Studies on Mentha arvensis L.

Abstract

Metntha arvensis L. grows in the temperate regions of the northern hemisphere such as Europe, North America, and Asia (including Japan), and is abundant in variation. Collecting 29 strains belonging to 3 subspecies of M. arvensis from their original habitats and artificially raising 5 F1's belonging to 2 intervarietal hybrids, the authors carried out their cytogenetical investigations as part of their fundamental studies of mint breeding. The results were as follows : ( 1 ) M. arvensis var. canadensis is distributed in North America and M. arvensis var. piperascens is distributed in East Asia. Each has 96 chromosomes in the somatic cells. M. arvensis var. agrestis and var. praecox which are distributed in Europe have 72 chromosomes in the somatic cells. The former 2 varieties are octoploid and the latter 2 varieties are hexaploid, considering the basic number of chromosomes of the genus Mentha to be 12. ( 2 ) There seem to be no structural variations of chromosomes between var. canadensis and var. piperascens, because F1 plants have nearly as high a fertility as that of the parental plants and are complete in the conjugation of chromosomes in meiosis and no univalent chromosomes can be seen. ( 3 ) Artificially raised F1 hybrids of var. canadensis and var. agrestis have 84 chromosomes in the somatic cells, and 36 bivalent chromosomes and 12 univalents appear at MI in meiosis. The behavior of chromosomes in the meiosis of the F1 hybrids mentioned above is just the same as that in F1 plants from var. piperasc, eles and var. agrestis which the authors raised and about which they have already published. So it is clear that var. canadensis and also var. piperasceles must have appeared naturally from hexaploid varieties with 72 chromosomes by the addition of two sets of twelve chromosomes. ( 4 ) A variety of M. arvelesis which was collected in the vicinity of Rock Lake in the U.S.A. have 84 chromosomes in the root-tip cells, and 36 bivalents and 12 univalents at MI in meiosis just as the artiflcially raised F1 hybrids mentioned above. This variety can be assumed to be a natural heptaploid hybrid between the octoploid variety from the U.S.A. and a hexaploid variety similar to the European mint. A strain of M. arvensis var. agrestis [152-9] is male sterile and its meiotic cell divisions of PMC's cannot be observed. The cause of this abnormality was sought histologically and it was found that the degeneration of archesporial tissue occurred about the time when pollen mother cells differentiated and before their meiotic cell divisions began. A similar phenomenon was observed in four other strains of our collection, so it seems to be a common occurrence in this species of mint.