We investigated the effect of inorganic salt concentration on the in vitro organogenesis of spearmint (Mentha spicata L.) by using Murashige and Skoog culture medium of different strength (full, half-and quarter strength). The highest number of shoots and roots induced per explant (3.5 and 10, respectively), as well as the maximum average shoot length (16 cm) was observed on half strength medium. The maximum leaf number (35 per explant) and average root length (7 cm) were associated with regeneration on full medium. The highest average phenolic content was observed on shoot extracts (7.20 mg/g f.w.) and root extracts (5.93 mg/g f.w.) cultured onto quarter and half strength medium, respectively. The strength of the culture medium was inversely correlated with the antioxidant activity of plant extracts. Therefore, for spearmint organogenesis, half strength MS medium offers a compromise between optimum growth in vitro and antioxidant phenolic accumulation.

One hundred spearmint (Mentha spicata) plantlets were regenerated from apical shoot segments of ten fieldgrown donor plants. Although the accumulation of rosmarinic acid and total phenolics in vitro was almost half than in vivo, regenerants demonstrated a tenfold-higher hydrogen peroxide compared to the donor plants. This finding may have been associated with the increased activity of hydroxyphenylpyruvate reductase, a key enzyme of the phenolic biosynthetic pathway and the increased production of yet unidentified phenolic compounds in vitro. This process of in vitro culture associated with a reduction of rosmarinic acid and total phenolics and with an increase of the antioxidant capacity indicated the possible promotion of in vitro-specific biosynthetic pathways.

In vitro micropropagation of spearmint plants (Mentha spicata Linn.