Flower preformation in the nival plant Ranunculus glacialis L.: shoot architecture and impact of the growing season length on floral morphogenesis and developmental dynamics

Flower preformation is a widespread phenomenon in perennial plants from temperate and cold regions. An advanced preformation status reduces the prefloration period and thus increases the chance to mature seeds in time. Despite the particular importance of this strategy for high-mountain plants, studies are rare. Here we investigated how the length of the growing season impacts floral development, and to what extent floral development is synchronized with reproductive phenophases in the arctic-alpine species Ranunculus glacialis L. The study was carried out in the alpine-nival ecotone in the European Central Alps at sites with different snowmelt dates. Individuals were sampled at regular intervals throughout the growing season, and shoot architecture and changes in floral structures were analysed in detail using different microscopic techniques. R. glacialis individuals consist of a cluster of independent ramets, comprising 3 sympodia each. Floral initiation terminates the vegetative growth of each sympodium 2–3 years before flowers become functional. A specific feature is that basal and distal leaves on a sympodium mature in different years. The date of snowmelt did not affect the speed of development but flower size and the number of lateral flowers within an inflorescence. Belowground floral preformation is closely linked to aboveground reproductive processes, however, continues below the snow in case winter conditions set in too early. The staggered preformation of architectural units creates a permanent belowground reserve pool of floral structures which might be advantageous in the climatically harsh and unpredictable high-mountain environment.