Relevance of chromosome number variation to yield and quality of essential oil in Mentha arvensis L.

Abstract

Six morphologically distinct genotypes of Mentha arvensis L. consisting of four clones introduced from Japan (CIMAP/MA-2), Thailand (CIMAP/MA-3), Taiwan (CIMAP/MA-4) and Brazil (CIMAP/MA-5) and two bud-sports (CIMAP/MAS-1 and CIMAP/MAS-2) of a Thailand clone (CIMAP/MA-3) were studied under uniform environmental conditions for somatic chromosome numbers, yielding ability and essential oil quality. The somatic chromosome number in these genotypes ranged from 2n=64 to 108. Three clones viz. CIMAP/MA-2 (2n=84), CIMAP/MA-5 (2n=72) and CIMAP/MAS-2 (2n=108) possessed chromosome numbers multiple of the base number (x=12) while the remaining three clones (CIMAP/MA-3, CIMAP/MA-4 and CIMAP/MAS-1) contained aneuploid numbers such as 2n=98, 64 and 90, respectively. The results on chromosome number variation in the two bud-sports-CIMAP/MAS-1 (2n=90) and CIMAP/MAS-2 (2n=108) suggested that these two cytotypes might have originated by addition or reduction of chromosomes due to irregular mitosis in the bud meristem of the parental clone-CIMAP/MA-3 with 2n=98. It is likely that a similar mechanism for reduction of chromosome number from 2n=96 to 2n=84 might have operated in CIMAP/MA-2. The relevance of a large variation in chromosome number brought about by introgression and sporting and its subsequent fixation through vegetative propagation in evolving the observed cytotypes in M. arvensis is discussed. The variation among the six genotypes for various characters like plant height, leaf/stem ratio, fresh herbage yield, oil content, oil yield, oil composition and congealing point of oil indicated a genotypic control for these traits. CIMAP/MAS-1 and CIMAP/MAS-2 with significantly high oil content, oil yeild and exceptionally greater amount of menthol in the oil proved significantly superior over rest of the genotypes. The absence of menthyl acetate in the oil of CIMAP/MAS-1 and CIMAP/MAS-2 quite contrary to the remaining four clones is interpreted in the sence that these two genotypes might be in possession of homozygous recessive gene ‘e’ responsible for completely blocking the conversion of menthol to menthyl acetate.