Differential effects of enhanced UV-B radiation on reproductive effort in two chemotypes of Mentha spicata under field conditions

Abstract

Seedlings of two chemotypes of mint (Mentha spicata L.), i.e. the piperitone and piperitenone oxide rich wild mint (chemotype I) and the carvone and dihydrocarvone rich spearmint (chemotype II), were grown to maturity in the field under ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion over Patras (38.3°N, 29.1°E). Enhanced UV-B radiation had no effect on total stem length, photochemical efficiency of PS II, chlorophyll content, UV-B absorbing capacity of epicuticular and internal compounds, photosynthetic capacity at 5% CO₂ and relative water content of both chemotypes. However, specific leaf mass in chemotype II, and leaf number in chemotype I were significantly increased under UV-B supplementation, while total leaf area remained unaffected. Dry mass accumulation measured at plant harvest showed a slight, non-significant trend for increased above- and below-ground biomass in both chemotypes, but a particularly significant increase in the biomass allocated to inflorescences in chemotype II was found. Seed yield was significantly improved by supplemental UV-B radiation in both chemotypes, but, again, chemotype II was particularly responsive, increasing the number of seeds by 100%. We conclude that the aromatic species M. spicata is not only resistant to enhanced UV-B radiation, but some chemotypes may be selectively benefited through increased reproductive effort.