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Four spearmint, and two peppermint clonal lines, selected for enhanced rosmarinic acid content (50-120 mg g-1 rosmarinic acid DW), where up to 80% of the antioxidant activity was correlated to rosmarinic acid content, were examined to determine the effects of environmental and physiological conditions on the accumulation of rosmarinic acid in leaf tissues. Exposure to a short photoperiod of 12 hours in comparison to 16 hours reduced rosmarinic acid accumulation in two mint lines, but no significant difference was found between photoperiods of 14 and 16 hours. The physiological age of the plant strongly influenced the accumulation of rosmarinic acid with the highest levels recorded in the vegetative state, and a significant reduction in the concentration of rosmarinic acid in the leaves in both the bud initiation and flowering stages in the mint lines. Cold stress, impacted over a six week period had no effect on rosmarinic acid production. A field study of the commercial chemotype 700B indicated that soil type plays an essential role in the accumulation of rosmarinic acid in the leaf tissue, probably due to retention of moisture which favours rosmarinic acid production. For producers and extractors, taking these factors into account would significantly increase rosmarinic acid accumulation in commercially high rosmarinic acid mint and increase the quality control of plant extracts for the natural products industry.