The Effects of Solar Radiation on Plant Growth, Oil Yield and Oil Quality of Japanese Mint

Abstract

Plants of Japanese mint (<it>Mentha arvensis L. subsp. haplocalyx</it> Briquet var. <it>piperascens</it> Holmes) were grown under 100, 64, 49 and 28% conditions of prevailing radiation for 10 weeks, with harvests at 4, 6.8 and 10 weeks. Measurements were made on plant growth, yields of essential oil and chemical composition of oil. The greatest morphological responses to increased shading intensity were increases in stem length and leaf area. Little response to shading was shown by mean relative growth rate (RGR) or mean net assimilation rate (NAR) while that of mean leaf area ratio (LAR) was marked. No significant differences in oil yield were found among different treatments at the final harvest nor were there great differences in amounts of menthol and menthone, two important constituents of Japanese mint oil. The experiments indicate that, within the experimental limits imposed, Japanese mint tends to compensate in growth and oil production for shading effects. This work may also explain the different and sometimes contradictory results reported in the literature on the effects of shading on mint oil production