Nitrogen allocation and trade-off among growth and defense strategies of ornamental herbaceous plants exposed to increasing ozone concentrations

Nitrogen (N) allocation and trade-offs across primary and secondary metabolic pathways help to evaluate plant adaptive capacity to harsh environmental conditions. Tropospheric ozone (O3) is a major concern for ecosystem health, particularly in highly polluted cities like those of China. Here, Salvia splendens, Zinnia elegans and Helianthus annuus plants were exposed to ambient air (AA), moderate (AA+40 ppb, AA+40) and elevated (AA+80 ppb, AA+80) O3 concentrations to explore interspecific differences in N allocation and O3-response strategies. Photosynthetic rate (A) of S. splendens increased at 15 and 32 days from the beginning of the exposure (FBE; +24 % on average) under AA+ 40, whereas decreased in Z. elegans under both AA+ 40 and AA+ 80 (-24 %, on average, at 15 days FBE), and in H. annuus under AA+ 80 (-18 % at 32 days FBE). AA+ 80 reduced N based on leaf area (Narea) in all the three species. In addition, in S. splendens, AA+ 40 promoted N allocation to photosynthesis (Nphoto; +10 % at 32 days FBE) and decreased the N allocation to cell wall (NCW, -24 % throughout the entire exposure period), whereas in Z. elegans Nphoto was decreased at 15 and 48 days FBE (-38 %, on average) and no change was reported in NCW. Differently, both Nphoto and NCW decreased under AA+ 80 in all the three species. AA+40 maintained or improved the biomass production of S. splendens and Z. elegans, whereas H. annuus growth was inhibited. Overall, S. splendens and Z. elegans better adapted to AA+ 40 compared to H. annuus, which was attributed to increased contributions of Nphoto and N to chemical defense, highlighting species-specific strategies in adapting to O3-stress conditions.