Testing unified theories for ozone response in C 4 species.

There is tremendous interspecific variability in O 3  sensitivity among C 3  species, but variation among C 4  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 O 3 across species. In this study, we investigated leaf morphological, chemical, and photosynthetic responses of 22 genotypes of four C 4 bioenergy species (switchgrass, sorghum, maize, and miscanthus) to elevated O 3 in side-by-side field experiments using free-air O 3 concentration enrichment (FACE). The C 4  species varied largely in leaf morphology, physiology, and nutrient composition. Elevated O 3 did not alter leaf morphology, nutrient content, stomatal conductance, chlorophyll fluorescence, and respiration in most genotypes but reduced net CO 2 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 O 3 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 O 3 were not affected by LMA directly but negatively influenced by LMA indirectly through stomatal conductance. These results demonstrate that there is significant variation in O 3  sensitivity among C 4  species with maize and sorghum showing greater sensitivity of photosynthesis to O 3 than switchgrass and miscanthus. Interspecific variation in O 3  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 O 3  sensitivity in C 4 bioenergy grasses. These findings advance understanding of O 3 tolerance in C 4  grasses and could aid in optimal placement of diverse C 4 bioenergy feedstock across a polluted landscape.