Chromosome number and ploidy level of balm (Melissa officinalis)

BACKGROUND: Lemon balm (Melissa officinalis L.) is of increasing importance resulting in rising growth area. Improved knowledge on the genome structure, number of chromosomes in connection with the taxonomical structure of balm is indispensable for improved new varieties. RESULTS: A collection of 40 balm accessions (M. officinalis) was characterized by flow cytometry and FISH (18/25S and 5S rDNA) to determine the chromosome number and ploidy level. Three different types were found: diploid genotypes with 2n = 2× = 32 chromosomes; tetraploid 2n = 4× = 64 chromosomes and triploid 2n = 3× = 48 chromosomes. A haploid base number of × = 16 chromosomes is likely. First time described triploid accessions are sterile but cytologically and morphologically stable for many years. Triploids express better winter hardiness and regeneration after harvesting cuts as well as bigger leaves and internodes. CONCLUSIONS: A basic chromosome number of x = 16 is reported for the first time for the species M. officinalis.