Selected high-phenolic lines of spearmint were subjected to a constant 30 • C heat regimen for a period of 4 weeks to determine the effects of heat stress on soluble phenolics, phenols and rosmarinic acid biosynthesis and antioxidant capacity. Heat stress significantly reduced levels of total phenolic acids (71-87%) and soluble phenols (75-87%). This loss was concomitant with a loss of total antioxidant capacity of 21-60% after week 1 and up to 95% by week 4. High-performance liquid chromatography profiling of heat-stressed plants at 270 and 320 nm detected nearly a complete loss of rosmarinic acid in all seven chemotypes. High-temperature drying of non-heat-stressed plants at 80 • C resulted in a similar loss of total antioxidant capacity and rosmarinic acid content an effect not observed in material that was subjected to low-temperature drying first, followed by exposure to high temperature. This suggests that heat stress negatively regulates rosmarinic acid biosynthesis and causes a potential rapid biological breakdown of rosmarinic acid in tissues. 2,2-Diphenyl-1-picrylhydrazyl radical assays of heat-stressed and non-stressed plants clearly show that rosmarinic acid is the major contributor to the antioxidant capacity in spearmint.

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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.

The effect of a non-indole compound, phenylacetic acid (PAA), on the induction of somatic embryogenesis in tissue cultures of geranium (Pelargonium x hortorum Bailey cv. Scarlet Orbit Improved) was investigated. Hypocotyl explants derived from young, dark-grown seedlings were cultured on Murashige and Skoog (1962) medium (MS) supplemented with PAA or IAA (0.01-120 μM) alone or in combination with BAP (8 μM). Somatic embryogenesis was induced by both PAA and IAA at 0.01-20 μM with 8 μM BAP, however, the optima differed considerably for the two compounds. Maximal activity of IAA for somatic embryogenesis was found at 0.1-2.5 μM, whereas PAA gave best results at 10 and 20 μM under identical culture conditions. Higher concentrations (30-120 μM) of IAA or PAA in the medium induced callusing in the explants, but the callus was neither embryogenic nor morphogenic.

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