Ozone effects on photosynthesis of ornamental species suitable for urban green spaces of China

Urban green spaces (UGS) offer several ecosystemical benefits to the urban environment. However, these advantages may be weakened by alterations of plants photophysiological mechanisms due to increasing tropospheric ozone (O3) concentrations, a serious problem for China. To evaluate their utilization in UGS, we selected three widely-used urban plant species (smoke tree, Cotinus coggygria Scop.; marigold, Tagetes erecta Linn.; rose, Rosa chinensis Jacp.) to investigate their biometric and photophysiological responses to (i) ambient air (AA), (ii) AA + 60 ppb O3 (AA + 60), and (iii) AA + 120 ppb O3 (AA + 120) (9 h d−1, from 8:30 am to 5:30 pm). Considering visible injury and biomass production, smoke tree and marigold seem to be O3-sensitive, whereas rose should be considered more tolerant. The exposure to the pollutant gas reduced photosynthetic efficiency in all seedlings. However, different features were shown throughout our study by the three species here monitored. In smoke tree, stomatal limitations seemed to be its principal weakness. In marigold, the reduction of the photosynthetic performance was mainly ascribable to impairments of both light and dark reactions of photosynthesis. Here, stomatal closure maybe not the cause to limit the photosynthetic rate, but a down-regulated response. Unexpectedly, CO2 assimilation increased in roses exposed to AA + 60 and did not change in those treated with AA + 120 after one month from the beginning of the exposure (FBE). This seemed due to a better efficacy of these plants in dark reactions of photosynthesis. This feature was confirmed also a month later. In conclusion, the results of this study indicate that non-invasive methods such as gas exchange and chlorophyll fluorescence for monitoring photosynthetic performance of urban plants can be useful to give guidelines to manage UGS in the “climate change era”. Generally, in severe O3-polluted areas as those of several cities of China, the plants with high-efficient biochemical processes driving a well photosynthetic performance (such as rose) might be a recommended choice.