Timing of floral evocation in the grapevine ( Vitis vinifera L. cv. Chardonnay) is identified by cyto‐histological changes in the vegetative shoot apical meristem

Background and Aims The grapevine floral development pathway commences in early spring with floral evocation in the shoot apical meristem (SAM) of the sylleptic latent bud. Grapevine floral evocation is defined as the irreversible biochemical and cellular changes in the SAM, which commit the plant to the subsequent formation of flowers. These changes are induced by non‐leaf derived stimuli acting directly on the SAM. The timing of floral evocation, however, between budburst and floral initiation, is unknown. The aims of this study were: (i) to establish a precise timing for floral evocation; and (ii) to identify reliable reproducible markers for floral evocation. Methods and Results Sylleptic primary latent buds of Vitis vinifera L. cv. Chardonnay were harvested at 9 am and 5 pm daily, from budburst to 5 days after floral initiation. The SAMs were examined first by stereomicroscope dissection to determine the timing of floral initiation, and then by histological sectioning of those SAMs with nil floral initiation, to determine the timing of floral evocation. Floral initiation occurred 43 days after budburst. A classic cyto‐histological zonation pattern of three regions was observed in all vegetative SAMs up to floral evocation, which occurred 32 h prior to floral initiation. At floral evocation, a ‘mantle over core’ cyto‐histological zonation pattern of two regions was observed. Conclusions The precise timing of floral evocation has been identified and reliable markers for evocation described. Significance of the Study Targeted transcriptomic analysis of the pre‐evocation SAM, to investigate the cohort of genes controlling floral evocation, can now be readily undertaken.