The effects of different light–temperature conditions on leaf growth and essential oil yield and oil composition of Mentha×piperita L, M spicata L, M longifolia L, M rubra L and Chamomilla recutita (L) Rausch were studied in a biotron experiment. A majority of the Mentha L species exposed to a 21–3 h photoperiod, simulating conditions typical of the July environment in northern Sweden, produced significantly higher concentrations of menthol than treatments with shorter photoperiods. Night temperatures had little effects on the menthol levels under the 21 h photoperiod regime. The total yields of menthol and menthone in the Mentha L species were significantly higher in the third to fifth leaf pairs compared to the topmost leaf pair position. The different environmental treatments had no evident effects on leaf growth (measurements based on the five topmost leaf pairs). The combination of 21–3 h photoperiod and 25–18°C thermoperiod resulted in the highest levels of α-bisabolol in C recutita (L) Rausch. Also, bud or flower formation was only induced in treatments with photoperiods ⩾17 h.
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Fahlén et al. (1997) studied this question.