Ozone exposure alters nutrients and stoichiometric ratios in different organs of four urban tree species despite limited negative effects on leaf physiology and plant growth and biomass

Abstract To better understand the effects of ground-level ozone (O 3 ) on nutrients and stoichiometry in different plant organs, urban tree species Celtis sinensis , Cyclocarya paliurus , Quercus acutissima , and Quercus nuttallii were subjected to a constant exposure to charcoal-filtered air (CF), nonfiltered air (NF), or NF + 40, 60, or 80 nmol O 3 mol –1 (NF40, NF60, and NF80) starting early in the summer of the growing season. At the end of summer, net CO 2 assimilation rate ( A ), stomatal conductance ( g s ), leaf mass per area (LMA), and/or leaf greenness (SPAD) either were not significantly affected by elevated O 3 or were even higher in some cases during the summer compared with the CF or NF controls. LMA was significantly lower in autumn only after the highest O 3 exposures. Compared to NF, NF40 caused a large increase in g s across species in late summer and more K and Mn in stems. At the end of the growing season, nutrient status and stoichiometric ratios in different organs were variously altered under O 3 stress; many changes were large and often species-specific. Across O 3 treatments, LMA was primarily associated with C and Mg levels in leaves and Ca levels in leaves and stems. NF40 enriched K, P, Fe, and Mn in stems, relative to NF, and NF60 enhanced Ca in leaves relative to CF and NF40. Moreover, NF resulted in a higher Ca/Mg ratio in leaves of Q. acutissima only, relative to the other O 3 regimes. Interestingly, across species, O 3 stress led to different nutrient modifications in different organs (stems + branches vs leaves). Thus, ambient and/or elevated O 3 exposures can alter the dynamics and distribution of nutrients and disrupt stoichiometry in different organs in a species-specific manner. Changes in stoichiometry reflect an important defense mechanism in plants under O 3 , and O 3 pollution adds more risk to ecological stoichiometries in urban areas.