stage. Despite cultivar-specific differences in onset and duration, all genotypes followed a similar developmental sequence, progressing from a vegetative shoot apical meristem to differentiated floral organs, indicating a conserved developmental pattern in almond. The timing of floral developmental stages also showed a slight variation depending on the type of reproductive shoot along the branch, with buds on mixed shoots generally developing one week later than those on spurs and brindles. Measurement of pistil length revealed that buds on brindles tended to produce longer pistils at particular stages, buds on spurs generally exhibited shorter pistils, and mixed shoots consistently showed intermediate values. Altogether, this work provides the most comprehensive temporal and morphological characterization of flower bud development in almond to date, establishing a developmental framework for comparing genotypes and offering valuable insights for understanding how cultivar-specific bud phenology may respond to different environments and temperatures.