Variation in the Vernalization Response of a Geographically Diverse Collection of Timothy Genotypes

Timothy (Phleum pratense L.) has earlier been characterized as a long-day plant, which neither requires vernalization to induce flowering nor shows a vernalization response. Variation in flowering time of timothy has thus been ascribed to differences in critical photoperiods. We studied vernalization response in a geographically diverse collection of timothy and found that vernalization accelerates heading in all 38 genotypes included in this study. In addition, considerable variation in vernalization response, ranging from 711 to >1400 growing degree days, was observed between genotypes, and a vernalization requirement to induce flowering was indicated for five genotypes of Nordic origin. While heading time, irrespective of vernalization treatment, was found to be correlated to geographic origin of genotypes, a strong vernalization response was identified in genotypes of different geographic origin. Genotypes of non-Nordic origin with a strong vernalization response were either di- or tetraploid, whereas respective Nordic genotypes were hexaploid. The ploidy level in relation to vernalization response is discussed. This study clearly demonstrates the presence of considerable genetic variation for vernalization response within timothy, and future studies will elucidate the underlying causative genetic variation.