The response of marigold (Tagetes erecta Linn.) to ozone: impacts on plant growth and leaf physiology.

Progressively increasing ozone (O3) concentrations pose a potential threat to the value of marigold (Tagetes erecta Linn.), a plant widely used in urban landscaping. The response of marigold to elevated O3 has been reported earlier, but the mechanisms underlying the O3 effect have not been clearly elucidated. In the present study, we exposed marigold "Moonsong Deep Orange" plants to elevated O3, including ambient non-filtered air (NF) plus 60 ppb (NF+60) and 120 ppb (NF+120) O3, to assess visible injury and the possible physiological consequences of this pollutant. Yellow lesions appeared after 4 days under NF+120 treatment and 12 days under NF+60 treatment, with 85.6% and 36.8% of the leaves being injured at harvest time, respectively. Compared with NF, NF+60 inhibited leaf photosynthesis, stem-diameter growth, and biomass production significantly, while the parameters were decreased more by NF+120. Although the stomatal conductance decreased under elevated O3 exposure, the O3 flux into leaves increased by 28.0-104.8% under NF+60 treatment and 57.5-145.6% under NF+120 treatment. The total ascorbic acid (ASA) content increased due to elevated O3 exposure, while the reduced ASA content did not, resulting in a decreased ratio of reduced to total ASA. A lower level of jasmonic acid (JA) was observed under elevated O3 exposure. In conclusion, the impacts of elevated O3 on marigold plants may be ascribed to increased O3 flux into leaves and reduced protective capacity of leaves to convert oxidized to reduced ASA and synthesize endogenous JA.