Testing unified theories for ozone response in C4 species

There is tremendous interspecific variability in O₃ sensitivity among C₃ species, but variation among C₄ species has been less clearly documented. It is also unclear whether stomatal conductance and leaf structure such as leaf mass per area (LMA) determine the variation in sensitivity to O₃ across species. In this study, we investigated leaf morphological, chemical, and photosynthetic responses of 22 genotypes of four C₄ bioenergy species (switchgrass, sorghum, maize, and miscanthus) to elevated O₃ in side‐by‐side field experiments using free‐air O₃ concentration enrichment (FACE). The C₄ species varied largely in leaf morphology, physiology, and nutrient composition. Elevated O₃ did not alter leaf morphology, nutrient content, stomatal conductance, chlorophyll fluorescence, and respiration in most genotypes but reduced net CO₂ assimilation in maize and photosynthetic capacity in sorghum and maize. Species with lower LMA and higher stomatal conductance tended to show greater losses in photosynthetic rate and capacity in elevated O₃ compared with species with higher LMA and lower stomatal conductance. Stomatal conductance was the strongest determinant of leaf photosynthetic rate and capacity. The response of both area‐ and mass‐based leaf photosynthetic rate and capacity to elevated O₃ were not affected by LMA directly but negatively influenced by LMA indirectly through stomatal conductance. These results demonstrate that there is significant variation in O₃ sensitivity among C₄ species with maize and sorghum showing greater sensitivity of photosynthesis to O₃ than switchgrass and miscanthus. Interspecific variation in O₃ sensitivity was determined by direct effects of stomatal conductance and indirect effects of LMA. This is the first study to provide a test of unifying theories explaining variation in O₃ sensitivity in C₄ bioenergy grasses. These findings advance understanding of O₃ tolerance in C₄ grasses and could aid in optimal placement of diverse C₄ bioenergy feedstock across a polluted landscape.