Plant growth traits and allergenic potential of Ambrosia artemisiifolia pollen as modified by temperature and NO2

Ongoing global warming favors the spread of thermo-tolerant plant species including plants with a high allergenic risk such as common ragweed (Ambrosia artemisiifolia). To assess effects of temperature increase on plant traits and biochemical processes potentially associated with allergenicity, we performed a phytotron chamber experiment which included two levels of temperature (control, elevated by 4 °C) simulated at 10 min resolution, and NO2 (0 ppb, 80 ppb added at a constant rate), respectively, and their combination in a full factorial design. The aim was to evaluate effects of temperature on various parameters related to ragweed allergenicity relative to effects of a known enhancer of allergenicity such as NO2, including the interaction between temperature and NO2. As plant growth parameters, we assessed plant height, male inflorescence size, start and end date of pollen release as well as average pollen release per inflorescence. As pollen traits, we determined pollen allergenicity by dot blot and biochemical factors that may enhance (oxidative stress indicators) or attenuate (flavonoids) pollen allergenicity. We tested the hypotheses that elevated temperatures do not increase the stress level of thermophilic ragweed and thus do not affect allergenicity, whereas NO2 increases allergenicity independent of temperature. Two-way ANOVA results revealed that elevated temperature significantly enhanced the plant height, the size of male inflorescences, and the duration of pollen release, in addition, significantly delayed pollen release independent of NO2. Pollen release per inflorescence was significantly induced by elevated NO2. As single factors, elevated temperature or NO2 affected allergenicity, moreover, the combined treatment of elevated temperature plus NO2 resulted in increased levels of NAD(P)H oxidase activity, H2O2 content, and highest allergenicity. Concentrations and composition of pollen flavonoids were significantly changed by NO2. Irrespective of treatments, pollen flavonoids were dominated by quercetins, but their overall composition was significantly changed by treatments. Flavonoids from lower temperature treatments showed significantly reduced allergen-IgE binding activities. A common trend observed was that elevated temperatures particularly in combination with NO2, increased the pollen allergenicity.