Long-day increase of HvVRN2 expression marks the deadline to fulfill the vernalization requirement in winter barley

Vernalization and photoperiod cues are integrated in winter barley plants to flower in the right conditions. We hypothesize that there is a timeframe to satisfy the vernalization needs in order to flower in the optimum moment. Growth and expression of different flowering promoters ( HvVRN1 , HvCO2 , Ppd-H1 , HvFT1 , HvFT3 ) and repressors ( HvVRN2 , HvCO9 and HvOS2 ) were evaluated in two winter barley varieties under: (1) natural increasing photoperiod, without vernalization, and (2) under short day conditions in three insufficient vernalization treatments. Here, we provide evidence of the existence of a day-length threshold, around 12 h 30 min in our latitudes (Zaragoza, Spain, 41°43′N), marked by the rise of HvVRN2 expression, which defines the moment in which cold requirement must be satisfied to acquire competency to flower. Before that, expression of HvCO2 was induced and might be promoting HvFT1 in both inductive and non-inductive conditions. HvFT3 , to be effectively expressed, must receive induction of cold or plant development, through downregulation of HvVRN2 and HvOS2 . We emphasize the contribution of HvOS2 , together with HvVRN2 , in the delay of flowering in vernalization-responsive cultivars. Understanding this complex mechanism of flowering might be useful for breeders to define varieties, particularly in a climate change scenario.